As I mentioned last time, I’ve been porting some of the Java Android SDK demos over to Oxygene for Java. This is for various purposes:

• to ensure I am on top of various Java code layout approaches. The one that requires the most faffing about is nested classes. In Java the nested class gets access to members of the outer class, thanks to having an implicit this parameter passed through the constructor. That requires a bit of manual rewiring, which is a bit of a bore, but no big deal.
• to get some exposure to various additional Android programming areas.
• to run more and more code through Oxygene for Java and report any latent bugs that show up when trying it out.
• to build up more useful Oxygene for Java examples that can be packaged in with the product and thus be of reference use to Oxygene users
• to find out what issues and bugs are lying in wait for unsuspecting newcomers to Android programming within the recesses of those SDK demos.

I’ve uploaded a bunch of them to Google Play now (see the end of this post) so anyone interested can check them out and report any problems left in there so I can try and remedy them. Of course they are demo apps and so you shouldn’t expect too much of them. They’re just there to demonstrate certain Android programming techniques and so aren’t professional-level applications.

In the last post I looked at the poor support for the exit-the-app-and-come-back situation with Lunar Lander and JetBoy. Well, poor support isn’t really accurate as the apps just crash out when you try and resume them. Poor implementation, poor planning, something like that perhaps.

Anyway, other common themes that come out of more demos are as follows:

1) Background image resources are not resized to accommodate screen sizes. Perhaps back when these demos were written, the various bitmaps used in AccelerometerPlay, JetBoy were larger than the largest of screens. Not now with tablets and large display phones.

In the case of an image on a game that will get an initial size on startup and not change, for example a game that is fixed to landscape or portrait orientation, then you can size the bitmap once after having loaded it. JetBoy is fixed to landscape orientation thanks to this attribute in its activity declaration in the Android manifest file:

android:screenOrientation=”landscape”

Similarly the AccelerometerPlay demo is forced to portrait with this attribute in the activity declaration in the Android manifest:

android:screenOrientation=”portrait”

If you draw in a View descendant, then override the onSizeChanged() method and you can resize the bitmap by passing it to the static Bitmap method Bitmap.createScaledBitmap() along with the target width and height, typically as passed in parameters to onSizeChanged(). For example, in the accelerometer demo I now have this:

method SimulationView.onSizeChanged(w, h, oldw, oldh: Integer);
begin
  // other stuff is here
  …
  // Resize the background bitmap
  // NOTE: original code did not resize the background bitmap
  // so it did not cover the background on larger displays
  mWood := Bitmap.createScaledBitmap(mWood, w, h, true)
end;

If you’re working with a SurfaceView and have implemented the SurfaceHolder.Callback interface then you can do much the same thing in the surfaceChanged() method.

You can also choose to size the bitmap on-demand when it’s needed from its original size to the required size for the target canvas. This will be a little less performant than a one-off resize but can be useful. For example, the JetBoy app has a ready screen before play starts. The background to that screen was not resized appropriately. If you didn’t want to resize the background bitmap once ahead of time you could identify the current size and target size as Rect objects and call an appropriate override of the Canvas object’s drawBitmap() method:

method JetBoyThread.doDrawReady(aCanvas: Canvas);
begin
  var src := new Rect(0, 0, mTitleBG.Width, mTitleBG.Height);
  var dest := new Rect(0, 0, aCanvas.Width, aCanvas.Height);
  aCanvas.drawBitmap(mTitleBG, src, dest, nil);
end;

2) Older games were coded to assume the presence of a directional touchpad (D-Pad). Most modern devices come without one of these, so to make them usable on current devices I’ve had to add in onTouchEvent() overrides in either the View or Activity for simple screen touch responses. In the case of the Snake demo app I implemented GestureDetector.SimpleOnGestureListener to handle directional swipes.

3) Some demos have been updated over the Android releases to take on newer behaviour. For example, there is a simple Bluetooth Chat demo. You can install it and run it on two devices and the users can connect and do a simple Instant Message type chat. In the most recent SDKs for Android 4.0.x (Ice Cream Sandwich) this demo has been tweaked compared to what you see in the samples for the older Android versions. This newer version uses the so-called Holographic theme introduced in Android 3, which adds an ActionBar to the app as a side-effect. What are menu options in the earlier version are now ActionBar actions when installed on a more recent OS.

