Android應用程式註冊廣播接收器 registerReceiver 的過程分析
前面我們介紹了Android系統的廣播機制,從本質來說,它是一種訊息訂閱/釋出機制,因此,使用這種訊息驅動模型的第一步便是訂閱訊息;而對Android應用程式來說,訂閱訊息其實就是註冊廣播接收器,本文將探討Android應用程式是如何註冊廣播接收器以及把廣播接收器註冊到哪裡去的。
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在Android的廣播機制中,ActivityManagerService扮演著廣播中心的角色,負責系統中所有廣播的註冊和釋出操作,因此,Android應用程式註冊廣播接收器的過程就把是廣播接收器註冊到ActivityManagerService的過程。Android應用程式是通過呼叫ContextWrapper類的registerReceiver函式來把廣播接收器BroadcastReceiver註冊到ActivityManagerService中去的,而ContextWrapper類本身又藉助ContextImpl類來註冊廣播接收器。
在Android應用程式框架中,Activity和Service類都繼承了ContextWrapper類,因此,我們可以在Activity或者Service的子類中呼叫registerReceiver函式來註冊廣播接收器。Activity、Service、ContextWrapper和ContextImpl這四個類的關係可以參考前面Android系統在新程序中啟動自定義服務過程(startService)的原理分析一文中描述的Activity類圖。
這篇文章還是繼續以例項來進行情景分析,所用到的例子便是上一篇文章Android系統中的廣播(Broadcast)機制簡要介紹和學習計劃
開始進入主題了,在Android系統中的廣播(Broadcast)機制簡要介紹和學習計劃一文所介紹的例子中,註冊廣播接收器的操作是MainActivity發起的,我們先來看看註冊過程的序列圖:
在分析這個序列圖之前,我們先來看一下MainActivity是如何呼叫registerReceiver函式來註冊廣播接收器的:
public class MainActivity extends Activity implements OnClickListener { ...... @Override public void onResume() { super.onResume(); IntentFilter counterActionFilter = new IntentFilter(CounterService.BROADCAST_COUNTER_ACTION); registerReceiver(counterActionReceiver, counterActionFilter); } ......}
MainActivity在onResume函式裡,通過其父類ContextWrapper的registerReceiver函式註冊了一個BroadcastReceiver例項counterActionReceiver,並且通過IntentFilter例項counterActionFilter告訴ActivityManagerService,它要訂閱的廣播是CounterService.BROADCAST_COUNTER_ACTION型別的,這樣,ActivityManagerService在收到CounterService.BROADCAST_COUNTER_ACTION型別的廣播時,就會分發給counterActionReceiver例項的onReceive函式。接下來,就開始分析註冊過程中的每一個步驟了。
Step 1. ContextWrapper.registerReceiver
這個函式實現在frameworks/base/core/java/android/content/ContextWrapper.java檔案中:
public class ContextWrapper extends Context { Context mBase; ...... @Override public Intent registerReceiver( BroadcastReceiver receiver, IntentFilter filter) { return mBase.registerReceiver(receiver, filter); } ......}
這裡的成員變數mBase是一個ContextImpl例項,想知道為什麼,可以回過頭去看看Android應用程式啟動過程原始碼分析這篇文章>~<。Step 2. ContextImpl.registerReceiver
這個函式實現在frameworks/base/core/java/android/app/ContextImpl.java檔案中:
class ContextImpl extends Context { ...... @Override public Intent registerReceiver(BroadcastReceiver receiver, IntentFilter filter) { return registerReceiver(receiver, filter, null, null); } @Override public Intent registerReceiver(BroadcastReceiver receiver, IntentFilter filter, String broadcastPermission, Handler scheduler) { return registerReceiverInternal(receiver, filter, broadcastPermission, scheduler, getOuterContext()); } private Intent registerReceiverInternal(BroadcastReceiver receiver, IntentFilter filter, String broadcastPermission, Handler scheduler, Context context) { IIntentReceiver rd = null; if (receiver != null) { if (mPackageInfo != null && context != null) { if (scheduler == null) { scheduler = mMainThread.getHandler(); } rd = mPackageInfo.getReceiverDispatcher( receiver, context, scheduler, mMainThread.getInstrumentation(), true); } else { ...... } } try { return ActivityManagerNative.getDefault().registerReceiver( mMainThread.getApplicationThread(), rd, filter, broadcastPermission); } catch (RemoteException e) { return null; } } ......}
