Service是什麼?Service又不是什麼?

在Android王國中,Service是一個勞動模範,總是默默的在後臺運行,無怨無悔,且總是乾最臟最累的活,比如下載文件,傾聽音樂,網路操作等這些耗時的操作,所以我們請尊重的叫他一聲:"勞模,您辛苦了". 帶著這份好尊重,我又重新研讀了API的文檔,發現老外寫東西還是很靠譜的,人家在文檔中告訴你Se ...

在Android王國中,Service是一個勞動模範,總是默默的在後臺運行,無怨無悔,且總是乾最臟最累的活,比如下載文件,傾聽音樂,網路操作等這些耗時的操作,所以我們請尊重的叫他一聲:"勞模,您辛苦了".

帶著這份好尊重,我又重新研讀了API的文檔,發現老外寫東西還是很靠譜的,人家在文檔中告訴你Service是什麼,又告訴你Service又不是什麼,我覺得這種思路不錯,那就從這兩個個方面開始談起吧:

Service是什麼?

A Service is an application component. ☆ Service 是一個應用程式組件
that can perform long-running operations in the background. ☆ 它能在後臺執行一些耗時較長的操作.
and does not provide a user interface. ☆ 並且不提供用戶界面
Another application component can start a service and it will continue to run in the background event if the user switches to another application. ☆ 服務能被其它的應用程式組件啟動,即使用戶切換到其他的應用程式時還能保持在後臺運行.
Additionally,a component can bind to a service to interact with it and even perform interprocess communcation(IPC). ☆ 此外,組件還能綁定服務,並與服務交互,甚至執行進程間通信(IPC).
For example,a service might handle network transactions,play music,perform file I/O,or interact with a content provider,all from the background. ☆ 比如,一個服務可以處理網路傳輸,聽音樂,執行文件操作,或者與內容提供者進行交互,所有這些都在後臺進行.
A service can essentially tack two forms.☆ 服務有以下兩種基本類型

Started --> startService()
Bound --> bindService()

Service又不是什麼?

A service is not a separate process.☆ 服務不是一個單獨的進程.
A service is not a thread.it runs in the main thread of its hosting process. ☆ 服務不是一個線程,它運行在主線程.
the service does not create its own thread and does not run in a separate process(unless you specify otherwise). ☆ 服務不能自己創建並且不能運行在單獨的進程中(除非你明確指定).
This means that, if your service is going to do any CPU intensive work ot blocking operations(such as MP3 playback or network). ☆ 這意味著如果你的服務要執行一些很耗CUP的工作或者阻塞的操作(比如播放mp3或網路操作),you should create a new thread within the service to do that work. ☆ 你應該在服務中創建一個新的線程來執行這些工作.
By using a separate thread, you will reduce the risk of Application Not Responding(ANR) errors and the application's main thread can remain dedicated to user interaction with your activities. ☆ 利用一個分離的進程,將減少你的activities發生應用程式停止響應(ANR)錯誤的風險.

如何創建一個Started服務

繼承service
1. publicclassFirstServiceextendsService{
2. privatestaticfinalString TAG ="--FirstService-->";
3. publicFirstService(){
4. Log.i(TAG,"Service is running.");
5. }
7. @Override
8. publicvoid onCreate(){
9. Log.i(TAG,"onCreate is running.");
10. super.onCreate();
11. }
13. @Override
14. publicint onStartCommand(Intent intent,int flags,int startId){
15. Log.i(TAG,"onStartCommand is running.");
16. returnsuper.onStartCommand(intent, flags, startId);
17. }
19. @Override
20. publicIBinder onBind(Intent intent){
21. Log.i(TAG,"IBinder is running.");
22. returnnull;
23. }
24. }
四大組件都需要在manifests.xml中註冊,這個也不例外.
如何啟動它
1. Intent intent =newIntent(ServiceActivity.this,FirstService.class);
2. startService(intent);
生命周期onCreate(), onStartCommand(), onDestory()就這三個生命周期
1. --FirstService-->:Service is running.
2. --FirstService-->: onCreate is running.
3. --FirstService-->: onStartCommand is running.
我們在onStartCommand方法中列印下當前線程
1. @Override
2. publicint onStartCommand(Intent intent,int flags,int startId){
3. Log.i(TAG,"onStartCommand is running.Thread:"+Thread.currentThread());
4. returnsuper.onStartCommand(intent, flags, startId);
5. }
列印結果如下:
1. onStartCommand is running.Thread:Thread[main,5,main]
驗證了兩點:①由於是第二次運行,構造方法和onStart都沒有列印,說明服務一旦啟動是默默運行在後臺的;②服務是運行在主線程的
如何結束服務:①調用stopService()方法 ,會回調service的onDestory()方法;
1. Intent intent2 =newIntent(ServiceActivity.this,FirstService.class);
2. stopService(intent2);
還可以在Android系統設置-->應用-->正在運行-->找到後是如下樣式,會告訴我們有一個服務在運行;
onStartCommand的返回值:

