Android消息处理流程(handler)
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说到消息的处理流程就要涉及到Handler、Message、MessageQueue、Looper、HandlerThread这几个类:
- Handler的作用有两个:在新线程中发送消息,在主线程中接受并处理消息。通过handler发送的消息必须被送入到指定的MessageQueue中,所以当前线程中必须有一个MessageQueue对象。MessageQueue对象是在Looper的构造方法中创建的
- MessageQueue:消息队列,负责对消息进行管理。
- Looper:负责管理MessageQueue中的消息
如果是主线程:系统已经初始化了一个Looper对象,程序直接创建Handler,就可以通过hangler发送消息了。
如果是子线程:必须手动创建Looper对象并启动它。创建Looper对象调用Looper的prepare方法即可,通过looper的loop方法启动。
下面我们就从消息的创建开始介绍消息的处理流程。
一.消息的创建:
借助Message类的obtain方法创建消息
Message message = Message.obtain();message.what = POST;message.obj = response.body().string();
二.消息的发送:send和post
不管是调用send类型的哪个方法,都会首先调用sendMessageDelayed方法,然后sendMessageDelayed中又会调用sendMessageAtTime方法。而post类型的方法又是对send类型的方法的封装。
send类型的方法:
public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0); }public final boolean sendEmptyMessage(int what) { return sendEmptyMessageDelayed(what, 0); }public final boolean sendEmptyMessageDelayed(int what, long delayMillis) { Message msg = Message.obtain(); msg.what = what; return sendMessageDelayed(msg, delayMillis); }public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) { Message msg = Message.obtain(); msg.what = what; return sendMessageAtTime(msg, uptimeMillis); }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); }public final boolean sendMessageAtFrontOfQueue(Message msg) { 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, 0); }
post类型的方法
public final boolean post(Runnable r) { return sendMessageDelayed(getPostMessage(r), 0); }public final boolean postAtTime(Runnable r, long uptimeMillis) { return sendMessageAtTime(getPostMessage(r), uptimeMillis); }public final boolean postAtTime(Runnable r, Object token, long uptimeMillis) { return sendMessageAtTime(getPostMessage(r, token), uptimeMillis); }public final boolean postDelayed(Runnable r, long delayMillis) { return sendMessageDelayed(getPostMessage(r), delayMillis); }public final boolean postAtFrontOfQueue(Runnable r) { return sendMessageAtFrontOfQueue(getPostMessage(r)); }
而在sendMessageAtTime方法中又会调用enqueueMessage方法:
这个方法中的mQueue是通过与Handler有关的Looper获得的。
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); }
下面来看enqueueMessage方法,在enqueueMessage方法中将Message的target设置为当前的handler并将Message添加到消息队列。
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }
三.消息的接受
一方面我们将message添加到MessageQueue,另一方面HandlerThread类中的run方法开始执行:
我们重点看 Looper.prepare()和Looper.loop()
public void run() { mTid = Process.myTid(); Looper.prepare(); synchronized (this) { mLooper = Looper.myLooper(); notifyAll(); } Process.setThreadPriority(mPriority); onLooperPrepared(); Looper.loop(); mTid = -1; }
Looper.prepare()方法:prepare中先判断当前线程中是否已经存在Looper对象,如果不存在则用new Looper创建Looper对象,从而保证当前线程中只有一个Looper对象。
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)); }
接下来我们来看Looper的构造函数:
可以看到在Looper的构造函数中创建了MessageQueue对象,所以当前线程中Looper对象的唯一性保证了MessageQueue对象的唯一性。
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }
然后看Looper.loop方法:
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(); } }
我们重点看for循环中的内容:
1.Message msg = queue.next();
在循环中不断的调用MessageQueue的next方法取出消息队列中的消息。
2.调用msg.target.dispatchMessage(msg);方法,按照发送消息时target的不同将消息交给不同的handler处理,target就是发送message时跟message关联的handler,也就是执行我们重写的Handler类的handleMessage方法。
3.最后对消息进行回收。
Handler handler = new Handler() { @Override public void handleMessage(Message msg) { switch (msg.what) { case 100: break; case 200: break; } super.handleMessage(msg); } };
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