Android 消息處理源碼分析（2）

 Android 消息處理源碼分析（1）點擊打開鏈接

繼續接着分析剩下的類文件

Looper.java
public final class Looper {
    final MessageQueue mQueue;   //消息隊列
    final Thread mThread;   //Looper聯繫的線程
    
    public static void prepare() {
        prepare(true);
    }
    
    private static void prepare(boolean quitAllowed) {   //先會檢查是否有Looper，若有則拋出異常，沒有的話則創建一個Looper實例保存起來
        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();
        }
    }
   //在這個線程中運行消息隊列，調用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(); // 從消息隊列中取出一條消息
            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);   //交給msg的handler分發消息處理

	...
   }
    //取出當前線程的Looper，返回空則表示當前線程沒有Looper
    public static Looper myLooper() {
        return sThreadLocal.get();
    }
}

Handler.java
public class Handler {
     //定義Callback接口
     public interface Callback {
   	   public boolean handleMessage(Message msg);
     }
	
     //子類要實現的消息處理方法
     public void handleMessage(Message msg) {
     }

     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {        //分發信息
        if (msg.callback != null) {          //若指定了msg.callback，則由它處理
            handleCallback(msg);
        } else {
            if (mCallback != null) {        //若指定了Handler.mCallback，則由它處理
                if (mCallback.handleMessage(msg)) {    //調用mCallback接口的實現方法
                    return;
                }
            }
            handleMessage(msg);   最後才調用Handler自身重載的handleMessage方法
        }
    }
    分發消息函數中，消息先會檢查自身有沒有處理自身的回調Runnable，若有則由它處理，若沒有則會檢查該handler有無自身的回調處理，若有則調用，若沒有則調用自身重載的handleMessage方法

    //Handler的生成總是和它當前所處線程有關的，如果當前線程中沒有一個Looper，則會報錯，UI線程中默認有產生Looper的函數
    public Handler() {
        this(null, false);
    }    

    //使用指定的Looper（可以處理那個Looper線程中的消息），不用默認的從當前線程中取出Looper
    public Handler(Looper looper) {
        this(looper, null, false);
    }
   ...
}