当我们保存图片后就会发个通知告诉系统让sdcard重新挂载，这样其他程序就会立即找到这张图片。

            Intent intent = new Intent();
            intent.setAction(Intent.ACTION_MEDIA_MOUNTED);
            intent.setData(Uri.fromFile(Environment
                    .getExternalStorageDirectory()));
            sendBroadcast(intent);

但是到了Android4.4就不灵了，Google将MEDIA_MOUNTED的权限提高了，于是就报了一个下面的错误。

W/System.err﹕java.lang.SecurityException: Permission Denial: not allowed to send broadcast android.intent.action.MEDIA_MOUNTED from pid=18338, uid=10087

在stackoverfollow中找到了一个办法，在高版本中使用ACTION_MEDIA_SCANNER_SCAN_FILE去通知系统重新扫描文件。代码如下：

                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
                    Intent mediaScanIntent = new Intent(
                            Intent.ACTION_MEDIA_SCANNER_SCAN_FILE);
                    Uri contentUri = Uri.fromFile(mPhotoFile); //out is your output file
                    mediaScanIntent.setData(contentUri);
                    CameraActivity.this.sendBroadcast(mediaScanIntent);
                } else {
                    sendBroadcast(new Intent(
                            Intent.ACTION_MEDIA_MOUNTED,
                            Uri.parse("file://"
                                    + Environment.getExternalStorageDirectory())));
                }
            } catch (FileNotFoundException e) {
                Log.d(TAG, "File not found: " + e.getMessage());
            } catch (IOException e) {
                Log.d(TAG, "Error accessing file: " + e.getMessage());
            }