Android开发——MediaProvider源码分析（2）

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在上一篇文章中说到系统当接收到扫描请求广播的时候就会调用scan或者scanFile去扫描手机（手机内存和sdcard）中的媒体文件。这两个方法都是启动MediaScannerService这个服务来完成扫描任务的。接下来我们来看看MediaScannerService是怎么工作的……

4.MediaScannerService（MSS）

MSS实现了Runnable，所以必然的需要实现run方法了，代码如下：

1: public void run()

2:    {

3:        // reduce priority below other background threads to avoid interfering

4:        // with other services at boot time.

5:        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND +

6:                Process.THREAD_PRIORITY_LESS_F***ORABLE);

7:        Looper.prepare();

8:

9:        mServiceLooper = Looper.myLooper();

10:        mServiceHandler = new ServiceHandler();

11:

12:        Looper.loop();

13:    }

在run方法中设置了线程的优先级，优先级比较低，主要为了避免跟其他服务抢夺资源。还有就是利用looper对ServiceHandler的消息进行循环控制。
接着看一下ServiceHandler的实现代码：

1: private final class ServiceHandler extends Handler

2:    {

3:        @Override

4:        public void handleMessage(Message msg)

5:        {

6:            Bundle arguments = (Bundle) msg.obj;

7:            //获取文件路径

8:          String filePath = arguments.getString("filepath");

9:

10:            try {

11:              if (filePath != null) {

12:                     //文件路径不为空，则调用扫面当个文件的方法

13:                    IBinder binder = arguments.getIBinder("listener");

14:                    IMediaScannerListener listener =

15:                            (binder == null ? null : IMediaScannerListener.Stub.asInterface(binder));

16:                    Uri uri = scanFile(filePath, arguments.getString("mimetype"));//扫描单个文件

17:                    if (listener != null) {

18:                         //执行扫描完成方法

19:                        listener.scanCompleted(filePath, uri);

20:                    }

21:                } else {

22:                     //如果文件路径为空，则获取扫面手机内存或者sdcard

23:                    String volume = arguments.getString("volume");

24:                    String[] directories = null;

25:                    //内置卡

26:                    if (MediaProvider.INTERNAL_VOLUME.equals(volume)) {

27:                        // scan internal media storage

28:                        directories = new String[] {

29:                                Environment.getRootDirectory() + "/media",

30:                        };

31:                    }//外置卡

32:                    else if (MediaProvider.EXTERNAL_VOLUME.equals(volume)) {

33:                        // scan external storage

34:                        directories = new String[] {

35:                                Environment.getExternalStorageDirectory().getPath(),

36:                                };

37:                    }

38:

39:                    if (directories != null) {

40:                        if (Config.LOGD) Log.d(TAG, "start scanning volume " + volume);

41:                         //扫描

42:                        scan(directories, volume);

43:                        if (Config.LOGD) Log.d(TAG, "done scanning volume " + volume);

44:                    }

45:                }

46:            } catch (Exception e) {

47:                Log.e(TAG, "Exception in handleMessage", e);

48:            }

49:

50:            stopSelf(msg.arg1);

51:        }

52:    };

在ServiceHandler中主要根据相关参数来调用不同的扫描方法。


那是在哪里调用ServiceHandler发送消息的呢？请看如下代码：

1: @Override

2: public void onCreate() {

3:     PowerManager pm = (PowerManager) getSystemService(Context.POWER_SERVICE);

4:     mWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, TAG);

5:     //启用新线程，这样就可以避免阻塞，执行run，初始化成员变量loop和handler

6:     Thread thr = new Thread(null, this, "MediaScannerService");

7:     thr.start();

8: }

9:

10: @Override

11: public int onStartCommand(Intent intent, int flags, int startId) {

12:     while (mServiceHandler == null) {

13:         synchronized (this) {

14:             try {

15:                 wait(100);

16:             } catch (InterruptedException e) {

17:             }

18:         }

19:     }

20:

21:     if (intent == null) {

22:         Log.e(TAG, "Intent is null in onStartCommand: ", new NullPointerException());

23:         return Service.START_NOT_STICKY;

24:     }

25:

26:     Message msg = mServiceHandler.obtainMessage();

27:     msg.arg1 = startId;

28:     msg.obj = intent.getExtras();

29:     //ServiceHandler发送消息

30:     mServiceHandler.sendMessage(msg);

31:

32:     // Try again later if we are killed before we can finish scanning.

