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Android Handler 详解

栏目:互联网时间:2014-09-24 21:31:26

Android开发中经常使用Handler来实现“跨越线程(Activity)更新UI”。本文将从源码角度回答:为什么使用Handler能够跨线程更新UI?为什么跨线程更新UI一定要用Handler?

Demo

Demo1. 用Handler更新UI

下面这个Demo完全是为了演示“跨线程更新UI”而写的。界面上只有一个TextView和一个Button,按下Button创建一个后台线程,该后台线程每隔一秒更新一次TextView,连续更新10次,结束。

Activity的代码如下:

public class MainActivity extends Activity { static final String TAG = "MainActivity"; Handler handler = null; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); final TextView text = (TextView)findViewById(R.id.txtHello); Button button = (Button)findViewById(R.id.btnRun); button.setOnClickListener(new OnClickListener(){ @Override public void onClick(View v) { Log.d(TAG, "clicked!"); new Thread() { public void run() { for(int i=0; i<10; i++) { Message msg = new Message(); msg.what = 1; msg.obj = "item-"+i; handler.sendMessage(msg); Log.d(TAG, "sended "+"item-"+i); try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } } } }.start(); } }); handler = new Handler() { @Override public void handleMessage(Message msg) { String str = "unknow"; switch(msg.what) { case 1: str = (String)msg.obj; break; default: break; } Log.d(TAG, "recv " + str); text.setText(str); super.handleMessage(msg); } }; } @Override public boolean onCreateOptionsMenu(Menu menu) { // Inflate the menu; this adds items to the action bar if it is present. getMenuInflater().inflate(R.menu.main, menu); return true; } }


布局文件较为简单:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" android:paddingBottom="@dimen/activity_vertical_margin" android:paddingLeft="@dimen/activity_horizontal_margin" android:paddingRight="@dimen/activity_horizontal_margin" android:paddingTop="@dimen/activity_vertical_margin" tools:context=".MainActivity" > <TextView android:id="@+id/txtHello" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="@string/hello_world" /> <Button android:id="@+id/btnStart" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Start" /> </RelativeLayout>

这里展示的是Handler的典型用法――用来更新UI控件。

下面再展示一个非典型用法,仅仅是为了后面的分析方便。


Demo2. 自制ActivityThread模拟Activity

本例是为了分析方便而创建的;使用一个线程LooperThread来模拟Activity。

后面阐述为什么要这么做,代码如下:

package com.example.handlerdemo; import android.os.Bundle; import android.os.Message; import android.app.Activity; import android.util.Log; import android.view.Menu; import android.view.View; import android.widget.Button; import android.widget.TextView; public class MainActivity extends Activity { static final String TAG = "MainActivity"; ActivityThread acitivityThread = null; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); setupViews(); } private void setupViews() { TextView tv = (TextView)findViewById(R.id.txtHello); Button bt = (Button)findViewById(R.id.btnStart); Log.d(TAG, String.format("[MainActivity] Thread %s(%d)", Thread.currentThread().getName(), Thread.currentThread().getId())); acitivityThread = new ActivityThread(); acitivityThread.start(); acitivityThread.waitForHandlerReady(); bt.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { new Thread() { @Override public void run() { for(int i=0; i<10; i++) { Message msg = new Message(); msg.what = i; acitivityThread.mHandler.sendMessage(msg); try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } } } }.start(); } }); } @Override public boolean onCreateOptionsMenu(Menu menu) { // Inflate the menu; this adds items to the action bar if it is present. getMenuInflater().inflate(R.menu.main, menu); return true; } }
MainActivity.java


package com.example.handlerdemo; import android.os.Handler; import android.os.Looper; import android.os.Message; import android.util.Log; public class ActivityThread extends Thread { static final String TAG = "LooperThread"; public Handler mHandler = null; public ActivityThread() { super("LooperThread"); } @Override public void run() { Looper.prepare(); synchronized(this) { mHandler = new Handler() { @Override public void handleMessage(Message msg) { Log.d(TAG, String.format("recv msg.what: %d in Thread: %s(%d)", msg.what,   Thread.currentThread().getName(),Thread.currentThread().getId())); } }; this.notify(); } Looper.loop(); } public void waitForHandlerReady() { try { synchronized(this) { while(mHandler == null) this.wait(); } } catch (InterruptedException e) { e.printStackTrace(); } } }
ActivityThread.java

这个Demo的布局文件很简单,就不贴出来了。


为什么使用Handler能够跨线程更新UI?

