Here is another approach. Instead of propagating the first event of a quick succession of events and suppressing all that follow, now all are suppressed except from the last one. I think that the scenarios that can benefit from this approach are more common.
To make this happen we must use a sliding delay. Every incoming event cancels the timer that would fire the previous event, and restarts the timer. This opens the possibility that a never-ending series of events will delay the propagation forever. To keep things simple, there is no provision for this abnormal case in the extension methods below.
public static class FileSystemWatcherExtensions
{
public static IDisposable OnAnyEvent(this FileSystemWatcher source,
WatcherChangeTypes changeTypes, FileSystemEventHandler handler, int delay)
{
var cancellations = new Dictionary<string, CancellationTokenSource>(
StringComparer.OrdinalIgnoreCase);
var locker = new object();
if (changeTypes.HasFlag(WatcherChangeTypes.Created))
source.Created += FileSystemWatcher_Event;
if (changeTypes.HasFlag(WatcherChangeTypes.Deleted))
source.Deleted += FileSystemWatcher_Event;
if (changeTypes.HasFlag(WatcherChangeTypes.Changed))
source.Changed += FileSystemWatcher_Event;
if (changeTypes.HasFlag(WatcherChangeTypes.Renamed))
source.Renamed += FileSystemWatcher_Event;
return new Disposable(() =>
{
source.Created -= FileSystemWatcher_Event;
source.Deleted -= FileSystemWatcher_Event;
source.Changed -= FileSystemWatcher_Event;
source.Renamed -= FileSystemWatcher_Event;
});
async void FileSystemWatcher_Event(object sender, FileSystemEventArgs e)
{
var key = e.FullPath;
var cts = new CancellationTokenSource();
lock (locker)
{
if (cancellations.TryGetValue(key, out var existing))
{
existing.Cancel();
}
cancellations[key] = cts;
}
try
{
await Task.Delay(delay, cts.Token);
// Omitting ConfigureAwait(false) is intentional here.
// Continuing in the captured context is desirable.
}
catch (TaskCanceledException)
{
return;
}
lock (locker)
{
if (cancellations.TryGetValue(key, out var existing)
&& existing == cts)
{
cancellations.Remove(key);
}
}
cts.Dispose();
handler(sender, e);
}
}
public static IDisposable OnAllEvents(this FileSystemWatcher source,
FileSystemEventHandler handler, int delay)
=> OnAnyEvent(source, WatcherChangeTypes.All, handler, delay);
public static IDisposable OnCreated(this FileSystemWatcher source,
FileSystemEventHandler handler, int delay)
=> OnAnyEvent(source, WatcherChangeTypes.Created, handler, delay);
public static IDisposable OnDeleted(this FileSystemWatcher source,
FileSystemEventHandler handler, int delay)
=> OnAnyEvent(source, WatcherChangeTypes.Deleted, handler, delay);
public static IDisposable OnChanged(this FileSystemWatcher source,
FileSystemEventHandler handler, int delay)
=> OnAnyEvent(source, WatcherChangeTypes.Changed, handler, delay);
public static IDisposable OnRenamed(this FileSystemWatcher source,
FileSystemEventHandler handler, int delay)
=> OnAnyEvent(source, WatcherChangeTypes.Renamed, handler, delay);
private struct Disposable : IDisposable
{
private readonly Action _action;
internal Disposable(Action action) => _action = action;
public void Dispose() => _action?.Invoke();
}
}
Usage example:
myWatcher.OnAnyEvent(WatcherChangeTypes.Created | WatcherChangeTypes.Changed,
MyFileSystemWatcher_Event, 100);
This line combines the subscription to two events, the Created and the Changed. So it is roughly equivalent to these:
myWatcher.Created += MyFileSystemWatcher_Event;
myWatcher.Changed += MyFileSystemWatcher_Event;
The difference is that the two events are regarded as a single type of event, and in case of a quick succession of these events only the last one will be propagated. For example if a Created event is followed by two Changed events, and there is no time gap larger than 100 msec between these three events, only the second Changed event will be propagated by invoking the MyFileSystemWatcher_Event handler, and the previous ones will be discarded.
