Handler 系列二:通信
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承接上一篇Handler系列一,上篇主要总结了Handler如何通信,这篇来介绍Handler怎么通信。
Handler的通信机制
Handler,Looper,MessageQueue如何关联
Handler 通信机制
创建Handler,并采用当前线程的Looper创建消息循环系统;
Handler通过sendMessage(Message)或Post(Runnable)发送消息,调用enqueueMessage把消息插入到消息链表中;
Looper循环检测消息队列中的消息,若有消息则取出该消息,并调用该消息持有的handler的dispatchMessage方法,回调到创建Handler线程中重写的handleMessage里执行。
Handler 如何关联 Looper、MessageQueue
Handler及其关联的类图
以上类图可以快速帮助我们理清Handler与Looper、MessageQueue的关系,以下从源码的角度慢慢分析:
1、Handler 发送消息
上一段很熟悉的代码:
Message msg =Message.obtain(); //从全局池中返回一个message实例,避免多次创建message(如new Message) msg.obj = data; msg.what=1; //标志消息的标志 handler.sendMessage(msg);
从sendMessageQueue开始追踪,函数调用关系:sendMessage -> sendMessageDelayed ->sendMessageAtTime,在sendMessageAtTime中,携带者传来的message与Handler的mQueue一起通过enqueueMessage进入队列了。
对于postRunnable而言,通过post投递该runnable,调用getPostMessage,通过该runnable构造一个message,再通过 sendMessageDelayed投递,接下来和sendMessage的流程一样了。
2、消息入队列
在enqueueMessage中,通过MessageQueue入队列,并为该message的target赋值为当前的handler对象,记住msg.target
很重要,之后Looper取出该消息时,还需要由msg.target.dispatchMessage
回调到该handler中处理消息。
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis);}
在MessageQueue中,由Message的消息链表进行入队列
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;}
3、Looper 处理消息
再说处理消息之前,先看Looper是如何构建与获取的:
构造Looper时,构建消息循环队列,并获取当前线程
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }
但该函数是私有的,外界不能直接构造一个Looper,而是通过Looper.prepare来构造的:
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)); }
这里创建Looper,并把Looper对象保存在sThreadLocal中,那sThreadLocal是什么呢?
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
它是一个保存Looper的TheadLocal实例,而ThreadLocal是线程私有的数据存储类,可以来保存线程的Looper对象,这样Handler就可以通过ThreadLocal来保存于获取Looper对象了。
TheadLocal 如何保存与获取Looper?
public void set(T value) { Thread currentThread = Thread.currentThread(); Values values = values(currentThread); if (values == null) { values = initializeValues(currentThread); } values.put(this, value); } public T get() { // Optimized for the fast path. Thread currentThread = Thread.currentThread(); Values values = values(currentThread); if (values != null) { Object[] table = values.table; int index = hash & values.mask; if (this.reference == table[index]) { return (T) table[index + 1]; } } else { values = initializeValues(currentThread); } return (T) values.getAfterMiss(this); }
在 set 中都是通过 values.put
保存当前线程的 Looper 实例,通过 values.getAfterMiss(this)
获取,其中put
和getAfterMiss
都有key
和value
,都是由Value对象的table数组保存的,那么在table数组里怎么存的呢?
table[index] = key.reference; table[index + 1] = value;
很显然在数组中,前一个保存着ThreadLocal对象引用的索引,后一个存储传入的Looper实例。
接下来看Looper在loop中如何处理消息
在loop
中,一个循环,通过next
取出MessageQueue中的消息
若取出的消息为null,则结束循环,返回。
设置消息为空,可以通过MessageQueue的quit和quitSafely方法通知消息队列退出。
若取出的消息不为空,则通过msg.target.dispatchMessage回调到handler中去。
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 Printer logging = me.mLogging; if (logging != null) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } msg.target.dispatchMessage(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(); }}
4、handler处理消息
Looper把消息回调到handler的dispatchMessage中进行消息处理:
若该消息有callback,即通过Post(Runnable)的方式投递消息,因为在投递
runnable
时,把runnable
对象赋值给了message的callback
。若handler的mCallback不为空,则交由通过
callback
创建handler方式去处理。否则,由最常见创建handler对象的方式,在重写handlerMessage中处理。
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); }}
总结
以一个时序图来总结handler的消息机制,包含上述如何关联Looper和MessageQueue的过程。
Handler-Looper-MessageQueue时序图
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