Handler、Looper与MessageQueue分析

public static void main(String[] args) {
        // 代码省略
        Process.setArgV0("<pre-initialized>");

        Looper.prepareMainLooper(); //1.创建消息循环Looper

        ActivityThread thread = new ActivityThread();
        thread.attach(false);

        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler(); // UI线程的Handler
        }

        if (false) {
            Looper.myLooper().setMessageLogging(new
                    LogPrinter(Log.DEBUG, "ActivityThread"));
        }

        Looper.loop(); // 2.执行消息循环

        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

public Handler() {
    //代码省略
    mLooper = Looper.myLooper(); //获取Looper
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread that has not called Looper.prepare()");
    }
    
    mQueue = mLooper.mQueue; //获取消息队列
    mCallback = null;
}

public static @Nullable Looper myLooper() {
    return sThreadLocal.get();
}

public static @NonNull MessageQueue myQueue() {
    return myLooper().mQueue;
}
public static void prepare() {
    prepare(true);
}

//为当前线程设置一个Looper
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));
}

//设置UI线程的Looper
public static void prepareMainLooper() {
    prepare(false);
    synchronized (Looper.class) {
        if (sMainLooper != null) {
            throw new IllegalStateException("The main Looper has already been prepared.");
        }
        sMainLooper = myLooper();
    }
}

// Looper的私有构造方法
private Looper(boolean quitAllowed) {
    mQueue = new MessageQueue(quitAllowed);
    mThread = Thread.currentThread();
}

/**
 * Run the message queue in this thread. Be sure to call
 * {@link #quit()} to end the loop.
 */
public static void loop() {
    final Looper me = myLooper();
    if (me == null) {
        throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
    }
    final MessageQueue queue = me.mQueue;  //1.获取消息队列

    // 代码省略
    for (;;) { //2.死循环，即消息循环
        //3.获取消息，可能阻塞
        Message msg = queue.next(); // might block
        if (msg == null) {
            // No message indicates that the message queue is quitting.
            return;
        }

        // 代码省略
        try {
            msg.target.dispatchMessage(msg); //4.处理消息
            end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
        } finally {
            if (traceTag != 0) {
                Trace.traceEnd(traceTag);
            }
        }
        // 代码省略
        msg.recycleUnchecked(); //回收消息
    }
}

public final class Message implements Parcelable {
    //target处理
    Handler target;
    //Runnable类型的callback
    Runnable callback;
    //下一条消息，消息队列是链式存储的
    Message next;

    //代码省略
}

//消息处理函数，子类覆写
public void handleMessage(Message msg) {
}

private static void handleCallback(Message message) {
    message.callback.run();
}

//分发消息
public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
        handleCallback(msg);
    } else {
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}

public final boolean post(Runnable r)
{
    return  sendMessageDelayed(getPostMessage(r), 0);
}

private static Message getPostMessage(Runnable r) {
    Message m = Message.obtain();
    m.callback = r;
    return m;
}

public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
    if (delayMillis < 0) {
        delayMillis = 0;
    }
    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
    MessageQueue queue = mQueue;
    if (queue == null) {
        RuntimeException e = new RuntimeException(
                this + " sendMessageAtTime() called with no mQueue");
        Log.w("Looper", e.getMessage(), e);
        return false;
    }
    return enqueueMessage(queue, msg, uptimeMillis); // 将消息插入到消息队列
}

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    msg.target = this; // Message的target就是该Handler本身
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);// 调用MessageQueue的加入消息方法
}

public final boolean sendMessage(Message msg)
{
    return sendMessageDelayed(msg, 0);
}

new Thread(){
    Handler handler = null;
    public void run () {
        //为当前线程创建Looper，并且绑定到ThreadLocal中
        Looper.prepare()
        handler = new Handler();
        //启动消息循环
        Looper.loop();
    };
}.start();

java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
        at android.os.Handler.<init>(Handler.java:200)
        at android.os.Handler.<init>(Handler.java:114)
        at android.widget.Toast$TN$2.<init>(Toast.java:345)
        at android.widget.Toast$TN.<init>(Toast.java:345)
        at android.widget.Toast.<init>(Toast.java:103)
        at android.widget.Toast.makeText(Toast.java:262)
        at com.agehua.loveprints.view.main.MainActivity$ShowExceptionThread.run(MainActivity.java:127)

private class ShowExceptionThread extends Thread {
    private String ex;

    ShowExceptionThread(String ex) {
        this.ex = ex;
    }

    @Override
    public void run() {
        Looper.prepare();
        Toast.makeText(AndroidApplication.getInstance().getApplicationContext(), "Error Encountered: " + ex, Toast.LENGTH_LONG).show();
        Looper.loop();
    }
}