Android官方開發文檔Training系列課程中文版：後台服務之響應IntentService的處理結果

這節課主要學習如何將IntentService中的執行結果返回給請求點。一種推薦的方式就是使用 LocalBroadcastManager來實現，它會將所廣播的Intent限制在APP內部。

發送IntentService的處理結果

為了可以將IntentService的處理結果發送給其它組件，首先需要創建一個Intent對象，並將執行結果放入該Intent內。

接下來要做的就是：將剛才創建好的Intent通過LocalBroadcastManager.sendBroadcast()發送出去。但凡是對該Intent注冊了的，那麼發送該Intent都會收到結果。可以通過getInstance()獲取LocalBroadcastManager的實例。

例子如下：

public final class Constants {
    ...
    // Defines a custom Intent action
    public static final String BROADCAST_ACTION =
        "com.example.android.threadsample.BROADCAST";
    ...
    // Defines the key for the status "extra" in an Intent
    public static final String EXTENDED_DATA_STATUS =
        "com.example.android.threadsample.STATUS";
    ...
}
public class RSSPullService extends IntentService {
...
    /*
     * Creates a new Intent containing a Uri object
     * BROADCAST_ACTION is a custom Intent action
     */
    Intent localIntent =
            new Intent(Constants.BROADCAST_ACTION)
            // Puts the status into the Intent
            .putExtra(Constants.EXTENDED_DATA_STATUS, status);
    // Broadcasts the Intent to receivers in this app.
    LocalBroadcastManager.getInstance(this).sendBroadcast(localIntent);
...
}

下一步就是如何處理接收到的Intent對象了。

接收IntentService的處理結果

如果需要接收廣播出來的Intent，那麼就需要用到BroadcastReceiver了。在BroadcastReceiver的實現類中重寫onReceive()方法。當LocalBroadcastManager將相應的Intent對象廣播出來後，那麼該方法就會被自動回調。

舉個例子：

// Broadcast receiver for receiving status updates from the IntentService
private class ResponseReceiver extends BroadcastReceiver
{
    // Prevents instantiation
    private DownloadStateReceiver() {
    }
    // Called when the BroadcastReceiver gets an Intent it's registered to receive
    @
    public void onReceive(Context context, Intent intent) {
...
        /*
         * Handle Intents here.
         */
...
    }
}

一旦定義好了BroadcastReceiver，那麼就可以為其定義指定的意圖過濾器了。要做到這些，需要創建一個IntentFilter。下面的代碼演示了如何定義一個過濾器：

// Class that displays photos
public class DisplayActivity extends FragmentActivity {
    ...
    public void onCreate(Bundle stateBundle) {
        ...
        super.onCreate(stateBundle);
        ...
        // The filter's action is BROADCAST_ACTION
        IntentFilter mStatusIntentFilter = new IntentFilter(
                Constants.BROADCAST_ACTION);

        // Adds a data filter for the HTTP scheme
        mStatusIntentFilter.addDataScheme("http");

為了將BroadcastReceiver以及IntentFilter注冊到系統，需要先獲取LocalBroadcastManager的實例，然後再調用它的registerReceiver()方法。下面的代碼演示了這個過程：

        // Instantiates a new DownloadStateReceiver
        DownloadStateReceiver mDownloadStateReceiver =
                new DownloadStateReceiver();
        // Registers the DownloadStateReceiver and its intent filters
        LocalBroadcastManager.getInstance(this).registerReceiver(
                mDownloadStateReceiver,
                mStatusIntentFilter);
        ...

BroadcastReceiver可以同時處理多種類型的Intent對象，這項特性可以為每種Action定義不同的代碼，而不需要專門去定義BroadcastReceiver。為同一個BroadcastReceiver定義另外的IntentFilter，只需再創建一個IntentFilter，然後再次注冊一下就好：

        /*
         * Instantiates a new action filter.
         * No data filter is needed.
         */
        statusIntentFilter = new IntentFilter(Constants.ACTION_ZOOM_IMAGE);
        ...
        // Registers the receiver with the new filter
        LocalBroadcastManager.getInstance(getActivity()).registerReceiver(
                mDownloadStateReceiver,
                mIntentFilter);

發送廣播Intent並不會啟動或者恢復Activity。就算是APP處於掛起狀態(處於後台)也同樣會接收到Intent。如果APP處於掛起狀態的話，有任務完成需要通知到用戶，那麼可以使用Notification做到。絕不要啟動Activity來響應接收到的Intent廣播。

原文地址：https://developer.android.com/training/run-background-service/report-status.html