BroadcastReceiver

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BroadcastReceiver，似乎很简单！很多书籍上都有描述，从书中例子来看百多行代码就搞定！但是真的如此吗？

假如有个需求，要求大概如下：当蓝牙开始搜索设备时，点亮LED；搜索停止时，关掉LED。这个需求开始检讨时，认为新规一个BroadcastReceiver子类，收到蓝牙开始搜索/停止的广播通知时，点亮/关闭LED即可。可是经过一番调查后，木子发现：BroadcastReceiver没那么简单。让我们一起来看看吧！

第一， sendBroadcast / sendOrderedBroadcast / sendStickyBroadcast是异步函数，它们发出广播通知后即返回，不会等到所有receiver收到通知后才继续往下走，所以控制LED的receiver动作就不会和真正的蓝牙搜索同步，甚至可能搜索完了时，LED才开始点灯在；

涉及的代码比较多，来张时序图吧！

第二，为什么对sendOrderedBroadcast发出的broadcast，receiver在处理时能重新赋值（通过setResultCode / setResultData / setResultExtras），但是对sendBroadcast发出的broadcast就不行；

看看源代码吧！打开BroadReceiv.java文件，可发现如下代码：

public final void setResultExtras(Bundle extras) {

checkSynchronousHint();

mResultExtras = extras;

}

void checkSynchronousHint() {

// Note that we don't assert when receiving the initial sticky value,

// since that may have come from an ordered broadcast. We'll catch

// them later when the real broadcast happens again.

if (mOrderedHint || mInitialStickyHint) {

return;

}

RuntimeException e = new RuntimeException(

"BroadcastReceiver trying to return result during a non-ordered broadcast");

e.fillInStackTrace();

Log.e("BroadcastReceiver", e.getMessage(), e);

}

在这里，我们发现可否调用setResultExtras之类的函数取决于mOrderedHint || mInitialStickyHint的值，那么它们是何时赋值的呢？

继续看代码，打开ActivityThread.java，发现以下代码：

private final void handleReceiver(ReceiverData data) {

……

ContextImpl context = (ContextImpl)app.getBaseContext();

receiver.setOrderedHint(true);

receiver.setResult(data.resultCode, data.resultData,

data.resultExtras);

receiver.setOrderedHint(data.sync);

receiver.onReceive(context.getReceiverRestrictedContext(),

data.intent);

……

}

还有，ReceiverDispatcher. InnerReceiver. Args. Run() {

……

receiver.setOrderedHint(true);

receiver.setResult(mCurCode, mCurData, mCurMap);

receiver.clearAbortBroadcast();

receiver.setOrderedHint(mCurOrdered);

receiver.setInitialStickyHint(mCurSticky);

receiver.onReceive(mContext, intent);

……

}

现在是不是很清楚原因了？

第三，什么是sendStickyBroadcast？SDK上说的简单了点，至少木子没看懂！

打开ActivityManagerService.java文件，找到如下代码：

public Intent registerReceiver(IApplicationThread caller,

IIntentReceiver receiver, IntentFilter filter, String permission) {

……

// 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 {

allSticky = getStickiesLocked(null, filter, allSticky);

}

……

}

private final List getStickiesLocked(String action, IntentFilter filter,

List cur) {

final ContentResolver resolver = mContext.getContentResolver();

final ArrayList<Intent> list = mStickyBroadcasts.get(action);

if (list == null) {

return cur;

}

int N = list.size();

for (int i=0; i<N; i++) {

Intent intent = list.get(i);

if (filter.match(resolver, intent, true, TAG) >= 0) {

if (cur == null) {

cur = new ArrayList<Intent>();

}

cur.add(intent);

}

return cur;

}

从这里可以看出调用 Intent android.content.ContextWrapper.registerReceiver(BroadcastReceiver receiver, IntentFilter filter) 返回的intent是从mStickyBroadcasts里得到的。那么mStickyBroadcasts里的内容从何而来？

接着看：

private final int broadcastIntentLocked(ProcessRecord callerApp,

String callerPackage, Intent intent, String resolvedType,

IIntentReceiver resultTo, int resultCode, String resultData,

Bundle map, String requiredPermission,

boolean ordered, boolean sticky, int callingPid, int callingUid) {

……

// Add to the sticky list if requested.

