Practice English Speaking&Listening with: Cross-Compiling Android Applications to the iPhone

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LEA: Today, we have Professor Arno Puder from San Francisco State University. Professor

Puder--prior to his current position at SFSU, he worked for AT&T Labs Research and German

Telecom. His research interests include middleware, ubiquitous computing, and applications for

sensor networks. Today, he is going to talk about Cross-Compiling Android Applications

to the iPhone. And I'll give it to Professor Puder, then.

>> PUDER: Okay, thank you, Lea, for the introduction. And, well, thank you for inviting me to talk

about my research. Smartphones had become very popular and particular are those two

that I've mentioned on my slide here, on the title slide; Android and iPhone. That certainly

created quite some buzz. And as more and more of the smartphone are released, of course,

the problem also increases in terms of deploying your application on different smartphones.

So the technology I'm going to talk about today is helping you porting applications

from one smartphone to another. And in particular, I will be talking today about, well, as the

title suggests; Cross-Compiling Androids Applications to the iPhone. Now before I go into any technical

details, I wanted to just briefly introduce the team. Now of course with any open source

project and XMLVM is an open source project, you stand on the shoulder of giants. So XMLVM

as an open source project, it's making use of over half a dozen other open source projects.

But it almost boils down to the people. So within our project, there are a number of

people who have made significant contributions to XMLVM. So the core team besides myself;

Sascha Haeberling who is actually another Googler, he works out of the Zurich office;

and Wolfgang Korn, who works for a consulting company in Germany, blueCarat. And blueCarat

is also a corporate sponsor of our project. Now, as a growing open source project, we

also have now a growing number of people who make contributions to XMLVM. And at least

as of today, I list those names that I've--of people who have done some working on our project.

Well, a bit of background. I guess it goes without saying that smartphones certainly

are getting very, very interesting. If you look at the growth rates, so they have double-digit

growth rates, and that I'm sure is going to persist for quite sometime to come. Now if

you are an application developer, then you are faced with the problem of rewriting your

application for the various platforms. And that can consume a lot of time and resources.

Now, while the programming environment is a little bit different, the smartphones do

have similar capabilities when it comes to their hardware features. So, again, the problem

here is that that you as a developer are faced with this problem of deploying your application

cross-platform. And on the next slide here, I've put down some, well, a kind of a little

snapshot here. So on the first column, I have the HTC G1. I have, on the second column,

the iPhone 3GS. And the third column, I have the Palm Pre. Now, Android is releasing more

and more devices and Google has just recently released its Nexus One. So, of course, even

with an Android, there is now a multitude of different devices out there. But, again,

like for the slide here, I guess, the important point is if you look at the upper part of

that slide, I gave you a little background on the hardware details on the various devices.

And you can see, there are almost, at least, very similar. And if I keep on adding more

and more devices here, they probably will be the same in terms of speed, in terms of

CPU, in terms of RAM. But then if you look on the bottom half of the slides, you will

see that they're actually quite different when it comes to how we have to develop an

application for them. So, I mean, you as a developer, of course, you look at the smartphone

from a SDK perspective. So how do you actually write an application for the phone? And if

you look at Android--well, Android is using Java as the language to write an Android application.

And the GUI is based on specific Android API that you make use of writing an Android application.

You have virtual machine technology that's allowed on it's--on their phone and everything

is open source. Well, I mean, there's a little bit of a caveat of course. There are some

proprietary parts that Google does not turn into open source. But at least for the Base

SDK, everything is open source. Now if you look at the iPhone, that's completely radically

different. So we have a language called Objective-C. And I will say a few more things about Objective-C

on the following slides. Furthermore, you have a proprietary API called Cocoa Touch

providing iPhone Applications. And of course the complete tool chain is proprietary. Actually,

you need even a Mac platform in order to write an iPhone application. Now I also want to

say a few things about the Palm Pre. Actually, it's a device I like very much. I think it's

unfortunately a little bit underrated. It should be given a bit more attention if you

ask me. But if you look at how to write Palm Pre applications, they actually have decided

to make use of WAP technology. So if you want to write a native Palm Pre application, you

end up writing HTML, CSS, and JavaScript. Almost as if you are writing a web application.

So again, like the point of that slide is here that while the phones are somewhat similar

on the hardware side, they are radically different from the way you actually write applications

for these phones. So the propose solution that I'm going to go into some detail here

in this presentation is that we say, "Well, as kind of a frame of reference, we take Android."

So we expect people to use Android and we do that for various reasons. Well first of

all, I mean, I'm in education business. And there are certainly a lot more skill set out

there for Java than there is, for example, for Objective-C. So if you look at how much

innovation you can achieve, certainly, I would argue that basing on Java is certainly a smarter

choice than Objective-C. So I'm not looking at the current number of applications you

have currently in the App Store, I'm looking overall at the skill set that they have out

in the industry. The second thing is that Android was built from day one or it was designed

from day one to serve heterogeneous set of devices. So in particular, Android copes very

well with different screen resolutions. And the iPhone, for example, is only doing that

now. Apple has recently announced the iPad and only now Apple has faced with the problem

of having two different devices; the iPhone and the iPad that have different screen resolutions.

So only now actually they are beefing up there, enhancing their SDK to also support different

screen resolutions. So from that perspective, it makes a lot of sense to take Android as

the frame of reference. And then what we do in our tool chain, that kind of--is also the

outline for the rest of this presentation. So the first step that we do is--oh, that

we did rather--is to design a Java API for Cocoa Touch. Remember that Cocoa Touch is

based on Objective-C. So the first time will be to create a Java abstraction on Cocoa Touch.

The next step is going to be to cross-compile Java to Objective-C. Since we're starting

from Androids then we want to go over to Objective-C, we have to cross-compile that somehow. And

maybe I should also stress this point because the iPhone or Apple rather prohibits by licensed

agreement the use of virtual machine technology on the iPhone. So you cannot run a virtual

machine on the iPhone. And--but by cross-compiling, we actually end up with native--a native application

that does not require a virtual machine. So our toolset is within the legal framework

of the Apple SDK. Now, the last step that we have to do is to deal with the API mapping.

