Android MediaScanner 扫描流程

android MediaScanner 扫描调用的时序图：

android MediaScanner 代码具体分析：

引言：

 * The way the scan currently works is:
 * - The Java MediaScannerService creates a MediaScanner (this class), and calls
 *   MediaScanner.scanDirectories on it.
 * - scanDirectories() calls the native processDirectory() for each of the specified directories.
 * - the processDirectory() JNI method wraps the provided mediascanner client in a native
 *   'MyMediaScannerClient' class, then calls processDirectory() on the native MediaScanner
 *   object (which got created when the Java MediaScanner was created).
 * - native MediaScanner.processDirectory() calls
 *   doProcessDirectory(), which recurses over the folder, and calls
 *   native MyMediaScannerClient.scanFile() for every file whose extension matches.
 * - native MyMediaScannerClient.scanFile() calls back on Java MediaScannerClient.scanFile,
 *   which calls doScanFile, which after some setup calls back down to native code, calling
 *   MediaScanner.processFile().
 * - MediaScanner.processFile() calls one of several methods, depending on the type of the
 *   file: parseMP3, parseMP4, parseMidi, parseOgg or parseWMA.
 * - each of these methods gets metadata key/value pairs from the file, and repeatedly
 *   calls native MyMediaScannerClient.handleStringTag, which calls back up to its Java
 *   counterparts in this file.
 * - Java handleStringTag() gathers the key/value pairs that it's interested in.
 * - once processFile returns and we're back in Java code in doScanFile(), it calls
 *   Java MyMediaScannerClient.endFile(), which takes all the data that's been
 *   gathered and inserts an entry in to the database.

从上面可以看出media scanner流程的设计逻辑，下面有几点需要注意：

1，接收mount 广播

mediaScannerReceiver 接收外部设备广播后，通过mediaScannerService每次扫描外部的都从SD卡目录扫描，然后递归到U盘，若加快扫描速度，可以考虑只扫描U盘目录或者固定目录。

2，预扫描 prescan

该函数的作用就是在扫描之前把数据库中的信息提取并保存（包括媒体文件的路径，Metadata,所属表的URI）,这个主要是为了扫描后更好的更新数据库。

3，processDirectory媒体扫描

该函数是耗时最长的，主要分两种，如果是目录的话，就递归调用，如果是文件的话就

a,先调用beginFile,该函数用来保存到数据库之前的缓存中，还用来判断当前文件是否被修改过，若修改过，继续调用扫描。

b,再调用processFile,该函数通过调用mediaRetriver获取ID3信息

c，最好调用endfile 讲扫描到的信息插入到数据库中