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Handler源码 一

Handler源码 一

作者: 王灵 | 来源:发表于2019-02-20 13:12 被阅读0次

    Handler主要涉及到Handler、Message、MessageQueue、Looper四个类
    四个类主要功能:

    Handler

    1. sendXXXMessage 最终调用 enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis)
    /**
     *消息队列
     *消息
     *实际执行的时间点(如果当前时间小于执行时间则需要阻塞一段时间)
     */
    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
            msg.target = this;
            if (mAsynchronous) {
                msg.setAsynchronous(true);
            }
            return queue.enqueueMessage(msg, uptimeMillis);
        }
    
    1. handleMessage(Message msg) 我们新建Handler都会重写这个函数
    /**
         * Subclasses must implement this to receive messages.
         */
        public void handleMessage(Message msg) {
        }
    
    1. dispatchMessage(Message msg) 会在Looper.loop()里被调用
    /**
         * Handle system messages here.
         */
        public void dispatchMessage(Message msg) {
            if (msg.callback != null) {
                handleCallback(msg);
            } else {
                if (mCallback != null) {
                    if (mCallback.handleMessage(msg)) {
                        return;
                    }
                }
                handleMessage(msg);
            }
        }
    

    Message

    在主流程中用到两个变量

    /*package*/ Handler target; //用于保存发生消息的handler
    /*package*/ Message next; //用于下一个Message(看源码MessageQueue使用链表来保存Message)
    

    MessageQueue

    1. enqueueMessage(Message msg, long when) 向队列添加消息
      //注意:mMessages是一个全局变量,时间戳最小的msg
      needWake 标记是否需要唤醒
    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) {
                    IllegalStateException e = new IllegalStateException(
                            msg.target + " sending message to a Handler on a dead thread");
                    Log.w(TAG, e.getMessage(), e);
                    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 {
                    // 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.
                    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; // invariant: p == prev.next
                    prev.next = msg;
                }
    
                // We can assume mPtr != 0 because mQuitting is false.
                if (needWake) {
                    nativeWake(mPtr);
                }
            }
            return true;
        }
    
    
    1. Message next()从队列取出一个消息
     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;
                    }
    
                    // 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;
    
                // 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;
            }
        }
    

    Looper

    1.loop() 开始在MessageQueue里取出消息,并处理
    for 循环
    {
    Message msg = queue.next();获得消息
    msg.target.dispatchMessage(msg);最终调用handler的handleMessage(msg)
    }

    /**
         * 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;
    
            // 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(); // might block
                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
                final Printer logging = me.mLogging;
                if (logging != null) {
                    logging.println(">>>>> Dispatching to " + msg.target + " " +
                            msg.callback + ": " + msg.what);
                }
    
                final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
    
                final long traceTag = me.mTraceTag;
                if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                    Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
                }
                final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
                final long end;
                try {
                    msg.target.dispatchMessage(msg);
                    end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
                } finally {
                    if (traceTag != 0) {
                        Trace.traceEnd(traceTag);
                    }
                }
                if (slowDispatchThresholdMs > 0) {
                    final long time = end - start;
                    if (time > slowDispatchThresholdMs) {
                        Slog.w(TAG, "Dispatch took " + time + "ms on "
                                + Thread.currentThread().getName() + ", h=" +
                                msg.target + " cb=" + msg.callback + " msg=" + msg.what);
                    }
                }
    
                if (logging != null) {
                    logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
                }
    
                // 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();
            }
        }
    

    下面就分两个部分来写:消息传递流程和问题处理

    消息传递流程 (不是很理想,但是我弄了半天。。。画不出来)


    20180308193346613.jpeg

    我自己画的,我真的尽力了


    IMG_20190220_165440.jpg

    基本的流程就走完了,问题怎么解决的放在下一页吧!写个东西还真是费力

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