So if you write an Android application that has different kind of API than the iPhone

obviously, and the last step that I will be talking about today is how we deal with that

through what we call the Android Compatibility Library. So as a programmer, as a geek, the

first thing you do in any program language is "Hello World". So what does "Hello World"

look like for the iPhone? Well, as I mentioned before, the iPhone is based on Objective-C.

So if you want to write a native application for the iPhone, you have to do that in Objective-C.

Now, I tell my student sometimes that Objective-C is like the Latin--are like the language Latin,

you know. There's exactly one country in this world where Latin is being spoken, that is

the Vatican, of course. So Objective-C is used inside of Apple, but not used much outside

of Apple. And it is a dynamically-typed language. It's a strict superset of the C programming

language, so every legal C programmer is also a legal Objective-C programmer. And what you

see here is, again, is the "Hello World" program, and if it's at any kind of GUI programming

then probably you will make sense of it rather quickly. One thing you often notice or the

first thing you often notice when you look at your first Objective-C program is that

you see all these square brackets. And these square brackets denote method invocations.

So whenever you see a square bracket, the first argument is actually then the target,

which can either be a class object or an instance and anyhow, the list of named parameters that

you are sending to that object. So for example, if you look at the first thing here, "[UIScreen

mainScreen]", basically, "UIScreen" is a class of the Cocoa framework, Cocoa Touch framework,

and you ask it to give back the "mainScreen", so what you get back is a pointer to this

object. And you can actually see here C shimmering through a little bit because you still have

these pointers that you have to deal with in Objective-C. Now, if you look at to the

rest of this program, I think you can probably make a sense rather quickly if you done, for

example, some Swing programming. So if you look at class "UIWindow" that is being used

here a little bit down. That kind of corresponds to a "JFrame" if you have--in the Swing programming.

"UIView", it might correspond or it might take an analogy to "JPanel". "UILabel", then

again, correspond to something like a "JLabel" in Swing. So with that, I think you can probably

understand what is happening here. Just maybe a few side notes here. First of all, the entry

point of an iPhone application is a method that you have to overload that is called "applicationDidFinishLaunching".

So that is--I didn't come up with that name. That is the official API as defined by the

iPhone SDK. So that is going to be the first thing that's going to be called of your iPhone

application. Of course, you have to derive your application class from base class of

"UIApplication" in order to overwrite this method. But that will basically then kickoff

your program. That's the first thing I want to point out. The second thing I want to point

out--I already mentioned that Objective-C is a superset of C. So that's why you have

these pointers. I want to point out the second line here of my implementation. And I'm not

sure if you can see the mouse here. So basically, I asked the screen object for its resolution

and I get back something of type "CGRect". Now, you will notice that there is no pointer

here. Actually "CGRect" is a simple struct. And unlike C++, struct is not a class. A struct

is just simply a value type. So what is done--what is happening here, actually you are being

given back a struct that is copied by a deep copy mechanism here. I want to point that

out because Java, as you know, does not have value types. So when we do API mapping, we

have to also kind of breach this--well, you might call it a problem when we have value

types in Objective-C, we have to represent it somehow on the Java side. So this here

is now the Objective-C version of "Hello World" for the iPhone. So the first thing that we

have done in our framework is to come up with a Java API. So on this slide now, what you

see is a Java program that is basically using our API mapping for Cocoa Touch, but it's--well,

first of all, 100% Java and--but it has the same semantics. So with all I've said on the

previous slide, I guess, you can immediately understand what is happening here. So we've

cut the naming convention. So you see? We have to derive your application class from

base class called "UIApplication". You have to overwrite a method called "applicationDidFinishLaunching",

and then also for the other classes; "UIScreen", "UIWindow". We just have taken the same names,

of course, as we have found them on the Objective-C site. So we are trying to mimic the API from

Cocoa Touch that we see on the Objective-C side as closely as possible on the Java side

to make it more easier for a programmer to make that transition. So ideally, you can

even go to the Cocoa Touch API documentation and then kind of understand what these classes

are doing. But of course, we have taken some design decisions here in terms of staying

closer to the nature of Java. So we have made use of strongly-typed interfaces for one thing,

so wherever we see something that never get typed on the Objective-C side, we'll make

use of Java interfaces. And also like, for example, your constructors. We are using constructors

a little bit more of the way they are supposed to in Java. And it's also a little bit different

the way it's done in Objective-C. So if you take a look at this slide here now, so if

you take this Java source code here, then basically what happens in our tool chain is

that we cross-compile this Java source codes over to Objective-C source code. And how we

do that, I will explain on the next few slides. But once we have the cross-compile to Objective-C

source code, we link it against a little library that we call the Objective-C Wrapper Classes.

And then what we end up with is a native executable on the iPhone. So there's no need for a virtual

machine. So you can deploy that on your iPhone and, again, since we are within the legal

frameworks of the Apple SDK, we can also push it on the App Store, of course. Now, if you

remember, the--even a little "Hello World" application I had on the previous slide was

making use of a class called "UIScreen", "UIWindow", "UILabel". Now, what it can do is--and that

is what--I have no--on the slide here--what it can do is it can actually provide a Java-only

implementation of these classes. So if you have a class like "UIWindow", instead of just

having a skeleton, why not just implement it also based, for example, on Java 2D. What

you end up with is a 100% pure Java version of your iPhone application that you can run

on any platform. So any platform that supports Java, you can now run. Basically, it's out

own Java-based iPhone emulator that allows you to run your Java-based iPhone applications.

And I want to switch over for the first demo to explain how this works. And I'm in Eclipse

here. And I'm going to launch now a little application that is kind of our trademark

application called Fireworks. And it does--not a whole lot. It just switches into full screen

mode and then it does the little fireworks. And I have my iPod touch here, so if I call

fireworks here on this device then, of course, you see the same thing. Now, there's one little

gimmick to the application, when we turn it around, we use the accelerometer so you will

notice on the device here, if you can see that the sparks always follow gravity. So

no matter how I turn the device, the stars will always go to the bottom. So--and we have

mimic this behavior in our own Java emulator here by the scroll bars. So if I modify these

scroll bars here by feeding different values into the accelerometer, then you can see that

the stars start flying into different directions. Now, again, I want to stress what the application

you see here that--the thing is that what is called the iPhone simulator. It's a 100%

pure Java application. You can run that on the Windows, in the Linux. And the application

that you have seen on my iPod touch is just the cross-compiled version of the application.

