Android中Handler,Looper详解

//【Handler】Looper 原理详解示例总结

核心知识点

1、相关名词

UI线程：就是我们的主线程，系统在创建UI线程的时候会初始化一个Looper对象，同时也会创建一个与其关联的MessageQueue
Handler：作用就是发送与处理信息，如果希望Handler正常工作，在当前线程中必须要有一个Looper对象
Message：Handler接收与处理的对象。Handler也能接收与处理Runnable对象
MessageQueue：消息队列，先进先出管理Message，在初始化Looper对象时会创建一个与之关联的MessageQueue
Looper：每个线程只能够有一个Looper，Looper负责创建并管理当前线程中的MessageQueue，调用loop方法后就会在一个无限循环体中不断地从MessageQueue中取出Message并分发给对应的Handler，最后回调handleMessage()方法处理此消息。Looper才是整个机制的核心！

2、基本使用过程：

在主线程中创建Handler并重写handleMessage()方法
在任何线程中都可以利用此Handler发送消息，消息会被发送到主线程的MessageQueue中
一旦MessageQueue中有新消息，主线程中的 Looper 就会发现此消息，然后就会调用Handler的handleMessage()方法处理此消息

3、一些细节：

调用Looper.prepare()后首先会在本线程中保存唯一的一个Looper实例，然后会在该实例中创建并保存一个MessageQueue对象；Looper.prepare()在一个线程中只能调用一次，MessageQueue在一个线程中也只存在一个。
调用Looper.loop()方法后会让当前线程进入一个无限循环中，Looper不断从MessageQueue中读取消息，然后回调msg.target.dispatchMessage(msg)方法。
Handler的构造方法中，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相关联。
Handler的sendMessage方法，会给msg.target赋值为handler自身，然后加入MessageQueue中。
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。

Handler面试点

1、Handler的产生背景

Handler是线程间通讯的机制，Android中，网络访问、文件处理等耗时操作必须放到子线程中去执行，否则将会造成ANR异常。

ANR异常：Application Not Response 应用程序无响应

产生ANR异常的原因：在主线程执行了耗时操作，对Activity来说，主线程阻塞5秒将造成ANR异常，对BroadcastReceiver来说，主线程阻塞10秒将会造成ANR异常。

解决ANR异常的方法：耗时操作都在子线程中去执行

但是，Android不允许在子线程去修改UI，可我们又有在子线程去修改UI的需求，因此需要借助Handler（但是，一定要明白，Handler的作用不是为了在子线程去修改UI，而是为了实现线程间通讯！一定要明白理念与表现的区别）。

2、Handler机制相关概念

Looper：一个线程可以产生一个Looper对象，由它来管理此线程里的MessageQueue(消息队列)。
Handler：你可以构造Handler对象来与Looper沟通，以便push新消息到MessageQueue里；或者接收Looper从Message Queue取出发送来的消息。
Message Queue(消息队列)：用来存放线程放入的消息。
线程：UIthread 通常就是main thread，而Android启动程序时会替它建立一个MessageQueue。

3、基本过程：

UI线程创建时，就创建了一个Looper，Looper内部维护这一个MessageQueue。

Looper通过开启一个while(true)死循环来轮询MessageQueue中的Message。

当Looper轮询到Message时，就分发此Message。

4、Looper和Handler之间的关联

Handler在子线程发送消息到MessageQueue，Message被Looper取出来后，分发给handler的handleMessage方法来处理。

那么，为什么说一个Looper可以对应多个Handler，Looper如何保证哪个Handler发出去的Message将交由哪个handler来处理？

因为Handler在发送Message的时候，在Message的成员变量target上标记了当前handler的引用：

highlighter- code-theme-dark kotlin

message.target = this;//this即这个发送此Message的handler对象

当Looper取到message时，通过下面的方法分发Message：

highlighter- code-theme-dark Java

"background-color:#2b2b2b">"color:#bababa">message.target.dispatchMessage（message）;  "color:#7f7f7f">//message.target即发送此Message的handler对象

