Observing changes in the MockFileSystem

[SuperJMN]

Describe the solution you'd like
I'd like to be able to observe common/interesting events (from a testing perspective) that happen into a MockFileSystem.

Why? I'm creating a cache service that uses an IFileSystem and I need to test that a file is only written once.

Describe alternatives you've considered
I've tried to implement my own MockFileSystem using System.Reactive, but the work is a bit overwhelming.

[fgreinacher]

This is the same as #168 which has gone stale.

As mentioned over there I generally like the idea and would accept a contribution here unless it's getting overly complex.

[mistial-dev]

This is the same as #168 which has gone stale.

As mentioned over there I generally like the idea and would accept a contribution here unless it's getting overly complex.

What do you think of an approach like this?

#1313

[mistial-dev]

The neat thing about this kind of approach is that it makes testing certain scenarios easier. For example:

  var fileSystem = new MockFileSystem(new MockFileSystemOptions { EnableEvents = true });

  // Make everything read-only with one subscription
  using (fileSystem.Events.Subscribe(args =>
  {
      if (args.Phase == OperationPhase.Before &&
          (args.Operation == FileOperation.Write ||
           args.Operation == FileOperation.Delete ||
           args.Operation == FileOperation.Create))
      {
          args.SetResponse(new OperationResponse
          {
              Exception = new UnauthorizedAccessException("File system is read-only")
          });
      }
  }))
  {
      // This will throw UnauthorizedAccessException
      fileSystem.File.WriteAllText(@"C:\test.txt", "data");
  }

Or generic logging, etc.

  var fs = new MockFileSystem(new MockFileSystemOptions { EnableEvents = true });
  var log = new List<string>();

  using (fs.Events.Subscribe(args =>
  {
      if (args.Phase == OperationPhase.After)
          log.Add($"{args.Operation} on {args.Path}");
  }))
  {
      // Your test code here
      MyService.ProcessFiles(fs);

      // Now you can assert on what actually happened
      Assert.That(log, Contains.Item("Delete on C:\\temp\\cache.tmp"));
  }

  // Simulate disk full on the third write
  var writeCount = 0;
  using (fs.Events.Subscribe(FileOperation.Write, args =>
  {
      if (++writeCount >= 3 && args.Phase == OperationPhase.Before)
      {
          args.SetResponse(new OperationResponse
          {
              Exception = new IOException("Disk full")
          });
      }
  }))
  {
      // Test your retry logic, error handling, etc.
  }

  // Prevent deletion of important files
  using (fs.Events.Subscribe(FileOperation.Delete, args =>
  {
      if (args.Phase == OperationPhase.Before && args.Path.EndsWith(".config"))
      {
          args.SetResponse(new OperationResponse
          {
              Exception = new UnauthorizedAccessException("Config files are protected")
          });
      }
  }))
  {
      // Deletion is prevented
  }

[mistial-dev]

The actual syntax to add events is pretty minimal. For example:

        return mockFileDataAccessor.Events.WithEvents(fullPath, FileOperation.Read, ResourceType.File, () =>
        {
            // ... existing code
            return ReadAllTextInternal(path, encoding);
        });