Android Handler原理介绍
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Andriod提供了Handler 和 Looper 来满足线程间的通信。Handler先进先出原则。Looper类用来管理特定线程内对象之间的消息交换(MessageExchange)。UIthread 通常就是main thread,而Android启动程序时会替Looper实例建立一个MessageQueue消息队列。
1 Looper
Activity启动的时候(OnCreate之前)就会创建一个Looper线程,由它来管理Looper线程里的MessageQueue(消息队列)。 UIThread线程通过Looper.prepare()创建唯一的Looper实例,然后创建该Looper实例中会同时创建一个MessageQueue对象;
对于Looper主要是prepare()和loop()两个方法。
1.1prepare()
因为Looper.prepare()在一个UiThread线程中只能调用一次,所以MessageQueue在一个UIThread线程中只会存在一个。
在Looper.java中
/** Initialize the current thread as a looper. * This gives you a chance to create handlers that then reference * this looper, before actually starting the loop. Be sure to call * {@link #loop()} after calling this method, and end it by calling * {@link #quit()}. */ 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)); }
可以看出prepare()方法会首先判断主线程中是否存在Looper线程(对象),如果有,则抛出异常,如果没有则创建一个新的Looper对象(线程).
1.2loop()
接下来UIThread主线程中会调用Looper.loop()会让当前Looper线程进入一个无限循环,不断的从MessageQueue的队列中读取消息,然后调用msg.target.dispatchMessage(msg)方法分发消息。
在Looper.java文件代码如下
/** * Run the message queue in this thread. Be sure to call * {@link #quit()} to end the loop. */ 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;//1.此处是获取MessageQueue对象. //其实就是和MessageQueue产生一个关联,或者关系, //为后续操作MessageQueue提供一个对象。 // 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 (;;) { //2.此处for无限循环很重要,也就是此处说明了Looper对象是一直无限的在操作MessageQueue Message msg = queue.next(); // might block 每次循环都先从消息队列中取出一个消息 if (msg == null) { //如果取出的消息为空,则表明当前MessageQueue没有消息。如果不为就将消息分发出去,进行处理。 // No message indicates that the message queue is quitting. return; //消息为空返回,进行下一次取消息判断消息,然后处理消息 } // 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);//3.此处是将消息分发出去进行处理,可能你会想msg.target是谁呢?接下来的代码会展示。 //其实就是你在UIThread中创建的handler对象本身。 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(); } }
这段代码太长了,看起来真是很麻烦,所以我觉着只要抓住我文中的1、2、3三处代码就足够了。总的来说Looper中的loop()方法就是无限的从Message中去取消息,然后判断消息是否为null,如果为空则继续去取消息,再判断是否为null。如果消息不为空就通过dispatchMessage方法将消息分发出去处理。
接下来我们看一下msg.target是个什么东西,首先从msg的定义我们可以看出是Message的一个实例,我们就看Message类
在Message.java中有如下的代码
/** * * Defines a message containing a description and arbitrary data object that can be * sent to a {@link Handler}. This object contains two extra int fields and an * extra object field that allow you to not do allocations in many cases. * * <p class="note">While the constructor of Message is public, the best way to get * one of these is to call {@link #obtain Message.obtain()} or one of the * {@link Handler#obtainMessage Handler.obtainMessage()} methods, which will pull * them from a pool of recycled objects.</p> */public final class Message implements Parcelable {...../*package*/ Handler target;.....}
由此我们可以看出,原来msg.target就是Handler,也就是你在UIThread中创建的handler对象本身.至于这个msg.target在什么时候被赋值为handler对象本身的呢?后续讲解会解答这个问题.接下来我们继续看dispatchMessage。从上知道msg.target.dispatchMessage(msg)自然就是Handler.dispatchMessage(msg)。那么我们看一下Handler中的dispatchMessage方法
在Handler.java中
/** * 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);//有没有觉着此方法很熟悉? //其实这就是你在创建Handler对象的时候所重写的handleMessage方法 //到此处我们知道了,原来是消息被取出来之后, //就是调用我们的重写的handleMessage方法 } }
其实MessageQueue没有啥可说的,就是在创建Looper对象实例的时候,创建了该消息队列。他就是类似于水管的东西,只不过水管中装的是水,但是MessageQueue装的是一个一个的Message对象罢了。
Looper.java中
/** * Return the {@link MessageQueue} object associated with the current * thread. This must be called from a thread running a Looper, or a * NullPointerException will be thrown. */ public static @NonNull MessageQueue myQueue() { return myLooper().mQueue; } private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }
在Looper的构造方法中,可以看出在创建Looper对象的时候,创建了MessageQueue对象。
Looper主要作用:
1、Looper与UIThread线程绑定,并且一个UIThread线程只有一个Looper实例。创建一个Looper实例过程中会创建一个MessageQueue对象。所以一个Looper实例也保证了只有一个MessageQueue实例。
2、Looper的loop()方法,不断从MessageQueue中取消息,交给消息的target属性的dispatchMessage去处理。
好了,我们的异步消息处理线程已经有了消息队列(MessageQueue),也有了在无限循环体中取出消息的Looper,现在缺的就是发送消息的对象了。接下来就是Handler的创建了.
