为了解决Android媒体数据库更新不及时的问题，我想打算通过FileObserver监听SD卡根目录下所有文件的变化，然后根据文件的变化对Android媒体数据库进行更新。而FileObserver无法做到递归监听。通过参考FileObserver 研究及其递归监听初步实现这篇博客，在其基础上，主要解决了几个问题：
无法监听目录的创建、删除开启监听之后，新创建的目录无法监听冗余注册监听的问题冗余注册监听我是通过用ArrayMap键值对来解决的，以监听目录的绝对路径作为key，以监听器作为value。（也可以使用ArraySet集合实现）而开启监听之后，新创建的目录无法监听的问题则是通过监听FileObserver.CREATE事件，当创建新目录且该目录还没注册监听时，就注册并启动监听。 接下来，干货来了，代码如下：
public class RecursiveFileObserver extends FileObserver { Map<String, SingleFileObserver> mObservers; String mPath; int mMask; public RecursiveFileObserver(String path) { this(path, ALL_EVENTS); } public RecursiveFileObserver(String path, int mask) { super(path, mask); mPath = path; mMask = mask; } @Override public void startWatching() { if (mObservers != null) return ; mObservers = new ArrayMap<>(); Stack stack = new Stack(); stack.push(mPath); while (!stack.isEmpty()) { String temp = (String) stack.pop(); mObservers.put(temp, new SingleFileObserver(temp, mMask)); File path = new File(temp); File[] files = path.listFiles(); if (null == files) continue; for (File f: files) { // 递归监听目录 if (f.isDirectory() && !f.getName().equals(".") && !f.getName() .equals("..")) { stack.push(f.getAbsolutePath()); } } } Iterator<String> iterator = mObservers.keySet().iterator(); while (iterator.hasNext()) { String key = iterator.next(); mObservers.get(key).startWatching(); } } @Override public void stopWatching() { if (mObservers == null) return ; Iterator<String> iterator = mObservers.keySet().iterator(); while (iterator.hasNext()) { String key = iterator.next(); mObservers.get(key).stopWatching(); } mObservers.clear(); mObservers = null; } @Override public void onEvent(int event, String path) { int el = event & FileObserver.ALL_EVENTS; switch (el) { case FileObserver.ATTRIB: Log.i("RecursiveFileObserver", "ATTRIB: " + path); break; case FileObserver.CREATE: File file = new File(path); if(file.isDirectory()) { Stack stack = new Stack(); stack.push(path); while (!stack.isEmpty()) { String temp = (String) stack.pop(); if(mObservers.containsKey(temp)) { continue; } else { SingleFileObserver sfo = new SingleFileObserver(temp, mMask); sfo.startWatching(); mObservers.put(temp, sfo); } File tempPath = new File(temp); File[] files = tempPath.listFiles(); if (null == files) continue; for (File f: files) { // 递归监听目录 if (f.isDirectory() && !f.getName().equals(".") && !f.getName() .equals("..")) { stack.push(f.getAbsolutePath()); } } } } Log.i("RecursiveFileObserver", "CREATE: " + path); break; case FileObserver.DELETE: Log.i("RecursiveFileObserver", "DELETE: " + path); break; case FileObserver.DELETE_SELF: Log.i("RecursiveFileObserver", "DELETE_SELF: " + path); break; case FileObserver.MODIFY: Log.i("RecursiveFileObserver", "MODIFY: " + path); break; case FileObserver.MOVE_SELF: Log.i("RecursiveFileObserver", "MOVE_SELF: " + path); break; case FileObserver.MOVED_FROM: Log.i("RecursiveFileObserver", "MOVED_FROM: " + path); break; case FileObserver.MOVED_TO: Log.i("RecursiveFileObserver", "MOVED_TO: " + path); break; } } class SingleFileObserver extends FileObserver { String mPath; public SingleFileObserver(String path) { this(path, ALL_EVENTS); mPath = path; } public SingleFileObserver(String path, int mask) { super(path, mask); mPath = path; } @Override public void onEvent(int event, String path) { if(path != null) { String newPath = mPath + "/" + path; RecursiveFileObserver.this.onEvent(event, newPath); } } } }这算是一个比较好的递归监听解决方案，当然还可以进行优化，比如监听目录的删除事件，并取消该目录的监听。最后，吐槽一下，博客就是个人感想和收获的总结，到处抄来抄去没什么意思。看完别人的博客之后，我们应该写出自己的见解，提出更好的解决方案，而不是收藏、转载和抄袭。最后，借鉴别人的东西，请注明出处。