Also the newer version of the app offers both secure and insecure Bluetooth connections, something the original version didn’t as insecure Bluetooth connections were introduced in Android 2.3.3 (Gingerbread MR1).

The initial question was which version of the demo to bring over to Oxygene. In the end I got the translated app to operate on any Android version from 2.0 (Eclair) onwards.

To get the Holographic theme to be applied for Android 3 (Honeycomb) and later (and thus get the ActionBar) I took advantage of Android’s resource system (as mentioned here). I defined an app theme to be based on Android’s default old-school Theme in a resource file in res\values as per usual.

<resources
    xmlns:android=”http://schemas.android.com/apk/res/android”>
  <style name=”AppTheme” parent=”@android:style/Theme”>
  </style>
</resources>

But then I defined the same app theme to be based on Theme.Holo in a resource file in res\values-v11.

<resources
    xmlns:android=http://schemas.android.com/apk/res/android>
  <style name=”AppTheme” parent=”@android:style/Theme.Holo”>
  </style>
</resources>

If running on API 11 (Android 3.0 aka Honeycomb) or later the theme definition will be pulled from this latter directory. All that’s left to finish the job is to apply the app theme to the application in the Android manifest file:

<application android:label=”@string/app_name”
             android:icon=”@drawable/app_icon”
             android:theme=”@style/AppTheme”
             android:debuggable=”true” >
  …
</application>

That’s the appearance. But it was also important to ensure the option for establishing an insecure Bluetooth connection was removed from the menu if running on Android earlier than 2.3.3. Since the menu is defined in a menu resource file this simply involved having two menu definitions, one in the usual res\menu directory and one in res\menu-v10. The menu definition in res\menu has the problem option removed; the menu definition in menu-v10 will be used on Android API 11 (2.3.3) or later and has all menu options defined.

The final aspect of keeping the app version-agnostic is to check the code and what relies on the newer aspects of the source. Anything that needs to be skipped on Android versions earlier than 2.3.3 can be conditionalised using Build.VERSION.SDK_INT, something like this:

if Build.VERSION.SDK_INT >=
     Build.VERSION_CODES.GINGERBREAD_MR1 then
begin
  …
end;

Uploaded demos

Ok, so I’m done pulling Java code over to Oxygene and making it work properly for the time being. I’ve tidied up the demos that I’ve ported and uploaded them to Google Play, giving as useful a description as I can for each of them. The following are online – if you encounter any problems or have any simple improvement suggestions (bear in mind they are meant to just be ports of existing demos) I would appreciate hearing from you.

• AccelerometerPlay: demonstrates the accelerometer sensor by representing a slab of wood with three shiny balls on it. Tilt the device and the balls roll in a quite realistic manner.
• Bluetooth Chat: install on 2 Android phones and you can establish a connection (secure or, if on Android 2.3.3 or later, insecure) and have an IM-style chat
• Cube Live Wallpaper: two live wallpapers. One is a rotating wire cube. One has a settings page to allow to to choose between a rotating wire cube and a rotating wire dodecahedron. On Android home screens that support it (not HTC Sense 3.0 or later, or Android 4.0 or later) the wallpaper responds to offset messages, meaning that the shape will rotate as you move from home screen to home screen.
• JetBoy: a scroller game that demonstrates Android’s JET audio engine. Destroy a certain number of asteroids in a certain amount of time by firing in time to the music.
• Lunar Lander: use direction and thrusters to safely land on the landing pad. I’ve mapped touches on areas of the screen (as per the app description on Google Play) to the D-Pad controls, but apparently it’s still a but tricky. This could just be a function of the game implementation, or maybe the D-Pad to screen mappings. Any thoughts, let me know!
• Snake: The classic snake game. Swipe the screen to change the snake’s direction and reach the apples. Mind yourself and mind the walls!
• Wiktionary ‘Word of the Day’ Widget: install on your home screen and you get Wiktionary’s word of the day displayed, along with the word’s definition. Touch the widget to open up a Wiktionary browser where you can follow links to other words, search for words or go to a random word.

The full set of Oxygene-ported Android SDK apps can be found at this link.

Alternatively all my Oxygene-powered Android apps can be found at this link.

I’ve been porting some of the Android SDK samples from their original Java over to Oxygene for Java, with the hope they’ll be included in a forthcoming update to the product.

