Androidhandler详解

一、Looper
looper是关联message与handler的重要部分。

你在Android的子线程中new handler为啥报错&＃xff0c;和looper关系不浅&＃xff0c;据如下个例子&＃xff0c;在子线程中调用handler

new Thread(new Runnable() { public void run() { Looper.prepare(); Handler handler &＃61; new Handler(){ &＃64;Override public void handleMessage(Message msg) { Toast.makeText(getApplicationContext(), "handler msg", Toast.LENGTH_LONG).show(); } }; handler.sendEmptyMessage(1); Looper.loop(); }; }).start();
或者在子线程直接使用Looper.getMainLooper()&＃xff1b;new Thread(new Runnable() { public void run() { Handler handler &＃61; new Handler(Looper.getMainLooper()){ &＃64;Override public void handleMessage(Message msg) { Toast.makeText(getApplicationContext(), "handler msg", Toast.LENGTH_LONG).show(); } }; handler.sendEmptyMessage(1); }; }).start();


Looper.getMainLooper()实际就是主线程的looper&＃xff0c;主线程默认都会创建looper

创建looper时会创建messagequeue

注&＃xff1a;每一个线程只能有一个looper&＃xff0c;looper是线程持有

Looper实际上是给当前子线程建立的looper&＃xff0c;为了能够遍历当前子线程的消息队列&＃xff0c;

源码

public static void loop() {final Looper me &＃61; myLooper();if (me &＃61;&＃61; null) {throw new RuntimeException("No Looper; Looper.prepare() wasn&＃39;t called on this thread.");}final MessageQueue queue &＃61; 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 &＃61; Binder.clearCallingIdentity();for (;;) {Message msg &＃61; queue.next(); // might blockif (msg &＃61;&＃61; 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 loggerfinal Printer logging &＃61; me.mLogging;if (logging !&＃61; null) {logging.println(">>>>> Dispatching to " &＃43; msg.target &＃43; " " &＃43;msg.callback &＃43; ": " &＃43; msg.what);}final long slowDispatchThresholdMs &＃61; me.mSlowDispatchThresholdMs;final long traceTag &＃61; me.mTraceTag;if (traceTag !&＃61; 0 && Trace.isTagEnabled(traceTag)) {Trace.traceBegin(traceTag, msg.target.getTraceName(msg));}final long start &＃61; (slowDispatchThresholdMs &＃61;&＃61; 0) ? 0 : SystemClock.uptimeMillis();final long end;try {msg.target.dispatchMessage(msg);end &＃61; (slowDispatchThresholdMs &＃61;&＃61; 0) ? 0 : SystemClock.uptimeMillis();} finally {if (traceTag !&＃61; 0) {Trace.traceEnd(traceTag);}}if (slowDispatchThresholdMs > 0) {final long time &＃61; end - start;if (time > slowDispatchThresholdMs) {Slog.w(TAG, "Dispatch took " &＃43; time &＃43; "ms on "&＃43; Thread.currentThread().getName() &＃43; ", h&＃61;" &＃43;msg.target &＃43; " cb&＃61;" &＃43; msg.callback &＃43; " msg&＃61;" &＃43; msg.what);}}if (logging !&＃61; null) {logging.println("<<<< &＃43; msg.target &＃43; " " &＃43; msg.callback);}// Make sure that during the course of dispatching the// identity of the thread wasn&＃39;t corrupted.final long newIdent &＃61; Binder.clearCallingIdentity();if (ident !&＃61; newIdent) {Log.wtf(TAG, "Thread identity changed from 0x"&＃43; Long.toHexString(ident) &＃43; " to 0x"&＃43; Long.toHexString(newIdent) &＃43; " while dispatching to "&＃43; msg.target.getClass().getName() &＃43; " "&＃43; msg.callback &＃43; " what&＃61;" &＃43; msg.what);}msg.recycleUnchecked();}}


二、MessageQueue
MessageQueue是一个消息队列&＃xff0c;用于存放message&＃xff0c;looper来把其中的message发送到handlermessage

采用先进先出的方式来管理message&＃xff0c;内部采用单链表来进行维护&＃xff0c;插入删除比较快

三、Message
就是咱最常用的消息了&＃xff0c;按照我上面的说法建议使用Message.obtain()来获取Message实例&＃xff0c;应为message有个pool

源码

public static Message obtain() {synchronized (sPoolSync) {if (sPool !&＃61; null) {Message m &＃61; sPool;sPool &＃61; m.next;m.next &＃61; null;m.flags &＃61; 0; // clear in-use flagsPoolSize--;return m;}}return new Message();}