概览

以Demo2为例,这个Demo至少涉及三个线程:GodActivity线程,ActivityThread线程(模拟UI),匿名线程(GodActivity创建的,叫他aThread)。暂且把GodActivity当做上帝,把ActivityThread看做Demo1里的Activity。现在,我们先预览一下为什么aThread可以通过Handler来更新ActivityThread的UI(纯属虚构),这两个线程的交互关系如下图所示:


(PS:此前的版本画了很多对象的生命线,结果很混乱,删了一堆无关紧要的之后,立刻清晰了,^_^)

这个序列图(Sequence Diagram)已经简洁明了地给出了答案:

  1. Activity线程的幕后还有一个MessageQueue;MessageQueue故名思议是一个Message组成的Queue;
  2. aThread只是将数据以Message的形式挂到了Activity幕后的MessageQueue上了;
  3. Activity线程从MessageQueue上取Message并调用Handler.handlerMessage,所以实际的“更新动作”还是发生在Activity线程内;


详解

下面将从Android 4.4.4源码的角度分析Handler的“幕后黑手”。

几个关键类

Demo2中和Handler有关的类除了MessageQueue还有Message和Looper,这几个类的关系如下:

关键点:

  • MessageQueue通过Message.next维护链表结构(java引用即指针);
  • ActivityThread的消息循环被封装在Looper.loop()内,Looper.prepare()用于创建属于当前线程的Looper和MessageQueue;
  • 每个Message可以通过target指向一个Handler,Handler实际上就是一个用来处理Message的callback

接下来的代码,只贴代码片段(方法),如果对各类的属性有所疑惑,可以回头查看此图。

Looper.prepare()

根据Looper的注释可以看到,Looper线程“三部曲”:

  1. Looper.prepare()
  2. new Handler() { /* override handleMessage() */ }
  3. Looper.loop();

下面逐渐切入Looper.prepare():

public static void prepare() { prepare(true); }
Looper.java

无参数版本调用了有参数版本:

private static void prepare(boolean quitAllowed) { if (sThreadLocal.get() != null) { throw new RuntimeException("Only one Looper may be created per thread"); } sThreadLocal.set(new Looper(quitAllowed)); // 放入“单例”中 }
Looper.java

这段代码中引用了sThreadLocal,它被定义为ThreadLocal类型,即线程私有数据类型(或者叫做线程级别单例)

ThreadLocal<T>可以理解为Map<Thread,T>的一层包包装(实际上Android,JVM都是按Map实现的,感兴趣的同学可自行研究;set(value)时,以当前线程对象为key,所以每个线程能够保存一份value。)

可见Looper.prepare()调用使得AcitivityThread通过Looper.sThreadLocal<Looper>持有了一个Looper对象。


继续看Looper的构造方法Looper(quitAllowed):

private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); // 和当前线程关联 }
Handler.java

可以看到Looper的构造函数中创建了一个MessageQueue。


流程又转到了MessageQueue的构造函数MessageQueue(quitAllowed):

MessageQueue(boolean quitAllowed) { mQuitAllowed = quitAllowed; mPtr = nativeInit(); }
MessageQueue.java


Handler()

首先看上面调用的默认构造方法:

/** * Default constructor associates this handler with the {@link Looper} for the * current thread. 将当前线程的Looper与此handler关联。 * 如果当前线程没有looper,这个handler将不能接收消息,从而导致异常抛出 * If this thread does not have a looper, this handler won't be able to receive messages * so an exception is thrown. */ public Handler() { this(null, false); }
Handler.java


