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Android

Requirements:

Android SDK (version 12 or later) http://developer.android.com/sdk/index.html

Android NDK r7 or later http://developer.android.com/tools/sdk/ndk/index.html

Minimum API level supported by SDL: 10 (Android 2.3.3) Joystick support is available for API level >=12 devices.

================================================================================ How the port works

  • Android applications are Java-based, optionally with parts written in C
  • As SDL apps are C-based, we use a small Java shim that uses JNI to talk to the SDL library
  • This means that your application C code must be placed inside an Android Java project, along with some C support code that communicates with Java
  • This eventually produces a standard Android .apk package

The Android Java code implements an "Activity" and can be found in: android-project/src/org/libsdl/app/SDLActivity.java

The Java code loads your game code, the SDL shared library, and dispatches to native functions implemented in the SDL library: src/core/android/SDL_android.c

Your project must include some glue code that starts your main() routine: src/main/android/SDL_android_main.c

================================================================================ Building an app

For simple projects you can use the script located at build-scripts/androidbuild.sh

There's two ways of using it:

androidbuild.sh com.yourcompany.yourapp < sources.list
androidbuild.sh com.yourcompany.yourapp source1.c source2.c ...sourceN.c

sources.list should be a text file with a source file name in each line Filenames should be specified relative to the current directory, for example if you are in the build-scripts directory and want to create the testgles.c test, you'll run:

./androidbuild.sh org.libsdl.testgles ../test/testgles.c

One limitation of this script is that all sources provided will be aggregated into a single directory, thus all your source files should have a unique name.

Once the project is complete the script will tell you where the debug APK is located. If you want to create a signed release APK, you can use the project created by this utility to generate it.

Finally, a word of caution: re running androidbuild.sh wipes any changes you may have done in the build directory for the app!

For more complex projects, follow these instructions:

  1. Copy the android-project directory wherever you want to keep your projects and rename it to the name of your project.
  2. Move or symlink this SDL directory into the "/jni" directory
  3. Edit "/jni/src/Android.mk" to include your source files
  4. Run 'ndk-build' (a script provided by the NDK). This compiles the C source

If you want to use the Eclipse IDE, skip to the Eclipse section below.

  1. Create "/local.properties" and use that to point to the Android SDK directory, by writing a line with the following form:

    sdk.dir=PATH_TO_ANDROID_SDK

  2. Run 'ant debug' in android/project. This compiles the .java and eventually creates a .apk with the native code embedded

  3. 'ant debug install' will push the apk to the device or emulator (if connected)

Here's an explanation of the files in the Android project, so you can customize them:

android-project/
    AndroidManifest.xml	- package manifest. Among others, it contains the class name
    			  of the main Activity and the package name of the application.
    build.properties	- empty
    build.xml		- build description file, used by ant. The actual application name
    			  is specified here.
    default.properties	- holds the target ABI for the application, android-10 and up
    project.properties	- holds the target ABI for the application, android-10 and up
    local.properties	- holds the SDK path, you should change this to the path to your SDK
    jni/			- directory holding native code
    jni/Android.mk		- Android makefile that can call recursively the Android.mk files
    			  in all subdirectories
    jni/SDL/		- (symlink to) directory holding the SDL library files
    jni/SDL/Android.mk	- Android makefile for creating the SDL shared library
    jni/src/		- directory holding your C/C++ source
    jni/src/Android.mk	- Android makefile that you should customize to include your 
                              source code and any library references
    res/			- directory holding resources for your application
    res/drawable-*		- directories holding icons for different phone hardware. Could be
    			  one dir called "drawable".
    res/layout/main.xml	- Usually contains a file main.xml, which declares the screen layout.
    			  We don't need it because we use the SDL video output.
    res/values/strings.xml	- strings used in your application, including the application name
    			  shown on the phone.
    src/org/libsdl/app/SDLActivity.java - the Java class handling the initialization and binding
    			  to SDL.  Be very careful changing this, as the SDL library relies
    			  on this implementation.

================================================================================ Build an app with static linking of libSDL

This build uses the Android NDK module system.

Instructions:

  1. Copy the android-project directory wherever you want to keep your projects and rename it to the name of your project.

  2. Rename "/jni/src/Android_static.mk" to "/jni/src/Android.mk" (overwrite the existing one)

  3. Edit "/jni/src/Android.mk" to include your source files

  4. create and export an environment variable named NDK_MODULE_PATH that points to the parent directory of this SDL directory. e.g.:

    export NDK_MODULE_PATH="$PWD"/..

  5. Edit "/src/org/libsdl/app/SDLActivity.java" and remove the call to System.loadLibrary("SDL2").

  6. Run 'ndk-build' (a script provided by the NDK). This compiles the C source

================================================================================ Customizing your application name

To customize your application name, edit AndroidManifest.xml and replace "org.libsdl.app" with an identifier for your product package.

