老罗Looper(23)例子讲述Looper与Message,线程和Handler关系

package com.example.handler3;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends Activity {

private Button mButton;
private TextView mTextView;
private MyHandler mHandler;
//子线程发送消息，主线程更新UI
private class MyRunnable implements Runnable{
@Override
public void run() {
Message msg = Message.obtain();
msg.what = 3;
msg.obj = "xiaoxi";
mHandler.sendMessage(msg);
}
}
private class MyHandler extends Handler{
//空的构造方法
public MyHandler(){
}
//构造方法
public MyHandler (Looper loop){
super(loop);
}
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
mTextView.setText("接受消息" + msg.obj);
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mButton = (Button)findViewById(R.id.button1);
mTextView = (TextView)this.findViewById(R.id.textView1);
//Activity中有系统Looper对象来处理线程中发送的消息，一下两段写不写都行
//      Looper l = Looper.myLooper();
//      mHandler = new MyHandler(l);
mButton.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View arg0) {
MyRunnable r = new MyRunnable();
Thread t = new Thread(r);
t.start();
}
});
}
}

验证Activity中有系统Looper对象来处理线程中发送的消息

error代码

package com.example.handler3;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends Activity {

private Button mButton;
private TextView mTextView;
private Handler mHandler;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mButton = (Button)findViewById(R.id.button1);
mTextView = (TextView)this.findViewById(R.id.textView1);
//启动线程
new Thread(new MyRunnable()).start();
//Activity中有系统Looper对象来处理线程中发送的消息，一下两段写不写都行
mButton.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View arg0) {
Message msg = Message.obtain();
msg.obj = "xiaoxi";
mHandler.sendMessage(msg);
}
});
}
private class MyRunnable implements Runnable{
@Override
public void run() {
mHandler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
System.out.println("接收到消息：" + msg.obj);
}
};
}
}
}

java.lang.RuntimeException: Can’t create handler inside thread that has not called Looper.prepare()

error原因，线程默认是与Message loop无关联的。

正确写法：

package com.example.handler3;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends Activity {

private Button mButton;
private TextView mTextView;
private Handler mHandler;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mButton = (Button)findViewById(R.id.button1);
mTextView = (TextView)this.findViewById(R.id.textView1);
//启动线程
new Thread(new MyRunnable()).start();
//Activity UI主线程中有系统Looper对象来处理线程中发送的消息，一下两段写不写都行
mButton.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View arg0) {
Message msg = Message.obtain();
msg.obj = "xiaoxi";
mHandler.sendMessage(msg);
}
});
}
private class MyRunnable implements Runnable{
@Override
public void run() {
//在子线程实例化handler,子线程中没有Looper对象来处理线程中发送的消息
Looper.prepare();//循环消息队列
mHandler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
System.out.println("接收到消息：" + msg.obj);
}
};
Looper.loop();//直到消息队列循环结束
}
}
}