Android官方开发文档Training系列课程中文版：后台服务之响应IntentService的处理结果

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

这节课主要学习如何将IntentService中的执行结果返回给请求点。一种推荐的方式就是使用 LocalBroadcastManager来实现，它会将所广播的Intent限制在APP内部。

发送IntentService的处理结果

为了可以将IntentService的处理结果发送给其它组件，首先需要创建一个Intent对象，并将执行结果放入该Intent内。

接下来要做的就是：将刚才创建好的Intent通过LocalBroadcastManager.sendBroadcast()发送出去。但凡是对该Intent注册了的，那么发送该Intent都会收到结果。可以通过getInstance()获取LocalBroadcastManager的实例。

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);
...
}

下一步就是如何处理接收到的Intent对象了。

接收IntentService的处理结果

如果需要接收广播出来的Intent，那么就需要用到BroadcastReceiver了。在BroadcastReceiver的实现类中重写onReceive()方法。当LocalBroadcastManager将相应的Intent对象广播出来后，那么该方法就会被自动回调。

举个例子：

// 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.
         */
...
    }
}

一旦定义好了BroadcastReceiver，那么就可以为其定义指定的意图过滤器了。要做到这些，需要创建一个IntentFilter。下面的代码演示了如何定义一个过滤器：

// 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");

为了将BroadcastReceiver以及IntentFilter注册到系统，需要先获取LocalBroadcastManager的实例，然后再调用它的registerReceiver()方法。下面的代码演示了这个过程：

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

BroadcastReceiver可以同时处理多种类型的Intent对象，这项特性可以为每种Action定义不同的代码，而不需要专门去定义BroadcastReceiver。为同一个BroadcastReceiver定义另外的IntentFilter，只需再创建一个IntentFilter，然后再次注册一下就好：

        /*
         * 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);

发送广播Intent并不会启动或者恢复Activity。就算是APP处于挂起状态(处于后台)也同样会接收到Intent。如果APP处于挂起状态的话，有任务完成需要通知到用户，那么可以使用Notification做到。绝不要启动Activity来响应接收到的Intent广播。