手动通知扫描SD卡主动生成缩略图

最近做项目遇到的难题，调用系统拍照获取不到缩略图，非得关机重启才会生成，所以我们要主动通知系统扫描SD卡生成缩略图，

在Android4.4之前也就是以发送一个Action为“Intent.ACTION_MEDIA_MOUNTED”的广播通知执行扫描。如下：

this.sendBroadcast(new Intent(Intent.ACTION_MEDIA_MOUNTED, Uri.parse("file://" + Environment.getExternalStorageDirectory())));

但在Android4.4中，则会抛出以下异常：

W/ActivityManager(  498): Permission Denial: not allowed to send broadcast android.intent.action.MEDIA_MOUNTED from pid=2269, uid=20016 
那是因为Android4.4中限制了系统应用才有权限使用广播通知系统扫描SD卡。 
解决方式： 
使用MediaScannerConnection执行具体文件或文件夹进行扫描。

 Intent mediaScanIntent = new Intent(
                     Intent.ACTION_MEDIA_SCANNER_SCAN_FILE);
             String path = "file://"
                             + out.getPath();
             Uri contentUri = Uri.parse(path); //out is your output file
             mediaScanIntent.setData(contentUri);
             this.sendBroadcast(mediaScanIntent);

MediaScannerReceiver 源码中的广播接收的部分代码：

  @Override
     public void onReceive(Context context, Intent intent) {
         String action = intent.getAction();
         Uri uri = intent.getData();
         String externalStoragePath = Environment.getExternalStorageDirectory().getPath();  

         if (action.equals(Intent.ACTION_BOOT_COMPLETED)) {
             // scan internal storage
             scan(context, MediaProvider.INTERNAL_VOLUME);
         } else {
             if (uri.getScheme().equals("file")) {
                 // handle intents related to external storage
                 String path = uri.getPath();
                 if (action.equals(Intent.ACTION_MEDIA_MOUNTED) &&
                         externalStoragePath.equals(path)) {
                     scan(context, MediaProvider.EXTERNAL_VOLUME);
                 } else if (action.equals(Intent.ACTION_MEDIA_SCANNER_SCAN_FILE) &&
                         path != null && path.startsWith(externalStoragePath + "/")) {
                     scanFile(context, path);
                 }
             }
         }
     }  

所以最后的代码是：

 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
             Intent mediaScanIntent = new Intent(
                     Intent.ACTION_MEDIA_SCANNER_SCAN_FILE);
             String path = "file://"
                             + out.getPath();
             Uri contentUri = Uri.parse(path); //out is your output file
             mediaScanIntent.setData(contentUri);
             this.sendBroadcast(mediaScanIntent);         } else {
             sendBroadcast(new Intent(
                     Intent.ACTION_MEDIA_MOUNTED,
                     Uri.parse("file://"
                             + Environment.getExternalStorageDirectory())));
         }