Of course on the device, the classes are linked against the Cocoa Touch classes so to yield

the behavior that you've seen. There's a little bit more that's kind of a fun thing. So here

actually I have on my iPod touch, a little remote control application. And what I can

do is I have created a talk network between my iPod touch and my laptop here. So what

I'm going to do now is I'm going to punch in the IP address of my laptop. I hope I get

this right here. So what happens now is my iPod touch acts as a remote control for my

Java-based emulator. And what happens is when I turn my iPod touch, you will see that--well,

you can see that the scroll bars kind of turn by magic. So basically the readings from my

iPod touch are being sent via HTTP request to our Java-based emulator. So when I turn

the device, my iPod touch, it kind of--if I turn it upside down, my device and the stars

in my Java emulator start flying up. Now, this application that I've used here on my

iPod touch, we've actually implemented using our own technology. So what I can do now is

I can go back to Eclipse and I can just simply run the second instance of our Java emulator.

And what you see now here is actually the interface I've just seen on my iPod touch.

So I still have the fireworks running here in parallel, but what I can do now is since

I'm running both emulators on the same laptop, I can just simply type in the loop IP address.

I can turn this on. And if I now use the controls on the one emulator, you can see that it reacts

over on the second emulator. So if, for example, I go up here with X, it starts going left

and right. So we can see that actually for our Java-based emulator, we have already pushed

it to some extent here, so all that you see here, again, on the second emulator is just

a Java implementation. So we have mimicked a little bit the look and feel of the iPhone

by providing a Java 2D implementation of the Cocoa GUI elements. So let me go back to my

presentation. And--so let me talk about some of the challenges that we're facing. Before

I actually tell you how we do certain things, let me tell you, first of all, the problems

that we have in trying to do so. Well, first of all as I've mentioned before, as a design

goal, we try to mimic the Java API as closely 1:1 as we see from the Objective-C API, but

we make use of strongly-typed interfaces so just you stay in the spirit of the Java programming

language. Now when it comes to challenges, there are challenges related to the language

cross compilation, there are challenges related to the API. And let me first talk about the

Objective-C challenges. Well, first of all, Objective-C has no name spaces. Objective-C

does not permit method overloading. So they allow method overwriting but not method overloading.

So a Java supports both. So when you cross-compile, you have to deal with that somehow. The version

of Objective-C that's being used on the iPhone does not have a garbage collector. So if you

write an application, you actually have to make use of a reference counting mechanism

that is given to you by the iPhone framework. But it's your responsibility as a developer

to actually correctly retain/release object references. So since we don't have a garbage

collector on the iPhone, what we do is, actually doing our cross-compilation, we insert these

retain and releases during the cross-compilation phase. Now of course, if you're familiar with

garbage collection, you will know that if you do reference counting--reference counting,

although, it's more efficient than garbage collection but it does have some limitations.

So, for example, if you have a cyclic data structure that cannot be reclaimed by reference

counting, so that is one of the limitations that we have right now on our tool chain.

So if you have a cycle in your data structures that you build up, then you need to break

that cycle manually by assigning another point somewhere. Once you do that, of course, then

we can reclaim memory. But if you just have a cycle in your data structures, at this point,

we cannot garbage collect that. Now, the challenges we have on the API level, well, they're quite

a few. Well, first of all, the Cocoa API is not as cleanly designed as you might think.

So for one thing, it's not always object orientation that they have used in designing the API.

So you actually will see quite a few C functions there for doing certain things. So, for example,

I've--there's one here, "CGColorCreate", it creates a color object but that actually is

a C function. So you have to provide it the RGB values, and then what you get back is

something that it can use as color value. But this function here is actually a function

in the C programming language sense. So it's not--like I said, a method of a class, as

you might--would expect in a clean object-oriented design but it's just a C function. There's

a question here? >> What is that--particularly the worse?

>> PUDER: I wouldn't say it's worse, it's just--I mean, it comes from historic background,

so of course, the Quartz framework has been there for quite sometime and...

>> I mean, does that--they produce some sort of challenge to the cross-compilation [INDISTINCT]?

>> PUDER: Okay. The question is does that creates challenge in cross-compilation? Well,

it does because, remember we cross-compile from Java to Objective-C, so whatever we see

on the Objective-C side we have to create some Java API for it. So since there are no

functions in Java, we have to, you know, create some set of method of some class. So we have

to basically turn this more procedural interface into an optical interface. So that's the challenge

that we had to, you know, deal with. And I'm not saying that what Apple has done is as

bad here. It's just legacy basically, you know. The other things I'd already mentioned

before, like value type. If you look at "CGRect", that is actually not a class, it's just simply

a value type. So there are no value types in Java. So I didn't point that out in that

slide earlier, but you probably, you might remember that actually there was a Java class

called "CGRect". So, again, value types don't--they don't have something equivalent on the Java

sides. So, here, we again map it to a proper Java class. But during the API mapping, of

course, we have to map a Java class to a value type on the Objective-C side. So that happens

inside these Wrapper Classes. The next thing that kind of cause us some headaches is that,

well, at least, the design of the Cocoa API is using some old little tricks that people

might remember, the older people in the audience at least from the C programming language so

that if you need additional output parameters, there's a common trick to have a pointer to

a pointer basically to pass back additional outputs and arguments. Now, Java only supports

call by values so you cannot do call by reference. So again from the Java API and design perspective,

we have to deal with that. And what we have done here is to create some holder classes

that basically are just act as a wrapper around an output argument. And the last part is again,

the delegation that is heavily being used in Cocoa Touch. And as I mentioned before,

here, generally, we have turned that into a strongly-typed Java interface, also to stay

truthful to the spirit of Java. So let me explain to you how we actually cross-compile

