Android开发-Handler消息机制记录

背景

Handler在Android中非常重要，它可以实现线程之间的通信，Android中对于UI的更新只能在主线程中进行，所以Handler的地位非常重要，包括可以处理一些多个线程同时更新UI时的操作，Handler就是为了解决线程间通信而出现的。

Handler的使用

Handler和MessageQueue, Looper, Message共同构成一个消息处理机制，如下是Handler基本使用代码示例

// 创建Handler对象，获取主线程Looper
val mHandler = object : Handler(Looper.getMainLooper()) {override fun handleMessage(msg: Message) {when (msg.what) {1 -> {// 处理消息}}}}// 其他线程需要通知时调用sendMessage发消息通知目标线程thread { val msg = Message()msg.what = 1mHandler.sendMessage(msg)}

接下来解析下Handler消息处理机制的每一个组成部分的作用

• Handler: 用于消息的发送和处理
• Message: 携带消息的内容，参数有what、arg1、arg2（储存整型数据）、obj（储存对象类型）
• MessageQueue: 管理消息的队列，每个线程拥有一个
• Looper: 每个线程都有一个自己的Looper，管理MessageQueue，不断的循环从消息队列中取出消息并分发给相应的Handler

Handler原理

Handler机制过程

• 发送消息：需要通信的线程创建一个Handler对象用于发送消息传递信息，通过调用Handler的发送消息的方法来发送Message到MessageQueue，根据Handler源码可以发现如下发送消息的方法：

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//public class Handler {...public final boolean post(@NonNull Runnable r) {throw new RuntimeException("Stub!");}public final boolean postAtFrontOfQueue(@NonNull Runnable r) {throw new RuntimeException("Stub!");}public final boolean postAtTime(@NonNull Runnable r, @Nullable Object token, long uptimeMillis) {throw new RuntimeException("Stub!");}public final boolean postAtTime(@NonNull Runnable r, long uptimeMillis) {throw new RuntimeException("Stub!");}public final boolean postDelayed(@NonNull Runnable r, @Nullable Object token, long delayMillis) {throw new RuntimeException("Stub!");}public final boolean postDelayed(@NonNull Runnable r, long delayMillis) {throw new RuntimeException("Stub!");}public final boolean sendEmptyMessage(int what) {throw new RuntimeException("Stub!");}public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {throw new RuntimeException("Stub!");}public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {throw new RuntimeException("Stub!");}public final boolean sendMessage(@NonNull Message msg) {throw new RuntimeException("Stub!");}public final boolean sendMessageAtFrontOfQueue(@NonNull Message msg) {throw new RuntimeException("Stub!");}public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) {throw new RuntimeException("Stub!");}public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {throw new RuntimeException("Stub!");}
}

可以发现以上源码中post开头的方法都有一个Runnable参数，所以使用post传递信息时，当Handler从队列取出一个Message后执行到dispatchMessage()方法进行分发时会自动处理post中的自定义Runnable，而不会去执行Handler中重写的handleMessage方法
同理，send开头的方法可以将数据封装到Bundle中绑定到Message或者设置msg.what的值来进行区分然后分发到对应传入的回调接口的handleMessage方法中处理（需要先在接收的线程中实现一个Handler类然后重写下handleMessage方法）

• Looper事件循环：在线程中先调用Looper.prepare()，然后通过Looper.loop()让线程开始循环处理消息，如果一段时间后如果不需要了也可以调用Looper.end()结束当前所在线程的循环器，可以通过Looper.myQueue()获取到线程中的MessageQueue
• 处理消息：需要接收通信的线程创建一个Handler对象用于接收信息，创建Handler对象后需要重写handleMessage()方法来处理其他线程获取到的信息并进行相应操作处理

源码解析

Handler源码

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//package android.os;import android.annotation.NonNull;
import android.annotation.Nullable;
import android.util.Printer;public class Handler {/** @deprecated */@Deprecatedpublic Handler() {throw new RuntimeException("Stub!");}/** @deprecated */@Deprecatedpublic Handler(@Nullable Callback callback) {throw new RuntimeException("Stub!");}public Handler(@NonNull Looper looper) {throw new RuntimeException("Stub!");}public Handler(@NonNull Looper looper, @Nullable Callback callback) {throw new RuntimeException("Stub!");}@NonNullpublic static Handler createAsync(@NonNull Looper looper) {throw new RuntimeException("Stub!");}@NonNullpublic static Handler createAsync(@NonNull Looper looper, @NonNull Callback callback) {throw new RuntimeException("Stub!");}public void dispatchMessage(@NonNull Message msg) {throw new RuntimeException("Stub!");}public final void dump(@NonNull Printer pw, @NonNull String prefix) {throw new RuntimeException("Stub!");}@NonNullpublic final Looper getLooper() {throw new RuntimeException("Stub!");}@NonNullpublic String getMessageName(@NonNull Message message) {throw new RuntimeException("Stub!");}public void handleMessage(@NonNull Message msg) {throw new RuntimeException("Stub!");}public final boolean hasCallbacks(@NonNull Runnable r) {throw new RuntimeException("Stub!");}public final boolean hasMessages(int what) {throw new RuntimeException("Stub!");}public final boolean hasMessages(int what, @Nullable Object object) {throw new RuntimeException("Stub!");}@NonNullpublic final Message obtainMessage() {throw new RuntimeException("Stub!");}@NonNullpublic final Message obtainMessage(int what) {throw new RuntimeException("Stub!");}@NonNullpublic final Message obtainMessage(int what, int arg1, int arg2) {throw new RuntimeException("Stub!");}@NonNullpublic final Message obtainMessage(int what, int arg1, int arg2, @Nullable Object obj) {throw new RuntimeException("Stub!");}@NonNullpublic final Message obtainMessage(int what, @Nullable Object obj) {throw new RuntimeException("Stub!");}public final boolean post(@NonNull Runnable r) {throw new RuntimeException("Stub!");}public final boolean postAtFrontOfQueue(@NonNull Runnable r) {throw new RuntimeException("Stub!");}public final boolean postAtTime(@NonNull Runnable r, @Nullable Object token, long uptimeMillis) {throw new RuntimeException("Stub!");}public final boolean postAtTime(@NonNull Runnable r, long uptimeMillis) {throw new RuntimeException("Stub!");}public final boolean postDelayed(@NonNull Runnable r, @Nullable Object token, long delayMillis) {throw new RuntimeException("Stub!");}public final boolean postDelayed(@NonNull Runnable r, long delayMillis) {throw new RuntimeException("Stub!");}public final void removeCallbacks(@NonNull Runnable r) {throw new RuntimeException("Stub!");}public final void removeCallbacks(@NonNull Runnable r, @Nullable Object token) {throw new RuntimeException("Stub!");}public final void removeCallbacksAndMessages(@Nullable Object token) {throw new RuntimeException("Stub!");}public final void removeMessages(int what) {throw new RuntimeException("Stub!");}public final void removeMessages(int what, @Nullable Object object) {throw new RuntimeException("Stub!");}public final boolean sendEmptyMessage(int what) {throw new RuntimeException("Stub!");}public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {throw new RuntimeException("Stub!");}public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {throw new RuntimeException("Stub!");}public final boolean sendMessage(@NonNull Message msg) {throw new RuntimeException("Stub!");}public final boolean sendMessageAtFrontOfQueue(@NonNull Message msg) {throw new RuntimeException("Stub!");}public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) {throw new RuntimeException("Stub!");}public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {throw new RuntimeException("Stub!");}public String toString() {throw new RuntimeException("Stub!");}public interface Callback {boolean handleMessage(@NonNull Message var1);}
}