Android(安卓)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);