Android 消息机制
一、消息机制概述
Android的消息机制主要是指Handler的运行机制以及Handler所附带的MessageQueue和Looper的工作过程,Handler的主要作用是将一个任务切换到当前Handler所在的线程中去执行。
Android提供这个功能主要是它规定了访问UI只能在主线程中进行,如果在子线程中访问UI,那么程序就会抛出异常。
以上规定的原因是:Android的UI控件不是线程安全的,如果在多线程并发访问时可能会导致UI控件处于不可预期的状态。但是加锁又会让UI的访问逻辑变得复杂;而且锁会阻塞某些线程的执行,因而锁机制会降低UI访问的效率。所以Android采用了最简单高效的一种方法——采用单线程模型来处理UI操作。
二、消息机制的整体流程
Handler调用sendMessage()向MessageQueue发送一条消息,这时会调用一个MessageQueue的enqueueMessage()将消息加入MessageQueue中,MessageQueue中有消息后通过MessageQueue.next()取走消息交给Looper,当Looper发现有新消息到来后,Looper通过Looper.loop()方法不断地从消息队列中读取消息,然后将这个消息交由Handler,在Handler中的handleMessage中对消息进行处理。
三、概念释义
Handler:消息辅助类,主要功能向消息池发送各种消息事件(Handler.sendMessage)和处理相应消息事件(Handler.handleMessage)。
MessageQueue:消息队列,但是内部存储结构并不是队列,而是采用单链表的数据结构来存储消息列表(因为单链表在插入和删除上比较有优势)。
为了能够更好地理解Looper,我们需要先了解一下ThreadLocal这个概念。
- ThreadLocal
ThreadLocal是一个线程内部的数据存储类,通过它可以在指定的线程中存储数据,数据存储以后,只有在指定线程中可以获取到存储的数据,对于其他线程来说则无法获取到数据。
为什么Looper需要用到ThreadLoacl?
因为Handler需要获取到当前线程的Looper,而Looper的作用域是线程,并且不同线程具有不同的Looper。所以这个时候通过ThreadLocal就可以轻松实现Looper在线程中的存取。如果不采用这个ThreadLocal的话那么系统就必须提供一个类似于LooperManager的类来管理Looper了,这时就体现出ThreadLocal的好处了。
Looper:消息循环,它会不停地从MessageQueue查看是否有新消息,如果有新消息就会处理,否则就会一直阻塞在那里。
四、工作原理
- Handler的工作原理:
Handler的主要工作包含消息的发送和接收,消息的发送可以通过post或send等一系列方法来实现,但最终都是用到了sendMessageAtTime()这个函数。
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); }
可以看到这个函数的返回的是enqueueMessage(),相当于Handler发送消息的过程就是向消息队列插入了一条消息。
在发送完消息后,MessageQueue的next()会返回这条消息给Looper,Looper收到消息就开始处理了,最终消息由Looper交由Handler处理,这时Handler的dispatchMessage()会被调用。
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }
从dispatchMessage中我们可以看到Handler就开始进入消息处理阶段了。
首先,会检查Message的callback是否为null,不为null就通过handleCallback来处理消息。Message的callback是一个Runnable对象,实际上就是Handler的post方法所传递的Runnable参数。
通过Callback可以采用如下方式来创建Handler对象:Handler handler = new Handler(callback)。这个Callback的作用就是去创建一个Handler的实例而不需要派生Handler子类。一般我们也可以直接派生一个Handler的子类,再重写hanldeMessage方法来处理具体的消息,这个Callback给我们提供了另外一种使用Handler的方式,可以让我们不派生Handler子类就可以创建Handler了。
- MessageQueue的工作原理:
MessageQueue主要包含两个操作:插入(enqueueMessage)和读取(next),读取的同时也包含着删除。因为单链表在插入和删除上比较有优势,所以内部是用的单链表进行的存储,这一点在上面也提到过,下面我们来看下这两个主要方法的源码。
enqueueMessage():
//这里的主要操作就是单链表的插入操作,没什么特别的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; }
next():
这里面有个for的死循环,是next()读取消息的关键。如果MessageQueue中有消息的话,next就会返回消息并从单链表中移除这条消息;如果MessageQueue中没有消息的话,它就会一直阻塞在这里,等待新消息的到来
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的工作原理:
之前说过,Looper在整个消息机制中扮演的是消息循环的角色,它会不停地从MessageQueue中查看是否有新消息,如果有新消息就会立刻处理,否则就一直阻塞在那里。
因为整个消息机制其实就是Handler的运行机制,而Handler的工作需要Looper,没有Looper的线程会报错,所以我们就需要在每个使用到Handler的线程中去创建Looper,一般在主线程当中,Looper是Android系统为我们创建好的,但是如果要在其他线程中创建Looper,就需要用到Looper的prepare()这个方法来为当前线程创建一个Looper,但是有一点需要注意,如果在子线程中手动创建了Looper,那么在所有事情处理完之后就应该调用quit()来终止消息循环,否则这个子线程会一直处于等待状态;在Looper成功创建之后,就可以通过Looper.loop()来开启消息循环了。
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(); } }
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