默认构造方法又调用了另一版本的构造方法,如下:

public Handler(Callback callback, boolean async) { if (FIND_POTENTIAL_LEAKS) { // FIND_POTENTIAL_LEAKS 为 false; final Class<? extends Handler> klass = getClass(); if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) && (klass.getModifiers() & Modifier.STATIC) == 0) { Log.w(TAG, "The following Handler class should be static or leaks might occur: " + klass.getCanonicalName()); } } mLooper = Looper.myLooper(); // 获取当前线程(调用者)的Looper if (mLooper == null) { // 如果当前线程没有Looper,则抛异常 throw new RuntimeException( "Can't create handler inside thread that has not called Looper.prepare()"); } mQueue = mLooper.mQueue; // 这里引用的MessageQueue是Looper()中创建的 mCallback = callback; mAsynchronous = async; }
Handler.java

Handler()调用了Looper.myLooper():

public static Looper myLooper() { return sThreadLocal.get(); // 从该线程的“单例”中取出Looper对象 }
Looper.java



Looper.loop()

Looper.loop()封装了消息循环,所以我们现在看看Looper.loop()的“真面目”:

public static void loop() { final Looper me = myLooper(); if (me == null) { throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } final MessageQueue queue = me.mQueue; // Make sure the identity of this thread is that of the local process, // and keep track of what that identity token actually is. Binder.clearCallingIdentity(); final long ident = Binder.clearCallingIdentity(); for (;;) { Message msg = queue.next(); // might block, 取出消息 if (msg == null) { // No message indicates that the message queue is quitting. return; } // This must be in a local variable, in case a UI event sets the logger Printer logging = me.mLogging; if (logging != null) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } // mLatencyLock is only initialized for non USER builds // (e.g., USERDEBUG and ENG) if ((!sLatencyEnabled) || (me != sMainLooper)) { msg.target.dispatchMessage(msg); // 通过msg.target分派消息 } else { // 记录性能数据 long t1 = SystemClock.uptimeMillis(); // 获得当前毫秒数(自启动) msg.target.dispatchMessage(msg); long t2 = SystemClock.uptimeMillis() - t1; // t2就是dispatchMessage(msg)所用时间 if (t2 < 50) { // We don't care about these from a latency perspective } else if (t2 < 250) { // Fast response that usually has low impact on user experience sLatencyCountFast++; sLatencySumFast += t2; if (sLatencyCountFast >= 100) { String name = getProcessName(); long avg = sLatencySumFast / sLatencyCountFast; EventLog.writeEvent(2731, "mainloop2_latency1", name, avg); sLatencyCountFast = 0; sLatencySumFast = 0; } } else if (t2 < 1000) { sLatencyCountSlow++; sLatencySumSlow += t2; if (sLatencyCountSlow >= 10) { String name = getProcessName(); long avg = sLatencySumSlow / sLatencyCountSlow; EventLog.writeEvent(2731, "mainloop2_latency2", name, avg); sLatencyCountSlow = 0; sLatencySumSlow = 0; } } else { String name = getProcessName(); EventLog.writeEvent(2731, "mainloop2_bad", name, t2); } } if (logging != null) { logging.println("<<<<< Finished to " + msg.target + " " + msg.callback); } // Make sure that during the course of dispatching the // identity of the thread wasn't corrupted. final long newIdent = Binder.clearCallingIdentity(); if (ident != newIdent) { Log.wtf(TAG, "Thread identity changed from 0x" + Long.toHexString(ident) + " to 0x" + Long.toHexString(newIdent) + " while dispatching to " + msg.target.getClass().getName() + " " + msg.callback + " what=" + msg.what); } msg.recycle(); } }
Looper.java

可以看到,Looper.loop()的for循环实际上就是“消息循环”,它负责从消息队列(MessageQueue)中不断地取出消息(MessageQueue.next),然后通过msg.target来派发(dispatch)消息。


How to dispatch?

