Android handler 機制詳解（doing）

##本文章基於AndroidAPI28，主要分析messageQueue 單鏈表實現的意義和用法、messageQueue.next() 以及 Looper.loop()方法

1. 相關調用圖

ps：不會畫圖簡單看一下吧

2. 相關方法介紹

1.1 Handler

使用方式不具體介紹了！（簡單說一下） 內部類重寫handMessage方法，調用handler.post(sendMessage 等其餘方法)，最終都會調用sendMessageAtTime() 方法

// uptimeMillis = SystemClock.uptimeMillis() + delayMillis 下面會用到
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);
    }

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }
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mQueue 在構造函數中注入，默認構造使用主線程Looper.mQueue (自行閱讀構造函數源碼)

1.2 MessageQueue

Low-level class holding the list of messages to be dispatched by a {@link Looper}. Messages are not added directly to a MessageQueue, but rather through {@link Handler} objects associated with the Looper.

You can retrieve the MessageQueue for the current thread with {@link Looper#myQueue() Looper.myQueue()}.

低級類，保存由Looper調度的消息列表。消息不是直接添加到MessageQueue，而是經過與Looper關聯的Handler對象添加。 您可使用Looper#myQueue() 檢索當前線程的MessageQueue。

1.2.1 enqueueMessage 方法（具體看中文註釋部分代碼）

boolean enqueueMessage(Message msg, long when) {
        if (msg.target == null) {
            throw new IllegalArgumentException("Message must have a target.");
        }
        if (msg.isInUse()) {
            throw new IllegalStateException(msg + " This message is already in use.");
        }

        synchronized (this) {
            if (mQuitting) {
                ...
                msg.recycle();
                return false;
            }

            msg.markInUse();
            msg.when = when;
            Message p = mMessages;
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;
            } else {
                //插入隊列中間。一般，除非隊列的開頭有障礙而且消息是隊列中最先的異步消息，不然咱們沒必要喚醒事件隊列。
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                // 按照when從小到大的順序將msg 插入 鏈表實現的隊列中；
                for (;;) {
                    ）
                    prev = p;
                    p = p.next;
                    // 依次移動指針，直到找到p==null（隊列尾部 ）或者 msg.when<p.when(要插入的msg的when比p指針的小)
                    if (p == null || when < p.when) {
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }

            // We can assume mPtr != 0 because mQuitting is false.
            if (needWake) {
                nativeWake(mPtr);
            }
        }
        return true;
    }
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須要注意的幾點：

1. nativeWake能夠喚醒由nativePollOnce方法引發的阻塞
2. msg.when = SystemClock.uptimeMillis() + delayMillis（相似postDelay等方法中傳入的delay參數）
3. nativePollOnce 能夠阻塞Android 主線程並且不會出現ANR，具體分析看這裏

1.2.2 MessageQueue#next()方法

Message next() {
        // Return here if the message loop has already quit and been disposed.
        // This can happen if the application tries to restart a looper after quit
        // which is not supported.
        final long ptr = mPtr;
        if (ptr == 0) {
            return null;
        }

        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        int nextPollTimeoutMillis = 0;
        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }

            nativePollOnce(ptr, nextPollTimeoutMillis);

            synchronized (this) {
                // Try to retrieve the next message. Return if found.
                final long now = SystemClock.uptimeMillis();
                Message prevMsg = null;
                Message msg = mMessages;
                if (msg != null && msg.target == null) {
                    // Stalled by a barrier. Find the next asynchronous message in the queue.
                    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 {
                        // Got a message.
                        mBlocked = false;
                        if (prevMsg != null) {
                            prevMsg.next = msg.next;
                        } else {
                            mMessages = msg.next;
                        }
                        msg.next = null;
                        if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                        msg.markInUse();
                        return msg;
                    }
                } else {
                    // No more messages.
                    nextPollTimeoutMillis = -1;
                }

                // Process the quit message now that all pending messages have been handled.
                if (mQuitting) {
                    dispose();
                    return null;
                }
            // 這裏是IdleHandler 取出Msg的邏輯，詳見下一節
            // While calling an idle handler, a new message could have been delivered
            // so go back and look again for a pending message without waiting.
            nextPollTimeoutMillis = 0;
        }
    }
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1.2.3 IdleHandler 簡單使用、分析

使用：Looper.myLoop().getQueue().addIdleHandler(),能夠用來作一些預加載的操做

public void addIdleHandler(@NonNull IdleHandler handler) {
        synchronized (this) {
            mIdleHandlers.add(handler);
        }
    }
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Q : next()方法中IdleHandler是怎麼被取出而後執行的？

A : 在MessageQueue#next方法死循環中使用pendingIdleHandlerCount（local variable）控制執行次數，能夠避免加鎖操做！

// 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);
            }

            // Run the idle handlers.
            // We only ever reach this code block during the first iteration.
            for (int i = 0; i < pendingIdleHandlerCount; i++) {
                final IdleHandler idler = mPendingIdleHandlers[i];
                mPendingIdleHandlers[i] = null; // release the reference to the handler

                boolean keep = false;
                try {
                    keep = idler.queueIdle();
                } catch (Throwable t) {
                    Log.wtf(TAG, "IdleHandler threw exception", t);
                }

                if (!keep) {
                    synchronized (this) {
                        mIdleHandlers.remove(idler);
                    }
                }
            }

            // Reset the idle handler count to 0 so we do not run them again.
            pendingIdleHandlerCount = 0;
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1.3 Looper

1.3.1 loop() 方法

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;
        for (;;) {
            // 調用next方法 取出
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }
            ......
            try {
                // 回調handler的dispatchMessage方法，造成一個閉環
                msg.target.dispatchMessage(msg);
                dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            // Make sure that during the course of dispatching the
            // identity of the thread wasn't corrupted.
            final long newIdent = Binder.clearCallingIdentity();
            if (ident != newIdent) {
                Log.wtf(TAG, "Thread identity changed from 0x"
                        + Long.toHexString(ident) + " to 0x"
                        + Long.toHexString(newIdent) + " while dispatching to "
                        + msg.target.getClass().getName() + " "
                        + msg.callback + " what=" + msg.what);
            }

            msg.recycleUnchecked();
        }
    }
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1.4 Native方法（nativePollOnce && nativeWake）

參考這裏

1.5 MessageQueue 單鏈表實現的意義

1.6 總結