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Handler的创建会关联一个Looper
对象,而Looper
对象关联着MessageQueen
对象,所以在Handler创建时,取出Looper
和MessageQueen
public Handler(Callback callback, boolean async) { ... //取出Looper mLooper = Looper.myLooper(); if (mLooper == null) { throw new RuntimeException( "Can't create handler inside thread that has not called Looper.prepare()"); } //取出Looper中的MessageQueen mQueue = mLooper.mQueue; mCallback = callback; mAsynchronous = async;}
Looper
是存放在ThreadLocal
里面的,可以看下面的源码
public static @Nullable Looper myLooper() { return sThreadLocal.get();}
整个创建的过程就完了,这里做了哪几件事:
Looper
Looper
中的MessageQueen
public final boolean sendEmptyMessage(int what){ return sendEmptyMessageDelayed(what, 0);}//往下追踪public final boolean sendEmptyMessageDelayed(int what, long delayMillis) { Message msg = Message.obtain(); msg.what = what; return sendMessageDelayed(msg, delayMillis);}//往下追踪public final boolean sendMessageDelayed(Message msg, long delayMillis){ if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);}//往下追踪public boolean sendMessageAtTime(Message msg, long uptimeMillis) { //直接获取MessageQueue 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);}//调用sendMessage方法其实最后是调用了enqueueMessage方法private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { //为msg.target赋值为this,也就是把当前的handler作为msg的target属性 //如果大家还记得Looper的loop()方法会取出每个msg然后执行msg.target.dispatchMessage(msg)去处理消息,其实就是派发给相应的Handler msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } //最终调用queue的enqueueMessage的方法,也就是说handler发出的消息,最终会保存到消息队列中去 return queue.enqueueMessage(msg, uptimeMillis);}
public final boolean post(Runnable r){ return sendMessageDelayed(getPostMessage(r), 0);}
其实post()方法最终也会保存到消息队列中去,和上面不同的是它传进来的一个Runnable
对象,执行了getPostMessage
()方法,我们往下追踪
private static Message getPostMessage(Runnable r) { Message m = Message.obtain(); m.callback = r; return m;}
实质上就是将这个Runnable
保存在Message的变量中,这就导致了我们下面处理消息的时候有两种不同方案
你还记得前面所说Looper
中msg.target.dispatchMessage()方法吗?这个方法就是调用Handler
的dispatchMessage()
public void dispatchMessage(Message msg) { if (msg.callback != null) { //1. post()方法的处理方法 handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } //2. sendMessage()方法的处理方法 handleMessage(msg); }}//1. post()方法的最终处理方法private static void handleCallback(Message message) { message.callback.run();}//2. sendMessage()方法的最终处理方法public void handleMessage(Message msg) { }
整个处理的过程就完了,这里做了哪几件事:
Runnable
执行run()方法sendMessage
()方法的处理方法就是执行handleMessage
()空方法,这也是我们为什么要在Handler重写这个方法的原因package mountain_hua.learn_handler1; import android.os.Handler;import android.support.v7.app.AppCompatActivity;import android.os.Bundle; public class MainActivity extends AppCompatActivity { //先在主线程中创建Handler,Handler会自动与主线程绑定 private Handler handler=new Handler(); @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); System.out.println("main thread id " + Thread.currentThread().getId()); test_handmessage(); } public void test_handmessage(){ new Thread(){ @Override public void run(){ //处理信息 try{ //模拟费时操作 System.out.println("run thread id " + Thread.currentThread().getId()); sleep(1000); //这里使用Runnable Runnable runnable=new Runnable() { @Override public void run() { System.out.println("Runnable thread id " + Thread.currentThread().getId()); } }; //这里执行post(Runnable)操作 handler.post(runnable); }catch (Exception e){ e.printStackTrace(); } } }.start(); } }
