README-android.txt
author Sam Lantinga <slouken@libsdl.org>
Fri, 13 Sep 2013 17:42:31 -0700
changeset 7740 415bb39617a1
parent 7501 b27825bb5879
child 7793 6463a850229d
permissions -rw-r--r--
Mac: Translate Ctrl-Left click to right click.
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Simple DirectMedia Layer for Android
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Requirements:
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Android SDK (version 10 or later)
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http://developer.android.com/sdk/index.html
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Android NDK r7 or later
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http://developer.android.com/sdk/ndk/index.html
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Minimum API level supported by SDL: 10 (Android 2.3.3)
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================================================================================
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 How the port works
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================================================================================
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- Android applications are Java-based, optionally with parts written in C
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- As SDL apps are C-based, we use a small Java shim that uses JNI to talk to 
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the SDL library
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- This means that your application C code must be placed inside an Android 
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Java project, along with some C support code that communicates with Java
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- This eventually produces a standard Android .apk package
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The Android Java code implements an "Activity" and can be found in:
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android-project/src/org/libsdl/app/SDLActivity.java
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The Java code loads your game code, the SDL shared library, and
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dispatches to native functions implemented in the SDL library:
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src/SDL_android.c
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Your project must include some glue code that starts your main() routine:
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src/main/android/SDL_android_main.c
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================================================================================
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 Building an app
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================================================================================
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Instructions:
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1. Copy the android-project directory wherever you want to keep your projects
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   and rename it to the name of your project.
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2. Move or symlink this SDL directory into the <project>/jni directory
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3. Edit <project>/jni/src/Android.mk to include your source files
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4. Run 'ndk-build' (a script provided by the NDK). This compiles the C source
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If you want to use the Eclipse IDE, skip to the Eclipse section below.
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5. Create <project>/local.properties and use that to point to the Android SDK directory, by writing a line with the following form:
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sdk.dir=PATH_TO_ANDROID_SDK
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6. Run 'ant debug' in android/project. This compiles the .java and eventually 
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   creates a .apk with the native code embedded
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7. 'ant debug install' will push the apk to the device or emulator (if connected)
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Here's an explanation of the files in the Android project, so you can customize them:
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android-project/
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	AndroidManifest.xml	- package manifest. Among others, it contains the class name
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				  of the main Activity and the package name of the application.
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	build.properties	- empty
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	build.xml		- build description file, used by ant. The actual application name
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				  is specified here.
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	default.properties	- holds the target ABI for the application, android-10 and up
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	project.properties	- holds the target ABI for the application, android-10 and up
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	local.properties	- holds the SDK path, you should change this to the path to your SDK
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	jni/			- directory holding native code
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	jni/Android.mk		- Android makefile that can call recursively the Android.mk files
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				  in all subdirectories
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	jni/SDL/		- (symlink to) directory holding the SDL library files
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	jni/SDL/Android.mk	- Android makefile for creating the SDL shared library
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	jni/src/		- directory holding your C/C++ source
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	jni/src/Android.mk	- Android makefile that you should customize to include your 
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                                  source code and any library references
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	res/			- directory holding resources for your application
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	res/drawable-*		- directories holding icons for different phone hardware. Could be
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				  one dir called "drawable".
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	res/layout/main.xml	- Usually contains a file main.xml, which declares the screen layout.
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				  We don't need it because we use the SDL video output.
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	res/values/strings.xml	- strings used in your application, including the application name
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				  shown on the phone.
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	src/org/libsdl/app/SDLActivity.java - the Java class handling the initialization and binding
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				  to SDL.  Be very careful changing this, as the SDL library relies
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				  on this implementation.
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================================================================================
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 Customizing your application name
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================================================================================
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To customize your application name, edit AndroidManifest.xml and replace
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"org.libsdl.app" with an identifier for your product package.
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Then create a Java class extending SDLActivity and place it in a directory
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under src matching your package, e.g.
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	src/com/gamemaker/game/MyGame.java
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Here's an example of a minimal class file:
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--- MyGame.java --------------------------
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package com.gamemaker.game;
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import org.libsdl.app.SDLActivity; 
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/* 
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 * A sample wrapper class that just calls SDLActivity 
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 */ 
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public class MyGame extends SDLActivity { }
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------------------------------------------
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Then replace "SDLActivity" in AndroidManifest.xml with the name of your
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class, .e.g. "MyGame"
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================================================================================
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 Customizing your application icon
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================================================================================
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Conceptually changing your icon is just replacing the "ic_launcher.png" files in
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the drawable directories under the res directory. There are four directories for
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different screen sizes. These can be replaced with one dir called "drawable",
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containing an icon file "ic_launcher.png" with dimensions 48x48 or 72x72.
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You may need to change the name of your icon in AndroidManifest.xml to match
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this icon filename.
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================================================================================
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 Loading assets
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Any files you put in the "assets" directory of your android-project directory
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will get bundled into the application package and you can load them using the
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standard functions in SDL_rwops.h.
