文章记录自己的学习过程，供日后参考。
首先要知道壁纸设置的大体流程：上层应用调用wallpaperManager.setStream()或其他接口进行设置壁纸，WallpaperManagerService首先将壁纸copy到/data/system/user/0/目录下，WallpaperManagerService对该目录注册了一个WallpaperObserver，拷贝壁纸成功后会触发调用WallpaperObserver.onEvent()函数，onEvent()函数首先调用notifyCallbacksLocked()触发回调，然后再调用bindWallpaperComponentLocked()做进行进一步的壁纸更换。虽然整个壁纸机制有WallpaperManager、WallpaperService、ImageWallpaper、WallpaperManagerService、Globals、DrawableEngine、WallpaperData、WallpaperConnection等数据结构，感觉挺复杂的，我们应该剥离他的外表，看清他的本质。我们只要搞清了他们之间怎么交互的？交流些什么东西？就搞清楚了整个壁纸机制，说白了只要搞清楚IWallpaperService.aidl、IWallpaperEngine.aidl、IWallpaperConnection.aidl这三个文件就OK了，因为这三个文件是通信的关键所在。
1.IWallpaperService.aidl
oneway interface IWallpaperService {
void attach(IWallpaperConnection connection,
IBinder windowToken, int windowType, boolean isPreview,
int reqWidth, int reqHeight);
}
aidl语言专为Binder设计，因此里面的函数可看做是通信接口。
(1).WallpaperService.IWallpaperServiceWrapper extends IWallpaperService.Stub；
(2).WallpaperService.onBind()
/**
* Implement to return the implementation of the internal accessibility
* service interface. Subclasses should not override.
*/
@Override
public final IBinder onBind(Intent intent) {
return new IWallpaperServiceWrapper(this);
}
从上面两点可以知道：IWallpaperService的Binder本地端在WallpaperService中；WallpaperService服务将是调用mContext.bindService()启动的；IWallpaperService的代理对象将在ServiceConnection.onServiceConnected()中获取。很容易验证WallPaperManagerService中确实是这么干的。
WallPaperManagerService.bindWallpaperComponentLocked():
boolean bindWallpaperComponentLocked(ComponentName componentName, boolean force,
boolean fromUser, WallpaperData wallpaper, IRemoteCallback reply) {
........
Intent intent = new Intent(WallpaperService.SERVICE_INTERFACE);
........
WallpaperConnection newConn = new WallpaperConnection(wi, wallpaper);
intent.setComponent(componentName);
intent.putExtra(Intent.EXTRA_CLIENT_LABEL,
com.android.internal.R.string.wallpaper_binding_label);
intent.putExtra(Intent.EXTRA_CLIENT_INTENT, PendingIntent.getActivityAsUser(
mContext, 0,
Intent.createChooser(new Intent(Intent.ACTION_SET_WALLPAPER),
mContext.getText(com.android.internal.R.string.chooser_wallpaper)),
0, null, new UserHandle(serviceUserId)));
if (!mContext.bindServiceAsUser(intent, newConn,
Context.BIND_AUTO_CREATE | Context.BIND_SHOWING_UI,
new UserHandle(serviceUserId))) {
..........
}
}
文章最开头就说了，每次设置壁纸都会调用bindWallpaperComponentLocked()函数，也就是说每次设置壁纸都会重新new WallpaperConnection，重新调用mContext.bindServiceAsUser()绑定启动WallpaperService，为啥要每次重新bind一次呢？防止systemUI挂掉，无法设置壁纸？不管了，反正记住每次设置壁纸时都会bind一次WallpaperService，并且调用WallpaperConnection.onServiceConnected()函数。onServiceConnected()函数中真的获取了IWallpaperService的Binder代理对象。
public void onServiceConnected(ComponentName name, IBinder service) {
synchronized (mLock) {
if (mWallpaper.connection == this) {
mWallpaper.lastDiedTime = SystemClock.uptimeMillis();
mService = IWallpaperService.Stub.asInterface(service);
attachServiceLocked(this, mWallpaper);
// XXX should probably do saveSettingsLocked() later
// when we have an engine, but I'm not sure about
// locking there and anyway we always need to be able to
// recover if there is something wrong.
saveSettingsLocked(mWallpaper);
}
}
}
mService = IWallpaperService.Stub.asInterface(service);语句获取了IWallpaperService的Binder代理对象。再看看WallpaperConnection.onServiceConnected()函数中调用WallPaperManagerService.attachServiceLocked()干什么？
void attachServiceLocked(WallpaperConnection conn, WallpaperData wallpaper) {
try {
conn.mService.attach(conn, conn.mToken,
WindowManager.LayoutParams.TYPE_WALLPAPER, false,
wallpaper.width, wallpaper.height);
} catch (RemoteException e) {
Slog.w(TAG, "Failed attaching wallpaper; clearing", e);
if (!wallpaper.wallpaperUpdating) {
bindWallpaperComponentLocked(null, false, false, wallpaper, null);
}
}
}
嗯，跨Binder调用IWallpaperService本地端的attach()。看看attach()做了什么？
public void attach(IWallpaperConnection conn, IBinder windowToken,
int windowType, boolean isPreview, int reqWidth, int reqHeight) {
new IWallpaperEngineWrapper(mTarget, conn, windowToken,
windowType, isPreview, reqWidth, reqHeight);
}
直接new IWallpaperEngineWrapper对象，注意传进来的参数，包含壁纸显示参数。IWallpaperEngineWrapper extends IWallpaperEngine.Stub implements HandlerCaller.Callback，所以又知道了IWallpaperEngine的Binder本地端保存在WallpaperService中，代理端返回给WallPaperManagerService.WallpaperConnection.mEngine变量的呢？这个研究到后边应该自然就知道了。IWallpaperEngineWrapper的构造函数将被调用，在构造函数中会post一个handle消息DO_ATTACH，
switch (message.what) {
case DO_ATTACH: {
try {
mConnection.attachEngine(this);
} catch (RemoteException e) {
Log.w(TAG, "Wallpaper host disappeared", e);
return;
}
Engine engine = onCreateEngine();
mEngine = engine;
mActiveEngines.add(engine);
engine.attach(this);
return;
}
感觉牵涉的越来越多了，没关系，一个个来分析。
先上班，下班再研究。