Java to Objective-C. And the first thing is that we do not have a Java Source Code level

cross-compiler, so what our tool chain, what our--what XMLVM actually cross-compiles is

not Java source code, but Java bytecode. And more specifically, we are making used of the

Dalvik virtual machine or the DEX instruction set. So you might know that Android is not

making used of the standard JVM but Android have chosen their own virtual machine called

Dalvik. And the bytecode instructions are called DEX or DEX instruction set. The big

advantage of DEX is that it's a register-based instruction set. So the Java virtual machine

is based on this DEX machine but Dalvik is based on a register machine. So we're making

use of that. And the first thing that we do in our tool chain is to take the Java source

code and turn that into an XML file that where the implementation is represented by the Dalvik

instruction set. So to show you how this works. On this slide here, you'll see a very simple

Java class. So you see a class called Calc. It has one static member. One static method,

it's called Add. It just takes two integers, and all it does is just simply return the

sum. So it'd returns "x+y". So in our tool chain, what happens is well, first, you just

take a standard Java compiler to turn it into a class file. And then by piping it through

our tool chain, and again by making use of the open source tool that we've access to

from Android, we turn that into an XML file that just simply represents the Dalvik instruction

set but marked up through XML. So what you see on the next slide is basically this class

Calc but now marked up as XML and making use of the Dalvik instruction set. So it looks

a little bit--a little bit complicated, but if you take a closer look, they're actually

not that--they're not that difficult to understand. On the top level, you have--you have a tag

called "class" that is your class definition. Then you have a method that is our--the "@method"

we had before. We have attributes to denote the various properties, the modifiers that

you have for this method like, in this case, it's a static method. You have a signature

so you can clearly see that this method takes two integers as input. It takes one--or it

returns one int as a result. And then most importantly down here, you see that in the

code tag "code", you actually have the DEX instructions that make up the implementation

of method "add" by means of Dalvik instructions. And if you can see, actually, there are only

two instructions here. There is an "add-int" instruction and there is a "return" instruction.

Now, the "add-int" instruction makes reference to three registers; vx, vy, and vz. And what

happens inside the Dalvik virtual machine that the certain registrars are being initialized

with the input argument. So actually by convention, and I won't go into too many details here,

but register number one and register number two correspond with the input arguments x

and y. So, basically, what this one instruction "add-int" does here, it will add the content

of register one with register two, and it will store the sum in register zero. That's

exactly what my Java class was doing that I had on the previous slide. And the second

Dalvik instruction here, then says a return, and we're returning the content of register

0 as the result of this method. So that's kind of like a little inside glimpse into

what actually happens inside of Dalvik that runs on any Android phone. So--but why do

we convert this to XML? Well, that actually is also where the project XMLVM gets its name

from because we're using XML technologies for--than generating high-level languages.

So in particular, what we do know is we're using XSL stylesheets for the code generation.

So in the next slide, I give you one example on how we map this "add-int" instruction to

source code again. And what you see on the bottom here is now the--an XSL template that

will always fire--will always match whenever it comes across the "add-int" instruction

that I explained on the previous slide. But then what happens is that whenever the XSL

processor is coming across that "add-int" instruction, it is going to omit this code

that I have here in the--as the children of this template. And basic--the code that will

be generated is what I have here in this yellow box here. So basically, this one "add-int"

instruction is going to result in this here; "r1.i + r2.i" and that is being assigned to

"r0.i". Now, this registers here; r0, r1, r2 are based on the union that is of type

XMLVMElem. So because a register in Dalvik can hold different data types, it can hold

an integer, it can hold a float, for example. So--and this union here represents all this

different possible types that can be used toward in a register. And by--since we have

the instruction "add-int", we know that what is in those registers are integers, that's

why we are accessing the value through the ".i" discriminator. But you can see in terms

of code generations, so for each Dalvik instruction, we have like a very small, very easy to understand

template that will then basically generate code in the target language. Now on the next

slide here, I'll show you the results. If you take all our templates and you if you

run it through the XML file--I had two slides earlier--then, here is the Objective-C source

code that you would end up with. And, again, you see, of course, the other templates that

define all these registers here. So we have two registrars. They are all of type XMLVMElem

as I explained before. You can see here the "add" method. And here, you can see we're

actually using name mangling to cope with this problem that Objective-C does not allow

overloading. So we have our two input arguments, x and y which here, again, on bytecode level,

are referred to as n1 and n2. We initialize the input parameters with the registers. Here

is the one line, so the addition data is basically the code that was contributed by the XSL template

I showed on the previous slide and then the return statement then will return the result.

Now, of course, if you run this through GCC here, then there will--some optimization will

be happening here. You will no longer see this actual assignment here, so you will do

the--add immediately on those and one and two. So actually, by cross-compiling DEX instructions,

we are capable of generating fairly efficient code on the--on the target language. Now,

we have different set of stylesheets that do the very same for JavaScript. I mentioned

earlier that we also support the Palm Pre. So, we also have stylesheets that just simply

take this XML file that contains DEX instructions and--but maps then the DEX instructions sets

to JavaScript. So we can actually then also cross-compile an Android application over

to the iPhone--sorry, to the Palm Pre. So up to this point, all I've talked about was

the iPhone. I have not said anything about Android, right? So, I come now to the next

part of my presentation because now that we have added a Java layer over the iPhone, basically,

the next step will be to take care of the API mapping from Android over to the iPhone.

And if you look at it the way you write an Android application, of course, you first

of all use the Java language and you use an Android's specific API. So if you look on

the far left of this slide, you can see the Android application that runs on top of Android

is only making use of Android API. So whatever you see published as part of the Android SDK,

that is what the application uses. Now, what we do now is for the iPhone. So we have now

what we call the Android compatibility library. That is built upon a common device layer.

So basically within our compatibility library, we have a little extraction that actually

captures the essence of the smartphone, of the particularities of the smartphone and

those portions of Android that actually are generic to all smartphones. And I'll give

you some examples on the next few slides. But the way this works is now that the Android

compatibility library, it's also written in Java, of course, and it offers exactly the

same API as the Android API gives you. And all of it is cross-compiled to Objective-C.