START_STICKY:粘性的,被意外中止後自動重啟,重新調用onStartCommand(),但會丟失原來激活它的Intent,會用一個null intent來調用onStartCommand(),可以用於播放器.值為1
START_NOT_STICKY:非粘性的,被意外中止後不會重啟,除非還存在未發送的Intent,這是避免服務運行的最安全選項; 值為2
START_REDELIVER_INTENT:粘性的且重新發送Intent,被意外中止後重新啟動,且該service組件將得到用於激活它的Intent對象,這中服務適用於需要立即恢復工作的活躍服務,比如下載文件; 值為3

onStartCommand的參數:
1. @Override //第一個參數:為我們傳入的intent;第二個flags:啟動服務的方式,與返回值有關;第三個為我們啟動service的次數.
2. publicint onStartCommand(Intent intent,int flags,int startId){
3. Log.i(TAG,"onStartCommand is running.Thread:"+Thread.currentThread());
4. Log.i(TAG,"flags:"+flags);
5. Log.i(TAG,"startId:"+startId);
6. returnsuper.onStartCommand(intent, flags, startId);
7. }
因為前面服務已經啟動了,這次我們連續點了三次啟動服務的按鈕,列印日誌如下:
1. 11-1602:56:28.3859039-9039/com.wanghx.androidstudy I/--FirstService-->: onStartCommand is running.Thread:Thread[main,5,main]
2. 11-1602:56:28.3859039-9039/com.wanghx.androidstudy I/--FirstService-->: flags:0
3. 11-1602:56:28.3859039-9039/com.wanghx.androidstudy I/--FirstService-->: startId:2
4. 11-1602:56:33.9859039-9039/com.wanghx.androidstudy I/--FirstService-->: onStartCommand is running.Thread:Thread[main,5,main]
5. 11-1602:56:33.9859039-9039/com.wanghx.androidstudy I/--FirstService-->: flags:0
6. 11-1602:56:33.9859039-9039/com.wanghx.androidstudy I/--FirstService-->: startId:3
7. 11-1602:56:35.5359039-9039/com.wanghx.androidstudy I/--FirstService-->: onStartCommand is running.Thread:Thread[main,5,main]
8. 11-1602:56:35.5359039-9039/com.wanghx.androidstudy I/--FirstService-->: flags:0
9. 11-1602:56:35.5359039-9039/com.wanghx.androidstudy I/--FirstService-->: startId:4
我們發現flags的值沒有發生改變,而startId再按順序增加.

如何啟動一個綁定服務

在activity中創建一個內部類,繼承ServiceConnection.
1. classMyServiceConnectionimplementsServiceConnection{
2. @Override
3. publicvoid onServiceConnected(ComponentName name,IBinder service){
4. Log.i(TAG,"onServiceConnected");
5. }
7. @Override
8. publicvoid onServiceDisconnected(ComponentName name){
9. Log.i(TAG,"onServiceDisconnected");
10. }
11. }
在activity中定義一個成員connection
1. privateMyServiceConnection connection =newMyServiceConnection();
在綁定服務按鈕中加入綁定代碼
1. Intent intent3 =newIntent(ServiceActivity.this,FirstService.class);
2. bindService(intent3, connection, BIND_AUTO_CREATE);
按鈕點擊後列印如下日誌:
1. I/--FirstService-->:Service is running.
2. I/--FirstService-->: onCreate is running.
3. I/--FirstService-->:IBinder is running.
在解綁服務中加入代碼,這裡的connection必須和上邊的綁定服務的connection實例一致.
1. unbindService(connection);

Service和Thread的關係

其實他兩個沒有一毛錢關係.只是因為service需要做耗時操作,需要重新建立一線程來處理工作,而不阻塞主線程;
service是運行在主線程的;
activity啟動service後,即使activity被銷毀了,如果沒有主動關閉服務,服務還是會在後臺默默運行的;

