Android中使用IntentService執行後台任務

IntentService提供了一種在後台線程中執行任務的方式，適合處理執行時間較長的後台任務。

優點：

（1）IntentService運行在單獨的線程中，不會阻塞UI線程

（2）IntentService不受生命周期的影響

缺點：

（1）不能與UI直接進行交互，可以用Broadcast

（2）順序執行請求，第二個請求只有在第一個請求執行完以後才能執行

（3）請求不能被中斷

使用IntentService的步驟：

（1）在Activity中通過startService啟動service，並傳遞參數。

（2）Service中接收參數，做耗時的處理，處理完畢，發送Broadcat，並把處理結果傳遞出來

（3）Activity中注冊BroadcastReceiver，監聽廣播，更新UI。

看一個例子：

public class MainActivity extends Activity {

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		Button btn = (Button) this.findViewById(R.id.btn);
		btn.setOnClickListener(new View.OnClickListener() {
			@Override
			public void onClick(View v) {//通過startService啟動service，並傳遞參數。
				Intent mServiceIntent = new Intent(MainActivity.this,RSSPullService.class);
				mServiceIntent.setData(Uri.parse(http://www.baidu.com/));
				MainActivity.this.startService(mServiceIntent);
			}
		});
		//注冊BroadcastReceiver，監聽廣播
		IntentFilter statusIntentFilter = new IntentFilter(Constants.BROADCAST_ACTION);
        // Sets the filter's category to DEFAULT
        statusIntentFilter.addCategory(Intent.CATEGORY_DEFAULT);
		DownloadStateReceiver mDownloadStateReceiver = new DownloadStateReceiver();
		// Registers the DownloadStateReceiver and its intent filters
		LocalBroadcastManager.getInstance(this).registerReceiver(mDownloadStateReceiver, statusIntentFilter);

	}

	private class DownloadStateReceiver extends BroadcastReceiver {
		@Override
		public void onReceive(Context context, Intent intent) {
			String data = intent.getStringExtra(Constants.EXTENDED_DATA);
			Log.e(test, data);
			Toast.makeText(context, data, Toast.LENGTH_SHORT).show();
		}
	}

}

public class RSSPullService extends IntentService {

	public RSSPullService() {
		super(RSSPullService);
	}
	
	@Override
	protected void onHandleIntent(Intent workIntent) {//接收參數，做耗時的處理，處理完畢，發送Broadcat
		String localUrlString = workIntent.getDataString();
		String data = download(localUrlString);
		Intent localIntent = new Intent(Constants.BROADCAST_ACTION);
	    // Puts the status into the Intent
		localIntent.putExtra(Constants.EXTENDED_DATA, data);
	    // Broadcasts the Intent to receivers in this app.
	    LocalBroadcastManager.getInstance(this).sendBroadcast(localIntent);
	}
	
	private String download(String localUrlString){
		try{
			URL url = new URL(localUrlString);
			HttpURLConnection conn = (HttpURLConnection)url.openConnection();
			InputStream in = conn.getInputStream();
			ByteArrayOutputStream out = new ByteArrayOutputStream();
			byte[] buff = new byte[1024];
			int len = 0;
			while((len = in.read(buff)) != -1){
				out.write(buff,0,len);
			}
			in.close();
			return new String(out.toByteArray());
		}catch(Exception e){
			e.printStackTrace();
			return ;
		}
	}
}

public 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 static final String EXTENDED_DATA = com.example.android.threadsample.DATA;

}