java - 从另一个线程访问上下文

Question

作为 Android 平台的新手，我开始将一些 PC Java 应用程序迁移到 Android 环境。

当我尝试将服务 引用用作Toast 的上下文时，我发现了一个问题消息。

这是我的服务代码的相关部分:

public class ServicePFPE extends Service {

    Timer messageSimulator;
    TimerTask messagePoll;

    private class MessageReceptionTask extends TimerTask
    {
        public MessageReceptionTask(Context c) {
            context = c;
        }

        @Override
        public void run() {
            String shownText = "Message received!! on " + (new Date(System.currentTimeMillis())).toString();    
            //doToast(shownText, context);    //THIS LINE MAKES THE APP CRASH!
            System.out.println(shownText);    //But I can do this
        }    

        private Context context;
    }

    public ServicePFPE() {
        super();
        messageSimulator = new Timer();
        messagePoll = new MessageReceptionTask(this);
    }

    @Override    
    public IBinder onBind(Intent intent)    
    {    
            doToast("Service: onBind");    
            return null;    
    }    

    ...
    ...
    ...

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {    
        doToast("Service: onStartCommand");    
        messageSimulator.schedule(messagePoll, 5000, 5000);    
        return super.onStartCommand(intent, flags, startId);    
    }    

    ...
    ...
    ...

    private void doToast(String msg) { doToast(msg, this); }
    private void doToast(String msg, Context con) {
           Toast.makeText(con,msg,Toast.LENGTH_SHORT).show(); 
    }
}    

当计划任务运行到 doToast 调用时，Android 通知“不幸的是，myAPP 已停止”。

我认为这与我在不同线程中使用服务上下文有关，但我不确定。

您能否确认是否是这种情况？从服务运行计时器并能够使用其上下文的正确方法是什么？如果那不可能，我能否获取该线程的上下文以便生成 Toasts 用户消息。

Answer 1

这取决于您真正需要什么，如果您打算显示简单的通知，也许您可​​以使用 Android 通知栏(这是显示它们的标准方式)而不是 toasts。例如你可以使用:

  /**
     * Show a notification while this service is running.
     */
    private void showNotification() {
        // In this sample, we'll use the same text for the ticker and the expanded notification
        CharSequence text = getText(R.string.local_service_started);

        NotificationManager mNM;
        mNM = (NotificationManager)getSystemService(NOTIFICATION_SERVICE);
        // Set the icon, scrolling text and timestamp
        Notification notification = new Notification(R.drawable.stat_sample, text,
                System.currentTimeMillis());

        // The PendingIntent to launch our activity if the user selects this notification
        PendingIntent contentIntent = PendingIntent.getActivity(this, 0,
                new Intent(this, LocalServiceActivities.Controller.class), 0);

        // Set the info for the views that show in the notification panel.
        notification.setLatestEventInfo(this, getText(R.string.local_service_label),
                       text, contentIntent);

        // Send the notification.
        mNM.notify(NOTIFICATION, notification);
    }

但是，如果您只想要 toast ，您可以从服务中显示它们，您的问题是 timertask 是在与 UI 线程(运行服务的地方)不同的线程中执行的。要将此代码“发布”到 UI 线程，您可以直接使用如下代码:

Handler handler;

    @Override
    public void onCreate() {
        // Handler will get associated with the current thread, 
        // which is the main thread.
        handler = new Handler();
        super.onCreate();
    }

    private void runOnUiThread(Runnable runnable) {
        handler.post(runnable);
    }

Source

最后，如果您想在服务和 Activity 之间进行完全交互，您有几种方法:

1. 使用 Binder ，用于简单的通信，这更不是您所需要的。
2. 使用 Messenger 进行更复杂的交流。
3. 如果您只需要对话，您始终可以在对话模式下启动新 Activity 。
4. AIDL...

关于 1 和 2 的文档 here和 here

1. 粘合剂: 它们允许您在应用程序中绑定(bind)不同的对象，让它们直接访问对象本身及其功能，来自 android 文档的示例:

   公共(public)类 LocalService 扩展服务 { //给客户端的绑定(bind)器 private final IBinder mBinder = new LocalBinder(); //随机数生成器 private final Random mGenerator = new Random();

       /**
        * Class used for the client Binder.  Because we know this service always
        * runs in the same process as its clients, we don't need to deal with IPC.
        */
       public class LocalBinder extends Binder {
           LocalService getService() {
               // Return this instance of LocalService so clients can call public methods
               return LocalService.this;
           }
       }
   
       @Override
       public IBinder onBind(Intent intent) {
           return mBinder;
       }
   