因此，哪个handler发送的Message，将由哪个Handler来处理此Message。

Looper详解

prepare()方法

highlighter- code-theme-dark Java


"background-color:#2b2b2b">"color:#bababa">"color:#be94bb">"color:#cb7832">public "color:#cb7832">static "color:#cb7832">final "color:#cb7832">void "color:#81a2be">prepare"color:#b9b9b9">() {
        "color:#cb7832">if (sThreadLocal.get() != "color:#cb7832">null) "color:#cb7832">throw "color:#cb7832">new RuntimeException("color:#6a8759">"Only one Looper may be created per thread");
        sThreadLocal.set("color:#cb7832">new Looper("color:#cb7832">true));
}

ThreadLocal可以在一个线程中存储变量，可以看到，我们将一个Looper的实例放入了ThreadLocal，并且判断了sThreadLocal是否存储过Looper对象，存储过（即.get()不为null）则抛出异常。这也就说明了Looper.prepare()方法不能被调用两次，同时也保证了一个线程中只有一个Looper实例。

下面看其构造方法：

highlighter- code-theme-dark Java


"background-color:#2b2b2b">"color:#bababa">"color:#be94bb">"color:#cb7832">private "color:#81a2be">Looper"color:#b9b9b9">("color:#cb7832">boolean quitAllowed) {
	mQueue = "color:#cb7832">new MessageQueue(quitAllowed);
	mRun = "color:#cb7832">true;
	mThread = Thread.currentThread();
}

在构造方法中，创建了一个消息队列MessageQueue。

loop()方法

highlighter- code-theme-dark Java


"background-color:#2b2b2b">"color:#bababa">"color:#be94bb">"color:#cb7832">public "color:#cb7832">static "color:#cb7832">void "color:#81a2be">loop"color:#b9b9b9">() {
	"color:#cb7832">final Looper me = myLooper();"color:#7f7f7f">//方法体为 return sThreadLocal.get();  即此方法直接返回了sThreadLocal存储的Looper实例
	"color:#cb7832">if (me == "color:#cb7832">null) "color:#cb7832">throw "color:#cb7832">new RuntimeException("color:#6a8759">"No Looper; Looper.prepare() wasn't called on this thread.");"color:#7f7f7f">//如果me为null则抛出异常，也就是说looper方法必须在prepar()之后运行。
	"color:#cb7832">final MessageQueue queue = me.mQueue;"color:#7f7f7f">//拿到该looper实例中的消息队列
	Binder.clearCallingIdentity(); "color:#7f7f7f">// Make sure the identity of this thread is that of the local process, and keep track of what that identity token actually is.  
	"color:#cb7832">final "color:#cb7832">long ident = Binder.clearCallingIdentity();
	"color:#7f7f7f">//无限循环
	"color:#cb7832">for (;;) {
		Message msg = queue.next(); "color:#7f7f7f">// might block，取出一条消息，如果没有消息则阻塞等待
		"color:#cb7832">if (msg == "color:#cb7832">null) "color:#cb7832">return;
		Printer logging = me.mLogging; "color:#7f7f7f">// This must be in a local variable, in case a UI event sets the logger  
		"color:#cb7832">if (logging != "color:#cb7832">null) logging.println("color:#6a8759">">>>>> Dispatching to " + msg.target + "color:#6a8759">" " + msg.callback + "color:#6a8759">": " + msg.what);
		msg.target.dispatchMessage(msg);"color:#7f7f7f">//Msg的target其实就是handler对象
		"color:#cb7832">if (logging != "color:#cb7832">null) logging.println("color:#6a8759">"<<<<< Finished to " + msg.target + "color:#6a8759">" " + msg.callback);
		"color:#cb7832">final "color:#cb7832">long newIdent = Binder.clearCallingIdentity();"color:#7f7f7f">// Make sure that during the course of dispatching the identity of the thread wasn't corrupted.  
		"color:#cb7832">if (ident != newIdent) {
			Log.wtf(TAG, "color:#6a8759">"Thread identity changed from 0x" + Long.toHexString(ident) + "color:#6a8759">" to 0x" + Long.toHexString(newIdent)
					+ "color:#6a8759">" while dispatching to " + msg.target.getClass().getName() + "color:#6a8759">" " + msg.callback + "color:#6a8759">" what=" + msg.what);
		}
		msg.recycle();"color:#7f7f7f">//释放消息占据的资源
	}
}

