如何使用HandlerThread？
HandlerThread本质上是一个线程类，继承自Thread类，但是HandlerThread有自己的Looper对象，可以进行looper循环，不断从MessageQueue中取消息。那么，我们如何使用HandlerThread呢？
- 创建HandlerThread实例
// 创建HandlerThread的实例，并传入参数来表示当前线程的名字，此处//将该线程命名为“HandlerThread”
private HandlerThread mHandlerThread = new HandlerThread("HandlerThread");
- 启动HandlerThread线程
mHandlerThread.start();
- 使用HandlerThread的Looper去构建Handler并实现其handlMessage的回调方法
private Handler mHandler;
// 该Callback方法运行于子线程，mHandler在创建时使用的是mHandlerThread的looper对象！！
mHandler = new Handler(mHandlerThread.getLooper()) {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
Log.d("Kathy"，"Received Message = " + msg.what + " CurrentThread = " + Thread
.currentThread().getName());
}
};
可以看出，我们将前面的创建的HandlerThread实例的Looper对象传递给了Handler，这使得该Handler拥有了HandlerThread的Looper对象，通过该Handler发送的消息，都将被发送到该Looper对象的MessageQueue中，且回调方法的执行也是执行在HandlerThread这个异步线程中。
值得注意的是，如果在handleMessage()执行完成后，如果想要更新UI，可以用UI线程的Handler发消息给UI线程来更新。
举个Simple的例子
请看以下代码：
/**
* Created by Kathy on 17-2-21.
*/
public class HandlerThreadActivity extends Activity {
private HandlerThread mHandlerThread;
private Handler mHandler;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// 创建HandlerThread实例并启动线程执行
mHandlerThread = new HandlerThread("HandlerThread");
mHandlerThread.start();
// 将mHandlerThread.getLooper()的Looper对象传给mHandler，之后mHandler发送的消息都将在
// mHandlerThread这个线程中执行
mHandler = new Handler(mHandlerThread.getLooper()) {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
Log.d("Kathy"，"Received Message = " + msg.what + " CurrentThread = " + Thread
.currentThread().getName());
}
};
//在主线程中发送一条消息
mHandler.sendEmptyMessage(1);
new Thread(new Runnable() {
@Override
public void run() {
//在子线程中发送一条消息
mHandler.sendEmptyMessage(2);
}
}).start();
}
@Override
protected void onDestroy() {
super.onDestroy();
mHandlerThread.quit();
}
}
创建mHandler时传入HandlerThread实例的Looper对象，然后分别在主线程和其他的子线程中发送空消息，猜测handleMessage()中的Log会输出什么？
执行结果如下：
02-21 19:37:30.395 19479-19506/? D/Kathy: Received Message = 1 CurrentThread = HandlerThread
02-21 19:37:30.396 19479-19506/? D/Kathy: Received Message = 2 CurrentThread = HandlerThread
可知，无论是在主线程中发送消息，还是在其他子线程中发送消息，handleMessage()方法都在HandlerThread中执行。
源码角度解析HandlerThread
HandlerThread的源码结构很简单，行数也不多，推荐大家自己去SDK中阅读。
HandlerThread有两个构造函数，在创建时可以根据需求传入两个参数：线程名称和线程优先级。代码如下：
public HandlerThread(String name) {
super(name);
mPriority = Process.THREAD_PRIORITY_DEFAULT;
}
/**
* Constructs a HandlerThread.
* @param name
* @param priority The priority to run the thread at. The value supplied must be from
* {@link android.os.Process} and not from java.lang.Thread.
*/
public HandlerThread(String name, int priority) {
super(name);
mPriority = priority;
}
在HandlerThread对象的start()方法调用之后，线程被启动，会执行到run()方法，接下来看看，HandlerThread的run()方法都做了什么事情？
/**
* Call back method that can be explicitly overridden if needed to execute some
* setup before Looper loops.