Then create a Java class extending SDLActivity and place it in a directory under src matching your package, e.g.

src/com/gamemaker/game/MyGame.java

Here's an example of a minimal class file:

--- MyGame.java --------------------------
package com.gamemaker.game;

import org.libsdl.app.SDLActivity; 

/**
 * A sample wrapper class that just calls SDLActivity 
 */ 

public class MyGame extends SDLActivity { }

------------------------------------------

Then replace "SDLActivity" in AndroidManifest.xml with the name of your class, .e.g. "MyGame"

================================================================================ Customizing your application icon

Conceptually changing your icon is just replacing the "ic_launcher.png" files in the drawable directories under the res directory. There are four directories for different screen sizes. These can be replaced with one dir called "drawable", containing an icon file "ic_launcher.png" with dimensions 48x48 or 72x72.

You may need to change the name of your icon in AndroidManifest.xml to match this icon filename.

================================================================================ Loading assets

Any files you put in the "assets" directory of your android-project directory will get bundled into the application package and you can load them using the standard functions in SDL_rwops.h.

There are also a few Android specific functions that allow you to get other useful paths for saving and loading data:

  • SDL_AndroidGetInternalStoragePath()
  • SDL_AndroidGetExternalStorageState()
  • SDL_AndroidGetExternalStoragePath()

See SDL_system.h for more details on these functions.

The asset packaging system will, by default, compress certain file extensions. SDL includes two asset file access mechanisms, the preferred one is the so called "File Descriptor" method, which is faster and doesn't involve the Dalvik GC, but given this method does not work on compressed assets, there is also the "Input Stream" method, which is automatically used as a fall back by SDL. You may want to keep this fact in mind when building your APK, specially when large files are involved. For more information on which extensions get compressed by default and how to disable this behaviour, see for example:

http://ponystyle.com/blog/2010/03/26/dealing-with-asset-compression-in-android-apps/

================================================================================ Pause / Resume behaviour

If SDL is compiled with SDL_ANDROID_BLOCK_ON_PAUSE defined (the default), the event loop will block itself when the app is paused (ie, when the user returns to the main Android dashboard). Blocking is better in terms of battery use, and it allows your app to spring back to life instantaneously after resume (versus polling for a resume message).

Upon resume, SDL will attempt to restore the GL context automatically. In modern devices (Android 3.0 and up) this will most likely succeed and your app can continue to operate as it was.

However, there's a chance (on older hardware, or on systems under heavy load), where the GL context can not be restored. In that case you have to listen for a specific message, (which is not yet implemented!) and restore your textures manually or quit the app (which is actually the kind of behaviour you'll see under iOS, if the OS can not restore your GL context it will just kill your app)

================================================================================ Threads and the Java VM

For a quick tour on how Linux native threads interoperate with the Java VM, take a look here: http://developer.android.com/guide/practices/jni.html

If you want to use threads in your SDL app, it's strongly recommended that you do so by creating them using SDL functions. This way, the required attach/detach handling is managed by SDL automagically. If you have threads created by other means and they make calls to SDL functions, make sure that you call Android_JNI_SetupThread() before doing anything else otherwise SDL will attach your thread automatically anyway (when you make an SDL call), but it'll never detach it.

================================================================================ Using STL

You can use STL in your project by creating an Application.mk file in the jni folder and adding the following line:

APP_STL := stlport_static

For more information check out CPLUSPLUS-SUPPORT.html in the NDK documentation.

================================================================================ Additional documentation

The documentation in the NDK docs directory is very helpful in understanding the build process and how to work with native code on the Android platform.

The best place to start is with docs/OVERVIEW.TXT

================================================================================ Using Eclipse

First make sure that you've installed Eclipse and the Android extensions as described here: http://developer.android.com/tools/sdk/eclipse-adt.html

Once you've copied the SDL android project and customized it, you can create an Eclipse project from it:

  • File -> New -> Other
  • Select the Android -> Android Project wizard and click Next
  • Enter the name you'd like your project to have
  • Select "Create project from existing source" and browse for your project directory
  • Make sure the Build Target is set to Android 3.1 (API 12)
  • Click Finish

================================================================================ Using the emulator

There are some good tips and tricks for getting the most out of the emulator here: http://developer.android.com/tools/devices/emulator.html

Especially useful is the info on setting up OpenGL ES 2.0 emulation.

Notice that this software emulator is incredibly slow and needs a lot of disk space. Using a real device works better.

================================================================================ Troubleshooting

You can create and run an emulator from the Eclipse IDE:

  • Window -> Android SDK and AVD Manager

You can see if adb can see any devices with the following command:

adb devices

You can see the output of log messages on the default device with:

adb logcat

You can push files to the device with:

adb push local_file remote_path_and_file

You can push files to the SD Card at /sdcard, for example:

adb push moose.dat /sdcard/moose.dat

You can see the files on the SD card with a shell command:

adb shell ls /sdcard/

You can start a command shell on the default device with:

adb shell

You can remove the library files of your project (and not the SDL lib files) with:

ndk-build clean

You can do a build with the following command:

ndk-build

You can see the complete command line that ndk-build is using by passing V=1 on the command line:

ndk-build V=1

If your application crashes in native code, you can use addr2line to convert the addresses in the stack trace to lines in your code.