下面看看Message到底是如何被dispatch的:

public void dispatchMessage(Message msg) { if (msg.callback != null) { // 方法 1 handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { // 方法 2 return; } } handleMessage(msg); // 方法 3 } }
Handler.java

从这段代码可以看出,实现正常的Message处理有三种方式:

  1. 为Message.callback注册一个Runnable实例。
  2. 为Handler.mCallback注册一个Handler.Callback实例。
  3. 重写Handler的handleMessage方法。

另外,这三种方法优先级依次降低,且一个Message只能有一种处理方式。


Message的发送与获取

对于一个后台线程,它要发出消息(Handler.sendMessage);对于Activity线程,它要得到其他线程发来的消息(MessageQueue.next);而这两种工作都是以MessageQueue为基础的。下面,分别分析发送和接收的具体流程:

Handler.sendMessage()

Demo中后台线程正是通过Handler.sendMessage实现向Activity发消息的,Handler.sendMessage方法的代码如下:

public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0); }
Handler.java

public final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); }
Handler.java
其中,其中SystemClock.uptimeMillis()返回自启动以来CPU经过的毫秒数。


public boolean sendMessageAtTime(Message msg, long uptimeMillis) { MessageQueue queue = mQueue; if (queue == null) { RuntimeException e = new RuntimeException( this + " sendMessageAtTime() called with no mQueue"); Log.w("Looper", e.getMessage(), e); return false; } return enqueueMessage(queue, msg, uptimeMillis); }
Handler.java

Handler.enqueMessage其实只是对MessageQueue.enqueueMessage的简单包装:

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; // 将当前Handler(通常已重写handleMessage方法)与该Message绑定(通过target) if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); // 调用MessageQueue.enqueueMessage }
Handler.java

这里看到了Looper.loop()里引用的target的来源。


流程转到了MessageQueue.enqueueMessage(),看命名基本知道它是入队操作,代码如下:

boolean enqueueMessage(Message msg, long when) { if (msg.isInUse()) { throw new AndroidRuntimeException(msg + " This message is already in use."); } if (msg.target == null) { throw new AndroidRuntimeException("Message must have a target."); } synchronized (this) { // 临界区 if (mQuitting) { RuntimeException e = new RuntimeException( msg.target + " sending message to a Handler on a dead thread"); Log.w("MessageQueue", e.getMessage(), e); return false; } msg.when = when; Message p = mMessages; // 链表头 boolean needWake; if (p == null || when == 0 || when < p.when) { // p == null 队列为空 // when == 0 由 Handler.sendMessageAtFrontOfQueue() 发出 // when < p.when 新消息的when比队头要早 // New head, wake up the event queue if blocked. msg.next = p; // 将msg放到队头,step 1 mMessages = msg; // 将msg放到队头,step 2 needWake = mBlocked; } else { // Inserted within the middle of the queue. Usually we don't have to wake 插到队列中间。通常我们不必唤醒 // up the event queue unless there is a barrier at the head of the queue 事件(event)队列,除非队头有一个barrier, // and the message is the earliest asynchronous message in the queue.且消息是队列中最早的同步消息。 needWake = mBlocked && p.target == null && msg.isAsynchronous(); Message prev; for (;;) { // 遍历链表 prev = p; p = p.next; if (p == null || when < p.when) { // 到“尾部”了 或 新消息比当前消息更早 break; } if (needWake && p.isAsynchronous()) { needWake = false; } } // 以下两行将msg插入prev和p之间 msg.next = p; // invariant: p == prev.next prev.next = msg; } // We can assume mPtr != 0 because mQuitting is false. if (needWake) { nativeWake(mPtr); // 通知前台线程“有消息来啦” } } return true; }
MessageQueue.java
根据这段代码可知,MessageQueue上的Message是按照when大小排列的。唯一可能让人疑惑的是最后的nativeWake,稍后讨论。