       /** method for clients */
       public int getRandomNumber() {
         return mGenerator.nextInt(100);
       }
   }
   
   public class BindingActivity extends Activity {
       LocalService mService;
       boolean mBound = false;
   
       @Override
       protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.main);
       }
   
       @Override
       protected void onStart() {
           super.onStart();
           // Bind to LocalService
           Intent intent = new Intent(this, LocalService.class);
           bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
       }
   
       @Override
       protected void onStop() {
           super.onStop();
           // Unbind from the service
           if (mBound) {
               unbindService(mConnection);
               mBound = false;
           }
       }
   
       /** Called when a button is clicked (the button in the layout file attaches to
         * this method with the android:onClick attribute) */
       public void onButtonClick(View v) {
           if (mBound) {
               // Call a method from the LocalService.
               // However, if this call were something that might hang, then this request should
               // occur in a separate thread to avoid slowing down the activity performance.
               int num = mService.getRandomNumber();
               Toast.makeText(this, "number: " + num, Toast.LENGTH_SHORT).show();
           }
       }
   
       /** Defines callbacks for service binding, passed to bindService() */
       private ServiceConnection mConnection = new ServiceConnection() {
   
           @Override
           public void onServiceConnected(ComponentName className,
                   IBinder service) {
               // We've bound to LocalService, cast the IBinder and get LocalService instance
               LocalBinder binder = (LocalBinder) service;
               mService = binder.getService();
               mBound = true;
           }
   
           @Override
           public void onServiceDisconnected(ComponentName arg0) {
               mBound = false;
           }
       };
   }
   

2. 信使: 更高级和复杂，通过这种方式，您可以将消息从一个对象发送到另一个对象:

   公共(public)类 MessengerService 扩展服务 { /** 命令服务显示消息 */ static final int MSG_SAY_HELLO = 1;

       /**
        * Handler of incoming messages from clients.
        */
       class IncomingHandler extends Handler {
           @Override
           public void handleMessage(Message msg) {
               switch (msg.what) {
                   case MSG_SAY_HELLO:
                       Toast.makeText(getApplicationContext(), "hello!", Toast.LENGTH_SHORT).show();
                       break;
                   default:
                       super.handleMessage(msg);
               }
           }
       }
   
       /**
        * Target we publish for clients to send messages to IncomingHandler.
        */
       final Messenger mMessenger = new Messenger(new IncomingHandler());
   
       /**
        * When binding to the service, we return an interface to our messenger
        * for sending messages to the service.
        */
       @Override
       public IBinder onBind(Intent intent) {
           Toast.makeText(getApplicationContext(), "binding", Toast.LENGTH_SHORT).show();
           return mMessenger.getBinder();
       }
   }
   
   
   
    public class ActivityMessenger extends Activity {
           /** Messenger for communicating with the service. */
           Messenger mService = null;
   
           /** Flag indicating whether we have called bind on the service. */
           boolean mBound;
   
           /**
            * Class for interacting with the main interface of the service.
            */
           private ServiceConnection mConnection = new ServiceConnection() {
               public void onServiceConnected(ComponentName className, IBinder service) {
                   // This is called when the connection with the service has been
                   // established, giving us the object we can use to
                   // interact with the service.  We are communicating with the
                   // service using a Messenger, so here we get a client-side
                   // representation of that from the raw IBinder object.
                   mService = new Messenger(service);
                   mBound = true;
               }
   
               public void onServiceDisconnected(ComponentName className) {
                   // This is called when the connection with the service has been
                   // unexpectedly disconnected -- that is, its process crashed.
                   mService = null;
                   mBound = false;
               }
           };
   
           public void sayHello(View v) {
               if (!mBound) return;
               // Create and send a message to the service, using a supported 'what' value
               Message msg = Message.obtain(null, MessengerService.MSG_SAY_HELLO, 0, 0);
               try {
                   mService.send(msg);
               } catch (RemoteException e) {
                   e.printStackTrace();
               }
           }
   
           @Override
           protected void onCreate(Bundle savedInstanceState) {
               super.onCreate(savedInstanceState);
               setContentView(R.layout.main);
           }
   
           @Override
           protected void onStart() {
               super.onStart();
               // Bind to the service
               bindService(new Intent(this, MessengerService.class), mConnection,
                   Context.BIND_AUTO_CREATE);
           }
   
           @Override
           protected void onStop() {
               super.onStop();
               // Unbind from the service
               if (mBound) {
                   unbindService(mConnection);
                   mBound = false;
               }
           }
       }
   

如果您想将 Activity 显示为精美的对话框以显示更新，您可以使用具有此主题的常规 Activity :

<activity android:theme="@android:style/Theme.Dialog" />