So what you end up with is, again, a native application on the iPhone. Now, we do the

same thing over for the Palm Pre. So now except, of course, here, we cross-compile from Java

to JavaScript. And we have a different webOS layer that kind of adapts the common layer

API to the way things are handled on the Palm Pre. So, to give you some examples on how

this looks like, let me give you three different examples. So first example, and the first

example, I want to show you how we map an Android button over to an iPhone button. That's

a fairly simple example. So, if you look at an Android button, there's a class called

"Button", it lives in name space "android.widget" and in our implementation of our XMLVM framework.

What we do is we just basically--our implementation of the Android button is basically nothing

more than a wrapper around a "UIButton". And this "UIButton" that you see here in the first

line of this class Button is of course, now the Java wrapper that we have created for

the Cocoa Touch "UIButton". So, "UIButton" is the name of the class of the iPhone button.

And I have one little example here. It's a little bit more sophisticated. If you have

a button, of course, you want to install a click listener. Well, it's called a click

listener in Androids. So a click listener basically allows you to install a callback

whenever the user presses the button then the application would be notified via this

callback. And there is a method called "setOnClickListener" and you pass an interface reference called

"OnClickListener". And what we do in our implementation--now of course, the "UIButton" has something equivalent.

There is actually a method called "addTarget" and that serves conceptually the same purpose

as the "setOnClickListener" on the Android side. Now, of course, there is a different--there's

a different way of defining this delegate and on the Cocoa touch side, there is something

called "UIControlDelegate". So, basically, what we do here is we instantiate an anonymous

class, we overwrite the "raiseEvent". And then inside the implementation of "raiseEvent",

we basically sent the click event back to--on the Android side. So, what happens is if your

Android application is using an Android button, if you cross-compile it over the iPhone, what

you have on your native iPhone application is actually the "UIButton" as given to you

by the Cocoa Touch Library. If you push that "UIButton" on the iPhone, what will happen

is it will call "raiseEvent" of that "UIButton", that "raiseEvent" is then forwarding that

to the--reference "theListener.onClick" which basically is the callback into your Android

application. It's that when you--when you push the iPhone button, it--what will happen

is here now that you will actually end up calling the callback inside your Android application.

Now remember, all of that is being cross-compiled over to Objective-C, so also this little piece

of code here will be cross-compiled over to Objective-C. Okay, second example, in Android,

there is this infamous R-class. And the way you reference resources in Android is via

this R-class. So if you have, for example, an external image, if you have a PNG file.

The way you reference it is by making use of this R-class. Now, Android will automatically

create this R-class for you. So what I have on the top here is--what you will see is a

little bit of that generated code that Android generates for you. So imagine if you have

an image resource, a file called "ball.png", then what Android is going to do, it is going

to create a member, a field of static final class called "drawable", that itself is nested

inside of a public final class called R, the R-class that I just mentioned. And it will

assign a some kind of unique ID that ideally you will not see but when in your Android

application, when you actually reference that, the way you do that is--so, if for example,

if you want--if you have an image view and you want to actually load it with that "ball.png",

what you do on Android is basically you say "setImageResource" and then you just simply

reference this field of this public static final class by saying "R.drawable.ball" and

you can see as exactly a "R.drawable.ball". So, so far so good. That's the way the Android

people have decided to design the API to reference external resources. Now, how do we deal with

that during cross-compilation because when you think about it, this method here "setImageResource".

Actually, it takes an integer. So at runtime, in our compatibility library, what we would

see is we will see this awkward hex number here. So how do we actually map this back

now to a file called "ball.png"? Well, what we do is we make use of Java reflection. And

on the bottom of this slide here, I'll show you a little bit of code snippet that also

comes out of our Android compatibility library. And basically what happens here is, you know,

you can see that we--first of all, we're asking for a class object of this "R.drawable". So,

this dollar sign here is just the internal way for Java to represent nested classes on

byte code level. So, "R.drawable" is just the way you--you reference the nested "drawable"

class on byte code level. But first thing we do is we just retrieve the class object.

They were using Java reflection to get back all of the declared fields which is just basically

an array of field and this API comes from J2SE. There's nothing magic. That is just

standard Java stuff. And then you can iterate over the--all the fields and then we can retrieve

the name and we can retrieve the integer that is associated. So in particular, if we do

this little for-loop here, eventually, we will see--for the ID, we will see this hex

number here. And for field name, we will see "ball" as a string. And from that, we can

use them the string to create the file name and then to actually load the image resource.

So, we resolve that those resources, that runtime, by making use of Java reflection.

Now once again this piece of code on the bottom here is also cross-compiled over to Objective-C.

So, this gives you a bit of an idea that our cross-compiler is actually fairly powerful,

in the way that it can deal with this kind of Java reflection. Third example, Androids

has layout managers. So, Android--I mentioned before that Android is actually very well

equipped to deal with different types of device and one nice feature about Android is that

they have a way to describe these interfaces by a declare definitions. So, there are some

XML files that's--that basically describe a user interface. And on the bottom here,

I've just listed one of those XML files. So that is basically the schema of that XML file

on the bottom as defined by the Android team and what you see here is "LinearLayout" is

one of the many layout managers that you have in Android. And basically, it just defines

a vertical arrangement of different interface elements and what you can see is--here is

"LinearLayout" is it just--it has an "EditText", it has a "TextView", it has a "Button" and

then another "LinearLayout" that is--simply is wrapper around a little logo. And here

the screenshot on the right, you can actually see what it would look like in the preview

of Eclipse. Now, Android is Open Source. Android has an implementation, has Java code that

takes this XML file and basically parses this XML file and you will render this user interface.