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、执行loop()方法，不断从MessageQueue中取消息，交给消息的target属性的dispatchMessage方法去处理。

大家可能还会问，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，已经默默的在当前UI线程中调用了Looper.prepare()和Looper.loop()方法。

而如果我们要想在子线程中创建Handler，则必须先调用Looper.prepare()方法，Handler创建后再调用Looper.loop()方法。

Handler详解

使用Handler之前，我们都是初始化一个实例，我们可以在声明的时候直接初始化，或者在onCreate中初始化。

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（当然可以在任何线程）是怎么发送到MessageQueue中的。

highlighter- code-theme-dark JavaScript


"background-color:#2b2b2b"><span style="color:#bababa">public <span style="color:#be94bb"><span style="color:#81a2be">Handlerspan>()span> {<span style="color:#7f7f7f">//我们一般都是使用此无参的构造方法span>
	<span style="color:#e0c46c">thisspan>(<span style="color:#6896ba">nullspan>, <span style="color:#6896ba">falsespan>);
}
public <span style="color:#be94bb"><span style="color:#81a2be">Handlerspan>(<span style="color:#b9b9b9">Callback callback, boolean <span style="color:#cb7832">asyncspan>span>)span> {
	<span style="color:#cb7832">ifspan> (FIND_POTENTIAL_LEAKS) {
		final Class<span style="color:#cb7832">extendsspan> Handler> klass = getClass();
		<span style="color:#cb7832">ifspan> ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) && (klass.getModifiers() & Modifier.STATIC) == <span style="color:#6896ba">0span>) {
			Log.w(TAG, <span style="color:#6a8759">"The following Handler class should be static or leaks might occur: "span> + klass.getCanonicalName());
		}
	}
	mLooper = Looper.myLooper();<span style="color:#7f7f7f">//获取当前线程保存的Looper实例span>
	<span style="color:#cb7832">ifspan> (mLooper == <span style="color:#6896ba">nullspan>) <span style="color:#cb7832">throwspan> <span style="color:#cb7832">newspan> RuntimeException(<span style="color:#6a8759">"Can't create handler inside thread that has not called Looper.prepare()"span>);<span style="color:#7f7f7f">//在初始化Handler之前必须先通过Looper.prepare()方法创建Looper的实例span>
	mQueue = mLooper.mQueue;<span style="color:#7f7f7f">//获取这个Looper实例中保存的MessageQueue（消息队列）span>
	mCallback = callback;
	mAsynchronous = <span style="color:#cb7832">asyncspan>;
}span>

然后看我们最常用的发送消息相关的几个方法

highlighter- code-theme-dark Java


"background-color:#2b2b2b">"color:#bababa">"color:#be94bb">"color:#cb7832">public "color:#cb7832">final "color:#cb7832">boolean "color:#81a2be">sendMessage"color:#b9b9b9">(Message msg) {
	"color:#cb7832">return sendMessageDelayed(msg, "color:#6896ba">0);
}
 
"color:#be94bb">"color:#cb7832">public "color:#cb7832">final "color:#cb7832">boolean "color:#81a2be">sendEmptyMessageDelayed"color:#b9b9b9">("color:#cb7832">int what, "color:#cb7832">long delayMillis) {
	Message msg = Message.obtain();
	msg.what = what;
	"color:#cb7832">return sendMessageDelayed(msg, delayMillis);
}
 
