关于Handler你不知道的事

提到Handler，我们都会想到可以用来在子线程给UI线程发送消息，常用来子线程刷新UI。
而往深了问，你一定还知道Handler会绑定到一个Looper，而每个Looper会和一个MessageQ关联，从而达到向指定线程发送消息的功能。除此之外，还了解多少？
提到MessageQueue，就不得不聊到Android的消息处理机制。

Handler消息处理机制

Android的消息机制，涉及到Handler、Looper、Message和MessageQueue。

Handler是用于处理Message的对象。它通常与一个特定的线程(通常是主线程)关联。通过Handler开发者可以将Message发送到与其关联的线程的消息队列中,以便在那个线程中执行处理。
Message是一个包含要传递的数据和指令的的对象。当需要在不同线程之间传递数据或执行行任务时,通常会创建一个Message并将其发送给Handler。
Looper是一个用于管理线程的消息队列别的对象。每个线程都可以有一个Looper，它会在线程上创建一个消息队列，允许该线程接收并处理Message。主线程通常已经具有一个默认的Looper，而其他线程需要显式创建一个Looper。
MessageQueue即是消息队列。是一个FIFO(先进先出)队列,用于存储待处理的Message。每个Looper都有一个关联的MessageQueue，Handler将Message发送到这个队列中,然后由Looper依次处理队列中的Message。

简易的消息处理机制流程图

MessageQueue

从上面的描述，我们可以MessageQueue会按照一定的规则取出要执行的Message，当消息执行完之后Message就会被回收。

那么下面我们就通过代码来验证几种情况：

Message#Obtain创建消息的消息和直接New Message的方法创建消息，是否都会被回收
如果消息对象被持有，是否消息也会被回收
如果接受消息在主线程，那么在View#post方法执行后，消息是否会被回收
如果接收消息在主线程，那么当切换到其他线程执行任务后，消息是否会被回收
如果接受消息在主线程，通过Kotlin Coroutine执行任务在 Dispatchers.Main或 Dispatchers.Default后，消息是否会被回收

通过Message#Obtain创建消息并发送

 Message.obtain(). also { it.what = 1  mObtainMsg = it mHandler.sendMessage(it) }

通过直接New的方法创建消息并发送

Message().also {
it.what = 2mHandler.sendMessage(it)
}

发送消息后打印消息信息

class MyHandler(private val actRef: WeakReference<TestMessageActivity>) :Handler(Looper.getMainLooper()) {override fun handleMessage(msg: Message) {super.handleMessage(msg)val act = actRef.get() ?: returnLog.d(TAG, "msg:$msg}")Log.d(TAG, "mObtainMsg:${act.mObtainMsg}}")act.mSWThread.post {
Log.d(TAG, "msg--post:$msg}")Log.d(TAG, "mObtainMsg--post:${act.mObtainMsg}}")}
if (act.mSWThread.isChecked) {//是否开启了子线程Thread {
Log.d(TAG, "thread--msg:$msg}")Log.d(TAG, "thread--mObtainMsg:${act.mObtainMsg}}")Thread.sleep(3000L)Log.d(TAG, "threadAfter--msg:$msg}")Log.d(TAG, "threadAfter--mObtainMsg:${act.mObtainMsg}}")} .start()}if (act.mSWCoroutine.isChecked) {//是否开启协程GlobalScope.launch(Dispatchers.Main) {
Log.d(TAG, "launchMain----msg:$msg}")Log.d(TAG, "launchMain--mObtainMsg:${act.mObtainMsg}}")}
GlobalScope.launch {
Log.d(TAG, "launch--msg:$msg}")Log.d(TAG, "launch--mObtainMsg:${act.mObtainMsg}}")}
}}
}

