Service(八)

Android應用程式學習筆記

Running a Service in the Foreground

Service不僅只在後台執行，也可以讓service在前台運行，只是這樣的用法比較少。

只要呼叫startForeground()啟動service，就可以讓service在前台執行。這個方法有兩個參數，第一個參數是整數，只用來標識notification。第二個參數是Notification物件。

Notification notification = new Notification(R.drawable.icon, getText(R.string.ticker_text),
        System.currentTimeMillis());
Intent notificationIntent = new Intent(this, ExampleActivity.class);
PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, notificationIntent, 0);
notification.setLatestEventInfo(this, getText(R.string.notification_title),
        getText(R.string.notification_message), pendingIntent);
startForeground(ONGOING_NOTIFICATION, notification);

停止service只要呼叫stopForeground()。

Managing the Lifecycle of a Service

service的生命週期比起activity的簡單多了，然而，密切關注你的service如何產生及如何銷毀的甚至更重要，因為service可以在後台執行，不會被使用者意識到。

service生命週期可以分成兩部分來看:

• A started service
  Service由其他組件在呼叫startService()方法產生，service會無限地運行，藉著自己呼叫stopSelf()停止自己，或者其他組件呼叫stopService()來停止service，當service停止後，系統銷毀。
• A bound service
  service由其它組件呼叫bindService()產生，客戶端通過IBinder與service通訊，客戶端可以透過呼叫unbindService()關閉與service的連結。假設許多客戶端綁定在同一個service，當它們都不再與service連結時，系統就會銷毀service。

以下你可以練習來幫助了解生命週期

public class ExampleService extends Service {
    int mStartMode;       // indicates how to behave if the service is killed
    IBinder mBinder;      // interface for clients that bind
    boolean mAllowRebind; // indicates whether onRebind should be used

    @Override
    public void onCreate() {
        // The service is being created
    }
    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        // The service is starting, due to a call to startService()
        return mStartMode;
    }
    @Override
    public IBinder onBind(Intent intent) {
        // A client is binding to the service with bindService()
        return mBinder;
    }
    @Override
    public boolean onUnbind(Intent intent) {
        // All clients have unbound with unbindService()
        return mAllowRebind;
    }
    @Override
    public void onRebind(Intent intent) {
        // A client is binding to the service with bindService(),
        // after onUnbind() has already been called
    }
    @Override
    public void onDestroy() {
        // The service is no longer used and is being destroyed
    }
}