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Simple DirectMedia Layer for Android

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Requirements:

Android SDK (version 10 or later)

http://developer.android.com/sdk/index.html

Android NDK r7 or later

http://developer.android.com/sdk/ndk/index.html

Minimum API level supported by SDL: 10 (Android 2.3.3)

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How the port works

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- 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/SDL_android.cpp

Your project must include some glue code that starts your main() routine:

src/main/android/SDL_android_main.cpp

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Building an app

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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 <project>/jni directory

3. Edit <project>/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.

5. Edit <project>/local.properties to point to the Android SDK directory

6. Run 'ant debug' in android/project. This compiles the .java and eventually

creates a .apk with the native code embedded

7. '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.

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.

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Customizing your application name

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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 { }

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Then replace "SDLActivity" in AndroidManifest.xml with the name of your

class, .e.g. "MyGame"

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Customizing your application icon

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Conceptually changing your icon is just replacing the icon.png files in the

drawable directories under the res directory. There are 3 directories for

different screen sizes. These can be replaced with 1 dir called 'drawable',

containing an icon file 'icon.png' with dimensions 48x48 or 72x72.

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

this icon filename.

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Loading assets

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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/

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Pause / Resume behaviour

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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)

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Threads and the Java VM

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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.

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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

folder and adding the following line:

APP_STL := stlport_static

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

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Additional documentation

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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

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Using Eclipse

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First make sure that you've installed Eclipse and the Android extensions as described here:

http://developer.android.com/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 2.0

* Click Finish

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Using the emulator

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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.

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Troubleshooting

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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

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Memory debugging

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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 $*

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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 ""

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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

available on older devices and some for building which is not in older NDK/SDKs.

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).

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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,

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

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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