3 Handler
3.1 创建Handler的实例的时候,在Handler构造方法,会首先得到当前线程中保存的Looper实例,进而与Looper实例中的MessageQueue关联。
在Handler.java中
/** * Use the {@link Looper} for the current thread with the specified callback interface * and set whether the handler should be asynchronous. * * Handlers are synchronous by default unless this constructor is used to make * one that is strictly asynchronous. * * Asynchronous messages represent interrupts or events that do not require global ordering * with respect to synchronous messages. Asynchronous messages are not subject to * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}. * * @param callback The callback interface in which to handle messages, or null. * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for * each {@link Message} that is sent to it or {@link Runnable} that is posted to it. * * @hide */ 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; }
3.2 Handler的sendMessage方法,会给msg的target赋值为handler自身,然后加入MessageQueue中。
在Handler.java中
/** * Pushes a message onto the end of the message queue after all pending messages * before the current time. It will be received in {@link #handleMessage}, * in the thread attached to this handler. * * @return Returns true if the message 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 sendMessage(Message msg) { return sendMessageDelayed(msg, 0); } /** * Enqueue a message into the message queue after all pending messages * before (current time + delayMillis). You will receive it in * {@link #handleMessage}, in the thread attached to this handler. * * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. Note that a * result of true does not mean the message will be processed -- if * the looper is quit before the delivery time of the message * occurs then the message will be dropped. */ public final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); } /** * Enqueue a message into the message queue after all pending messages * before the absolute time (in milliseconds) <var>uptimeMillis</var>. * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b> * Time spent in deep sleep will add an additional delay to execution. * You will receive it in {@link #handleMessage}, in the thread attached * to this handler. * * @param uptimeMillis The absolute time at which the message should be * delivered, using the * {@link android.os.SystemClock#uptimeMillis} time-base. * * @return Returns true if the message was successfully placed in to the * message queue. Returns false on failure, usually because the * looper processing the message queue is exiting. Note that a * result of true does not mean the message will be processed -- if * the looper is quit before the delivery time of the message * occurs then the message will be dropped. */ 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; //赋值为本身 if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }
走了这么大一圈,代码这么多,但是最终就是将消息加到了消息队列中。此处也刚好回到了刚才我们所说的msg.target是什么时候赋值为Handler对象本身的,就是在你调用sendMessage的时候,将msg.target赋值为handler的对象.
3.3 在构造Handler实例时,我们会重写handleMessage方法,也就是msg.target.dispatchMessage(msg)最终调用的方法。此部分代码就是我们要实现的handlermessage内容了,不同的需求实现的代码不同,所以接下来的就看各位自己怎么实现了。
总结一下
1、首先Looper.prepare()在UIThread线程中创建一个Looper实例,然后该实例中创建一个MessageQueue对象;因为Looper.prepare()在一个线程中只能调用一次,所以MessageQueue在一个线程中只会存在一个。
2、Looper.loop()会让当前Looper线程进入一个无限循环,不停地从MessageQueue的实例中取消息msg,然后调msg.target.dispatchMessage(msg)方法将消息分发出去。
3、Handler的构造方法,会首先关联当前UIThread线程中保存的Looper实例,进而与Looper实例中的MessageQueue关联。
4、Handler的sendMessage方法,会给msg的target赋值为handler自身,然后将消息msg加入MessageQueue中。
5、创建对象Handler时,我们会重写handleMessage方法,也就是msg.target.dispatchMessage(msg)最终调用的方法。
好了,总结完成,大家可能还会问,那么在Activity中,我们并没有显示的调用Looper.prepare()和Looper.loop()方法,为啥Handler可以成功创建呢,这是因为在Activity的启动代码中,已经在当前UI线程调用了Looper.prepare()和Looper.loop()方法。
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