Android开发——MediaProvider源码分析（二）

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_FAVORABLE);

   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。

----------------------END------------------------------