通過兩個函式的中轉,最終就進入到ContextImpl.registerReceiverInternal這個函式來了。這裡的成員變數mPackageInfo是一個LoadedApk例項,它是用來負責處理廣播的接收的,在後面一篇文章講到廣播的傳送時(sendBroadcast),會詳細描述。引數broadcastPermission和scheduler都為null,而引數context是上面的函式通過呼叫函式getOuterContext得到的,這裡它就是指向MainActivity了,因為MainActivity是繼承於Context類的,因此,這裡用Context型別來引用。由於條件mPackageInfo != null和context != null都成立,而且條件scheduler == null也成立,於是就呼叫mMainThread.getHandler來獲得一個Handler了,這個Hanlder是後面用來分發ActivityManagerService傳送過的廣播用的。這裡的成員變數mMainThread是一個ActivityThread例項,在前面Android應用程式啟動過程原始碼分析這篇文章也描述過了。我們先來看看ActivityThread.getHandler函式的實現,然後再回過頭來繼續分析ContextImpl.registerReceiverInternal函式。
Step 3. ActivityThread.getHandler
這個函式實現在frameworks/base/core/java/android/app/ActivityThread.java檔案中:
public final class ActivityThread { ...... final H mH = new H(); private final class H extends Handler { ...... public void handleMessage(Message msg) { ...... switch (msg.what) { ...... } ...... } ...... } ...... final Handler getHandler() { return mH; } ......}
有了這個Handler之後,就可以分發訊息給應用程式處理了。再回到上一步的ContextImpl.registerReceiverInternal函式中,它通過mPackageInfo.getReceiverDispatcher函式獲得一個IIntentReceiver介面物件rd,這是一個Binder物件,接下來會把它傳給ActivityManagerService,ActivityManagerService在收到相應的廣播時,就是通過這個Binder物件來通知MainActivity來接收的。
我們也是先來看一下mPackageInfo.getReceiverDispatcher函式的實現,然後再回過頭來繼續分析ContextImpl.registerReceiverInternal函式。
Step 4. LoadedApk.getReceiverDispatcher
這個函式實現在frameworks/base/core/java/android/app/LoadedApk.java檔案中:
final class LoadedApk { ...... public IIntentReceiver getReceiverDispatcher(BroadcastReceiver r, Context context, Handler handler, Instrumentation instrumentation, boolean registered) { synchronized (mReceivers) { LoadedApk.ReceiverDispatcher rd = null; HashMap<BroadcastReceiver, LoadedApk.ReceiverDispatcher> map = null; if (registered) { map = mReceivers.get(context); if (map != null) { rd = map.get(r); } } if (rd == null) { rd = new ReceiverDispatcher(r, context, handler, instrumentation, registered); if (registered) { if (map == null) { map = new HashMap<BroadcastReceiver, LoadedApk.ReceiverDispatcher>(); mReceivers.put(context, map); } map.put(r, rd); } } else { rd.validate(context, handler); } return rd.getIIntentReceiver(); } } ...... static final class ReceiverDispatcher { final static class InnerReceiver extends IIntentReceiver.Stub { final WeakReference<LoadedApk.ReceiverDispatcher> mDispatcher; ...... InnerReceiver(LoadedApk.ReceiverDispatcher rd, boolean strong) { mDispatcher = new WeakReference<LoadedApk.ReceiverDispatcher>(rd); ...... } ...... } ...... final IIntentReceiver.Stub mIIntentReceiver; final Handler mActivityThread; ...... ReceiverDispatcher(BroadcastReceiver receiver, Context context, Handler activityThread, Instrumentation instrumentation, boolean registered) { ...... mIIntentReceiver = new InnerReceiver(this, !registered); mActivityThread = activityThread; ...... } ...... IIntentReceiver getIIntentReceiver() { return mIIntentReceiver; } } ......}
