Android開發——MediaProvider原始碼分析（2）

lostinai發表於2013-07-30
AndroidIDE原始碼

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在上一篇文章中說到系統當接收到掃描請求廣播的時候就會呼叫scan或者scanFile去掃描手機（手機記憶體和sdcard）中的媒體檔案。這兩個方法都是啟動MediaScannerService這個服務來完成掃描任務的。接下來我們來看看MediaScannerService是怎麼工作的……

4.MediaScannerService（MSS）

MSS實現了Runnable，所以必然的需要實現run方法了，程式碼如下：

   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_FAVORABLE);
   7:        Looper.prepare();
   8:  
   9:        mServiceLooper = Looper.myLooper();
  10:        mServiceHandler = new ServiceHandler();
  11:  
  12:        Looper.loop();
  13:    }
在run方法中設定了執行緒的優先順序，優先順序比較低，主要為了避免跟其他服務搶奪資源。還有就是利用looper對ServiceHandler的訊息進行迴圈控制。

接著看一下ServiceHandler的實現程式碼：

   1: private final class ServiceHandler extends Handler
   2:    {
   3:        @Override
   4:        public void handleMessage(Message msg)
   5:        {
   6:            Bundle arguments = (Bundle) msg.obj;
   7:            //獲取檔案路徑
   8:            String filePath = arguments.getString("filepath");
   9:            
  10:            try {
  11:                if (filePath != null) {
  12:                     //檔案路徑不為空，則呼叫掃面當個檔案的方法
  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"));//掃描單個檔案
  17:                    if (listener != null) {
  18:                         //執行掃描完成方法
  19:                        listener.scanCompleted(filePath, uri);
  20:                    }
  21:                } else {
  22:                     //如果檔案路徑為空，則獲取掃面手機記憶體或者sdcard
  23:                    String volume = arguments.getString("volume");
  24:                    String[] directories = null;
  25:                    //內建卡
  26:                    if (MediaProvider.INTERNAL_VOLUME.equals(volume)) {
  27:                        // scan internal media storage
  28:                        directories = new String[] {
  29:                                Environment.getRootDirectory() + "/media",
  30:                        };
  31:                    }//外接卡
  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:                         //掃描
  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中主要根據相關引數來呼叫不同的掃描方法。大笑

那是在哪裡呼叫ServiceHandler傳送訊息的呢？請看如下程式碼：

   1: @Override
   2: public void onCreate() {
   3:     PowerManager pm = (PowerManager) getSystemService(Context.POWER_SERVICE);
   4:     mWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, TAG);
   5:     //啟用新執行緒，這樣就可以避免阻塞，執行run，初始化成員變數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傳送訊息
  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: }
以上三個方法是屬於Service的生命週期的。當我們呼叫startService的時候，如果對應的Service還未建立就會呼叫onCreate方法===方法。每次startService的時候就呼叫onStartCommand，所以ServiceHandler就在此傳送訊息了。

最後，稍微看一下MSS裡面掃描方面。主要是呼叫MediaScanner對媒體檔案進行掃描分析的。至於MediaScanner的實現以後在分析。

   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: //建立掃描器
  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: //掃描目錄
  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: //掃描檔案
  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:         //掃描單個檔案
  77:         return scanner.scanSingleFile(path, volumeName, mimeType);
  78:     }

在MediaProvider中還有一個類：MediaThumbRequest，用來建立預覽圖的，比如視訊的預覽圖，圖片的預覽圖，音訊的專輯圖片…這些圖片的資訊也是儲存在資料庫的，有興趣的同學可以自己開啟資料庫看看裡面的表。如下圖：

media_db_tables

囉哩囉唆的寫了兩篇文章，希望對大家有所幫助。

其中應該有不少是錯誤的觀點，望大家指正。

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