最经面试中，技术面试中有一个是Handler的消息机制，细细想想，我经常用到的Handler无非是在主线程（或者说Activity）新建一个Handler对象，另外一个Thread是异步加载数据，同时当他加载完数据后就send到主线程中的那个Handler对象，接着Handler来处理，刚才发送的一些消息。
1 public class HandlerTestActivity extends Activity { 2 private TextView tv; 3 private static final int UPDATE = 0; 4 private Handler handler = new Handler() { 5 6 @Override 7 public void handleMessage(Message msg) { 8 // TODO 接收消息并且去更新UI线程上的控件内容 9 if (msg.what == UPDATE) { 10 // Bundle b = msg.getData(); 11 // tv.setText(b.getString("num")); 12 tv.setText(String.valueOf(msg.obj)); 13 } 14 super.handleMessage(msg); 15 } 16 }; 17 18 /** Called when the activity is first created. */ 19 @Override 20 public void onCreate(Bundle savedInstanceState) { 21 super.onCreate(savedInstanceState); 22 setContentView(R.layout.main); 23 tv = (TextView) findViewById(R.id.tv); 24 25 new Thread() { 26 @Override 27 public void run() { 28 // TODO 子线程中通过handler发送消息给handler接收，由handler去更新TextView的值 29 try { 30 for (int i = 0; i < 100; i++) { 31 Thread.sleep(500); 32 Message msg = new Message(); 33 msg.what = UPDATE; 34 // Bundle b = new Bundle(); 35 // b.putString("num", "更新后的值：" + i); 36 // msg.setData(b); 37 msg.obj = "更新后的值：" + i; 38 handler.sendMessage(msg); 39 } 40 } catch (InterruptedException e) { 41 e.printStackTrace(); 42 } 43 } 44 }.start(); 45 } 46 47 }
如图所示，每个Thread都一个Looper，这个Looper类是用于管理其中的消息队列（MessageQueue）的,那Handler是干嘛的呢，他是用来传递消息队列的。
那下面就分析Looper、Hanlder方法吧。
Looper方法是用来处理消息队列的，注意了，它和线程是绑定的。
要是想在子线程中获取一个Looper该怎么做呢：
Looper.prepare();
Looper looper = Looper.myLooper();
那么这些都干了哪些工作呢???
来看下它的源码吧:
Looper:
……
//准备Looper相关事宜 public static void prepare() { //只能有一个对象哦 if (sThreadLocal.get() != null) { throw new RuntimeException("Only one Looper may be created per thread"); } sThreadLocal.set(new Looper()); } //构造函数 /*新建一个消息队列 * 把当前运行的线程作为运行线程 */ private Looper() { mQueue = new MessageQueue(); mRun = true; mThread = Thread.currentThread(); }
public static final Looper myLooper() {
//这个方法是从当前线程的ThreadLocal中拿出设置的looper
return (Looper)sThreadLocal.get();
}
/** * Run the message queue in this thread. Be sure to call * {@link #quit()} to end the loop. */ public static void loop() { Looper me = myLooper(); if (me == null) { throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } 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(); while (true) { Message msg = queue.next(); // might block if (msg != null) { if (msg.target == null) { // No target is a magic identifier for the quit message. return; } long wallStart = 0; long threadStart = 0; // 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); wallStart = SystemClock.currentTimeMicro(); threadStart = SystemClock.currentThreadTimeMicro(); } msg.target.dispatchMessage(msg); if (logging != null) { long wallTime = SystemClock.currentTimeMicro() - wallStart; long threadTime = SystemClock.currentThreadTimeMicro() - threadStart; logging.println("<<<<< Finished to " + msg.target + " " + msg.callback); if (logging instanceof Profiler) { ((Profiler) logging).profile(msg, wallStart, wallTime, threadStart, threadTime); } } // 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(); } } }
下面就来看下Handler:
public Handler() { if (FIND_POTENTIAL_LEAKS) { 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()); } } //先获得一个Looper对象，这个要是在子线程里，是需要先prepare（）的 mLooper = Looper.myLooper(); if (mLooper == null) { throw new RuntimeException( "Can't create handler inside thread that has not called Looper.prepare()"); } mQueue = mLooper.mQueue; mCallback = null; } /** * Returns a new {@link android.os.Message Message} from the global message pool. More efficient than * creating and allocating new instances. The retrieved message has its handler set to this instance (Message.target == this). * If you don't want that facility, just call Message.obtain() instead. 会从消息池里面取得消息队列 */ public final Message obtainMessage() { return Message.obtain(this); }
那我现在写个小例子，是在子线程实现的消息的传递。
@Override public void onClick(View v) { int id = v.getId(); if (id == R.id.btn1) { new Thread() { public void run() { Log.i("log", "run"); Looper.prepare(); // Looper looper = Looper.myLooper(); Toast.makeText(MainActivity.this, "toast", 1).show(); Handler h = new Handler() { @Override public void handleMessage(Message msg) { // TODO Auto-generated method stub super.handleMessage(msg); if (msg != null) { String strMsg = (String) msg.obj; System.out.println(strMsg); } } }; //获取到Handler对象的消息 Message msg = h.obtainMessage(); msg.obj = "add"; msg.sendToTarget(); Looper.loop();// 进入loop中的循环，查看消息队列 }; }.start(); } }
不知你是否理解，这个小Demo中，我们需要注意：
1 子线程也是可以有Handler的，其实Handler只是从当前的线程中获取到Looper来监听和操作MessageQueue的。
2 子线程需要先prepare（）才能获取到Looper的，是因为在子线程只是一个普通的线程，其ThreadLoacl中没有设置过Looper，所以会抛出异常，而在Looper的prepare（）方法中sThreadLocal.set(new Looper())是设置了Looper的。
而对于主线程里面的Handler，是没有以上的麻烦的，因为这个在Activity创建时，就已经初始化了Looper等其他工作了。
另外可以看下参考文章中的子线程中Toast。
参考文章：