if (sticky) {

if (checkPermission(android.Manifest.permission.BROADCAST_STICKY,

callingPid, callingUid)

!= PackageManager.PERMISSION_GRANTED) {

String msg = "Permission Denial: broadcastIntent() requesting a sticky broadcast from pid="

+ callingPid + ", uid=" + callingUid

+ " requires " + android.Manifest.permission.BROADCAST_STICKY;

Slog.w(TAG, msg);

throw new SecurityException(msg);

}

if (requiredPermission != null) {

Slog.w(TAG, "Can't broadcast sticky intent " + intent

+ " and enforce permission " + requiredPermission);

return BROADCAST_STICKY_CANT_HAVE_PERMISSION;

}

if (intent.getComponent() != null) {

throw new SecurityException(

"Sticky broadcasts can't target a specific component");

}

ArrayList<Intent> list = mStickyBroadcasts.get(intent.getAction());

if (list == null) {

list = new ArrayList<Intent>();

mStickyBroadcasts.put(intent.getAction(), list);

}

int N = list.size();

int i;

for (i=0; i<N; i++) {

if (intent.filterEquals(list.get(i))) {

// This sticky already exists, replace it.

list.set(i, new Intent(intent));

break;

}

if (i >= N) {

list.add(new Intent(intent));

}

……

}

从这里，我们可以知道mStickyBroadcasts的内容是系统处理sendStickyBroadcast时添加的，现在很清楚了吧！所以如果你调用Intent android.content.ContextWrapper.registerReceiver的时机点在调用sendStickyBroadcast后，你就可以直接得到该intent，从而可以快速的得到intent的数据，继续你的处理。

注意事项，调用sendStickyBroadcast，需要在app的AndroidManifest.xml里添加

<uses-permission android:name="android.permission.BROADCAST_STICKY" />

第四，接收sendOrderedBroadcast发出的broadcast的receivers是如何排序的？

首先，在app安装时，PackageManager会根据intentFilter给activity、service和receiver建立列表，具体代码位于intentResolver.java文件，调用顺序如下：

addFilter 》register_mime_types

其次，broadcast发送过程中，有以下代码：

private final int broadcastIntentLocked(ProcessRecord callerApp,

String callerPackage, Intent intent, String resolvedType,

IIntentReceiver resultTo, int resultCode, String resultData,

Bundle map, String requiredPermission,

boolean ordered, boolean sticky, int callingPid, int callingUid) {

……

// Figure out who all will receive this broadcast.

List receivers = null;

List<BroadcastFilter> registeredReceivers = null;

try {

if (intent.getComponent() != null) {

// Broadcast is going to one specific receiver class...

ActivityInfo ai = ActivityThread.getPackageManager().

getReceiverInfo(intent.getComponent(), STOCK_PM_FLAGS);

if (ai != null) {

receivers = new ArrayList();

ResolveInfo ri = new ResolveInfo();

ri.activityInfo = ai;

receivers.add(ri);

}

} else {

// Need to resolve the intent to interested receivers...

if ((intent.getFlags()&Intent.FLAG_RECEIVER_REGISTERED_ONLY)

== 0) {

receivers =

ActivityThread.getPackageManager().queryIntentReceivers(

intent, resolvedType, STOCK_PM_FLAGS);

}

registeredReceivers = mReceiverResolver.queryIntent(intent, resolvedType, false);

}

} catch (RemoteException ex) {

……

}

让我们看一下函数queryIntentReceivers，其核心处理在intentResolver.java文件

public List<R> queryIntent(Intent intent, String resolvedType, boolean defaultOnly) {

……

sortResults(finalList);

……

}

protected void sortResults(List<R> results) {

Collections.sort(results, mResolvePrioritySorter);

}

// Sorts a List of IntentFilter objects into descending priority order.

private static final Comparator mResolvePrioritySorter = new Comparator() {

public int compare(Object o1, Object o2) {

float q1 = ((IntentFilter)o1).getPriority();

float q2 = ((IntentFilter)o2).getPriority();

return (q1 > q2) ? -1 : ((q1 < q2) ? 1 : 0);

}

};

从这里receiver的优先级取决于intentFilter的属性Priority，它可以在AndroidManifest.xml里指明，例如，

<intent-filter android:priority="100">

</intent-filter>

</receiver>

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