I’d already contributed a port of the AccelerometerPlay demo, which is a neat little demo of using the accelerometer to control several balls on a surface (the screen).

Among other examples I’ve been looking at are the Lunar Lander and JetBoy demo games. Now, if you wanted to start writing a graphics frame-based game that used a SurfaceView as its output you’d think these would be a great starting point, right?

Wrong!

Well, kind of wrong anyway.

The games work ok and you can play them, although they both have the shortcoming that they were written back in the days when most devices had a D-Pad (directional pad) and/or physical keyboard. Consequently the original examples are unplayable on many current devices. That’s no problem – it’s not much trouble to add in support for touches on bits of the screen or swipes across or up and down the screen. Actually I had to add in swipe support for the Snake sample game, which also relied on a D-Pad, but since that one’s not based on a thread (being a simpler game it uses a Handler and its sendMessageDelayed() method).

The problem with Lunar Lander and JetBoy is with how their thread-based operation has been laid out. The JetBoy architecture was based on the simpler Lunar Lander and so both games suffer from the same problem. The symptom is that if you switch away, maybe by pressing Home or by a phone call coming through, and then switch back the game crashes out. If you look at a logcat from either app running and crashing you’ll find that the problem is an IllegalThreadStateException being thrown with the message:

java.lang.IllegalThreadStateException: Thread already started.

This doesn’t look like a very good basis for a couple of evidently appealing sample apps, being designed with a thread crash in… Clearly people will base their own games on these apps and be rather frustrated by this poor design.

Anyway, enough of the criticism. What’s the issue here? Well, a cursory search on the web shows this has (unsurprisingly) cropped up a lot of times and there have been various ways it has been responded to. So let’s look at the problem and the solution.

The games have an Activity whose UI is made up of a SurfaceView descendant. The game logic is contained in a Thread descendant. When the app starts up and the activity’s onCreate() method runs it creates the custom surface view and asks that view to create the thread.

The surface view has a couple of methods that mark the lifetime of the actual drawable surface: surfaceCreated() and surfaceDestroyed(). In surfaceCreated() the thread is set in motion with a call to its start() method. When the surface is destroyed, for example by switching back to the home screen, two things occur. Firstly a member field flag is set to indicate that the thread’s body (its run() method) should stop whirring round updating state and drawing frames. Then the thread’s join() method is called, which blocks the surface view thread until that run() method has done its job and exited.

Ok, with me so far? No evident problem so far. What happens when we switch back to the app?

Well, when the app’s UI resumes the surface view’s surfaceCreated() method is called, which calls the thread’s start() method to start it off again. And therein lies the nub of the problem. As the documentation states, start() will throw an IllegalThreadStateException if the thread has been started before.

Because of this coding scheme the Lunar Lander and JetBoy apps are set up to fail. So, what’s a good solution?

Well, I’ve tried a couple of options, some of which are frowned upon as not being the right way and settled for the suggestion of StackOverflow user Pompe de velo in this post. The changes required are as follows:

• in the thread define member variables:
  • var mPauseLock: Object := new Object;
  • var mPaused: Boolean;
• have the thread’s run() method keep iterating endlessly (while true do… instead of while mRun do…).
• in this loop, after all the individual iteration’s physics update and frame drawing add:
  

  locking mPauseLock do
     while mPaused do
       try
         mPauseLock.wait
       except
         on e: InterruptedException do
           { whatever };
       end

• if your thread already has pause() and unpause()/resume() type methods implemented then great. If not add them.
• have the pause() method do this:
  

  locking mPauseLock do
    mPaused := true;

• have the unpause() method do this:
  

  locking mPauseLock do
  begin
    mPaused := false;
    mPauseLock.notifyAll
  end;

• in the activity’s onPause() method call the thread’s pause() method
• declare a member field in the surface view:
  
  

  var mGameIsRunning: Boolean := false;

• in the surface view’s surfaceDestroyed() method remove all the code relating to managing the thread
• have the surface view’s surfaceCreated() method change the call to thread.start() to this code:
  

  if not mGameIsRunning then
  begin
    thread.start;
    mGameIsRunning := true
  end
  else
    thread.unpause;

Yeah, so there’s a few changes there, but once done the app can survive being switched away from and back to.

It would probably be a good move to take the fixed app, strip all the logic out and use it as a skeleton gaming loop app, which I may well do.