MessageQueue.next()

前文的Looper.loop方法通过MessageQueue.next()取出消息,现在看看它是如何实现的:

Message next() { int pendingIdleHandlerCount = -1; // -1 only during first iteration int nextPollTimeoutMillis = 0; for (;;) { if (nextPollTimeoutMillis != 0) { Binder.flushPendingCommands(); } // We can assume mPtr != 0 because the loop is obviously still running. // The looper will not call this method after the loop quits. nativePollOnce(mPtr, nextPollTimeoutMillis); // 等待通知,可能阻塞 synchronized (this) { // Try to retrieve the next message. Return if found. final long now = SystemClock.uptimeMillis(); Message prevMsg = null; Message msg = mMessages; // 链表头 if (msg != null && msg.target == null) { // Stalled by a barrier. Find the next asynchronous message in the queue. do { // 遍历链表 prevMsg = msg; msg = msg.next; } while (msg != null && !msg.isAsynchronous()); } if (msg != null) { if (now < msg.when) { // Next message is not ready. Set a timeout to wake up when it is ready. nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE); } else { // Got a message. mBlocked = false; if (prevMsg != null) { prevMsg.next = msg.next; // 将msg节点摘下 } else { // prevMsg == null, msg是链表头 mMessages = msg.next; } msg.next = null; // msg与MessageQueue“断绝关系” if (false) Log.v("MessageQueue", "Returning message: " + msg); msg.markInUse(); return msg; // 退出点1 到这为止,是常规逻辑 } } else { // No more messages. nextPollTimeoutMillis = -1; } // Process the quit message now that all pending messages have been handled. if (mQuitting) { dispose(); return null; // 退出点2 } // If first time idle, then get the number of idlers to run. // Idle handles only run if the queue is empty or if the first message // in the queue (possibly a barrier) is due to be handled in the future. if (pendingIdleHandlerCount < 0 && (mMessages == null || now < mMessages.when)) { pendingIdleHandlerCount = mIdleHandlers.size(); } if (pendingIdleHandlerCount <= 0) { // No idle handlers to run. Loop and wait some more. mBlocked = true; continue; } if (mPendingIdleHandlers == null) { mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)]; } mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers); } // Run the idle handlers. // We only ever reach this code block during the first iteration. for (int i = 0; i < pendingIdleHandlerCount; i++) { final IdleHandler idler = mPendingIdleHandlers[i]; mPendingIdleHandlers[i] = null; // release the reference to the handler boolean keep = false; try { keep = idler.queueIdle(); } catch (Throwable t) { Log.wtf("MessageQueue", "IdleHandler threw exception", t); } if (!keep) { synchronized (this) { mIdleHandlers.remove(idler); } } } // Reset the idle handler count to 0 so we do not run them again. pendingIdleHandlerCount = 0; // While calling an idle handler, a new message could have been delivered // so go back and look again for a pending message without waiting. nextPollTimeoutMillis = 0; } }

MessageQueue.java

MessageQueue.next()同样让人疑惑的是nativePollOnce,稍后也将见分晓。

小结

MessageQueue.next()和MessageQueue.sendMessage()分别被Activity线程、后台线程调用,而他们两个线程可能同时在调用这两个方法,所以他们共享并修改的成员变量需要加锁,这就是synchronized (this)出现的原因。

至此,已经能够完整的回答“为什么用Handler能够实现跨线程更新UI”。简单的说,Activity线程的背后都有一个消息队列(MessageQueue),后台线程通过Handler的sendMessage方法向这个消息队列上放消息;Activity线程将消息从消息队列上取下来之后,通过具体Handler的handleMessage方法处理消息,而更新UI的代码就在这个handleMessage中;所以,后台线程并没有做实际的“更新”,只是将要更新的内容以借助MessageQueue告诉了Activity线程,Activity线程才是实际做“更新”动作的人。

