Handler主要是主线程和子线程通信。一般子线程中做一些耗时操作做完之后通知主线程来修改UI。
实际上android系统在Activity启动或者状态变化等都是通过Handler机制实现的。
首先进入到ActivityThread的main方法中
public static void main(String[] args) {……Looper.prepareMainLooper();ActivityThread thread = new ActivityThread();thread.attach(false);if (sMainThreadHandler == null) {sMainThreadHandler = thread.getHandler();}……Looper.loop();……}
下面主要分析上面几句代码。
1.Looper.prepareMainLooper();
public static void prepareMainLooper() {prepare(false);synchronized (Looper.class) {if (sMainLooper != null) {throw new IllegalStateException("The main Looper has already been prepared.");}sMainLooper = myLooper();}}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));//为这个线程会新建一个Looper对象}
Looper的构造函数如下
private Looper(boolean quitAllowed) {mQueue = new MessageQueue(quitAllowed);//Looper维护了一个消息队列mRun = true;mThread = Thread.currentThread();}
小结：在调用完Loop.prepare后，，就会为当前线程创建一个消息泵Looper，这个Looper维护了一个消息队列MessageQueue
2. sMainThreadHandler =thread.getHandler();
sMainThreadHandler是Handler对象，getHandler方法如下：
final Handler getHandler() {return mH;}
看到mH在前面定义为final H mH = new H();实际上H是继承自Handler。部分代码如下：
private class H extends Handler {public static final int LAUNCH_ACTIVITY= 100;public static final int PAUSE_ACTIVITY= 101;……public void handleMessage(Message msg) {if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));switch (msg.what) {case LAUNCH_ACTIVITY: {Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");ActivityClientRecord r = (ActivityClientRecord)msg.obj;r.packageInfo = getPackageInfoNoCheck(r.activityInfo.applicationInfo, r.compatInfo);handleLaunchActivity(r, null);Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);} break;……}……}
小结：这里相当于平时在UI线程中创建一个Handler实现他的handlerMessage方法。
3.Looper.loop();
public static void loop() {……final MessageQueue queue = me.mQueue;for (;;) {Message msg = queue.next(); // might blockif (msg == null) {return;}……msg.target.dispatchMessage(msg);……msg.recycle();}}
小结：调用Looper.loop()，可以看到for循环，不停地从消息队列中取消息，然后分发msg.target.dispatchMessage(msg); 这里的msg.target就是Handler对象，指的是处理该Message的Handler。
public void dispatchMessage(Message msg) {if (msg.callback != null) {handleCallback(msg);} else {if (mCallback != null) {if (mCallback.handleMessage(msg)) {return;}}handleMessage(msg);}}
调用dispatchMessage，如果Message设置了回调函数就执行回调，否则如果定义Handler的时候如果传进了回调函数就执行传进的回调，不然就会执行handlerMessage函数，可以看到是有优先级顺序的。系统处理的时候由于未设置回调，就会执行handlerMessage。
比如上面的当收到LAUNCH_ACTIVITY消息，就会执行handleLaunchActivity—- performLaunchActivity—- mInstrumentation.newActivity(cl,component.getClassName(), r.intent); —–mInstrumentation.callActivityOnCreate(activity,r.state); —– activity.performCreate(icicle);
这样就执行到了我们平时所谓的Activity的入口onCreate方法。
到现在我们看到了Handler机制在android中的应用。接下来分析一下handler和Looper是怎么关联起来的。
Handler的构造函数最终都会去执行
public Handler(Callback callback, boolean async) {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());}}mLooper = Looper.myLooper();//获取当前线程的Looperif (mLooper == null) {throw new RuntimeException("Can't create handler inside thread that has not called Looper.prepare()");}mQueue = mLooper.mQueue;//将Looper对象的消息队列传给Handler的成员，使得Handler就可以操作该消息循环mCallback = callback;mAsynchronous = async;}
Message类如下
public final class Message implements Parcelable {public int what;public int arg1;public int arg2;……Handler target;//每个消息都有一个成员保存和他关联的HandlerRunnable callback; }
接下来我们看一下调用handler的sendMessage送消息时发生了什么
public final boolean sendMessage(Message msg){return sendMessageDelayed(msg, 0);}最后会执行到：private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {msg.target = this;//在这里将this复制给Message的Handler成员，this也就是我们定义的handler对象。if (mAsynchronous) {msg.setAsynchronous(true);}return queue.enqueueMessage(msg, uptimeMillis);//然后添加到消息队列里面}造物之前，必先造人。