在LoadedApk.getReceiverDispatcher函式中,首先看一下引數r是不是已經有相應的ReceiverDispatcher存在了,如果有,就直接返回了,否則就新建一個ReceiverDispatcher,並且以r為Key值保在一個HashMap中,而這個HashMap以Context,這裡即為MainActivity為Key值儲存在LoadedApk的成員變數mReceivers中,這樣,只要給定一個Activity和BroadcastReceiver,就可以檢視LoadedApk裡面是否已經存在相應的廣播接收發布器ReceiverDispatcher了。
在新建廣播接收發布器ReceiverDispatcher時,會在建構函式裡面建立一個InnerReceiver例項,這是一個Binder物件,實現了IIntentReceiver介面,可以通過ReceiverDispatcher.getIIntentReceiver函式來獲得,獲得後就會把它傳給ActivityManagerService,以便接收廣播。在ReceiverDispatcher類的建構函式中,還會把傳進來的Handle型別的引數activityThread儲存下來,以便後面在分發廣播的時候使用。
現在,再回到ContextImpl.registerReceiverInternal函式,在獲得了IIntentReceiver型別的Binder物件後,就開始要把它註冊到ActivityManagerService中去了。
Step 5. ActivityManagerProxy.registerReceiver
這個函式實現在frameworks/base/core/java/android/app/ActivityManagerNative.java檔案中:
class ActivityManagerProxy implements IActivityManager{ ...... public Intent registerReceiver(IApplicationThread caller, IIntentReceiver receiver, IntentFilter filter, String perm) throws RemoteException { Parcel data = Parcel.obtain(); Parcel reply = Parcel.obtain(); data.writeInterfaceToken(IActivityManager.descriptor); data.writeStrongBinder(caller != null ? caller.asBinder() : null); data.writeStrongBinder(receiver != null ? receiver.asBinder() : null); filter.writeToParcel(data, 0); data.writeString(perm); mRemote.transact(REGISTER_RECEIVER_TRANSACTION, data, reply, 0); reply.readException(); Intent intent = null; int haveIntent = reply.readInt(); if (haveIntent != 0) { intent = Intent.CREATOR.createFromParcel(reply); } reply.recycle(); data.recycle(); return intent; } ......}
這個函式通過Binder驅動程式就進入到ActivityManagerService中的registerReceiver函式中去了。Step 6. ActivityManagerService.registerReceiver
這個函式實現在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java檔案中:
public final class ActivityManagerService extends ActivityManagerNative implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback { ...... public Intent registerReceiver(IApplicationThread caller, IIntentReceiver receiver, IntentFilter filter, String permission) { synchronized(this) { ProcessRecord callerApp = null; if (caller != null) { callerApp = getRecordForAppLocked(caller); if (callerApp == null) { ...... } } List allSticky = null; // Look for any matching sticky broadcasts... Iterator actions = filter.actionsIterator(); if (actions != null) { while (actions.hasNext()) { String action = (String)actions.next(); allSticky = getStickiesLocked(action, filter, allSticky); } } else { ...... } // The first sticky in the list is returned directly back to // the client. Intent