Obtain创建消息发送消息，收到的日志结果

2024-03-22 16:32:04.780 21621-21621/com.lotus.duode.myapplication D/TestMessageActivity: msg:{ when=-1ms what=1 target=com.lotus.duode.myapplication.TestMessageActivity$MyHandler }}
2024-03-22 16:32:04.780 21621-21621/com.lotus.duode.myapplication D/TestMessageActivity: mObtainMsg:{ when=-1ms what=1 target=com.lotus.duode.myapplication.TestMessageActivity$MyHandler }}
2024-03-22 16:32:04.781 21621-22106/com.lotus.duode.myapplication D/TestMessageActivity: thread--msg:{ when=-2ms what=1 target=com.lotus.duode.myapplication.TestMessageActivity$MyHandler }}
2024-03-22 16:32:04.781 21621-22106/com.lotus.duode.myapplication D/TestMessageActivity: thread--mObtainMsg:{ when=-2ms what=1 target=com.lotus.duode.myapplication.TestMessageActivity$MyHandler }}
2024-03-22 16:32:04.803 21621-22108/com.lotus.duode.myapplication D/TestMessageActivity: launch--msg:{ when=-19ms callback=android.view.Choreographer$FrameDisplayEventReceiver target=android.view.Choreographer$FrameHandler }}
2024-03-22 16:32:04.803 21621-22108/com.lotus.duode.myapplication D/TestMessageActivity: launch--mObtainMsg:{ when=-19ms callback=android.view.Choreographer$FrameDisplayEventReceiver target=android.view.Choreographer$FrameHandler }}
2024-03-22 16:32:04.806 21621-21621/com.lotus.duode.myapplication D/TestMessageActivity: msg--post:{ when=-2d2h37m50s923ms barrier=0 }}
2024-03-22 16:32:04.806 21621-21621/com.lotus.duode.myapplication D/TestMessageActivity: mObtainMsg--post:{ when=-2d2h37m50s923ms barrier=0 }}
2024-03-22 16:32:04.806 21621-21621/com.lotus.duode.myapplication D/TestMessageActivity: launchMain----msg:{ when=-2d2h37m50s923ms barrier=0 }}
2024-03-22 16:32:04.806 21621-21621/com.lotus.duode.myapplication D/TestMessageActivity: launchMain--mObtainMsg:{ when=-2d2h37m50s923ms barrier=0 }}
2024-03-22 16:32:07.782 21621-22106/com.lotus.duode.myapplication D/TestMessageActivity: threadAfter--msg:{ when=-2d2h37m53s899ms barrier=0 }}
2024-03-22 16:32:07.782 21621-22106/com.lotus.duode.myapplication D/TestMessageActivity: threadAfter--mObtainMsg:{ when=-2d2h37m53s899ms barrier=0 }}

New Message创建消息发送消息，收到的日志结果

2024-03-22 16:32:27.427 21621-21621/com.lotus.duode.myapplication D/TestMessageActivity: msg:{ when=-9ms what=2 target=com.lotus.duode.myapplication.TestMessageActivity$MyHandler }}
2024-03-22 16:32:27.428 21621-21621/com.lotus.duode.myapplication D/TestMessageActivity: mObtainMsg:{ when=-2ms callback=android.view.Choreographer$FrameDisplayEventReceiver target=android.view.Choreographer$FrameHandler }}
2024-03-22 16:32:27.428 21621-22116/com.lotus.duode.myapplication D/TestMessageActivity: thread--msg:{ when=-10ms what=2 target=com.lotus.duode.myapplication.TestMessageActivity$MyHandler }}
2024-03-22 16:32:27.428 21621-22116/com.lotus.duode.myapplication D/TestMessageActivity: thread--mObtainMsg:{ when=-3ms callback=android.view.Choreographer$FrameDisplayEventReceiver target=android.view.Choreographer$FrameHandler }}
2024-03-22 16:32:27.428 21621-22108/com.lotus.duode.myapplication D/TestMessageActivity: launch--msg:{ when=-2d2h38m13s545ms barrier=0 }}
2024-03-22 16:32:27.428 21621-22108/com.lotus.duode.myapplication D/TestMessageActivity: launch--mObtainMsg:{ when=-3ms callback=android.view.Choreographer$FrameDisplayEventReceiver target=android.view.Choreographer$FrameHandler }}
2024-03-22 16:32:27.430 21621-21621/com.lotus.duode.myapplication D/TestMessageActivity: msg--post:{ when=-1ms callback=android.view.Choreographer$$ExternalSyntheticLambda0 target=android.view.Choreographer$FrameHandler }}
2024-03-22 16:32:27.430 21621-21621/com.lotus.duode.myapplication D/TestMessageActivity: mObtainMsg--post:{ when=-2d2h38m13s547ms barrier=0 }}
2024-03-22 16:32:27.430 21621-21621/com.lotus.duode.myapplication D/TestMessageActivity: launchMain----msg:{ when=-2ms callback=android.view.Choreographer$$ExternalSyntheticLambda0 target=android.view.Choreographer$FrameHandler }}
2024-03-22 16:32:27.430 21621-21621/com.lotus.duode.myapplication D/TestMessageActivity: launchMain--mObtainMsg:{ when=-2d2h38m13s547ms barrier=0 }}
2024-03-22 16:32:30.429 21621-22116/com.lotus.duode.myapplication D/TestMessageActivity: threadAfter--msg:{ when=-2d2h38m16s546ms barrier=0 }}
2024-03-22 16:32:30.429 21621-22116/com.lotus.duode.myapplication D/TestMessageActivity: threadAfter--mObtainMsg:{ when=-2d2h38m16s546ms barrier=0 }}