我下面的测试应用程序大致基于 Cristian Baita 的这个线程教程。它按预期工作，但从 myThread 的 run() 方法发送的消息从未被 MainActivity 的处理程序接收到。
我将 MainActivity 的处理程序传递给线程构造函数中的 myThread。然后我使用该处理程序的 sendMessage() 方法将消息发送回 MainActivity，但处理程序似乎从未收到它。为什么是这样？
注意：我发现在 Eclipse 中使用断点进行调试对线程来说是一件痛苦的事情，所以我最终使用 Log 语句而不是帮助跟踪应用程序的执行。
我已将完整的代码放在这篇文章的末尾，但总结一下：
MyThread 类的构造函数从调用活动中获取处理程序，如下所示。
public class MyThread extends Thread {
// Reference to mainHandler from the mainThread
private Handler parentHandler;
// Constructor
public MyThread(Handler pHandler) {
parentHandler = pHandler;
}
当我在 MainActivity 的 onCreate() 方法中创建线程时，我将它传递给处理程序mainHandler：
myThread = new MyThread(mainHandler);
myThread.start();
然后在 MyThread 的 run() 方法中，我有：
Message messageToParent = Message.obtain();
messageToParent.what = 2;
Log.i("myThread", "About to send message to parent ...");
parentHandler.sendMessage(messageToParent);
然后应该通过mainHandlerMainActivity 中定义的方式接收消息：
public Handler mainHandler = new Handler() {
public void handleMessages(Message msg){
Log.i("MainActivity", "Message Received");
switch(msg.what) {
case 2:
Log.i("MainActivity", "Handled message. msg.what = " + msg.what);
....
如果您在运行时观察 LogCat 窗口，您将看到 MainActivity 从未记录“收到的消息”或“已处理的消息...”。所以消息永远不会到达它的目的地。
主要活动：
public class MainActivity extends Activity {
private MyThread myThread;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
myThread = new MyThread(mainHandler);
myThread.start();
// Message the thread
Message msgToThread = Message.obtain();
msgToThread.what = 4;
Log.i("MainActivity", "About to send message to thread...");
myThread.getHandler().sendMessage(msgToThread);
}
public Handler mainHandler = new Handler() {
public void handleMessages(Message msg){
Log.i("MainActivity", "Message Received");
switch(msg.what) {
case 2:
Log.i("MainActivity", "Handled message. msg.what = " + msg.what);
// Message the thread
Message msgToThread = Message.obtain();
msgToThread.what = 6;
myThread.getHandler().sendMessage(msgToThread);
break;
default:
Log.i("MainActivity", "Unhandled message. msg.what = " + msg.what);
break;
}
}
};
}
MyThread 类：
public class MyThread extends Thread {
// Reference to mainHandler from the mainThread
private Handler parentHandler;
// Constructor
public MyThread(Handler pHandler) {
parentHandler = pHandler;
}
// Local handler for messages to this thread
private Handler myThreadHandler = new Handler() {
public void handleMessage(Message msg) {
switch(msg.what) {
case 4:
Log.i("myThread", "Handled message. msg.what = " + msg.what);
break;
case 6:
Log.i("myThread", "Handled message. msg.what = " + msg.what);
break;
default:
Log.i("myThread", "Unhandled message. msg.what = " + msg.what);
break;
}
}
};
@Override
public void run() {
super.run();
int count = 0;
boolean keepGoing = true;
try {
while(true) {
Log.i("myThread", "run() method - while loop is ticking ..." + count);
// some arbitrary conditions to make stuff happen
switch(count) {
case 5:
Message messageToParent = Message.obtain();
messageToParent.what = 2;
Log.i("myThread", "About to send message to parent ...");
parentHandler.sendMessage(messageToParent);
break;
case 10:
keepGoing = false;
break;
}
if(!keepGoing) {
Log.i("myThread", "myThread is going to stop");
break;
}
count++;
sleep(500);
}
}
catch (Exception e) {
Log.e("My Log", "Thread Loop Exception - " + e);
}
Log.i("myThread", "myThread has reached the end of it's run() method");
}
public Handler getHandler() {
return myThreadHandler;
}
}