Service類別

Service類別
Service沒有提供與用戶進行互動的展示層，為一背景執行的程式，一般來說，若是不需要UI操作的程序則可使用Service來完成。

Service一般是由Activity或Context物件來啟動，啟動的方式分為二種：
    透過startService()啟動：啟動後直到Service呼叫自身的stopSelf或stopService時才停止
    透過bindService()啟動：當綁定的Context被銷毀時，停止執行

生命週期如下所示：

程式碼

    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

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