So, what we have done now is--since we are an Open Source project, one that's simply

leveraged on the Open Source. So, actually we take the original Android source code of

the layout implementation, layout engine implementation, we also just simply cross-compile it over

to Objective-C. So at runtime, on the iPhone, we also copy these layout files over to the

iPhone. So, what happens at runtime on the iPhone that we read in this XML file and based

on this--this declarative description of user interface, we actually then start building

up the UI. So here again, we make use of Open Source and, you know, also get a sense of,

you know, what is all contained in our Android compatibility library. So it's partially,

the things we had to implement ourselves like the parsing of the R-class or the wrapper

for the widgets like Button but we also leverage the Android source codes in the sense of bits

and pieces that are device independent. So if you download XMLVM, there is a little application

that we have done now. We are tool developers. We are not game developers. So, don't expect

too much of that application. It's a little application called Xokoban. It's actually--the

gaming ideas is fairly old, you have to--it's a little strategy game where you have to push--well,

in this case, apple to target areas and so the little guy in the middle has to push these

apples to the white outlined target areas. And the way we have done this game is by implementing

it in Android. So, we implemented it in Java and it's only making use of Android API. So

the screenshot in the lower right is just simply a screenshot of that original native

Android version that we have implemented for the Xokoban game. Now when we cross-compile

that to the iPhone, it looks then like on the screenshot on the lower left side. Now

I mentioned before that Android is actually a very well-equipped with dealing in different

device capabilities. So, actually the way we have written the Xokoban game is that we

actually look at what is the screen solution on the device. Now, on the iPhone, at least

so far, it only has a certain resolution, 480 x 320. But the Nexus One for example,

on the Android side has a higher resolution. So actually our implementation of Xokoban

renders nicely even on high-resolution screens. So, this application has been pushed out to

the App Store, Apple App Store, and also the Android market. So if you have an Android

phone, if you have a Nexus One, or if you have an iPhone, you can actually go to your

respective markets and you can look for Xokoban. You can actually download the application.

Now I should make one little disclaimer here, because this screenshot is not exactly accurate.

And if you actually do download the application, you will notice that it's a little bit different

from the screenshot here and the difference is this little guy here in the middle, the

Android. You know, initially, we thought it's kind of like little inside gag, you know,

like Android pushing apples, you know. But Apple didn't like that, so actually, I had

an App--the App Reviewer calling me and he said, "Well, thou shall not have other Gods

besides me." Well, I didn't said that literally but what we basically he said is they don't

allow any competing--they don't allow references to compete in products. So, we had to replace

the Android logo with something else, so... So, now we have--the version that you can

actually download now is now--we're using the Linux penguin instead. And I guess for

Apple, that is not really a threat. So, they have they have accepted the tux as the guy.

Okay, that's going to be the last analogy between Apple and the Vatican I'm going to

make here. So to give you an example of this--a much more complicated and a much more levered

application, so we have been working with an Australian company called Funky Mobile

Games. And this company specializes on games from mobile platforms. And they have created

a version of a game "Stratego" for Android. So basically, they have a fairly elaborate

implementation of this game for Android. And they have used our tool, XMLVM, to cross-compile

their implementation of Legion over to the iPhone. Now, this application is making use

of quite a few Android's features, so they have like--they're making heavy use of activity

life cycle, they have sound, they have animation, they have custom dialogues, they use a lot

of declarative layouts for the UI and also use file I/O for the serialization of the

game state. They also have a fairly sophisticated AI engine, so they--actually, there's a--a

three time world champion of--of this game Stratego, who implemented this AI engine.

So, what I want to do now is actually I want to do another demo and I want to actually

show you this game of Legions side-by-side in both the Android emulator and also in the

cross-compiled version for the iPhone. So, let me first--so what I'm launching here now

is the Android version and I can turn up the sound just for a second here. I don't know

if the microphone picks it up but actually they have put a lot of effort into this game.

So they have hired a composer for the music. They have hired a design artist for the graphics.

So let me just turn off the sound here.

So let me now launch the iPhone version. And here, I'm in Xcode. So actually here, I have

loaded the cross-compiled Objective-C source code and I'm just launching the same application

now in the iPhone emulator. Whoa, it's a lot louder. And you can see that's already like,

you know we're mapping the sound API here. So let me just turn down the--or also turn

it off here to show you that's with the same thing. So, the first little thing that I want

to point out, if you actually look a side by side, the difference of the screenshot,

on the Android version, you have this exit button here which we don't have on the iPhone

version. Apple does not permit exiting of an application on the iPhone other than pushing

on the home button. So, actually we offer a way to the Android developer to find out

if your application is running on the Android or the iPhone. And depending on which platform

you run, you can actually then do things differently. So the Android version is actually also adding

this exit button which you cannot do on--well, you could do it on the iPhone but Apple would

not pass that through their App Review. So, there's like a help screen. So if you click

on help, then you have like some little help text here. You can go back to the main menu,

and let me just do that here on the, on the iPhone. Go back. Let me start a new game.

So here, I actually--I don't, I don't know the rules of this game. I know that you have

to place these elements here, so you have to pick like little soldiers, you have to

place them on the game board. That's like the first step of setting up the board. But

what you--then can also do is there's a feature called AutoFill where you can just simply

pick a random setup. Here's a little [INDISTINCT] that says that you can go back. You can say

start game. It scrolls down the game field, and now we can start the game or we can play

the game and do the same thing here on the iPhone. So again, you know, I have like these

hovering pieces with the zoom feature. I can place them manually if I want to. I can also

pick the, the AutoFill feature. I can go back. I say start game. Again, it scrolls down here.

Now, I can, I can play it by making moves. And again, I'm not--I don't really know how

to play this game, so I'll just do something here. I just got killed. I know that. And

then--it's thinking. It's doing its countermove, and let me make something similar here on

the, on this side. So let me also pick maybe this here. So it does animation. It scrolls

over here. Again, I got killed. And then, let me just surrender here because I really

don't know how to play this game. So, if you want to surrender, there's a custom dialog.

