Android筆記（三十二） Android中線程之間的通訊（四）主線程給子線程發送訊息

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         之前的例子都是我們在子線程（WorkerThread）當中處理並發送訊息，然後在主線程（UI線程）中擷取訊息並修改UI，那麼可以不可以在由主線程發送訊息，子線程接收呢？我們按照之前的思路寫一下代碼：

package cn.lixyz.handlertest;import android.app.Activity;import android.os.Bundle;import android.os.Handler;import android.os.Message;import android.util.Log;import android.view.View;import android.widget.Button;public class MainActivity extends Activity {    private Button button;    private Handler handler;    private int i = 0;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        button = (Button) findViewById(R.id.button);        button.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                Log.d("TTTT", "onClick:" + Thread.currentThread().getName());                Message msg = handler.obtainMessage();                handler.sendMessage(msg);            }        });        WorkerThread wt = new WorkerThread();        wt.start();    }    class WorkerThread extends Thread {        @Override        public void run() {            super.run();            handler = new Handler() {                @Override                public void handleMessage(Message msg) {                    super.handleMessage(msg);                    Log.d("TTTT", "收到了訊息對象");                }            };        }    }}

MainActivity.java

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:tools="http://schemas.android.com/tools"    android:layout_width="match_parent"    android:layout_height="match_parent"    android:orientation="vertical"    tools:context=".MainActivity">    <Button        android:id="@+id/button"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:layout_gravity="center"        android:layout_marginTop="30dp"        android:text="Send Message" /></LinearLayout>

activity_main.xml

         上面的問題貌似沒什麼錯誤，但運行時卻出錯

         拋出異常：

09-17 04:01:31.655  15854-15869/? E/AndroidRuntime﹕ FATAL EXCEPTION: Thread-120    Process: cn.lixyz.handlertest, PID: 15854    java.lang.RuntimeException: Can‘t create handler inside thread that has not called Looper.prepare()            at android.os.Handler.<init>(Handler.java:200)            at android.os.Handler.<init>(Handler.java:114)            at cn.lixyz.handlertest.MainActivity$WorkerThread$1.<init>(MainActivity.java:48)            at cn.lixyz.handlertest.MainActivity$WorkerThread.run(MainActivity.java:48)

　　

         提示我們沒有調用Looper.prepare()，無法建立線程，原因是非主線程中預設是沒有建立Looper對象的，需要先調用Looper.prepare()啟用Looper。修改一下上面的代碼。

package cn.lixyz.handlertest;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.widget.Button;public class MainActivity extends Activity {    private Button button;    private Handler handler;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        button = (Button) findViewById(R.id.button);        button.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                Message msg = handler.obtainMessage();                msg.what = 100;                handler.sendMessage(msg);                Log.d("TTTT", "sendMessage:" + Thread.currentThread().getName());            }        });        WorkerThread wt = new WorkerThread();        wt.start();    }    class WorkerThread extends Thread {        @Override        public void run() {            super.run();            Looper.prepare();            handler = new Handler() {                @Override                public void handleMessage(Message msg) {                    super.handleMessage(msg);                    Log.d("TTTT", "handleMessage:" + Thread.currentThread().getName());                    int i = msg.what;                    Log.d("TTTT", "收到了訊息對象:" + i);                }            };            Looper.loop();        }    }}

MainActivity.java

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:tools="http://schemas.android.com/tools"    android:layout_width="match_parent"    android:layout_height="match_parent"    android:orientation="vertical"    tools:context=".MainActivity">    <Button        android:id="@+id/button"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:layout_gravity="center"        android:layout_marginTop="30dp"        android:text="Send Message" /></LinearLayout>

activity_main.xml

         運行結果：

09-17 04:29:48.669  28397-28397/cn.lixyz.handlertest D/TTTT﹕ sendMessage:main09-17 04:29:48.669  28397-28410/cn.lixyz.handlertest D/TTTT﹕ handleMessage:Thread-14109-17 04:29:48.669  28397-28410/cn.lixyz.handlertest D/TTTT﹕ 收到了訊息對象:100

         上面代碼錶示，訊息是從MainThread發送的，WorkerThread接收到了訊息。

         PS：非UI線程預設是沒有建立Looper對象的，所以需要主線程向子線程傳遞訊息時，需建立Looper對象並啟動迴圈，子線程才可以迴圈著從MaessageQueue中取出訊息。　　

          

 

 

         

 

Android筆記（三十二） Android中線程之間的通訊（四）主線程給子線程發送訊息