For example, if your crash looks like this:

I/DEBUG   (   31): signal 11 (SIGSEGV), code 2 (SEGV_ACCERR), fault addr 400085d0
I/DEBUG   (   31):  r0 00000000  r1 00001000  r2 00000003  r3 400085d4
I/DEBUG   (   31):  r4 400085d0  r5 40008000  r6 afd41504  r7 436c6a7c
I/DEBUG   (   31):  r8 436c6b30  r9 435c6fb0  10 435c6f9c  fp 4168d82c
I/DEBUG   (   31):  ip 8346aff0  sp 436c6a60  lr afd1c8ff  pc afd1c902  cpsr 60000030
I/DEBUG   (   31):          #00  pc 0001c902  /system/lib/libc.so
I/DEBUG   (   31):          #01  pc 0001ccf6  /system/lib/libc.so
I/DEBUG   (   31):          #02  pc 000014bc  /data/data/org.libsdl.app/lib/libmain.so
I/DEBUG   (   31):          #03  pc 00001506  /data/data/org.libsdl.app/lib/libmain.so

You can see that there's a crash in the C library being called from the main code. I run addr2line with the debug version of my code:

arm-eabi-addr2line -C -f -e obj/local/armeabi/libmain.so

and then paste in the number after "pc" in the call stack, from the line that I care about: 000014bc

I get output from addr2line showing that it's in the quit function, in testspriteminimal.c, on line 23.

You can add logging to your code to help show what's happening:

#include <android/log.h>

__android_log_print(ANDROID_LOG_INFO, "foo", "Something happened! x = %d", x);

If you need to build without optimization turned on, you can create a file called "Application.mk" in the jni directory, with the following line in it:

APP_OPTIM := debug

================================================================================ Memory debugging

The best (and slowest) way to debug memory issues on Android is valgrind. Valgrind has support for Android out of the box, just grab code using:

svn co svn://svn.valgrind.org/valgrind/trunk valgrind

... and follow the instructions in the file README.android to build it.

One thing I needed to do on Mac OS X was change the path to the toolchain, and add ranlib to the environment variables: export RANLIB=$NDKROOT/toolchains/arm-linux-androideabi-4.4.3/prebuilt/darwin-x86/bin/arm-linux-androideabi-ranlib

Once valgrind is built, you can create a wrapper script to launch your application with it, changing org.libsdl.app to your package identifier:

--- start_valgrind_app -------------------
#!/system/bin/sh
export TMPDIR=/data/data/org.libsdl.app
exec /data/local/Inst/bin/valgrind --log-file=/sdcard/valgrind.log --error-limit=no $*
------------------------------------------

Then push it to the device:

adb push start_valgrind_app /data/local

and make it executable:

adb shell chmod 755 /data/local/start_valgrind_app

and tell Android to use the script to launch your application:

adb shell setprop wrap.org.libsdl.app "logwrapper /data/local/start_valgrind_app"

If the setprop command says "could not set property", it's likely that your package name is too long and you should make it shorter by changing AndroidManifest.xml and the path to your class file in android-project/src

You can then launch your application normally and waaaaaaaiiittt for it. You can monitor the startup process with the logcat command above, and when it's done (or even while it's running) you can grab the valgrind output file:

adb pull /sdcard/valgrind.log

When you're done instrumenting with valgrind, you can disable the wrapper:

adb shell setprop wrap.org.libsdl.app ""

================================================================================ Why is API level 10 the minimum required?

API level 10 is the minimum required level at runtime (that is, on the device) because SDL requires some functionality for running not available on older devices. Since the incorporation of joystick support into SDL, the minimum SDK required to build SDL is version 12. Devices running API levels 10-11 are still supported, only with the joystick functionality disabled.

Support for native OpenGL ES and ES2 applications was introduced in the NDK for API level 4 and 8. EGL was made a stable API in the NDK for API level 9, which has since then been obsoleted, with the recommendation to developers to bump the required API level to 10. As of this writing, according to http://developer.android.com/about/dashboards/index.html about 90% of the Android devices accessing Google Play support API level 10 or higher (March 2013).

================================================================================ A note regarding the use of the "dirty rectangles" rendering technique

If your app uses a variation of the "dirty rectangles" rendering technique, where you only update a portion of the screen on each frame, you may notice a variety of visual glitches on Android, that are not present on other platforms. This is caused by SDL's use of EGL as the support system to handle OpenGL ES/ES2 contexts, in particular the use of the eglSwapBuffers function. As stated in the documentation for the function "The contents of ancillary buffers are always undefined after calling eglSwapBuffers". Setting the EGL_SWAP_BEHAVIOR attribute of the surface to EGL_BUFFER_PRESERVED is not possible for SDL as it requires EGL 1.4, available only on the API level 17+, so the only workaround available on this platform is to redraw the entire screen each frame.

Reference: http://www.khronos.org/registry/egl/specs/EGLTechNote0001.html

================================================================================ Known issues

  • The number of buttons reported for each joystick is hardcoded to be 36, which is the current maximum number of buttons Android can report.