33:     return Service.START_REDELIVER_INTENT;

34: }

35:

36: @Override

37: public void onDestroy() {

38:     // Make sure thread has started before telling it to quit.

39:     while (mServiceLooper == null) {

40:         synchronized (this) {

41:             try {

42:                 wait(100);

43:             } catch (InterruptedException e) {

44:             }

45:         }

46:     }

47:     mServiceLooper.quit();

48: }

以上三个方法是属于Service的生命周期的。当我们调用startService的时候，如果对应的Service还未创建就会调用onCreate方法===方法。每次startService的时候就调用onStartCommand，所以ServiceHandler就在此发送消息了。
最后，稍微看一下MSS里面扫描方面。主要是调用MediaScanner对媒体文件进行扫描分析的。至于MediaScanner的实现以后在分析。

1: private void openDatabase(String volumeName) {

2:        try {

3:            ContentValues values = new ContentValues();

4:            values.put("name", volumeName);

5:            getContentResolver().insert(Uri.parse("content://media/"), values);

6:        } catch (IllegalArgumentException ex) {

7:            Log.w(TAG, "failed to open media database");

8:      }         

9:  }

10:

11:  private void closeDatabase(String volumeName) {

12:        try {

13:            getContentResolver().delete(

14:                    Uri.parse("content://media/" + volumeName), null, null);

15:        } catch (Exception e) {

16:            Log.w(TAG, "failed to close media database " + volumeName + " exception: " + e);

17:        }

18:    }

19: //创建扫描器

20:  private MediaScanner createMediaScanner() {

21:        MediaScanner scanner = new MediaScanner(this);

22:        Locale locale = getResources().getConfiguration().locale;

23:        if (locale != null) {

24:            String language = locale.getLanguage();

25:          String country = locale.getCountry();

26:            String localeString = null;

27:            if (language != null) {

28:                if (country != null) {

29:                    scanner.setLocale(language + "_" + country);

30:                } else {

31:                  scanner.setLocale(language);

32:                }

33:            }

34:        }

35:      

36:        return scanner;

37:    }

38: //扫描目录

39:    private void scan(String[] directories, String volumeName) {

40:        // don't sleep while scanning

41:        mWakeLock.acquire();

42:

43:        ContentValues values = new ContentValues();

44:        values.put(MediaStore.MEDIA_SCANNER_VOLUME, volumeName);

45:        Uri scanUri = getContentResolver().insert(MediaStore.getMediaScannerUri(), values);

46:

47:        Uri uri = Uri.parse("file://" + directories[0]);

48:        sendBroadcast(new Intent(Intent.ACTION_MEDIA_SCANNER_STARTED, uri));

49:      

50:        try {

51:            if (volumeName.equals(MediaProvider.EXTERNAL_VOLUME)) {

52:                 openDatabase(volumeName);

53:            }

54:

55:            MediaScanner scanner = createMediaScanner();

56:            scanner.scanDirectories(directories, volumeName);

57:        } catch (Exception e) {

58:            Log.e(TAG, "exception in MediaScanner.scan()", e);

59:        }

60:

61:        getContentResolver().delete(scanUri, null, null);

62:

63:        sendBroadcast(new Intent(Intent.ACTION_MEDIA_SCANNER_FINISHED, uri));

64:        mWakeLock.release();

65:    }

66: //扫描文件

67: private Uri scanFile(String path, String mimeType) {

68:         String volumeName = MediaProvider.INTERNAL_VOLUME;

69:         String externalStoragePath = Environment.getExternalStorageDirectory().getPath();

70:

71:         if (path.startsWith(externalStoragePath)) {

72:             volumeName = MediaProvider.EXTERNAL_VOLUME;

73:             openDatabase(volumeName);

74:         }

75:         MediaScanner scanner = createMediaScanner();

76:         //扫描单个文件

77:         return scanner.scanSingleFile(path, volumeName, mimeType);

78:     }

在MediaProvider中还有一个类：MediaThumbRequest，用来创建预览图的，比如视频的预览图，图片的预览图，音频的专辑图片…这些图片的信息也是保存在数据库的，有兴趣的同学可以自己打开数据库看看里面的表。如下图：





啰哩啰唆的写了两篇文章，希望对大家有所帮助。

其中应该有不少是错误的观点，望大家指正。

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