简言之,Handler并没有真正的实现“跨线程”更新UI,而是将要更新的数据(Message携带)和如何更新(Handler携带)通过消息队列告诉了UI线程,UI线程才是真正的“幕后英雄”。


真正的ActivityThread

Demo2中的ActivityThread完全是虚构出来的,下面来看看Android的Activity到底是不是想我虚构的那样有一个Looper。

经过上面的分析,可以从两方面验证:

  1. 看看Activity源码中执行onCreate之前是否调用了Looper.prepare()。
  2. 执行onXXX方法时的CallStack上是否有Looper.loop();

第二点很容易验证,只需在任意onXXX方法中打一个断点,然后看程序的CallStack,就一面了然了:

根据这个调用栈,可以很明显的看到有Looper.loop;同时还能看到是ActivityThread.main调用它的,所以可以看看ActivityThread.main的源码:

public static void main(String[] args) { SamplingProfilerIntegration.start(); // CloseGuard defaults to true and can be quite spammy. We // disable it here, but selectively enable it later (via // StrictMode) on debug builds, but using DropBox, not logs. CloseGuard.setEnabled(false); Environment.initForCurrentUser(); // Set the reporter for event logging in libcore EventLogger.setReporter(new EventLoggingReporter()); Security.addProvider(new AndroidKeyStoreProvider()); Process.setArgV0("<pre-initialized>"); Looper.prepareMainLooper(); // 它和Looper.prepare类似 ActivityThread thread = new ActivityThread(); thread.attach(false); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } AsyncTask.init(); if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); } Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited"); }
ActivityThread.java

所以,上面提到的两方面都得到了验证。即真正的ActivityThread是有Looper的。


Native浮云

细心的朋友可能会发现,上面MessageQueue的代码中还遗留几个native开头方法:nativeInit,nativePollOnce,nativeWake。

下面就来扫清这些“遮眼”的浮云。和这几个native方法直接对应的是:

static JNINativeMethod gMessageQueueMethods[] = { /* name, signature, funcPtr */ { "nativeInit", "()I", (void*)android_os_MessageQueue_nativeInit }, { "nativeDestroy", "(I)V", (void*)android_os_MessageQueue_nativeDestroy }, { "nativePollOnce", "(II)V", (void*)android_os_MessageQueue_nativePollOnce }, { "nativeWake", "(I)V", (void*)android_os_MessageQueue_nativeWake }, { "nativeIsIdling", "(I)Z", (void*)android_os_MessageQueue_nativeIsIdling } };

android_os_MessageQueue.cpp


nativeInit

下面从adnroid_os_MessageQueue_nativeInit开始,顾名思义,nativeInit当然是完成一些初始化工作的。

static jint android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) { NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue(); // 创建了NativeMessageQueue if (!nativeMessageQueue) { jniThrowRuntimeException(env, "Unable to allocate native queue"); return 0; } nativeMessageQueue->incStrong(env); return reinterpret_cast<jint>(nativeMessageQueue); }
android_os_MessageQueue.cpp

看看NativeMessageQueue的声明:

class NativeMessageQueue : public MessageQueue { public: NativeMessageQueue(); virtual ~NativeMessageQueue(); virtual void raiseException(JNIEnv* env, const char* msg, jthrowable exceptionObj); void pollOnce(JNIEnv* env, int timeoutMillis); void wake(); private: bool mInCallback; jthrowable mExceptionObj; };
android_os_MessageQueue.cpp

NativeMessageQueue继承了MessageQueue,再来看看MessageQueue的声明:

class MessageQueue : public RefBase { public: /* Gets the message queue's looper. */ inline sp<Looper> getLooper() const { return mLooper; } /* Checks whether the JNI environment has a pending exception. * * If an exception occurred, logs it together with the specified message, * and calls raiseException() to ensure the exception will be raised when * the callback returns, clears the pending exception from the environment, * then returns true. * * If no exception occurred, returns false. */ bool raiseAn
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