Handler 使用

一、Handler和Looper关联

1、子线程内先调用Looper.prepare();
2、再创建Handler，
3、Looper.loop(); 让Looper开始工作

二、Looper

Looper创建

一个线程只能创建一个Looper，且把Looper 存到ThreadLocal中；

    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));
    }

Looper构造函数创建MassageQueue；

//Looper 关联主线程时为false   
private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

Handler构造函数会关联Looper

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());
            }
        }

        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread " + Thread.currentThread()
                        + " that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

其中mLooper = Looper.myLooper();获取本线程ThreadLocal中的Looper

/**
     * Return the Looper object associated with the current thread.  Returns
     * null if the calling thread is not associated with a Looper.
     */
    public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

Looper.loop是个死循环；

不断从消息队列中读取消息,取出消息进行分发；但阻塞是在queue.next()；
退出条件-msg=null（MessageQueue 返回空）;

/**
     * 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;
        ..........................
        for (;;) {
            Message msg = queue.next(); // might block
            //退出时，获取消息为null
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }

           ..........................
            try {
            //分发消息--【Handler target】;
                msg.target.dispatchMessage(msg);
                dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }
            
        }
    }

同步障碍器

作用：屏蔽后面同步消息，后面的异步消息会执行

queue.next(),是个死循环，直到取到一个消息；

闲置任务

执行完所有消息后会执行闲置Handler；没有闲置任务则进入阻塞状态。

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();
            }
            //nextPollTimeoutMillis 为0立即返回，为-1，永久等待（需要主动唤醒）；
            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) {//判断当前队首的消息是否是同步障碍器（target=nul，msg ！=null,普通消息为空入不了消息队列）
                    // Stalled by a barrier.  Find the next asynchronous message in the queue.
                  //取到下一异步消息
                    do {
                        prevMsg = msg;
                        msg = msg.next;
                    } while (msg != null && !msg.isAsynchronous());
                }
                if (msg != null) {//普通消息//非同步障碍器
                    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) {//跳过同步障碍器取消息；
                            prevMsg.next = msg.next;
                        } else {
                            mMessages = msg.next;//当前消息是队首
                        }
                        msg.next = null;
                        if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                        msg.markInUse();
                        return msg;
                    }
                } else {
                    // No more messages.
                  //无限等待
                    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.
                //执行完所有消息或执行闲置Handler任务
                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;//没有闲置任务，进入阻塞；
                    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.
            //执行闲置Handler
            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;

            // 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;
        }
    }

Message enqueueMessage 函数，往队列中插入消息，按时间执行顺序插入，链表头是距离执行时间最短的消息；

boolean enqueueMessage(Message msg, long when) {
       ..............
        synchronized (this) {
            if (mQuitting) {
               ..............
                return false;
            }

            msg.markInUse();
            msg.when = when;
            Message p = mMessages;
            boolean needWake;
          //队列首部为null /入队消息需要马上执行/入队消息早于队首消息，线程已阻塞，需要唤醒
            if (p == null || when == 0 || when < p.when) {
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;//mBlocked 标记消息循环是否阻塞
            } else {
                // Inserted within the middle of the queue.  Usually we don't have to wake
                // up the event queue unless there is a barrier at the head of the queue
                // and the message is the earliest asynchronous message in the queue.
                //根据队首是同步障碍器且插入消息是异步消息，
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
              //寻找插入位置
                for (;;) {
                    prev = p;
                    p = p.next;
                    if (p == null || when < p.when) {//待插入时间早于目标消息，或者是寻找到消息队列结尾；则确定了位置并退出；
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {//队列是阻塞，且插入异步消息执行时间更晚（否则上面会break），因此不需要唤醒。
                        needWake = false;
                    }
                }
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }

            // We can assume mPtr != 0 because mQuitting is false.
          //唤醒消息队列（ nativePollOnce处进入的睡眠）
            if (needWake) {
                nativeWake(mPtr);
            }
        }
        return true;
    }

调用Looper的退出，quit（）里面，调用消息队列推出；

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

消息推队列退出

   void quit(boolean safe) {
        if (!mQuitAllowed) {
            throw new IllegalStateException("Main thread not allowed to quit.");
        }

        synchronized (this) {
            if (mQuitting) {
                return;
            }
            mQuitting = true;

            if (safe) {
                removeAllFutureMessagesLocked();
            } else {
                removeAllMessagesLocked();
            }

            // We can assume mPtr != 0 because mQuitting was previously false.
            nativeWake(mPtr);
        }
    }