"color:#be94bb">"color:#cb7832">public "color:#cb7832">final "color:#cb7832">boolean "color:#81a2be">sendMessageDelayed"color:#b9b9b9">(Message msg, "color:#cb7832">long delayMillis) {
	"color:#cb7832">if (delayMillis < "color:#6896ba">0) delayMillis = "color:#6896ba">0;
	"color:#cb7832">return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
 
"color:#be94bb">"color:#cb7832">public "color:#cb7832">boolean "color:#81a2be">sendMessageAtTime"color:#b9b9b9">(Message msg, "color:#cb7832">long uptimeMillis) {
	MessageQueue queue = mQueue;
	"color:#cb7832">if (queue == "color:#cb7832">null) {
		RuntimeException e = "color:#cb7832">new RuntimeException("color:#cb7832">this + "color:#6a8759">" sendMessageAtTime() called with no mQueue");
		Log.w("color:#6a8759">"Looper", e.getMessage(), e);
		"color:#cb7832">return "color:#cb7832">false;
	}
	"color:#cb7832">return enqueueMessage(queue, msg, uptimeMillis);
}

辗转反侧最后都是调用了sendMessageAtTime方法，在此方法内部又直接获取MessageQueue，然后调用了enqueueMessage方法

highlighter- code-theme-dark Java


"background-color:#2b2b2b">"color:#bababa">"color:#be94bb">"color:#cb7832">private "color:#cb7832">boolean "color:#81a2be">enqueueMessage"color:#b9b9b9">(MessageQueue queue, Message msg, "color:#cb7832">long uptimeMillis) {
	msg.target = "color:#cb7832">this;
	"color:#cb7832">if (mAsynchronous) msg.setAsynchronous("color:#cb7832">true);
	"color:#cb7832">return queue.enqueueMessage(msg, uptimeMillis);
}

enqueueMessage中首先为meg.target赋值为this，最后调用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。【上面我们说了，Looper的loop方法会取出每个msg然后交给msg.arget.dispatchMessage(msg)去处理消息】，也就是说Looper的loop方法会把取出的每个msg通过当前的handler的dispatchMessage回调处理。

下面我们去看一看handler中的这个dispathMessage方法

highlighter- code-theme-dark Java


"background-color:#2b2b2b">"color:#bababa">"color:#be94bb">"color:#cb7832">public "color:#cb7832">void "color:#81a2be">dispatchMessage"color:#b9b9b9">(Message msg) {
	"color:#cb7832">if (msg.callback != "color:#cb7832">null) handleCallback(msg);
	"color:#cb7832">else {
		"color:#cb7832">if (mCallback != "color:#cb7832">null) {
			"color:#cb7832">if (mCallback.handleMessage(msg)) "color:#cb7832">return;
		}
		handleMessage(msg);
	}
}

可以看到，最后调用了handleMessage方法，可实际上这是一个空方法，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。

Handler的全部方法

highlighter- code-theme-dark Java


"background-color:#2b2b2b">"color:#bababa">mHandler.post("color:#cb7832">new Runnable() {
	"color:#7f7f7f">@Override
	"color:#be94bb">"color:#cb7832">public "color:#cb7832">void "color:#81a2be">run"color:#b9b9b9">() {
		mTxt.setText("color:#6a8759">"yoxi");"color:#7f7f7f">//在run方法中可以更新UI
	}
});  
"color:#be94bb">"color:#cb7832">public "color:#cb7832">final "color:#cb7832">boolean "color:#81a2be">post"color:#b9b9b9">(Runnable r) {
	"color:#cb7832">return sendMessageDelayed(getPostMessage(r), "color:#6896ba">0);
}
"color:#be94bb">"color:#cb7832">private "color:#cb7832">static Message "color:#81a2be">getPostMessage"color:#b9b9b9">(Runnable r) {
	Message m = Message.obtain();"color:#7f7f7f">//得到了一个Message对象
	m.callback = r;"color:#7f7f7f">//将我们创建的Runable对象作为callback属性，赋值给了此Message
	"color:#cb7832">return m;
}

