//保存Looper對(duì)象，在android中每創(chuàng)建一個(gè)消息隊(duì)列，就有一個(gè)并且是唯一一個(gè)與之對(duì)應(yīng)的Looper對(duì)象 
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
//主線程的Looper
private static Looper sMainLooper;
//消息隊(duì)列
final MessageQueue mQueue;
final Thread mThread;

//子線程中通過調(diào)用該方法來創(chuàng)建消息隊(duì)列
public static void prepare() {
    prepare(true);
}

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));
}


//主線程調(diào)用該方法來創(chuàng)建消息隊(duì)列
public static void prepareMainLooper() {
    prepare(false);
    synchronized (Looper.class) {
        if (sMainLooper != null) {
            throw new IllegalStateException("The main Looper has already been prepared.");
        }
        sMainLooper = myLooper();
    }
}

//實(shí)例化Looper，創(chuàng)建消息隊(duì)列，獲取當(dāng)前線程
private Looper(boolean quitAllowed) {
    mQueue = new MessageQueue(quitAllowed);
    mThread = Thread.currentThread();
}

//調(diào)用loop方法開啟消息循環(huán) 
public static void loop() { 
    //獲取當(dāng)前的Looper對(duì)象，若為null，拋出異常 
    final Looper me = myLooper();
    if (me == null) {
        throw new RuntimeException("No Looper; Looper.prepare() 
            wasn't called on this thread."); 
     } 
    //獲取當(dāng)前的消息隊(duì)列，進(jìn)入循環(huán) 
    final MessageQueue queue = me.mQueue; 
    for (;;) { 
        //調(diào)用next()方法從消息隊(duì)列中獲取消息，如果為null，結(jié)束循環(huán)；否則，繼續(xù)執(zhí)行(有可能會(huì)阻塞) 
        Message msg = queue.next(); 
        if (msg == null) { 
            return; 
        } 
        ...... 
        try { 
            //調(diào)用handler的dispatchMessage(msg)分發(fā)消息  
            msg.target.dispatchMessage(msg); 
        } finally {
        ...... 
        } 
        //回收消息資源  
        msg.recycleUnchecked(); 
    }
}

//消息循環(huán)退出
public void quit() {
    mQueue.quit(false);
}

public void quitSafely() {
    mQueue.quit(true);
}

/*
*MessageQueue中enqueueMessage方法的目的有兩個(gè)：
*1.插入消息到消息隊(duì)列
*2.喚醒Looper中等待的線程(如果是即時(shí)消息并且線程是阻塞狀態(tài))
*/
boolean enqueueMessage(Message msg, long when) {
    //發(fā)送該消息的handler為null，拋出異常
    if (msg.target == null) {
        throw new IllegalArgumentException("Message must have a target.");
    }
    //此消息正在被使用
    if (msg.isInUse()) {
        throw new IllegalStateException(msg + " This message is already in use.");
    }

    synchronized (this) {
        //此消息隊(duì)列已經(jīng)被放棄了
        if (mQuitting) {
            IllegalStateException e = new IllegalStateException(
                    msg.target + " sending message to a Handler on a dead thread");
            msg.recycle();
            return false;
        }
        msg.markInUse();
        msg.when = when;
        //消息隊(duì)列的第一個(gè)元素，MessageQueue中的成員變量mMessages指向的就是該鏈表的頭部元素。
        Message p = mMessages;
        boolean needWake;
        if (p == null || when == 0 || when < p.when) {
            //如果此隊(duì)列中頭部元素是null(空的隊(duì)列，一般是第一次)，或者此消息不是延時(shí)的消息，則此消息需要被立即處理，
            //將該消息作為新的頭部，并將此消息的next指向舊的頭部。如果是阻塞狀態(tài)則需要喚醒。
            msg.next = p;
            mMessages = msg;
            needWake = mBlocked;
        } else {
            //如果此消息是延時(shí)的消息，則將其添加到隊(duì)列中，
            //原理就是鏈表的添加新元素，按照時(shí)間順序來插入的，這樣就得到一條有序的延時(shí)消息鏈表  
            needWake = mBlocked && p.target == null && msg.isAsynchronous();
            Message prev;
            for (;;) {
                prev = p;
                p = p.next;
                if (p == null || when < p.when) {
                    break;
                }
                if (needWake && p.isAsynchronous()) {
                    needWake = false;
                }
            }
            msg.next = p;
            prev.next = msg;
        }
        if (needWake) {
            nativeWake(mPtr);
        }
    }
    return true;
}

