Skip to main content

Using CompositeDisposable in base classes

To help make an object eligible for collection by the GC (garbage collector) one would implement the IDisposable interface. Executing the dispose methods on muliple implmentations at the correct same time is often done using a custom DisposeWith extension method:

   1:  public static class DisposableExtensions
   2:  {
   3:      public static T DisposeWith<T>(this T disposable, CompositeDisposable disposables) where T : IDisposable
   4:      {
   5:          disposables.Add(disposable);
   6:   
   7:          return disposable;
   8:      }
   9:  }

Typically used as follows - explicitly creating a CompositeDisposable instance and then disposing the child view model with the instance:
Firstly can I get away without having to explicitly create an instance?

Also can I get to a point where I can just do something like this:
Inheritance seems the obvious choice but as you might already know, all the disposable types in the System.Reactive.Disposables namespace are sealed by design.

So inheriting directly from CompositeDisposable is a none starter, the next option would to create an intermediate class, @leeoades has a post showing the type of implementation we're using at the moment.

   1:  public interface IDisposableObject: IDisposable
   2:  {
   3:      void AddDisposable(IDisposable disposableObject);
   4:  }
   5:   
   6:  public class DisposableObject : IDisposableObject
   7:  {
   8:      CompositeDisposable _compositeDisposable = new CompositeDisposable();
   9:   
  10:      public void AddDisposable(IDisposable disposableObject)
  11:      {
  12:          _compositeDisposable.Add(disposableObject);
  13:      }
  14:   
  15:      public virtual void Dispose()
  16:      {
  17:          _compositeDisposable.Dispose();
  18:      }
  19:  }

This works well and does the job, but it does introduce another class into the inheritance hierarchy,e.g:

   1:  public abstract class ViewModel :  DisposableObject
   2:  {
   3:  }
   4:   
   5:  public class ParentViewModel : ViewModel
   6:  {
   7:  }

I always want to reduce the levels of inheritance as much as possible, especially when you're working on a framework that's going to be used by a wide variety of developers - keeping inheritance to a minimum helps new devs grok your code base quicker. It also introduces another line into the intellisense when using any derived classes, again this can be confusing when you're not au fait with the classes.
The other downside is having to change the DisposeWith extension method to have an instance of DisposableObject as the second parameter:

   1:  public static class DisposableExtensions
   2:  {
   3:      public static T DisposeWith<T>(this T disposable, IDisposableObject disposableObject) where T : IDisposable
   4:      {
   5:          disposableObject.AddDisposable(disposable);
   6:   
   7:          return disposable;
   8:      }
   9:  }

Is there away I can achieve the same functionality without introducing the DisposableObject class and not expose class methods that aren't relevant?

Yes - use a conversion operator, specifically use an implicit operator to convert from the source type to the destination type without having to specify the type using a dynamic cast. Below is taken from the MSDN example:
So now I'm able to remove the need for the DisposableObject class and prevent the need for any extra methods in my base class ViewModel. You'll notice I've also implemented the ICancelable interface from the  system.Reactive.Disposables namepsace, this exposes an IsDisposed method:

   1:  public abstract class ViewModel : ICancelable
   2:  {
   3:      private CompositeDisposable _disposable;
   4:   
   5:      protected ViewModel()
   6:      {
   7:          _disposable = new CompositeDisposable();
   8:      }
   9:   
  10:      public bool IsDisposed { get; private set; }
  11:   
  12:      public void Dispose()
  13:      {
  14:          if (!IsDisposed)
  15:          {
  16:              IsDisposed = true;
  17:   
  18:              _disposable.Dispose();
  19:              _disposable = null;
  20:          }
  21:      }
  22:   
  23:      public static implicit operator CompositeDisposable(ViewModel viewModel) 
  24:      {
  25:          return viewModel._disposable;
  26:      }
  27:  }

The parent & child view models haven't changed in declaration:

   1:  public class ParentViewModel : ViewModel
   2:  {
   3:  }
   4:   
   5:  public class ChildViewModel : ViewModel
   6:  {
   7:  }

 The intellisense has subtly changed, no longer exposing method declared on the DisposableObject class:
Testing produces the expected output - all view models disposed...

Labels:
Location: Stoke Newington, London Borough of Hackney, London N16 1AZ, UK

Comments

  1. Unknown12 June 2013 at 07:43

    Nice example. I like the code where you remove the implementation of a class, but provide an operator to do the conversion, and then return a backing field. I have never seen that before, or I never realized that it was doable like that. It explains some of the things I've seen in the past that seemed like 'magic' or the compiler being clever.

    ReplyDelete

Post a Comment

Popular posts from this blog

Implementing a busy indicator using a visual overlay in MVVM

This is a technique we use at work to lock the UI whilst some long running process is happening - preventing the user clicking on stuff whilst it's retrieving or rendering data. Now we could have done this by launching a child dialog window but that feels rather out of date and clumsy, we wanted a more modern pattern similar to the way <div> overlays are done on the web. Imagine we have the following simple WPF app and when 'Click' is pressed a busy waiting overlay is shown for the duration entered into the text box. What I'm interested in here is not the actual UI element of the busy indicator but how I go about getting this to show & hide from when using MVVM. The actual UI elements are the standard Busy Indicator coming from the WPF Toolkit : The XAML behind this window is very simple, the important part is the ViewHost. As you can see the ViewHost uses a ContentPresenter element which is bound to the view model, IMainViewModel, it contains 3 child v
Post a Comment
Read more

Showing a message box from a ViewModel in MVVM

I was doing a code review with a client last week for a WPF app using MVVM and they asked ' How can I show a message from the ViewModel? '. What follows is how I would (and have) solved the problem in the past. When I hear the words ' show a message... ' I instantly think you mean show a transient modal message box that requires the user input before continuing ' with something else ' - once the user has interacted with the message box it will disappear. The following solution only applies to this scenario. The first solution is the easiest but is very wrong from a separation perspective. It violates the ideas behind the Model-View-Controller pattern because it places View concerns inside the ViewModel - the ViewModel now knows about the type of the View and specifically it knows how to show a message box window: The second approach addresses this concern by introducing the idea of messaging\events between the ViewModel and the View. In the example below
5 comments
Read more

Custom AuthorizationHandler for SignalR Hubs

How to implement IAuthorizationRequirement for SignalR in Asp.Net Core v5.0 Been battling this for a couple of days, and eventually ended up raising an issue on Asp.Net Core gitHub  to find the answer. Wanting to do some custom authorization on a SignalR Hub when the client makes a connection (Hub is created) and when an endpoint (Hub method) is called:  I was assuming I could use the same Policy for both class & method attributes, but it ain't so - not because you can't, because you need the signatures to be different. Method implementation has a resource type of HubInnovationContext: I assumed class implementation would have a resource type of HubConnectionContext - client connects etc... This isn't the case, it's infact of type DefaultHttpContext . For me I don't even need that, it can be removed completely  from the inheritence signature and override implementation. Only other thing to note, and this could be a biggy, is the ordering of the statements in th
Post a Comment
Read more