Android开发:在后台服务中运行-报告工作状态

本节讲运行在后台服务里的工作请求，如何向发送请求者报告状态。推荐用LocalBroadcastManager发送和接收状态，它限制了只有本app才能接收到广播。

从IntentService汇报状态

从IntentService发送工作请求状态给其他组件，先创建一个包含状态和数据的Intent。也可以添加action和URI到intent里。 下一步，调用 LocalBroadcastManager.sendBroadcast()发送Intent,应用中所有注册了接收该广播的接收器都能收到。LocalBroadcastManager.getInstance()获取LocalBroadcastManager实例。

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);
...
}

下一步是接收广播并处理。

接收来自IntentService的广播

接收方式与普通的Broadcast一样，用一个BroadcastReceiver的子类，实现BroadcastReceiver.onReceive()方法

// 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.
         */
...
    }
}

定义好了接收器类以后，定义过滤器，匹配指定的action,categorie,data.

// 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");
        ...

注册方式稍有不同，用LocalBroadcastManager.registerReceiver()。

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

单个BroadcastReceiver可以处理多种类型的广播，这个特性允许你根据不同的action运行不同的代码，而无需为每个action定义一个BroadcastReceiver。

  /*
         * 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);

发送广播并不会启动或恢复Activity.BroadcastReceiver让Activity能够接收处理数据，包括应用在后台的时候，但不会强制app回到前台。如果你要在app在后台，对用户不可见时，通知用户一个事件发生，用Notification。绝对不要启动一个Activity来响应广播。