For now, I have more bugs to fix in SDK demo apps I’m porting over. Currently I’m getting on top of an issue with the BluetoothChat demo where exiting and restarting the app means connections from another device no longer show up on the UI. It would appear the Android SDK demos didn’t get a particularly good test through….. What’s more of a shame, though, is that despite numerous people reporting issues, the demo apps do not get updated to fix the problems.

<sigh>

A few weeks back I was browsing Paweł Głowacki’s blog and saw his write-up on implementing both a parallax starfield as well as a 3D starfield using Delphi and FireMonkey. Quite taken with the classic starfield demo idea I though it might be neat to have a go at putting together an Android implementation in Pascal using Oxygene for Java.

As Paweł did, I used the basic Python logic from http://codentronix.com/2011/04/27/a-cool-parallax-starfield-simulation-using-python to build up the parallax version from and then used the logic from http://codentronix.com/2011/05/28/3d-starfield-made-using-python-and-pygame for a basic 3D implementation. Then I bumped into a nice DHTML JavaScript implementation of an interactive starfield at http://www.chiptune.com/starfield/starfield.html, where moving the mouse changes the aspect of travel through the starfield.

So I knocked up some equivalent logic in Oxygene in an Android application and uploaded the result to the Android Market, oh, I mean Google Play. You find the free app through this link if you want to check out the results. The interactive starfield has some settings to customise the number and speed of stars as well as whether touching the screen changes the travel aspect.

Clearly the logic is reasonably straightforward – in each case it’s really just dots of various sizes drawn on the Canvas of a SurfaceView. This is done again and again after having moved the positions of the stars in ‘space’, but of course care must be taken to be CPU-friendly. To that end an Android Handler is used and each time a frame is drawn, the next one is scheduled in for some small time period later using a call to its postDelayed() method.

Anyway, I was quite pleased with how this little app turned out, and how it gave me an opportunity to go through the Google Play application publication process. And then I started thinking about other old effects from the demos that used to be quite popular in the late 80s and early 90s…

My favourite was the plasma effect: an undulating oozing flow of colours shifting through the screen. A bit of research helped me understand the general principle of how the plasma effect is implemented, with a plasma function at the heart of it. I found a selection of different plasma functions and rolled them into a configurable Android plasma app built with Oxygene for Java, available for free at this link:

These plasma effects are implemented by pre-assigning an interesting palette of colours and then iteratively calculating the value of each pixel of the plasma surface and plotting it on a bitmap. To keep things performant the plasma surface is typically rather less dense than the pixel count of a smart phone screen and so this bitmap is then stretched onto the Canvas of the underlying SurfaceView. However, to minimise the evident pixellation some of the effects actually render virtual pixels and use a random jitter (x & y offset) in conjunction with opacity to smooth out the edges.

Having been bitten by the bug of retro demo effects I then bumped into the Demo Effects Collection Open Source project. This shows, in C++, how a whole raft of old demo effects are implemented, so I picked a few other effects I had a nostalgic affection for and concocted Retro Effects, another free Android app built with Oxygene for Java and available at this link. This one has a fire effect, a particle explosion, shadebobs and copper bars (as well as a starfield and plasma implementation for completeness):

This has all been a terrific trip down memory lane for me, resurging my interest in the old demo effect scene, and using Oxygene for Java has allowed me to build the apps on my favourite mobile platform, Android, using my favourite language syntax, Pascal.

Oh, just in signing off I should mention that I also uploaded a couple of live wallpapers, one with the interactive starfield and one with the configurable plasma functions. Live wallpapers are quite a nice distraction and needn’t be as much of a battery hog as they are sometimes made out to be (especially if implemented carefully). The Android live wallpapers do, however, have a nominal download charge associated with them.

I’ve been porting some of the Android SDK demos from the original Java to Oxygene, including the live wallpaper demo (a rotating cube and a rotating dodecahedron), so I’ll probably blog some technical details on how these things are built at some point in the near future.

On 29th June, 2012 I’ll be travelling to Cambridge, UK to speak at Mobile East 2012, a mobile-focused conference. I’ll be delivering a session on Android development using RemObjects’ Oxygene for Java development tool and catching several of the other sessions during down-time on the day.

I’m looking forward to it – it’s been some months since I’ve spoken at a conference!