Yes. And then, we lose. It would tell you--it will actually open up all the fields. You

can actually uncover and see what you were playing against. Let me do the same thing

here on the iPhone. Surrender? Yes. And then, you'll see the same thing here. So, basically,

the--apart from this one little thing with the exit button, everything else is 100% pure

Android; 100% pure Java. All the across-compiled to Objective-C old met from Android API over

to the iPhone. So, going back to my presentation, of course, the question is then how far can

you push this? So, I already mentioned a few times now that Android certainly is very well-equipped

with dealing with the device heterogeneity. But what kind of applications can you actually

cross-compile? Well, games are certainly good candidates because games are not bound by

user interface guidelines. You know, like you have like your image buttons and you have

these custom dialogues, and a part from this exit button, actually, you can pretty much

do whatever you want. Now, if you have an application that is more based on widgets,

if you have a more business type application then, of course, things get a little bit more

difficult. And the reason is that, of course, every handset, every smartphone, has their

own proprietary sets of widgets. So, basically, if you ask me, so what did you do in that

case? Well, if you look at the model view control of paradigms, the way you should be

designing your application, what we can always cross-compile is the model. And we probably

can also cross-compile a fair amount of, of the controller. Now, the view you might want

to do a custom reimplementation based on the, on the particular smartphone. I mean the iPhone

is known for its very fancy widgets, so there's no point in trying to mimic an Android gooey

on, on the iPhone. So, probably a reasonable approach would be to actually then re-implement

the view portion of your application but making use of, of the smartphone specific widgets,

in that case, iPhone. Now, of course, I mentioned--as I've mentioned before, we have a Java obstruction

around the iPhone now, so which means that even though you may have to re-implement the,

the view for the iPhone, but at least, you can do so based in Java. And one thing we're

currently exploring is actually to, to mix iPhone and Android. So if you do a custom

implementation of, of a view for the iPhone, that basically, you can do--you can use an

iPhone widget but embed it into the Android framework. So you can take, for example, like

a picker, an "UIPicker", of the, the iPhone and you make use of it using the linear layout

from Android, so that that should be quite possible, and that's where we kind of going

to as a next step. So here's some a quick overview of the complete XMLVM tool-chain,

and there are lots of little boxes here. XMLVM as a research project began in the early 2000s

and we have many other use cases that we can do. Well, XMLVM, I already mentioned XMLVM

as a name is derived from the fact that we are--that we have a XML representation of

byte code and we're making use of XML technologies for doing cross-compilation. So, actually,

we do have a front end for .net. We have a front end for Ruby byte quote and we have

also have back ends for, for different languages. Now, all these different paths, you can take

through this tool chain of different maturity. Now, obviously, the smartphone portion is

what attracts attention now to XMLVM's that is the one that we're putting a lot of effort

in right now, and it is the most stable. So what I've shown so far right now is basically

this here. So we go from, from Java Class Files down to Objective-C, so that is what

I've shown you so far. Then we can also go down to JavaScript for the Palm Pre, but we

can also do different things. We can do something like, for example, we can go from .net, and

we can also go down to JavaScript. So it would be possible, for example, to write a C# application

and cross-compile over to JavaScript. Or we can also map .net byte codes to Java byte

code. Again, the various paths through these tool chain of different maturities. So, in

some cases, like this one in particular, I just have it on this slide, is more like a

proof of concept. But, at least, it's in, in the code repository, and I guess, the idea

of where the project get its name from. Well, I come to my last slides. And, basically,

you know, what I first told you about in the past hour is while we first have added the

Java layer on top of the iPhone and then we have created our own Android compatibility

library to be able to cross-compile Android applications over to the iPhone. And, I guess,

the benefits go without saying--well, first of all, we support the skill set of people.

I mean there are lot of Java programs is out there and you can tap into their skill set.

Android has a big momentum, so obviously, you know, supporting Android developers and

the only API that you have to know is Android. So we deal with all the device specific details

like iPhone and Palm Pre. So, overall, it reduces the development cost, and it reduces

the time to market. While since there's quite a bit of interest and--on XMLVM, at least,

the smartphone portion, we are in the process of building up commercial supports for XMLVM.

And well, what we plan to offer is a broad range of different services around mobile

applications because we have noticed that quite a few companies are in this kind of

situation where they are want to be present in this App space but they realized that they,

first of all, lacking the knowledge of the various platforms, but also they realized

the amount of effort they have to put in on targeting different devices. So, kind of to

fill that niche, and that is where we are in the process of building up the commercial

support. Well, that comes to the end of my presentation. I would like to thank you for

your attention. And if you have any questions, then I'll be happy to answer them.

>> I got a question here on VC. What is the support for things like OpenGL and things

like Compasses and some of the other hardware that's in Android but hasn't been in the iPhone,

at least, until recently. >> PUDER: Okay, so open GL. You're asking

about OpenGL, so OpenGL, we actually do map. There was one, one guy who map the OpenGL

API, so that's taken care of. The Compass API, we don't have yet. Now, I mentioned before

that Android actually is targeting different kinds of devices. So actually, Android allows

you to explore the capabilities of the device. So, if you write a good Android application,

it should cope with that situation where your application runs on the device that may not

have all the feature you need. How you'll react to it that is up to you. You may say,

well, sorry, I need--I really need the Compass and I--you don't have one on your device so

I cannot run, but we can certainly cross-compile that over to the iPhone. There's another question?

>> The choice to use XSLT for the code generation. Can you talk a little bit about that? I mean

it's a cute idea but, is it more than that? I mean, how would that compare to like an

ad-hoc just parsing the DEX code and...? >> PUDER: Well, so the question is a bit more

details about the stylesheets that we used. And, well, like I mentioned, the product XMLVM

itself came from the fact that I was trying to find ways of doing bicro manipulations

through XML technology. So, early I talked about .net. Actually, we do data flow analysis

using stylesheets. Maybe it's maybe pushing it a little bit too hard on the stylesheets

side but, I guess what attracts me for stylesheet is just the declarative way of expressing

code generation. So, basically, the little template I've shown you earlier for describing

the add-ins, it was like a very clean, and very nice, easy way to express, you know,

what should happen when you come across at an instruction on--in your byte code. Now,

of course, you can implement it in different ways, right? I mean there's no question that

you can also just implement it in Java, for example, and that will no doubt to be more

efficient than you if you do it stylesheets. But, remember, the stylesheets--running the

stylesheets happens at compilation time, not at run time.