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There are also a few Android specific functions that allow you to get other
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useful paths for saving and loading data:
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SDL_AndroidGetInternalStoragePath()
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SDL_AndroidGetExternalStorageState()
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SDL_AndroidGetExternalStoragePath()
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See SDL_system.h for more details on these functions.
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The asset packaging system will, by default, compress certain file extensions.
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SDL includes two asset file access mechanisms, the preferred one is the so
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called "File Descriptor" method, which is faster and doesn't involve the Dalvik
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GC, but given this method does not work on compressed assets, there is also the
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"Input Stream" method, which is automatically used as a fall back by SDL. You
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may want to keep this fact in mind when building your APK, specially when large
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files are involved.
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For more information on which extensions get compressed by default and how to
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disable this behaviour, see for example:
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http://ponystyle.com/blog/2010/03/26/dealing-with-asset-compression-in-android-apps/
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================================================================================
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 Pause / Resume behaviour
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================================================================================
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If SDL is compiled with SDL_ANDROID_BLOCK_ON_PAUSE defined (the default),
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the event loop will block itself when the app is paused (ie, when the user
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returns to the main Android dashboard). Blocking is better in terms of battery
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use, and it allows your app to spring back to life instantaneously after resume
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(versus polling for a resume message).
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Upon resume, SDL will attempt to restore the GL context automatically.
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In modern devices (Android 3.0 and up) this will most likely succeed and your
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app can continue to operate as it was.
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However, there's a chance (on older hardware, or on systems under heavy load),
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where the GL context can not be restored. In that case you have to listen for
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a specific message, (which is not yet implemented!) and restore your textures
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manually or quit the app (which is actually the kind of behaviour you'll see
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under iOS, if the OS can not restore your GL context it will just kill your app)
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================================================================================
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 Threads and the Java VM
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================================================================================
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For a quick tour on how Linux native threads interoperate with the Java VM, take
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a look here: http://developer.android.com/guide/practices/jni.html
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If you want to use threads in your SDL app, it's strongly recommended that you
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do so by creating them using SDL functions. This way, the required attach/detach
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handling is managed by SDL automagically. If you have threads created by other
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means and they make calls to SDL functions, make sure that you call
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Android_JNI_SetupThread before doing anything else otherwise SDL will attach
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your thread automatically anyway (when you make an SDL call), but it'll never
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detach it.
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================================================================================
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 Using STL
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You can use STL in your project by creating an Application.mk file in the jni
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folder and adding the following line:
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APP_STL := stlport_static
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For more information check out CPLUSPLUS-SUPPORT.html in the NDK documentation.
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================================================================================
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 Additional documentation
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================================================================================
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The documentation in the NDK docs directory is very helpful in understanding the
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build process and how to work with native code on the Android platform.
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The best place to start is with docs/OVERVIEW.TXT
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================================================================================
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 Using Eclipse
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================================================================================
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First make sure that you've installed Eclipse and the Android extensions as described here:
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	http://developer.android.com/sdk/eclipse-adt.html
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Once you've copied the SDL android project and customized it, you can create an Eclipse project from it:
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 * File -> New -> Other
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 * Select the Android -> Android Project wizard and click Next
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 * Enter the name you'd like your project to have
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 * Select "Create project from existing source" and browse for your project directory
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 * Make sure the Build Target is set to Android 2.0
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 * Click Finish
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================================================================================
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 Using the emulator
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================================================================================
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There are some good tips and tricks for getting the most out of the
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emulator here: http://developer.android.com/tools/devices/emulator.html
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Especially useful is the info on setting up OpenGL ES 2.0 emulation.
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Notice that this software emulator is incredibly slow and needs a lot of disk space.
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Using a real device works better.
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================================================================================
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 Troubleshooting
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================================================================================
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You can create and run an emulator from the Eclipse IDE:
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 * Window -> Android SDK and AVD Manager
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You can see if adb can see any devices with the following command:
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	adb devices
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You can see the output of log messages on the default device with:
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	adb logcat
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You can push files to the device with:
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	adb push local_file remote_path_and_file
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You can push files to the SD Card at /sdcard, for example:
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	adb push moose.dat /sdcard/moose.dat
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You can see the files on the SD card with a shell command:
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	adb shell ls /sdcard/
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You can start a command shell on the default device with:
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	adb shell
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You can remove the library files of your project (and not the SDL lib files) with:
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	ndk-build clean
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You can do a build with the following command:
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	ndk-build
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You can see the complete command line that ndk-build is using by passing V=1 on the command line:
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	ndk-build V=1
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If your application crashes in native code, you can use addr2line to convert the
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addresses in the stack trace to lines in your code.
