Android消息机制 Handler
在Android中解决子线程更新UI的三种方法:
-
Activity中 调用 runOnUiThread(Runnable action) {}
源码解析:/** * Runs the specified action on the UI thread. If the current thread is the UI * thread, then the action is executed immediately. If the current thread is * not the UI thread, the action is posted to the event queue of the UI thread. * * @param action the action to run on the UI thread */ public final void runOnUiThread(Runnable action) { if (Thread.currentThread() != mUiThread) { mHandler.post(action); } else { action.run(); } }
-
View 调用 View.post(Runnable action) {}
源码解析:/** *
Causes the Runnable to be added to the message queue. * The runnable will be run on the user interface thread.
* * @param action The Runnable that will be executed. * * @return Returns true if the Runnable was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. * * @see #postDelayed * @see #removeCallbacks */ public boolean post(Runnable action) { final AttachInfo attachInfo = mAttachInfo; if (attachInfo != null) { return attachInfo.mHandler.post(action); } // Postpone the runnable until we know on which thread it needs to run. // Assume that the runnable will be successfully placed after attach. getRunQueue().post(action); return true; } -
传统的用法: handler.post()
源码解析:/** * Causes the Runnable r to be added to the message queue. * The runnable will be run on the thread to which this handler is * attached. * * @param r The Runnable that will be executed. * * @return Returns true if the Runnable was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. */ public final boolean post(Runnable r) { return sendMessageDelayed(getPostMessage(r), 0); }
这三种方法最后都殊途同归的调用了 handler.post(Runnable r). 而 handler.post 内部却调用的是 sendMessageDelayed(Message m , 0);
所以最终的结论引入了 Handler 消息传播机制。
Handler.post(Runnbale r) 的调用
首先查看 post 源码
public final boolean post(Runnable r){ return sendMessageDelayed(getPostMessage(r), 0);}
调用到 sendMessageDelayed 方法
public final boolean sendMessageDelayed(Message msg, long delayMillis){ if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);}
延时发送消息,其实就是定时发送消息 sendMessageAtTime
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);}
最后执行到 MessageQueue 轮循消息
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis);}
引发的一些问题:
- Handler Looper Message MessageQueue 怎么发生联系?
- Looper是什么?
- Looper 和 MessageQueue 的关系?
- Handler 会发生内存泄露?为什么?
在handler创建的时候,handler 和 Looper 产生联系: handler 的构造函数如下:
public Handler() { this(null, false); }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 that has not called Looper.prepare()"); } mQueue = mLooper.mQueue; mCallback = callback; mAsynchronous = async;}
在handler创建的时候,和handler所在线程的 looper 产生了关联。在主线程中,Activity在创建的时候就帮我们创建了 looper 和 handler ,所以在子线程中更新ui的前两种方法可以行得通。第三种更新方式,实际原理和前两种一样,只不过是手动在主线程中创建了handler.
注意: 因此在子线程中创建 handler 时候会报错,只有开启了 looper 才可以创建 handler
Looper相关知识点
Looper类用来为一个线程开启一个消息循环
默认情况下android中新诞生的线程是没有开启消息循环的。(主线程除外,主线程系统会自动为其创建Looper对象,开启消息循环。)
Looper对象通过MessageQueue来存放消息和事件。一个线程只能有一个Looper,对应一个MessageQueue。通常是通过Handler对象来与 looper 进行交互的。
Handler可看做是Looper的一个接口,用来向指定的Looper发送消息及定义处理方法。
默认情况下Handler会与其被定义时所在线程的Looper绑定,比如,Handler在主线程中定义,那么它是与主线程的Looper绑定。
mainHandler = new Handler() 等价于new Handler(Looper.myLooper()).
Looper.myLooper():获取当前进程的looper对象,类似的 Looper.getMainLooper() 用于获取主线程的Looper对象。在非主线程中直接new Handler() 会报如下的错误:
E/AndroidRuntime( 6173): Uncaught handler: thread Thread-8 exiting due to uncaught exception
E/AndroidRuntime( 6173): java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
原因是非主线程中默认没有创建Looper对象,需要先调用Looper.prepare()启用Looper。-
Looper.loop(); 让Looper开始工作,从消息队列里取消息,处理消息。
注意:写在Looper.loop()之后的代码不会被执行,这个函数内部应该是一个循环,当调用mHandler.getLooper().quit()后,loop才会中止,其后的代码才能得以运行。
通过以上知识,可实现主线程给子线程(非主线程)发送消息。
Looper的使用过程以及和 Messagequeue 关系的建立:
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));}private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread();}
Looper的调用方法是 Looper.prepare() -- new handler()-- looper.loop();
轮询器在循环执行 Messagequeue 中的消息时候调用的:
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 traceTag = me.mTraceTag; if (traceTag != 0) { Trace.traceBegin(traceTag, msg.target.getTraceName(msg)); } try { msg.target.dispatchMessage(msg); } finally { if (traceTag != 0) { Trace.traceEnd(traceTag); } } 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(); }}
当执行到其中的一条message时候, 会回调携带 message 的handler 的方法 dispatchMessage
msg.target.dispatchMessage(msg);
而在 Handler的 dispatchMessage(msg) 中又执行了 callback.handleMessage(msg),
/** * 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); }}
在callback的handleMessage(msg)中又接着执行了 handler 的hanleMessage(msg) 方法;
/** * Subclasses must implement this to receive messages. */public void handleMessage(Message msg) {}
因此在创建 handler 的时候,一定要复写其中的 handleMessage 方法.
总结
首先Looper.prepare() 在本线程创建并保存一个实例,然后该Loop实例创建并维护一个 MessageQueue 对象,因为一个线程中 Looper.prepare()只能执行一次,因此 MessageQueue 只会存在一个。
Looper.loop() 会让当前线程进入一个无限循环,不断从消息队列中读取消息,并处理,此时会回调 msg.target.dispatchMessage(msg)
handler 的构造方法会得到当前线程中保存的 looper 实例。
Handler 的 sendMessage() 方法会给msg的 target 赋值为 handler 本身,然后加入到消息队列中。
在构造 handler 时候,我们会重写 handlemessage() 方法,也是 msg.target.dispatchMessage(msg)最终调用的方法。
三者的关系如下面这个丑图:
网上这个美图更漂亮一些:
补充
- 为什么主线程中的Looper.loop() 一直无限循环不会造成 ANR?
答: ANR 的原因就是有需要处理的事情未处理,或者在规定时间内未处理!
因为Android是由事件驱动的,looper.loop()不断地接收事件、处理事件,每一个点击、触摸等等都在主线程looper.loop()的控制之下,如果它停止了,应用也就停止了。只能说对消息的处理阻塞了主线程的looper.loop() 才会导致 ANR, looper.loop()会阻塞自己的运行???
- ANR的时间是多少?按键或者触摸是5s,广播是10s。官网的定义:
https://developer.android.com/training/articles/perf-anr.html
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