Android 消息处理源代码分析（1）点击打开链接

继续接着分析剩下的类文件

Looper.java

public final class Looper {

    final MessageQueue mQueue;   //消息队列

    final Thread mThread;   //Looper联系的线程

    public static void prepare() {

        prepare(true);

    }

    private static void prepare(boolean quitAllowed) {   //先会检查是否有Looper。若有则抛出异常。没有的话则创建一个Looper实例保存起来

        if (sThreadLocal.get() != null) {

            throw new RuntimeException("Only one Looper may be created per thread");

        }

        sThreadLocal.set(new Looper(quitAllowed));

    }

    public static void prepareMainLooper() {

        prepare(false);

        synchronized (Looper.class) {

            if (sMainLooper != null) {

                throw new IllegalStateException("The main Looper has already been prepared.");

            }

            sMainLooper = myLooper();

        }

    }

   //在这个线程中执行消息队列。调用quit（）停止

   public static void loop() {

	...

	final 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();

        for (;;) {

            Message msg = queue.next(); // 从消息队列中取出一条消息

            if (msg == null) {

                // No message indicates that the message queue is quitting.

                return;

            }

            // 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);

            }

            msg.target.dispatchMessage(msg);   //交给msg的handler分发消息处理

	...

   }

    //取出当前线程的Looper，返回空则表示当前线程没有Looper

    public static Looper myLooper() {

        return sThreadLocal.get();

    }

}

Handler.java

public class Handler {

     //定义Callback接口

     public interface Callback {

   	   public boolean handleMessage(Message msg);

     }

     //子类要实现的消息处理方法

     public void handleMessage(Message msg) {

     }

     * Handle system messages here.

     */

    public void dispatchMessage(Message msg) {        //分发信息

        if (msg.callback != null) {          //若指定了msg.callback，则由它处理

            handleCallback(msg);

        } else {

            if (mCallback != null) {        //若指定了Handler.mCallback。则由它处理

                if (mCallback.handleMessage(msg)) {    //调用mCallback接口的实现方法

                    return;

                }

            }

            handleMessage(msg);   最后才调用Handler自身重载的handleMessage方法

        }

    }

    分发消息函数中，消息先会检查自身有没有处理自身的回调Runnable，若有则由它处理。若没有则会检查该handler有无自身的回调处理，若有则调用，若没有则调用自身重载的handleMessage方法

    //Handler的生成总是和它当前所处线程有关的，假设当前线程中没有一个Looper。则会报错。UI线程中默认有产生Looper的函数

    public Handler() {

        this(null, false);

    }    

    //使用指定的Looper（能够处理那个Looper线程中的消息）。不用默认的从当前线程中取出Looper

    public Handler(Looper looper) {

        this(looper, null, false);

    }

   ...

}