老羅Looper(23)例子講述Looper與Message,線程和Handler關係

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;
    //子線程發送消息，主線程更新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{
        //空的構造方法
        public MyHandler(){
        }
        //構造方法
        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中有系統Looper對象來處理線程中發送的消息，一下兩段寫不寫都行
//      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();
            }
        });
    }
}

驗證Activity中有系統Looper對象來處理線程中發送的消息
error代碼

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);
        //啓動線程
        new Thread(new MyRunnable()).start();
        //Activity中有系統Looper對象來處理線程中發送的消息，一下兩段寫不寫都行
        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，線程默認是與Message loop無關聯的。
正確寫法：

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);
        //啓動線程
        new Thread(new MyRunnable()).start();
        //Activity UI主線程中有系統Looper對象來處理線程中發送的消息，一下兩段寫不寫都行
        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() {
            //在子線程實例化handler,子線程中沒有Looper對象來處理線程中發送的消息
            Looper.prepare();//循環消息隊列
            mHandler = new Handler(){
                @Override
                public void handleMessage(Message msg) {
                    super.handleMessage(msg);
                    System.out.println("接收到消息：" + msg.obj);
                }
            };
            Looper.loop();//直到消息隊列循環結束
        }
    }
}