sticky = allSticky != null ? (Intent)allSticky.get(0) : null; ...... if (receiver == null) { return sticky; } ReceiverList rl = (ReceiverList)mRegisteredReceivers.get(receiver.asBinder()); if (rl == null) { rl = new ReceiverList(this, callerApp, Binder.getCallingPid(), Binder.getCallingUid(), receiver); if (rl.app != null) { rl.app.receivers.add(rl); } else { ...... } mRegisteredReceivers.put(receiver.asBinder(), rl); } BroadcastFilter bf = new BroadcastFilter(filter, rl, permission); rl.add(bf); ...... mReceiverResolver.addFilter(bf); // Enqueue broadcasts for all existing stickies that match // this filter. if (allSticky != null) { ...... } return sticky; } } ......}
函式首先是獲得呼叫registerReceiver函式的應用程式程序記錄塊: ProcessRecord callerApp = null; if (caller != null) { callerApp = getRecordForAppLocked(caller); if (callerApp == null) { ...... } }
這裡得到的便是上一篇文章Android系統中的廣播(Broadcast)機制簡要介紹和學習計劃裡面介紹的應用程式Broadcast的程序記錄塊了,MainActivity就是在裡面啟動起來的。 List allSticky = null; // Look for any matching sticky broadcasts... Iterator actions = filter.actionsIterator(); if (actions != null) { while (actions.hasNext()) { String action = (String)actions.next(); allSticky = getStickiesLocked(action, filter, allSticky); } } else { ...... } // The first sticky in the list is returned directly back to // the client. Intent sticky = allSticky != null ? (Intent)allSticky.get(0) : null;
這裡傳進來的filter只有一個action,就是前面描述的CounterService.BROADCAST_COUNTER_ACTION了,這裡先通過getStickiesLocked函式查詢一下有沒有對應的sticky intent列表存在。什麼是Sticky Intent呢?我們在最後一次呼叫sendStickyBroadcast函式來發送某個Action型別的廣播時,系統會把代表這個廣播的Intent儲存下來,這樣,後來呼叫registerReceiver來註冊相同Action型別的廣播接收器,就會得到這個最後發出的廣播。這就是為什麼叫做Sticky Intent了,這個最後發出的廣播雖然被處理完了,但是仍然被粘住在ActivityManagerService中,以便下一個註冊相應Action型別的廣播接收器還能繼承處理。這裡,假設我們不使用sendStickyBroadcast來發送CounterService.BROADCAST_COUNTER_ACTION型別的廣播,於是,這裡得到的allSticky和sticky都為null了。
繼續往下看,這裡傳進來的receiver不為null,於是,繼續往下執行:
ReceiverList rl = (ReceiverList)mRegisteredReceivers.get(receiver.asBinder()); if (rl == null) { rl = new ReceiverList(this, callerApp, Binder.getCallingPid(), Binder.getCallingUid(), receiver); if (rl.app != null) { rl.app.receivers.add(rl); } else { ...... } mRegisteredReceivers.put(receiver.asBinder(), rl); }
這裡其實就是把廣播接收器receiver儲存一個ReceiverList列表中,這個列表的宿主程序是rl.app,這裡就是MainActivity所在的程序了,在ActivityManagerService中,用一個程序記錄塊來表示這個應用程式程序,它裡面有一個列表receivers,專門用來儲存這個程序註冊的廣播接收器。接著,又把這個ReceiverList列表以receiver為Key值儲存在ActivityManagerService的成員變數mRegisteredReceivers中,這些都是為了方便在收到廣播時,快速找到對應的廣播接收器的。再往下看:
BroadcastFilter bf = new BroadcastFilter(filter, rl, permission); rl.add(bf); ...... mReceiverResolver.addFilter(bf);
上面只是把廣播接收器receiver儲存起來了,但是還沒有把它和filter關聯起來,這裡就建立一個BroadcastFilter來把廣播接收器列表rl和filter關聯起來,然後儲存在ActivityManagerService中的成員變數mReceiverResolver中去。這樣,廣播接收器註冊的過程就介紹完了,比較簡單,但是工作又比較瑣碎,主要就是將廣播接收器receiver及其要接收的廣播型別filter儲存在ActivityManagerService中,以便以後能夠接收到相應的廣播並進行處理,在下一篇文章,我們將詳細分析這個過程,敬請關注。