android 消息机制浅析

android消息机制主要涉及到以下内容：Message， Looper，Handler。

其原理为：消息线程（默认是主线程，也就是UI线程），维护一个消息循环和一个消息队列，

工作线程向消息队列中添加消息，消息循环从队列中取出消息并处理。其中Looper类负责消息循环

和消息队列，Message类是消息的载体，Handler类负责消息队列中添加消息和消息的处理。

Looper的prepare接口负责为调用线程创建一个Looper对象，下面是android源码，汉字部分为

本文添加的注释：

public static final void prepare() {
if (sThreadLocal.get() != null) { //其中sThreadLocal为ThreadLocal对象，不熟悉ThreadLocal的同学可以google下
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper());
}

Looper的loop接口负责开启消息循环：

/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static final void loop() {
Looper me = myLooper();
MessageQueue queue = me.mQueue;//loop中维护的消息队列
while (true) {
Message msg = queue.next(); // 从消息队列中取消息
//if (!me.mRun) {
// break;
//}
if (msg != null) {
if (msg.target == null) {
// No target is a magic identifier for the quit message.
return;
}
if (me.mLogging!= null) me.mLogging.println(
">>>>> Dispatching to " + msg.target + " "
+ msg.callback + ": " + msg.what
);
msg.target.dispatchMessage(msg); //消息处理
if (me.mLogging!= null) me.mLogging.println(
"<<<<< Finished to " + msg.target + " "
+ msg.callback);
msg.recycle();
}
}
}

Handler中维护一个对Looper对象的引用，并直接获取它的消息队列，以便向消息队列中添加消息：

/**
* Default constructor associates this handler with the queue for the
* current thread.
*
* If there isn't one, this handler won't be able to receive messages.
*/
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());
}
}

mLooper = Looper.myLooper(); //获取线程的Looper对象
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue; //直接获取Looper对象的消息队列
mCallback = null;
}

Looper.loop方法中有一句代码msg.target.dispatchMessage(msg)（见上文）即是调用Handler的

dispatchMessage方法：

/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg); //调用子类的重写方法，故继承Handler时要重写该方法
}
}

清楚了消息机制后，就不难理解消息机制的用法了：

MyHandler extends Handler{

public void handleMessage(final Message msg){

switch(msg.what){

case 1:

//做一些具体处理，比如说更新UI等

break;

}

MyHandler mHandler=new MyHandler();

Message msg = Message.obtain(mHandler,1);

msg.sendToTarget();

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