>> Yeah >> PUDER: So, the inefficiency that we might

have in a slightly long of compilation times refers to compiling time not the run time.

The code is still efficient that we regenerate. >> Is it only slightly longer than the straightforward

parsing? >> PUDER: I didn't do--I mean, we don't have

any other reference implementations so I couldn't really tell you, you know, how we compare

in terms of benchmark. Yes, it is slower but by how much, I cannot tell you.

>> And one more question, the game you showed, the full game, so I'm assuming it didn't like

cross-compile out of the box. How much work was needed to change the original game that

they wrote for Android to actually pass the cross-compiler and work properly?

>> PUDER: Well, so your question was how much work that we have to put in to cross-compile

this Legions game? >> Yes.

>> PUDER: Or how changes were required in the Legions game itself?

>> Uh-hmm. >> PUDER: Well, first of all, we have to put

in quite a bit of effort to make--to push XML to this--to the point that it could cross-compile

Legions. >> Not counting the work on the cross-compile

itself? >> PUDER: Yes, but it's otherwise...

>> Yes. >> PUDER: ...the guys in Australia had not

to make any changes to their application. That is just the...

>> The exit like... >> PUDER: We just, we just go...

>> The exit button, get rid of that? I mean, it was...

>> PUDER: Well, of course, yeah. Well, that, I mean I--when I saw that, you know, I've

told them, okay, guys, you know, we cannot, we cannot cross-compile an exit button FL

won't allow that, so we introduced certain API. We told them about it but, to my recollection,

that was really the only that they had to change to make it work on the XMLVM. Everything

else, they were using from sounded API to activity life cycle, to the other layout managers.

We took exactly as is without requiring any kind of changes in their site.

>> They didn't need to tweak it like... >> PUDER: No, they didn't have to tweak it.

No. >> Very nice.

>> So what type of affordances do you have for debugging support?

>> PUDER: Yeah, debugging support is a good thing because the code that we regenerate,

of course, looks ugly, right? I mean if I can just go back here to x-quotes and I can

just simply click on one of the cross-compiled classes, you know, if you look at this year

that looks ugly, right? So, our philosophy is that Objective-C is kind of like the Assembly

language of smartphone development, right? I mean, ideally, you don't really want to

deal with it. But you are quite right when it comes to debugging, that--is a bit of a

problem because you cannot really correlate that back to the original Java source code.

Now, if you have a working Android application and it crashes on the iPhone, then the solution

is simple. There's a back end XMLVM. So, you know, I would suggest then, just post a back

report on XMLVM and we will fix it. Now, you--the debugging happens on the Android. And Android

gives you all the powerful debugging tools >> Right.

>> PUDER: So, if the iPhone version then behaved differently or crashes even, that just indicates

a back end in our tool chain, but, basically, we'll take care of that and you shield it

from that. So the good news is that the debugging, again, we leverage the Android STK.

>> The other question I had is what--how do you deal with--how do you let the developer

know when they're using a feature that's not really supported on iPhone? So, for example,

an Android like there's a text speech library that you can call from the APIs, which my

understanding is that API isn't quite available yet on iPhone, and...

>> PUDER: You would, you would notice during cross-compilation because it will be linking

our--you would try to link against something that is not present.

>> Okay. >> PUDER: We--so we don't have like, explicit

list, API list where you can look up and see which API we have covered. We should do that.

I do agree, but, at this point, if something is missing, something we do not support yet,

you would find out when you link because then if unresolved to externals.

>> Okay. Cool. Thanks. >> You mentioned support for the Palm Pre,

the Android and the iPhone. Is there any support for the BlackBerry?

>> PUDER: Well, of course, we get a lot of request. There's a request from--for Windows

mobile. There is request for BlackBerry. At this point, XMLVM does not support these as

back ends. But I think, you know, it's--we are in pretty good shape of very quickly targeting

also these platforms. So, the way we go about of this, you know, like if we have a client

who is interested in a certain feature then we certainly put that high in to-do list.

But at this point, our main focus is the iPhone. The second focus is the Palm Pre. And beyond

that, we just, you know, let customers decide on what they want.

>> Android has a SQL light database, I believe. It does the--this cross-compiling also support...?

>> PUDER: Okay. The question is SQL light. We don't have an API data for that at the

moment, but to my knowledge, the iPhone also has a SQL light database that you can also

access. >> So it's possible...

>> PUDER: So it would just be--it would be possible, yes. Again, the thing is, remember,

we don't have to have to re-implement the features, right? I mean when you talk about

Compass, when you talk about [INDISTINCT], when you talk about different features, usually

all these devices have these capabilities. So also a SQL light database is present on

all these devices, it's just creating this repose. So, actually, making this mapping

is not as complicated as it may sound, so we can very quickly--if there is a demand

for a certain feature, we can fill this gap if it's not present at the moment in XMLVM.

>> Android also has the OS itself. I think the framework or I guess it's--provides ways

to communicate between applications, like, you could register with the phone to be a

media player, and things like that. That's not possible.

>> PUDER: Yeah, well, okay. Of course, you know, if Android has this notion of activities,

and you can also call an activity from other application. So, for example, if you want

to select one of your contacts, you can call a self activity from another application.

Now, the way we plan to address that, and we haven't done it for contacts, but we have

done it actually for web views, we have implemented an activity that is--would be linked to your

application so that it looks like, on the iPhone, you're switching over to another application

when in fact, you're still within the same application that you have launched. So I think

in many cases, we can, we can deal with that, and so we actually, we do not do multitasking

which is not allowed on the iPhone anyways. But, we plan to implement special purpose

activities that kind of mimic what you have on the Android.

>> Do you kind of launch the dialer from--you link with the dialer, and when I click on

a button on my App, it'll actually show the dialer window that you linked with...

>> PUDER: Yes >> ...in my Apps?

>> PUDER: And then this special purpose activity then would mimic whatever you see on the...

>> Yeah. >> PUDER: ...on the Android, but it will also,

of course, make use of--it will make use of iPhone's specific features. Another questions?

Well, if not, then, again, thank you for coming today, and thanks for listening.

The Description of Cross-Compiling Android Applications to the iPhone