Handler机制浅析

本文主要是介绍Handler机制浅析，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

使用方法

实例化一个handler对象

val handler = object : Handler() {override fun handleMessage(msg: Message) {Log.e(TAG, "收到消息:" + msg.obj.toString())super.handleMessage(msg)}}

在需要发送消息的地方这样调用

val msg = Message()
msg.obj = "一条消息"
handler.sendMessage(msg)

打印日志

 我们一般是在主线程创建handler，子线程发消息，比如我们可以这样写

object : Thread() {override fun run() {super.run()val msg = Message()msg.obj = "子线程消息"handler.sendMessage(msg)}}.start()

打印日志

 有时候可能需要在子线程创建handler，主线程发消息,我们可能会这样写

class HandlerTestActivity : AppCompatActivity() {val TAG = "HandlerTestActivity"var handler:Handler?=nulloverride fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)setContentView(R.layout.activity_handler_test)object : Thread() {override fun run() {super.run()Looper.prepare()handler = object : Handler() {override fun handleMessage(msg: Message) {Log.e(TAG, "收到消息:" + msg.obj.toString())super.handleMessage(msg)}}Looper.loop()}}.start()}//按钮点击事件，点击后发送消息fun sendMessage(view: View) {val msg = Message()msg.obj = "主线程消息"handler?.sendMessage(msg)}
}

运行代码后，点击按钮会得到如下报错

 我们根据报错信息，到源码中寻找报错的原因，发现在Handler的构造方法中，有这个报错信息的代码

  public Handler(@Nullable 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 " + Thread.currentThread()+ " that has not called Looper.prepare()");}mQueue = mLooper.mQueue;mCallback = callback;mAsynchronous = async;}

可以看出，当mLooper为null时，就会抛出这个错误，mLooper是通过Looper.myLooper()获取的，

我们找到Looper.myLooper()方法

public static @Nullable Looper myLooper() {return sThreadLocal.get();}

发现是调用了sThreadLocal的get()方法获取的，而sThreadLocal是一个ThreadLocal对象，持有Looper的范型

 @UnsupportedAppUsagestatic final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

他的get方法是这样的

public T get() {Thread t = Thread.currentThread();ThreadLocalMap map = getMap(t);if (map != null) {ThreadLocalMap.Entry e = map.getEntry(this);if (e != null) {@SuppressWarnings("unchecked")T result = (T)e.value;return result;}}return setInitialValue();}

简单点说，就是从一个Map里取出一个T对象，也就是上面的范型Looper

有get就有set，set方法是这样的

public void set(T value) {Thread

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