D-LAN

h1. Watcher prototype

The cache file structure store all the hashes of the shared files. If a file is added or removed from a shared directory (or a subfolder) it should be added or removed from the cache.

As far as I know there is no way to watch changes recursively of a folder tree with Qt. The Qt class _QFileSystemWatcher_ cannot watch recursively.

The only way to synchronize is to periodically scan all the shared data and compare to the cached one. It can be a bit CPU consumer so it should be done with a low priority thread and maybe with a pause between each file hashing.

h2. Native solution

A better solution would be to use native platform API.

h3. Linux : inotify

On Linux it exists a function called "inotify":http://linux.die.net/man/7/inotify

* The big issue is that inotify doesn't support the recursive watch of a directory.

* There is a limitation of the number of watcher, see @/proc/sys/fs/inotify/max_user_watches@, for example : 8192 under Debian.

* There is no problem to create thousands of watcher, it's very light. 

h3. Windows : ReadDirectoryChangesW and WaitForMultiplesObjects

_ReadDirectoryChangesW_ can watch recursively and asynchronously any change from a given directory. See : http://msdn.microsoft.com/en-us/library/aa365465%28VS.85%29.aspx

Then, a thread can wait an event by calling _WaitForMultiplesObjects_, see : http://msdn.microsoft.com/en-us/library/aa365261%28VS.85%29.aspx

h4. Events when copying :

Beginning of the copy :

<pre>

waitEvent

Action =  1 (FILE_ACTION_ADDED)

filename =  "ACDC - Back In Black.mpg"

waitEvent

Action =  3 (FILE_ACTION_MODIFIED)

filename =  "ACDC - Back In Black.mpg"

</pre>

End of the copy :

<pre>

waitEvent

Action =  3 (FILE_ACTION_MODIFIED)

filename =  "ACDC - Back In Black.mpg"

<pre>