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之前的例子都是我们在子线程（WorkerThread）当中处理并发送消息，然后在主线程（UI线程）中获取消息并修改UI，那么可以不可以在由主线程发送消息，子线程接收呢？我们按照之前的思路写一下代码：
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", "收到了消息对象"); } }; } } }
<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>
上面的问题貌似没什么错误，但运行时却出错
抛出异常：
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)
提示我们没有调用Looper.prepare()，无法创建线程，原因是非主线程中默认是没有创建Looper对象的，需要先调用Looper.prepare()启用Looper。修改一下上面的代码。
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", "收到了消息对象:" + i); } }; Looper.loop(); } } }
<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>
运行结果：
09-17 04:29:48.669 28397-28397/cn.lixyz.handlertest D/TTTT﹕ sendMessage:main
09-17 04:29:48.669 28397-28410/cn.lixyz.handlertest D/TTTT﹕ handleMessage:Thread-141
09-17 04:29:48.669 28397-28410/cn.lixyz.handlertest D/TTTT﹕ 收到了消息对象:100
上面代码表示，消息是从MainThread发送的，WorkerThread接收到了消息。
PS：非UI线程默认是没有创建Looper对象的，所以需要主线程向子线程传递消息时，需创建Looper对象并启动循环，子线程才可以循环着从MaessageQueue中取出消息。
Android笔记（三十二） Android中线程之间的通信（四）主线程给子线程发送消息
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原文地址：http://www.cnblogs.com/xs104/p/4816743.html