演示代码

highlighter- code-theme-dark Java


"background-color:#2b2b2b">"color:#bababa">"color:#cb7832">public "color:#be94bb">"color:#cb7832">class "color:#81a2be">HandlerTestActivity "color:#cb7832">extends "color:#81a2be">ListActivity {
	"color:#cb7832">private TextView tv_info;
	"color:#cb7832">private Handler uiHandler;
	"color:#cb7832">private StaticThread thread;"color:#7f7f7f">//一个子线程
	
	"color:#cb7832">public "color:#cb7832">static "color:#cb7832">final "color:#cb7832">int MSG_WHAT_1 = "color:#6896ba">1;
	"color:#cb7832">public "color:#cb7832">static "color:#cb7832">final "color:#cb7832">int MSG_WHAT_2 = "color:#6896ba">2;
	"color:#cb7832">public "color:#cb7832">static "color:#cb7832">final "color:#cb7832">int MSG_WHAT_3 = "color:#6896ba">3;
	
	"color:#be94bb">"color:#cb7832">protected "color:#cb7832">void "color:#81a2be">onCreate"color:#b9b9b9">(Bundle savedInstanceState) {
		"color:#cb7832">super.onCreate(savedInstanceState);
		String[] array = {"color:#6a8759">"开启子线程，并在子线程中创建一个Handler", "color:#7f7f7f">//
				"color:#6a8759">"在主线程中，通过子线程的Handler[向子线程]发消息", "color:#7f7f7f">//
				"color:#6a8759">"演示Handler的post方法"};
		tv_info = "color:#cb7832">new TextView("color:#cb7832">this);
		tv_info.setText("color:#6a8759">"Handler、Looper、Message、MQ、Thread关系");
		getListView().addFooterView(tv_info);
		setListAdapter("color:#cb7832">new ArrayAdapter<>("color:#cb7832">this, android.R.layout.simple_list_item_1, "color:#cb7832">new ArrayList(Arrays.asList(array))));
		
		uiHandler = "color:#cb7832">new StaticUiHandler("color:#cb7832">this); "color:#7f7f7f">//系统启动时已经为主线程初始化了Looper、MQ等，我们可以直接创建Handler
		thread = "color:#cb7832">new StaticThread("color:#cb7832">this);
	}
	
	"color:#7f7f7f">@Override
	"color:#be94bb">"color:#cb7832">protected "color:#cb7832">void "color:#81a2be">onDestroy"color:#b9b9b9">() {
		"color:#cb7832">super.onDestroy();
		"color:#cb7832">if (uiHandler != "color:#cb7832">null) uiHandler.removeCallbacksAndMessages("color:#cb7832">null);
		"color:#cb7832">if (thread != "color:#cb7832">null && thread.getAnsyHandler() != "color:#cb7832">null) thread.getAnsyHandler().removeCallbacksAndMessages("color:#cb7832">null);
	}
	
