Service一般都被翻譯為服務的意思，在android裡，service是背景執行的元件，可用於長期作業或遠端處理，舉個例子:Service可以在執行其他app時在背景執行音樂的播放或者檔案的上傳/下載之類的動作

service有兩種呼叫的方式，簡單來說

1. startService():透過startService()啟動Service，即使當Activity退出，也不會影響Service的執行。

2. bindService():透過bindService()讓Activity綁定Service，當Activity退出那麼Service也會終止執行。

Service的生命週期

我用程式先呈現startService()的生命週期
首先new一個Service的類別

MyTestService

public class MyTestService extends Service {
    public MyTestService() {
    }
    //這是一開始就創建好的函式
    @Override
    public IBinder onBind(Intent intent) {
        // TODO: Return the communication channel to the service.
//        throw new UnsupportedOperationException("Not yet implemented");
        return null;
    }
    //下面新增
    @Override
    public void onCreate() {
        super.onCreate();
        Log.v("MyTestService","onCreate");
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Log.v("MyTestService","onStartCommand");
        return super.onStartCommand(intent, flags, startId);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.v("MyTestService","onDestroy");
    }
}

mainactivity

    //點click1按鈕會啟動Service，執行MyTestService內的onCreate()、onStartCommand()
    public void click1(View view) {
        Intent intent =new Intent(this,MyTestService.class);
        startService(intent);
    }
     //點click2按鈕會關閉Service後，執行MyTestService內的onDestroy()
    public void click2(View view) {
        Intent intent =new Intent(this,MyTestService.class);
        stopService(intent);
    }
}

Log結果，第一次click1會先執行MyTestService內的onCreate()、onStartCommand()，之後再點click1只會出現onStartCommand()

bindService()的生命週期:

首先創一個ServiceConnection函式，這個函式會接收到由 MyTestService 內的onBind() 所丟出來的 Ibinder 物件，利用這 IBinder 物件取得 Service 物件，就可以直接操作 Service 內各個 public 的 method。

 private ServiceConnection mServiceConnection =new ServiceConnection() { 
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            myTestService = ((MyTestService.LocalBinder)service).getTestService();
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {
        }
    };

MyTestService

public class MyTestService extends Service {
    public MyTestService() {
    }
    public class LocalBinder extends Binder //宣告一個繼承 Binder 的類別 LocalBinder
    {
        MyTestService getTestService()
        {
            return  MyTestService.this;
        }
    }

    private IBinder mBinder =new LocalBinder();

    public void Msg()
    {
        Log.d("MyTestService", "Msg()");
    }
    
    @Override
    public IBinder onBind(Intent intent) {
        // TODO: Return the communication channel to the service.
//        throw new UnsupportedOperationException("Not yet implemented");
        return mBinder;
    }
    @Override
    public boolean onUnbind(Intent intent){
        return super.onUnbind(intent);
    }  
}

MainActivity

public class MainActivity extends AppCompatActivity {
    private MyTestService myTestService;
    private Boolean mBoo;//判斷是否綁定
    private ServiceConnection mServiceConnection =new ServiceConnection() { 
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            myTestService = ((MyTestService.LocalBinder)service).getTestService();
            mBoo =true;
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {
            mBoo =false;
        }
    };
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
     @Override
    protected void onStart() {
        super.onStart();
        Intent intent =new Intent(this,MyTestService.class);
        bindService(intent,mServiceConnection, Context.BIND_AUTO_CREATE);
    }

    @Override
    protected void onStop() {
        super.onStop();
        unbindService(mServiceConnection);
        mBoo=false;
    }
    //當mBoo有綁定 且 myTestService !=null，就執行myTestService的Msg();
    public void click1(View view) { 
        if (mBoo && myTestService !=null){
            myTestService.Msg();
        }
    }
}

Log結果

那麼Service的介紹就到此結束~