Android App後台服務報告工作狀態實例

本節講運行在後台服務裡的工作請求，如何向發送請求者報告狀態。推薦用LocalBroadcastManager發送和接收狀態，它限制了只有本app才能接收到廣播。

從IntentService匯報狀態

從IntentService發送工作請求狀態給其他組件，先創建一個包含狀態和數據的Intent。也可以添加action和URI到intent裡。

下一步，調用 LocalBroadcastManager.sendBroadcast()發送Intent,應用中所有注冊了接收該廣播的接收器都能收到。LocalBroadcastManager.getInstance()獲取LocalBroadcastManager實例。

代碼如下：

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);
...
}

下一步是接收廣播並處理。

接收來自IntentService的廣播

接收方式與普通的Broadcast一樣，用一個BroadcastReceiver的子類，實現BroadcastReceiver.onReceive()方法
代碼如下：

// 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.
         */
...
    }
}
定義好了接收器類以後，定義過濾器，匹配指定的action,categorie,data.
代碼如下：

// 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");
        ...

注冊方式稍有不同，用LocalBroadcastManager.registerReceiver()。
代碼如下：

  // Instantiates a new DownloadStateReceiver
        DownloadStateReceiver mDownloadStateReceiver =
                new DownloadStateReceiver();
        // Registers the DownloadStateReceiver and its intent filters
        LocalBroadcastManager.getInstance(this).registerReceiver(
                mDownloadStateReceiver,
                mStatusIntentFilter);
        ...
單個BroadcastReceiver可以處理多種類型的廣播，這個特性允許你根據不同的action運行不同的代碼，而無需為每個action定義一個BroadcastReceiver。
代碼如下：

  /*
         * 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);
發送廣播並不會啟動或恢復Activity.BroadcastReceiver讓Activity能夠接收處理數據，包括應用在後台的時候，但不會強制app回到前台。如果你要在app在後台，對用戶不可見時，通知用戶一個事件發生，用Notification。絕對不要啟動一個Activity來響應廣播。