	"color:#7f7f7f">@Override
	"color:#be94bb">"color:#cb7832">protected "color:#cb7832">void "color:#81a2be">onListItemClick"color:#b9b9b9">(ListView l, View v, "color:#cb7832">int position, "color:#cb7832">long id) {
		"color:#cb7832">switch (position) {
			"color:#cb7832">case "color:#6896ba">0:"color:#7f7f7f">//开启子线程，并在子线程中创建一个Handler
				"color:#cb7832">if (thread != "color:#cb7832">null && !thread.isAlive()) thread.start();"color:#7f7f7f">//A thread is alive if it has been started and has not yet died.
				"color:#cb7832">break;
			"color:#cb7832">case "color:#6896ba">1:"color:#7f7f7f">//在主线程中，通过子线程的Handler[向子线程]发消息
				Message msg = Message.obtain("color:#cb7832">null, MSG_WHAT_1, "color:#6a8759">"消息内容"); "color:#7f7f7f">//第一个参数Handler的作用是指定msg.target
				"color:#7f7f7f">//这里设为null的原因是：后面调用sendMessage方法时重新指定了发送此消息的Handler为msg.target
				"color:#cb7832">if (thread != "color:#cb7832">null && thread.getAnsyHandler() != "color:#cb7832">null) thread.getAnsyHandler().sendMessage(msg);
				tv_info.append("color:#6a8759">"\n1、在UI线程中用子线程的Handler发消息，what=" + msg.what);
				"color:#cb7832">break;
			"color:#cb7832">case "color:#6896ba">2:
				"color:#7f7f7f">//其实这个Runnable并没有创建什么线程，而是发送了一条消息，当Handler收到此消息后回调run()方法
				uiHandler.post(() -> tv_info.append("color:#6a8759">"\n演示Handler的post方法"));
				"color:#cb7832">break;
		}
	}
	"color:#7f7f7f">//***********************************************静态内部类，防止内存泄漏*******************************************
	
	"color:#7f7f7f">/**
	 * 主线程使用的Handler
	 */
	"color:#cb7832">private "color:#cb7832">static "color:#be94bb">"color:#cb7832">class "color:#81a2be">StaticUiHandler "color:#cb7832">extends "color:#81a2be">Handler {
		"color:#cb7832">private SoftReference mSoftReference;
		
		"color:#be94bb">"color:#cb7832">public "color:#81a2be">StaticUiHandler"color:#b9b9b9">(HandlerTestActivity activity) {
			mSoftReference = "color:#cb7832">new SoftReference<>(activity);
		}
		
		"color:#7f7f7f">@Override
		"color:#be94bb">"color:#cb7832">public "color:#cb7832">void "color:#81a2be">handleMessage"color:#b9b9b9">(Message msg) {
			HandlerTestActivity activity = mSoftReference.get();
			"color:#cb7832">if (activity != "color:#cb7832">null && activity.thread != "color:#cb7832">null && activity.thread.getAnsyHandler() != "color:#cb7832">null) {
				activity.tv_info.append("color:#6a8759">"\n4、UI线程的Handler收到消息，what=" + msg.what);
				Message msg3 = Message.obtain("color:#cb7832">null, MSG_WHAT_3, msg.obj);
				activity.thread.getAnsyHandler().sendMessageAtTime(msg3, SystemClock.uptimeMillis() + "color:#6896ba">2000);
				activity.tv_info.append("color:#6a8759">"\n5、在UI线程中用子线程的Handler发消息，what=" + msg3.what);
			}
		}
	}
	
	"color:#7f7f7f">/**
	 * 异步线程(子线程)使用的Handler
	 */
	"color:#cb7832">private "color:#cb7832">static "color:#be94bb">"color:#cb7832">class "color:#81a2be">StaticAnsyHandler "color:#cb7832">extends "color:#81a2be">Handler {
		"color:#cb7832">private SoftReference mSoftReference;
		
		"color:#be94bb">"color:#cb7832">public "color:#81a2be">StaticAnsyHandler"color:#b9b9b9">(HandlerTestActivity activity) {
			mSoftReference = "color:#cb7832">new SoftReference<>(activity);
		}
		
		"color:#7f7f7f">@Override
		"color:#be94bb">"color:#cb7832">public "color:#cb7832">void "color:#81a2be">handleMessage"color:#b9b9b9">(Message msg) {
			HandlerTestActivity activity = mSoftReference.get();
			
