Handler详解
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参考
http://blog.csdn.net/u011240877/article/details/72892321
http://blog.csdn.net/lmj623565791/article/details/38377229/
一、Handler的定义
handler通过发送和处理Message和Runnable对象来关联相应线程的MessageQueue
1、可以让对应的message和runnable在未来的某个时间点进行相应的处理
2、让自己想要处理的耗时操作放到子线程,让UI操作放到主线程
二、Handler的使用方法
1、post(runnable)[底层还是调用了sendMessage方法]
2、sendMessage(Message)
三、Handler的机制原理
1、Activity的启动代码中,已经在当前UI线程调用了Looper.c和Looper.loop()方法
Looper
(对于Looper主要是prepare()和loop()两个方法。)
prepare()方法:
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)); }
从上面源码可以看出来
调用Looper.prepare()中创建Looper对象,并且把它保存在ThreadLocal中
(sThreadLocal是一个ThreadLocal对象,可以在一个线程中存储变量)并且一个线程中只能有一个Looper实例
再看Looper构造方法:
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }
可以看到在构造方法中,创建了一个MessageQueue(消息队列)。
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 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.isTagEnabled(traceTag)) { 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(); } }
public static @Nullable Looper myLooper() { return sThreadLocal.get(); }
loop()方法主要就是拿到当前线程的MessageQueue,然后进行死循环,不断的从消息队列中(先进先出的单向链表)取出消息,然后调用msg.target.dispatchMessage(msg);把消息交给msg的target的dispatchMessage方法去处理。Msg的target是什么呢?其实就是handler对象
Looper主要作用:
1、 与当前线程绑定,保证一个线程只会有一个Looper实例,同时一个Looper实例也只有一个MessageQueue。
2、 loop()方法,不断从MessageQueue中去取消息,交给消息的target属性的dispatchMessage去处理。
Handler登场
使用Handler之前,我们都是初始化一个实例
所以我们首先看Handler的构造方法,看其如何与MessageQueue联系上的,它在子线程中发送的消息(一般发送消息都在非UI线程)怎么发送到MessageQueue中的。Handle的构造方法:
在之前说过ThreadLocal会保存一个唯一的Looper
通过Looper.myLooper()获取了当前线程保存的Looper实例,然后又获取了这个Looper实例中保存的MessageQueue(消息队列),这样就保证了handler的实例与我们Looper实例中MessageQueue关联上了。
最后再看sendMessage()方法
public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0); }
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) { Message msg = Message.obtain(); msg.what = what; return sendMessageDelayed(msg, delayMillis); }
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); }
最后调用了sendMessageAtTime方法
在此方法内部有直接获取MessageQueue然后调用了enqueueMessage方法
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }
特别注意:enqueueMessage中首先为meg.target赋值为this,Looper的loop方法会取出每个msg然后交给msg,target.dispatchMessage(msg)去处理消息,也就是把当前的handler作为msg的target属性,最终会调用queue的enqueueMessage的方法,也就是说handler发出的消息,最终会保存到消息队列中去。
当loop()方法取出message时会回调msg,target.dispatchMessage(msg),我们再来看看Handler中dispatchMessage()方法
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }
调用了handleMessage方法,下面我们去看这个方法
public void handleMessage(Message msg) { }
可以看到这是一个空方法,为什么呢,因为消息的最终回调是由我们控制的,我们在创建handler的时候都是复写handleMessage方法,然后根据msg.what进行消息处理。
private Handler mHandler = new Handler() { public void handleMessage(android.os.Message msg) { switch (msg.what) { case value: break; default: break; } }; };
四、Handler引起的内存泄漏及解决办法
原因:
内部类持有外部类的引用导致Activity不能被回收
解决办法:
1、将Handler声明为static
2、在Activity的onDestory()中调用Handler的removeCallbacks()方法
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