Android MediaScanner源代码解析

1. 简介

MediaScanner向上和MediaProvider.MediaScannerService交互, 响应其请求;
向下和JNI交互.
MediaScanner主要工作内容:
（1）接收MediaScannerService的scanDirectories和scanSingleFile请求
（2）获取各个ContentProvider 环境（Audio，Video，Image，File） initialize
（3）获取需要扫描文件列表，移除已删除文件信息 prescan
（4）通过media_jni(android_media_MediaScanner.cpp)调用底层接口处理扫描
（5）接收media_jni处理结果，在android_media_MediaScanner.cpp中保存一个MyMediaScannerClient用于调用MediaScanner
（6）保存获取的文件信息 postscan
代码位置:
frameworks/base/media/java/android/media/MediaScanner.java

2. scanDirectories

scanDirectories(String[] directories, String volumeName)
扫描文件夹，被MSS调用
MediaScanner处理文件夹的具体流程是：
在MediaScanner中
核心代码如下：

    public void scanDirectories(String[] directories, String volumeName) {
            ......
            initialize(volumeName);
            prescan(null, true);
            ......
            for (int i = 0; i < directories.length; i++) {
                processDirectory(directories[i], mClient);
            }
            ......
            postscan(directories);
             ......
            releaseResources();
        }
    }

3. initialize

void initialize(String volumeName)
initialize和releaseResources相对应
功能就是初始化各个ContentProvider （Audio，Video，Image，File）

    private void initialize(String volumeName) {
        mMediaProvider = mContext.getContentResolver().acquireProvider("media");

        mAudioUri = Audio.Media.getContentUri(volumeName);
        mVideoUri = Video.Media.getContentUri(volumeName);
        mImagesUri = Images.Media.getContentUri(volumeName);
        mThumbsUri = Images.Thumbnails.getContentUri(volumeName);
        mFilesUri = Files.getContentUri(volumeName);
        mFilesUriNoNotify = mFilesUri.buildUpon().appendQueryParameter("nonotify", "1").build();

        if (!volumeName.equals("internal")) {
            // we only support playlists on external media
            mProcessPlaylists = true;
            mProcessGenres = true;
            mPlaylistsUri = Playlists.getContentUri(volumeName);

            mCaseInsensitivePaths = true;
        }
    }

4. prescan

void prescan(String filePath, boolean prescanFiles)
获取需要扫描文件列表，移除已删除文件信息

5. postscan

void postscan(String[] directories)
把获取的文件信息写回数据库
其主要工作在
void processPlayLists()
和
void processPlayList(FileEntry entry, Cursor fileList)
完成

5.1 processPlayLists()

    private void processPlayLists() throws RemoteException {
        Iterator<FileEntry> iterator = mPlayLists.iterator();
        Cursor fileList = null;
        try {
            // use the files uri and projection because we need the format column,
            // but restrict the query to just audio files
            fileList = mMediaProvider.query(mPackageName, mFilesUri, FILES_PRESCAN_PROJECTION,
                    "media_type=2", null, null, null);
            while (iterator.hasNext()) {
                FileEntry entry = iterator.next();
                // only process playlist files if they are new or have been modified since the last scan
                if (entry.mLastModifiedChanged) {
                    processPlayList(entry, fileList);
                }
            }
        }
    }

5.2 processPlayList(FileEntry entry, Cursor fileList)
processPlayList(FileEntry entry, Cursor fileList)的工作是把信息写回数据库

6. processDirectory

调用native层

private native void processDirectory(String path, MediaScannerClient client);