			"color:#cb7832">if (activity != "color:#cb7832">null) {
				"color:#cb7832">final Message tempMsg = Message.obtain(msg);"color:#7f7f7f">//把收到的消息保存起来
				"color:#7f7f7f">//注意，一定要注意！根据消息池机制，当此消息不再在【此子线程】中使用时，此msg会立即被重置(引用虽在，内容为空)
				"color:#7f7f7f">//所以，如果想把此消息转发到其他线程，或者想在其他线程中引用此消息，一定要手动把消息保存起来！
				
				activity.runOnUiThread(() -> {"color:#7f7f7f">//在子线程中创建Handler的目的是为了和其他线程通讯，绝对不是(也不能)更新UI
					activity.tv_info.append("color:#6a8759">"\n2、子线程的Handler收到消息，what=" + tempMsg.what);
					
					"color:#cb7832">if (activity.uiHandler != "color:#cb7832">null && tempMsg.what == MSG_WHAT_1) {
						Message msg2 = Message.obtain("color:#cb7832">null, MSG_WHAT_2, tempMsg.obj);
						activity.uiHandler.sendMessageDelayed(msg2, "color:#6896ba">2000);
						"color:#7f7f7f">//注意，不能直接把一条还在使用的消息转发出去，否则IllegalStateException: This message is already in use
						activity.tv_info.append("color:#6a8759">"\n3、在子线程中用UI线程的Handler发消息，what=" + msg2.what);
					}
				});
			}
		}
	}
	
	"color:#7f7f7f">/**
	 * 一个线程，用于执行耗时的操作
	 */
	"color:#cb7832">private "color:#cb7832">static "color:#be94bb">"color:#cb7832">class "color:#81a2be">StaticThread "color:#cb7832">extends "color:#81a2be">Thread {
		"color:#cb7832">private SoftReference mSoftReference;
		
		"color:#be94bb">"color:#cb7832">public "color:#81a2be">StaticThread"color:#b9b9b9">(HandlerTestActivity activity) {
			mSoftReference = "color:#cb7832">new SoftReference<>(activity);
		}
		
		"color:#cb7832">private Handler ansyHandler;
		
		"color:#be94bb">"color:#cb7832">public Handler "color:#81a2be">getAnsyHandler"color:#b9b9b9">() {
			"color:#cb7832">return ansyHandler;
		}
		
		"color:#be94bb">"color:#cb7832">public "color:#cb7832">void "color:#81a2be">run"color:#b9b9b9">() {
			HandlerTestActivity activity = mSoftReference.get();
			"color:#cb7832">if (activity != "color:#cb7832">null) {
				Looper.prepare(); "color:#7f7f7f">//在创建Handler【前】必须调用此方法初始化Looper，否则直接报否则报RuntimeException崩溃
				"color:#7f7f7f">//里面做的事情：①为当前线程创建唯一的Looper对象  ②在它的构造方法中会创建一个的MessageQueue对象
				"color:#7f7f7f">//此方法只能被调用一次，这保证了在一个线程中只有一个Looper实例以及只有一个与其关联的MessageQueue实例
				ansyHandler = "color:#cb7832">new StaticAnsyHandler(activity);  "color:#7f7f7f">//任何线程都可通过此Handler发送信息！
				Looper.loop(); "color:#7f7f7f">//若要能够接收到消息，创建Handler后，必须调用loop方法。当然此方法必须是在prepar之后执行
				"color:#7f7f7f">//里面做的事情：启动一个死循环，不断从MQ中取消息，没有则阻塞等待，有则将消息传给指定的Handler去处理
				activity.runOnUiThread(() -> Toast.makeText(activity, "color:#6a8759">"会一直阻塞在这里", Toast.LENGTH_SHORT).show());
			}
		}
	}
}

2017-8-25

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原文地址：https://blog.csdn.net/paladinzh/article/details/127432322

Android中Handler,Looper详解

//【Handler】Looper 原理 详解 示例 总结

核心知识点

Handler面试点

Looper详解

prepare()方法

loop()方法

Handler详解

Handler的全部方法

演示代码

//【Handler】Looper 原理详解示例总结