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For example, if your crash looks like this:
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I/DEBUG   (   31): signal 11 (SIGSEGV), code 2 (SEGV_ACCERR), fault addr 400085d0
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I/DEBUG   (   31):  r0 00000000  r1 00001000  r2 00000003  r3 400085d4
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I/DEBUG   (   31):  r4 400085d0  r5 40008000  r6 afd41504  r7 436c6a7c
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I/DEBUG   (   31):  r8 436c6b30  r9 435c6fb0  10 435c6f9c  fp 4168d82c
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I/DEBUG   (   31):  ip 8346aff0  sp 436c6a60  lr afd1c8ff  pc afd1c902  cpsr 60000030
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I/DEBUG   (   31):          #00  pc 0001c902  /system/lib/libc.so
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I/DEBUG   (   31):          #01  pc 0001ccf6  /system/lib/libc.so
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I/DEBUG   (   31):          #02  pc 000014bc  /data/data/org.libsdl.app/lib/libmain.so
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I/DEBUG   (   31):          #03  pc 00001506  /data/data/org.libsdl.app/lib/libmain.so
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You can see that there's a crash in the C library being called from the main code.
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I run addr2line with the debug version of my code:
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	arm-eabi-addr2line -C -f -e obj/local/armeabi/libmain.so
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and then paste in the number after "pc" in the call stack, from the line that I care about:
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000014bc
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I get output from addr2line showing that it's in the quit function, in testspriteminimal.c, on line 23.
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You can add logging to your code to help show what's happening:
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#include <android/log.h>
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	__android_log_print(ANDROID_LOG_INFO, "foo", "Something happened! x = %d", x);
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If you need to build without optimization turned on, you can create a file called
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"Application.mk" in the jni directory, with the following line in it:
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APP_OPTIM := debug
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================================================================================
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 Memory debugging
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================================================================================
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The best (and slowest) way to debug memory issues on Android is valgrind.
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Valgrind has support for Android out of the box, just grab code using:
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	svn co svn://svn.valgrind.org/valgrind/trunk valgrind
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... and follow the instructions in the file README.android to build it.
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One thing I needed to do on Mac OS X was change the path to the toolchain,
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and add ranlib to the environment variables:
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export RANLIB=$NDKROOT/toolchains/arm-linux-androideabi-4.4.3/prebuilt/darwin-x86/bin/arm-linux-androideabi-ranlib
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Once valgrind is built, you can create a wrapper script to launch your
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application with it, changing org.libsdl.app to your package identifier:
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--- start_valgrind_app -------------------
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#!/system/bin/sh
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export TMPDIR=/data/data/org.libsdl.app
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exec /data/local/Inst/bin/valgrind --log-file=/sdcard/valgrind.log --error-limit=no $*
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------------------------------------------
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Then push it to the device:
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	adb push start_valgrind_app /data/local
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and make it executable:
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	adb shell chmod 755 /data/local/start_valgrind_app
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and tell Android to use the script to launch your application:
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	adb shell setprop wrap.org.libsdl.app "logwrapper /data/local/start_valgrind_app"
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If the setprop command says "could not set property", it's likely that
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your package name is too long and you should make it shorter by changing
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AndroidManifest.xml and the path to your class file in android-project/src
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You can then launch your application normally and waaaaaaaiiittt for it.
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You can monitor the startup process with the logcat command above, and
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when it's done (or even while it's running) you can grab the valgrind
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output file:
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	adb pull /sdcard/valgrind.log
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When you're done instrumenting with valgrind, you can disable the wrapper:
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	adb shell setprop wrap.org.libsdl.app ""
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================================================================================
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 Why is API level 10 the minimum required?
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API level 10 is required because SDL requires some functionality for running not
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available on older devices and some for building which is not in older NDK/SDKs.
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Support for native OpenGL ES and ES2 applications was introduced in the NDK for
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API level 4 and 8. EGL was made a stable API in the NDK for API level 9, which
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has since then been obsoleted, with the recommendation to developers to bump the
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required API level to 10.
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As of this writing, according to http://developer.android.com/about/dashboards/index.html
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about 90% of the Android devices accessing Google Play support API level 10 or
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higher (March 2013).
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 A note regarding the use of the "dirty rectangles" rendering technique
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If your app uses a variation of the "dirty rectangles" rendering technique,
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where you only update a portion of the screen on each frame, you may notice a
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variety of visual glitches on Android, that are not present on other platforms.
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This is caused by SDL's use of EGL as the support system to handle OpenGL ES/ES2
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contexts, in particular the use of the eglSwapBuffers function. As stated in the
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documentation for the function "The contents of ancillary buffers are always 
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undefined after calling eglSwapBuffers".
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Setting the EGL_SWAP_BEHAVIOR attribute of the surface to EGL_BUFFER_PRESERVED
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is not possible for SDL as it requires EGL 1.4, available only on the API level
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17+, so the only workaround available on this platform is to redraw the entire
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screen each frame.
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Reference: http://www.khronos.org/registry/egl/specs/EGLTechNote0001.html
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 Known issues
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- TODO. I'm sure there's a bunch more stuff I haven't thought of