*/
protected void onLooperPrepared() {
}
@Override
public void run() {
mTid = Process.myTid();
Looper.prepare();
synchronized (this) {
mLooper = Looper.myLooper();
notifyAll();
}
Process.setThreadPriority(mPriority);
onLooperPrepared();
Looper.loop();
mTid = -1;
}
在run()方法中，执行了Looper.prepare()方法，mLooper保存并获得了当前线程的Looper对象，并通过notifyAll()方法去唤醒等待线程，最后执行了Looper.loop()开启looper循环语句。也就是说，run()方法的主要作用就是完成looper机制的创建。Handler可以获得这个looper对象，并开始异步消息传递了。
接下来看看Handler是如何获得这个Looper对象呢？
/**
* This method returns the Looper associated with this thread. If this thread not been started
* or for any reason is isAlive() returns false, this method will return null. If this thread
* has been started, this method will block until the looper has been initialized.
* @return The looper.
*/
public Looper getLooper() {
if (!isAlive()) {
return null;
}
// If the thread has been started, wait until the looper has been created.
synchronized (this) {
while (isAlive() && mLooper == null) {
try {
wait();
} catch (InterruptedException e) {
}
}
}
return mLooper;
}
Handler通过getLooper()方法获取looper对象。该方法首先判断当前线程是否启动?如果没有启动，返回null；如果启动，进入同步语句。如果mLooper为null，代表mLooper没有被赋值，则当前调用线程进入等待阶段。直到Looper对象被创建且通过notifyAll()方法唤醒等待线程，最后才会返回Looper对象。我们看到在run()方法中，mLooper在得到Looper对象后，会发送notifyAll()方法来唤醒等待的线程。
Looper对象的创建在子线程的run()方法中执行，但是调用getLooper()的地方是在主线程中运行，我们无法保证在调用getLooper()时Looper已经被成功创建，所以会在getLooper()中存在一个同步的问题，通过等待唤醒机制解决了同步的问题。
那么，HandlerThread是如何退出的呢？
/**
* Quits the handler thread's looper.
* <p>
* Causes the handler thread's looper to terminate without processing any
* more messages in the message queue.
* </p><p>
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
* </p><p class="note">
* Using this method may be unsafe because some messages may not be delivered
* before the looper terminates. Consider using {@link #quitSafely} instead to ensure
* that all pending work is completed in an orderly manner.
* </p>
*
* @return True if the looper looper has been asked to quit or false if the
* thread had not yet started running.
*
* @see #quitSafely
*/
public boolean quit() {
Looper looper = getLooper();
if (looper != null) {
looper.quit();
return true;
}
return false;
}
/**
* Quits the handler thread's looper safely.
* <p>
* Causes the handler thread's looper to terminate as soon as all remaining messages
* in the message queue that are already due to be delivered have been handled.
* Pending delayed messages with due times in the future will not be delivered.
* </p><p>
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
* </p><p>
* If the thread has not been started or has finished (that is if
* {@link #getLooper} returns null), then false is returned.
* Otherwise the looper is asked to quit and true is returned.
* </p>
*
* @return True if the looper looper has been asked to quit or false if the
* thread had not yet started running.
*/
public boolean quitSafely() {
Looper looper = getLooper();
if (looper != null) {
looper.quitSafely();
return true;
}
return false;
}
向下调用到Looper的quit()方法
public void quit() {
mQueue.quit(false);
}
public void quitSafely() {
mQueue.quit(true);
}
再向下调用到MessageQueue的quit()方法：
void quit(boolean safe) {
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true;
if (safe) {
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}
可以看到，无论是调用quit()方法还是quitSafely()，最终都将执行到MessageQueue中的quit()方法。
quit()方法最终执行的removeAllMessagesLocked()方法，该方法主要是把MessageQueue消息池中所有的消息全部清空，无论是延迟消息（延迟消息是指通过sendMessageDelayed或通过postDelayed等方法发送）还是非延迟消息。
quitSafely()方法，其最终执行的是MessageQueue中的removeAllFutureMessagesLocked方法，该方法只会清空MessageQueue消息池中所有的延迟消息，并将消息池中所有的非延迟消息派发出去让Handler去处理完成后才停止Looper循环，quitSafely相比于quit方法安全的原因在于清空消息之前会派发所有的非延迟消息。最后需要注意的是Looper的quit方法是基于API 1，而Looper的quitSafely方法则是基于API 18的。