Message next() {
    //與native方法相關(guān),當(dāng)mPtr為0時(shí)返回null,退出消息循環(huán)
    final long ptr = mPtr; 
    if (ptr == 0) {
        return null;
    }

    int pendingIdleHandlerCount = -1;
    //0不進(jìn)入睡眠,-1進(jìn)入睡眠 
    int nextPollTimeoutMillis = 0;  
    for (;;) {
        if (nextPollTimeoutMillis != 0) {
            //處理當(dāng)前線程中待處理的Binder進(jìn)程間通信請(qǐng)求
            Binder.flushPendingCommands();  
        }
        //native方法,nextPollTimeoutMillis為-1時(shí)進(jìn)入睡眠狀態(tài)
        //阻塞方法，主要是通過native層的epoll監(jiān)聽文件描述符的寫入事件來實(shí)現(xiàn)的。
        //如果nextPollTimeoutMillis=-1，一直阻塞不會(huì)超時(shí)。
        //如果nextPollTimeoutMillis=0，不會(huì)阻塞，立即返回。
        //如果nextPollTimeoutMillis>0，最長阻塞nextPollTimeoutMillis毫秒(超時(shí))，如果期間有程序喚醒會(huì)立即返回
        nativePollOnce(ptr, nextPollTimeoutMillis); 
        synchronized (this) {
            final long now = SystemClock.uptimeMillis();
            Message prevMsg = null;
            Message msg = mMessages;
            if (msg != null && msg.target == null) {
                do {
                    prevMsg = msg;
                    msg = msg.next;
                } while (msg != null && !msg.isAsynchronous());
            }
            if (msg != null) {
                if (now < msg.when) {
                    // Next message is not ready.  Set a timeout to wake up when it is ready.
                    nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                } else {
                    //正常取出消息，設(shè)置mBlocked = false代表目前沒有阻塞
                    mBlocked = false;
                    if (prevMsg != null) {
                        prevMsg.next = msg.next;
                    } else {
                        mMessages = msg.next;
                    }
                    msg.next = null;
                    msg.markInUse();
                    return msg;
                }
            } else {
                // No more messages.更新到睡眠狀態(tài)
                nextPollTimeoutMillis = -1;
            }

                // Process the quit message now that all pending messages have been handled.
                if (mQuitting) {
                    dispose();
                    return null;
                }

                // If first time idle, then get the number of idlers to run.
                // Idle handles only run if the queue is empty or if the first message
                // in the queue (possibly a barrier) is due to be handled in the future.
                if (pendingIdleHandlerCount < 0
                        && (mMessages == null || now < mMessages.when)) {
                    pendingIdleHandlerCount = mIdleHandlers.size();
                }
                if (pendingIdleHandlerCount <= 0) {
                    // No idle handlers to run.  Loop and wait some more.
                    mBlocked = true;
                    continue;
                }

                if (mPendingIdleHandlers == null) {
                    mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
                }
                mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
            }
        }            
        //非睡眠狀態(tài)下處理IdleHandler接口 
        for (int i = 0; i < pendingIdleHandlerCount; i++) { 
            final IdleHandler idler = mPendingIdleHandlers[i]; 
            // release the reference to the handler 
            mPendingIdleHandlers[i] = null; 
            boolean keep = false; 
            try { 
               keep = idler.queueIdle(); 
            } catch (Throwable t) { 
                Log.wtf(TAG, "IdleHandler threw exception", t); 
            } 
            if (!keep) { 
                synchronized (this) { 
                    mIdleHandlers.remove(idler); 
                } 
            } 
        } 
        pendingIdleHandlerCount = 0; 
        nextPollTimeoutMillis = 0;
    }        
}

/*
*通過handler類向線程的消息隊(duì)列發(fā)送消息，
*每個(gè)Handler對(duì)象中都有一個(gè)Looper對(duì)象和MessageQueue對(duì)象
*/
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());
        }
    }
    //獲取Looper對(duì)象
    mLooper = Looper.myLooper(); 
    if (mLooper == null) {...}
    //獲取消息隊(duì)列
    mQueue = mLooper.mQueue;  
    mCallback = callback;
    mAsynchronous = async;
}

/*
*多種sendMessage方法,最終都調(diào)用了同一個(gè)方法sendMessageAtTime()
*/
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;
    }
    //向消息隊(duì)列中添加消息
    return enqueueMessage(queue, msg, uptimeMillis); 
}
    
/*
*1.當(dāng)Message中的callback不為null時(shí)，執(zhí)行Message中的callback中的方法。這個(gè)callback時(shí)一個(gè)Runnable接口。
*2.當(dāng)Handler中的Callback接口不為null時(shí)，執(zhí)行Callback接口中的方法。
*3.直接執(zhí)行Handler中的handleMessage()方法。
*/
public void dispatchMessage(Message msg) {
    // 消息Callback接口不為null,執(zhí)行Callback接口
    if (msg.callback != null) {
        handleCallback(msg);
    } else {
        if (mCallback != null) {
            //Handler Callback接口不為null,執(zhí)行接口方法
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        //處理消息
        handleMessage(msg); 
    }
}