Reactive Extensions is ideally suited to the task of monitoring a directory for changes. While the events from FileSystemWatcher are ok, it isn’t efficient. The goal is to send as little data as possible to the server. If a file is moved or copied, there’s no need to upload the file again. Instead, you should recognize the event on the client and simply send a Moved or Copied message pointing to the original file on the server.

Here’s a simple prototype. I left out all the code to track the files with hashes. It gets a little trickier trying to track directories. If a directory is deleted, the entire subtree is deleted. You can use SQLite with the closure.c extension to track hierarchical data.

Anyway, this proof-of-concept is easy. Cut and paste lost some formatting. Stupid tabs.

    public class DropboxClient
    {
        private readonly FileSystemWatcher watcher;

        public IObservable<DropboxEventArg> FileSystemObservable { get; private set; } 

        public DropboxClient(string home)
        {
            watcher = new FileSystemWatcher
            {
                Path = home,
                EnableRaisingEvents = true,
                IncludeSubdirectories = true
            };

            SetupRx();
        }

        private void SetupRx()
        {
            var changed = Observable.FromEventPattern<FileSystemEventArgs>(watcher, "Changed");
            var created = Observable.FromEventPattern<FileSystemEventArgs>(watcher, "Created");
            var deleted = Observable.FromEventPattern<FileSystemEventArgs>(watcher, "Deleted");
            var renamed = Observable.FromEventPattern<FileSystemEventArgs>(watcher, "Renamed");

            // Often it repeats a change event for every little change (timestamp, file size, etc). 
            var dbchanged = changed
                .DistinctUntilChanged()
                //.Do(UpdateRecord)
                .Select(fe => new DropboxEventArg(fe.EventArgs));

            // This seems to work fine, I think
            var dbrenamed = renamed.Select(fe => new DropboxEventArg(fe.EventArgs));

            // Deleted is ok, too
            var dbdeleted = deleted
                //.Do(DeleteRecord)
                .Select(fe => new DropboxEventArg(fe.EventArgs));

            // If file already exists, then a created file is a copy of another file
            var dbcreated = created
                .Select(fe =>
                {
                    if (FileExists(fe.EventArgs.FullPath))
                        return new DropboxEventArg(fe.EventArgs, DropboxChangeTypes.Copied);
                    else
                    {
                        //CreateRecord(fe);
                        return new DropboxEventArg(fe.EventArgs);
                    }
                });

            FileSystemObservable = dbchanged.Merge(dbrenamed).Merge(dbdeleted).Merge(dbcreated);
        }

        private void CreateRecord(EventPattern<FileSystemEventArgs> fe) {
            // Create row in repo
            throw new NotImplementedException();
        }

        private void UpdateRecord(EventPattern<FileSystemEventArgs> obj)
        {
            // If file size is different, rehash and update
            // If dir, maybe do nothing. Not sure.
            throw new NotImplementedException();
        }

        private void DeleteRecord(EventPattern<FileSystemEventArgs> obj) {
            // Delete file from repository
            // If directory, delete entire subtree from repo
            throw new NotImplementedException();
        }

        private bool FileExists(string fpath)
        {
            // If file, hash and lookup 
            // If dir, maybe do nothing
            return false;
        }
    }

    public enum DropboxChangeTypes
    {
	Changed,
	Created,
	Deleted,
	Renamed,
	Moved,
	Copied
    }

    public class DropboxEventArg {
	public DropboxChangeTypes ChangeType;
	public string FullPath;
	public string Name;

	public DropboxEventArg()
	{
	}

	public DropboxEventArg(FileSystemEventArgs fe)
	{
		FullPath = fe.FullPath;
		Name = fe.Name;
		switch (fe.ChangeType)
		{
			case WatcherChangeTypes.Changed: 
				ChangeType = DropboxChangeTypes.Changed; break;
			case WatcherChangeTypes.Created:
				ChangeType = DropboxChangeTypes.Created; break;
			case WatcherChangeTypes.Deleted:
				ChangeType = DropboxChangeTypes.Deleted; break;
			case WatcherChangeTypes.Renamed:
				ChangeType = DropboxChangeTypes.Renamed; break;
		}
	}

	public DropboxEventArg(FileSystemEventArgs fe, DropboxChangeTypes ct)
	{
		FullPath = fe.FullPath;
		Name = fe.Name;
		ChangeType = ct;
	}
    }