首先在主线程中创建了一个handler,这时handler会自动绑定主线程,然后在一个新线程里面创建了Runnable对象,并且用了handler的post(Runnable)操作来发送消息给在主线程中的handler执行。我们看看打印出来的结果:
07-30 21:34:23.891 21509-21509/mountain_hua.learn_handler1 I/System.out: main thread id 107-30 21:34:23.893 21509-21560/mountain_hua.learn_handler1 I/System.out: run thread id 937807-30 21:34:24.894 21509-21509/mountain_hua.learn_handler1 I/System.out: Runnable thread id 1
结果可以看到Runnable与main线程id相同,说明了Runnable里面执行的事件是被handler放在主线程中执行的,与Run所在线程无关 。
package mountain_hua.learn_handler1; import android.os.Handler;import android.os.Message;import android.support.v7.app.AppCompatActivity;import android.os.Bundle; public class MainActivity extends AppCompatActivity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); test_handmessage(); test_handmessage1(); } public void test_handmessage(){ new Thread(){ @Override public void run(){ //处理信息 try{ //模拟费时操作 System.out.println("run thread id " + Thread.currentThread().getId()); sleep(1000); }catch (Exception e){ e.printStackTrace(); } Message msg=new Message(); // 也可以用下面两种方法获得Message // Message msg1=Message.obtain(); // Message msg2=handler.obtainMessage(); msg.what=0;//msg.what的类型是int型,作为msg的识别码 msg.arg1=1;//msg.arg1的类型是int型,可以传递简单的参数 msg.arg2=2;//msg.arg2的类型是int型,可以传递简单的参数 msg.obj="Object类型";//msg.obj是Object型,可以传递任意参数 //将Message发送给handler handler.sendMessage(msg); } }.start(); } public void test_handmessage1(){ new Thread(){ @Override public void run(){ //处理信息 try{ //模拟费时操作 System.out.println("run thread id " + Thread.currentThread().getId()); sleep(2000); }catch (Exception e){ e.printStackTrace(); } Message msg=new Message(); // 也可以用下面两种方法获得Message // Message msg1=Message.obtain(); // Message msg2=handler.obtainMessage(); msg.what=99;//msg.what的类型是int型,作为msg的识别码 msg.arg1=100;//msg.arg1的类型是int型,可以传递简单的参数 msg.arg2=101;//msg.arg2的类型是int型,可以传递简单的参数 msg.obj="Object类型";//msg.obj是Object型,可以传递任意参数 //将Message发送给handler handler.sendMessage(msg); } }.start(); } Handler handler=new Handler() { @Override public void handleMessage(Message msg) { switch (msg.what) { case 0: System.out.println("handleMessage thread id " + Thread.currentThread().getId()); System.out.println("msg.arg1:" + msg.arg1); System.out.println("msg.arg2:" + msg.arg2); break; case 99: System.out.println("handleMessage thread id " + Thread.currentThread().getId()); System.out.println("msg.arg1:" + msg.arg1); System.out.println("msg.arg2:" + msg.arg2); } } };}
这里用两个Messge给主线程中的Handler发送消息,打印出的信息为:
07-30 21:19:08.532 2461-2782/mountain_hua.learn_handler1 I/System.out: run thread id 931107-30 21:19:08.532 2461-2783/mountain_hua.learn_handler1 I/System.out: run thread id 931207-30 21:19:09.532 2461-2461/mountain_hua.learn_handler1 I/System.out: handleMessage thread id 107-30 21:19:09.532 2461-2461/mountain_hua.learn_handler1 I/System.out: msg.arg1:107-30 21:19:09.532 2461-2461/mountain_hua.learn_handler1 I/System.out: msg.arg2:207-30 21:19:10.532 2461-2461/mountain_hua.learn_handler1 I/System.out: handleMessage thread id 107-30 21:19:10.532 2461-2461/mountain_hua.learn_handler1 I/System.out: msg.arg1:10007-30 21:19:10.532 2461-2461/mountain_hua.learn_handler1 I/System.out: msg.arg2:101
可以看到handler处理消息队列是在主线程中处理的(handler在哪个线程中创建的就与哪个线程绑定), 与run存在的线程无关。
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