1.消息处理机制简介
主要用于进程内线程之间的通信，主线程一般调用looper()进行循环等待处理消息，其它线程向它发消息并指定消息的处理方法。
(1)涉及文件包括frameworks中的:
Looper.java
Handler.java
MessageQueue.java
Message.java
android_os_MessageQueue.cpp
...
涉及文件包括system中的:
Looper.h
Looper.cpp
(2)涉及native处理机制
使用pipe加epoll机制实现消息队列的睡眠和唤醒进程的功能，同时利用epoll的timeout特性可以指定消息延期多长时间后执行。
2.消息发送流程
enqueueMessage //MessageQueue.java
nativeWake(mPtr);
android_os_MessageQueue_nativeWake //android_os_MessageQueue.cpp
nativeMessageQueue->wake()
wake() //system/core/libutils/Looper.cpp
write(mWakeWritePipeFd, "W", 1); //只是向消息队列的写端写入一个字符唤醒epoll_wait监听而已
从发送流程可以看出：
Framework中的sendMessage其实并不是通过管道传输的，它只是把msg放到Looper的消息队列上，获取消息的时候是直接从消息队列上获取的，
整个消息传送的过程根本不涉及管道传输数据。native方法中使用pipe进行epoll的作用仅仅是想让java的队列具有睡眠唤醒功能。
3.消息获取流程
loop() //Looper.java
queue.next() //MessageQueue.java
nativePollOnce(ptr, nextPollTimeoutMillis); //会阻塞在这个函数它，它最终调用的是epoll_wait()进行阻塞。
android_os_MessageQueue_nativePollOnce //android_os_MessageQueue.cpp
pollOnce(int timeoutMillis) //system/core/include/utils/Looper.h
pollInner //system/core/libutils/Looper.cpp
epoll_wait(..., timeoutMillis); //调用系统的epoll_wait函数并设置等待时间为timeoutMillis
4.消息的处理流程
Handler.java中的dispatchMessage方法：
public void dispatchMessage(Message msg) {
//优先调用callback，注意它是一个Runnable线程!!执行结果就是启动这个线程去执行。
if (msg.callback != null) {
handleCallback(msg);
} else {
//否则若是创建handler的时候指定了callback,就调用这个callback的handleMessage()进行处理
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
//否则就调用handler的它，它是个空函数，但是可以被重写
handleMessage(msg);
}
}
5.实验Demo
下面的例子是分别使用这三种处理msg的方法的Demo，在MainActivity.java中实现的
package com.example.mm.app_message_01;
import android.os.Bundle;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.util.Log;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Button;
public class MainActivity extends AppCompatActivity {
private static final String TAG = "MessageTest";
private Button mButton = null;
private int mClickedCount = 0;
private int getMessageCount1 = 0;
private int getMessageCount2 = 0;
private int getMessageCount3 = 0;
/* Test1: success */
private MyThread mThread1 = null;
private Handler mHandler1 = null;
private MyThread mThread2 = null;
private Handler mHandler2 = null;
private MyThread mThread3 = null;
private Handler mHandler3 = null;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
mButton = (Button) findViewById(R.id.button);
mButton.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
// Code here executes on main thread after user presses button
Log.d(TAG, "Clicked: " + mClickedCount);
mClickedCount++;
mHandler1.sendMessage(new Message());
mHandler2.post(new Runnable() {
@Override
public void run() {
Log.d(TAG, "-2-: Get a new Message, getMessageCount = " + getMessageCount2);
getMessageCount2++;
}
});
mHandler3.sendMessage(new Message());
}
});
/*----------------------test1------------------------*/
mThread1 = new MyThread();
mThread1.start();
mHandler1 = new Handler(mThread1.getLooper(), new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
Log.d(TAG, "-1-: Get a new Message, getMessageCount = " + getMessageCount1);
getMessageCount1++;
return true;
}
});
/*----------------------test2------------------------*/
mThread2 = new MyThread();
mThread2.start();
mHandler2 = new Handler(mThread2.getLooper());
/*----------------------test3------------------------*/
mThread3 = new MyThread();
mThread3.start();
mHandler3 = new MyHandler(mThread3.getLooper());
}
class MyThread extends Thread {
private Looper mLooper;
public void run() {
Looper.prepare();
synchronized (this) {
mLooper = Looper.myLooper();
notifyAll();
}
Looper.loop();
}
Looper getLooper() {
if (!isAlive()) {
return null;
}
synchronized (this) {
if (mLooper == null) {
try {
wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
return mLooper;
}
}
class MyHandler extends Handler {
public MyHandler(Looper looper) {
super(looper);
}
@Override
public void handleMessage(Message msg) {
Log.d(TAG, "-3-: Get a new Message, getMessageCount = " + getMessageCount3);
getMessageCount3++;
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
}
测试结果：
/*每当点击屏幕上的Button都会打印下面一组log:*/ 01-01 13:15:22.310 13412-13412/com.example.mm.app_message_01 D/MessageTest: Clicked: 14 01-01 13:15:22.312 13412-13431/com.example.mm.app_message_01 D/MessageTest: -1-: Get a new Message, getMessageCount = 14 01-01 13:15:22.312 13412-13432/com.example.mm.app_message_01 D/MessageTest: -2-: Get a new Message, getMessageCount = 14 01-01 13:15:22.312 13412-13433/com.example.mm.app_message_01 D/MessageTest: -3-: Get a new Message, getMessageCount = 14
6.native的实现很复杂，好像是镜面映射了一个message的queue和java的queue对应。
7.Looper类中只有一个构造函数，它里面new了一个MessageQueue，也就是说MessageQueue是Looper对象的。
参考：
android消息处理机制原理解析：https://blog.csdn.net/chunqiuwei/article/details/52251242
原文地址：https://www.cnblogs.com/hellokitty2/p/10801429.html
时间： 2024-10-18 21:57:33