Android Handler 的工作原理解析
16lz
2021-01-23
前言
提到Handler大家应该都不会陌生，它是安卓中消息机制的主要核心类，配合MessageQueue和Looper一起使用构成了我们所熟悉的Android消息机制。Handler的主要工作是将一个任务切换到指定的线程去执行，因为Android中规定访问UI只能在主线程中进行，如果在子线程中访问UI，会抛出异常。而Android中不建议在主线程执行耗时操作否则会造成ANR，所以说。系统提供Handler的主要原因是为了解决在子线程中无法访问UI的问题。
本文深入分析 Android 的消息处理机制，了解 Handler 的工作原理。
Handler
创建一个handler的构造方法很多，不过最终都会调用下面这两个构造方法，贴源码：
public Handler(Looper looper, Callback callback, boolean async) { mLooper = looper; mQueue = looper.mQueue; mCallback = callback; mAsynchronous = async; } 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; }可以看到，主要完成了 Looper mLooper，MessageQueue mQueue，Callback mCallback 和 boolean mAsynchronous的初始化。通过调用 Looper.myLooper() 获得了 Looper 对象。如果 mLooper 为空，那么会抛出异常：”Can’t create handler inside thread that has not called Looper.prepare()”，意思是：不能在未调用 Looper.prepare() 的线程创建 handler。
Looper
1.在 Handler 的构造方法中调用 Looper.myLooper() 获得了 Looper 对象，方法源码如下：
// sThreadLocal.get() will return null unless you've called prepare(). static final ThreadLocal sThreadLocal = new ThreadLocal(); public static @Nullable Looper myLooper() { return sThreadLocal.get(); } 从 sThreadLocal 中获取了Looper对象，ThreadLocal是一个线程内部的数据存储类，通过它可以在指定线程中存储数据，存储后可以在指定线程中获取到存储的数据，而其他线程则无法获取到数据。这里用ThreadLocal说明Looper是线程独立的。
2.在Handler的构造方法中异常信息可知道如果没有调用Looper.prepare()是不能创建handler的，因为并没有获取Lopper，源码如下。
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)); }调用了Looper的构造方法创建了Looper对象，然后设置给sThreadLocal，这里需要注意如果已经设置过就会抛出异常，因为一个线程只会有一个Looper。下面看Looper构造方法：
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }在这里创建了MessageQueue消息队列对象，获取了当前线程。
3.有了Looper之后，还必须调用Looper.loop()才能开启消息轮询，这样才能不断从消息队列 MessageQueue 取出消息交由 Handler 处理。
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(); // Allow overriding a threshold with a system prop. e.g. // adb shell 'setprop log.looper.1000.main.slow 1 && stop && start' final int thresholdOverride = SystemProperties.getInt("log.looper." + Process.myUid() + "." + Thread.currentThread().getName() + ".slow", 0); boolean slowDeliveryDetected = false; 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 traceTag = me.mTraceTag; long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs; //......省略不关键代码 try { msg.target.dispatchMessage(msg); dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0; } finally { if (traceTag != 0) { Trace.traceEnd(traceTag); } } if (logSlowDelivery) { if (slowDeliveryDetected) { if ((dispatchStart - msg.when) <= 10) { Slog.w(TAG, "Drained"); slowDeliveryDetected = false; } } else { if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery", msg)) { // Once we write a slow delivery log, suppress until the queue drains. slowDeliveryDetected = true; } } } if (logSlowDispatch) { showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", msg); } 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(); } }这里主要是一直循环调用queue.next()拿到Message，然后通过msg.target.dispatchMessage(msg)方法处理，target是发送这条消息的handler对象（下面的分析会看到），这样交给handler的dispatchMessage(msg)处理。该循环只会在queue.next()为null时，才能跳出循环，否则一直阻塞下去。
MessageQueue
MessageQueue主要包含两个操作：插入和读取。读取的时候伴随着删除操作。
插入操作：enqueueMessage（），实现单链表的插入操作。
boolean enqueueMessage(Message msg, long when) { synchronized (this) { 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; }读取操作：next()方法，代码较长，看看就行了。无线循环，这个方法被Looper.loop()调用。
Message next() { 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; } }再回到Handler
Handler主要负责消息的发送和接收，发送可以通过一系列post方法和send方法，post方法最终也会调用send方法实现。
public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0); } public final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); } 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); }一步步调用可以发现Handler发送消息就是像MessageQueue中加入了一个新的Message。
之前在Looper介绍过，在loop方法中调用MessageQueue.next()方法拿到msg后，会调用handler的dispatchMessage()方法，dispatchMessage方法实现如下
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }这里的msg.callback就是一个Runnable对象，也就是Handler.post()传递的参数。
private static void handleCallback(Message message) { message.callback.run(); }mCallback是一个Callback接口，定义如下：
/** * Callback interface you can use when instantiating a Handler to avoid * having to implement your own subclass of Handler. */ public interface Callback { /** * @param msg A {@link android.os.Message Message} object * @return True if no further handling is desired */ public boolean handleMessage(Message msg); }作用和它的注释一样，可以用来创建一个Handler的实例，但并不需要派生Handler的子类。在Handler构造方法中讲过，可以传一个Callback进去，这样就不用重写Handler的handleMessage方法。配张图来解释下dispatchMessage流程。
总结
现在我们可以知道消息处理是通过 Handler 、Looper 以及 MessageQueue共同完成。 Handler 负责发送以及处理消息，Looper 创建消息队列并不断从队列中取出消息交给 Handler， MessageQueue 则用于保存消息。这三者是一个整体，缺一不可。
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