如何連接遠程的service,只需要在manifests.xml中這樣寫即可
1. <service
2. android:name="com.example.servicetest.MyService"
3. android:process=":remote">
4. </service>
如何讓activity與遠程的service進行通信呢?這就要使用AIDL進行跨進程通信(IPC)了.
AIDL:Android Interface Definition Language:Android介面定義語言,它可以用於讓多個service與多個應用程式組件之間進行跨進程通信;
這些都不是重點,我們還是弄一下在我們自己的程式中service與activity之間的通信吧;

activity-->service 通過intent傳遞數據給service;
activity調用onServiceConnected()中的IBind對象來訪問service中的方法;
IBinder通信的關鍵是利用activity中的IBinder對象獲得service對象,然後調用方法;
1. publicclassFirstServiceextendsService{
2. privatestaticfinalString TAG ="FirstService-->";
3. privateMyBinder myBinder =newMyBinder();
5. publicFirstService(){
6. Log.i(TAG,"Service is running.");
7. }
9. @Override
10. publicvoid onCreate(){
11. Log.i(TAG,"onCreate is running.");
12. super.onCreate();
13. }
15. @Override
16. publicint onStartCommand(Intent intent,int flags,int startId){
17. String name = intent.getStringExtra("name");
18. Log.i(TAG,"onStartCommand is running.Thread:"+Thread.currentThread());
19. Log.i(TAG,"flags:"+flags);
20. Log.i(TAG,"startId:"+startId);
21. Log.i(TAG,"name:"+name);
22. return START_STICKY;
23. }
25. @Override
26. publicvoid onDestroy(){
27. Log.i(TAG,"onDestroy is running.");
28. super.onDestroy();
29. }
31. @Override
32. publicIBinder onBind(Intent intent){
33. Log.i(TAG,"IBinder is running.");
34. return myBinder;
35. }
37. publicclassMyBinderextendsBinder{
38. publicFirstService getService(){
39. returnFirstService.this;
40. }
41. }
43. publicint getRandomNumber(){
44. returnnewRandom().nextInt(10)+1;
45. }
46. }
1. publicclassServiceActivityextendsAppCompatActivityimplementsView.OnClickListener{
2. privateMyServiceConnection connection =newMyServiceConnection();
3. privatestaticfinalString TAG ="ServiceActivity-->";
4. privateFirstService mFirstService;
5. privateboolean isBinder;// 服務是否綁定
7. @Override
8. protectedvoid onCreate(Bundle savedInstanceState){
9. super.onCreate(savedInstanceState);
10. setContentView(R.layout.activity_service);
11. findViewById(R.id.btn_start_service).setOnClickListener(this);
12. findViewById(R.id.btn_stop_service).setOnClickListener(this);
13. findViewById(R.id.btn_bound_service).setOnClickListener(this);
14. findViewById(R.id.btn_unbound_service).setOnClickListener(this);
15. findViewById(R.id.btn_get_number).setOnClickListener(this);
16. }
18. @Override
19. publicvoid onClick(View v){
20. switch(v.getId()){
21. case R.id.btn_start_service:
22. Intent intent =newIntent(ServiceActivity.this,FirstService.class);
23. intent.putExtra("name","Zhangsan");
24. startService(intent);
25. break;
26. case R.id.btn_stop_service:
27. Intent intent2 =newIntent(ServiceActivity.this,FirstService.class);
28. stopService(intent2);
29. break;
30. case R.id.btn_bound_service:
31. if(!isBinder){
32. Intent intent3 =newIntent(ServiceActivity.this,FirstService.class);
33. bindService(intent3, connection, BIND_AUTO_CREATE);
34. isBinder =true;
35. }
36. break;
37. case R.id.btn_unbound_service:
38. if(isBinder){
39. unbindService(connection);
40. isBinder =false;
41. }
42. break;
43. case R.id.btn_get_number:
44. if(mFirstService ==null){
45. Toast.makeText(getApplicationContext(),"請先綁定服務",Toast.LENGTH_SHORT).show();
46. }else{
47. Toast.makeText(getApplicationContext(),"得到的隨機數為:"+ mFirstService.getRandomNumber(),Toast
48. .LENGTH_SHORT).show();
49. }
50. break;
51. }
52. }
54. classMyServiceConnectionimplementsServiceConnection{
55. @Override
56. publicvoid onServiceConnected(ComponentName name,IBinder service){
57. Log.i(TAG,"onServiceConnected");
58. FirstService.MyBinder myBinder =(FirstService.MyBinder) service;
59. mFirstService = myBinder.getService();
60. }
62. @Override
63. publicvoid onServiceDisconnected(ComponentName name){
64. Log.i(TAG,"onServiceDisconnected");
65. }
66. }
67. }
這裡有一個小插曲:一起寫出來大家分享下:記得以前學java基礎時,老師曾說過一個java文件中只能有一個public類,類名稱必須與java文件名相同,為什麼這個FirstService中有兩個public類,只是因為MyBinder雖然是一個public class,但是MyBinder是一個內部類,這裡必須要用public修飾,否則其他包就訪問不到這個內部類了;

來自為知筆記(Wiz)