tracking file renaming/deleting with FSEvents on Lion

Question

I\'m trying to use FSEvents to detect when files were added/removed from a specific folder. For the moment, I implemented a simple wrapper around FSEvents, and it works fine

Answer 1

You are actually raising two issues related to FSEvents and renames. 1. A file is renamed and both the old and new file names are within the directory trees being monitored. 2. A file is renamed and one of the names is not in the directory trees being monitored.

You have solved (almost) the first issue. It is also necessary to provide your application with a means of knowing which events are being reported in the same FSEvent group of events. Your method of knowing that two renames are reported consecutively only works if they are within the same group of events being reported within the same latency period. If two rename events of type 2 occur one after another but are not within the same group of events being reported in the same latency group, they actually have nothing to do with each other - and you will mistakenly think one file has be renamed to another.

It is possible to handle the second type of rename by simply monitoring every directory in the system using the root, but this will flood you with many unnecessary events. You can determine if a "partial" rename is the result of a file being moved out of the directory tree being monitored or into the directory tree being monitored by doing a stat() on the file. If stat() fails with an errno of 2, then the file has been moved outside the directory being monitored, and it can be treated as if it has been deleted. If stat() succeeds, the event can be treated as if the file has been created.

Answer 2

Well, I didn't find the perfect answer to my question, but I found a solution which I eventually was really satisfied with, so I thought I might share ^^

As I said, when moving stuff to the trash, if you're only watching 1 folder, you won't catch the event generated when the image is put in the trash. So, I decided to do the following : I have a class which creates a stream on the root folder ("/") so that it will catch all the events -> this solves the problem of files being sent to the trash, and all such stuff. Then, this class allow to register delegates on certain pathes. So, instead of creating many streams, I create one big stream, then filter events as needed, and I create many delegates.

So all I have to do now when I want to watch events on a special folder is the following :

[[FSEventsListener instance] addListener:self forPath:somePath];

I just have to create an instance of FSEventListener at application start, and release it when the app stops. And I just need to implement the following 3 methods which will be automatically called :

-(void)fileWasAdded:(NSString *)file;
-(void)fileWasRemoved:(NSString *)file;
-(void)fileWasRenamed:(NSString *)oldFile to:(NSString *)newFile;

If you're interested in the source code of this little utility, you can check here : http://blog.pcitron.fr/tools/macosx-imageviewer/ (the utility was added at the version 0.8)

I developed it as part of a a little image viewer to keep the UI synchronized with the disk content (it displays the number of images contained in each directories, etc.) The source code is available, and the utility is in Utils/FSEventsListener.h/.m.

And if by any chance someone actually downloads the application and take a look at the sources, if you find anything usefull (performance / feature improvement, whatever) feel free to drop a comment / mail ^^