Rx Framework Part II - Observables From Events and ‘LINQ to Events’ – With a WPF Drawing demo

By Anoop Madhusudanan

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In my last post, I gave a high level overview about .NET Reactive Extensions (Rx), Push and Pull data sequences, and creating Observables from .NET events. In this post, we’ll explore more Observables from Events, and the ‘LINQ to Event’ aspect. We’ll create a small drawing application in WPF in declarative style. You may want to download the related source code to play with

A note on the source code. I’m using the System.Reactive.dll, that I rebased to .NET CLR using Reflexil. It is just for demo purposes, and is not expected to include in your production deployments.

Event To Observables

To recap, as described in my last post, you may use the Observable.FromEvent method to create Observables from events and subscribe to them, like,

var allKeyDowns = Observable.FromEvent<KeyEventHandler, KeyEventArgs>
                (   h => new KeyEventHandler(h), 
                    h => el.KeyDown += h, 
                    h=> el.KeyDown -= h
                 );

As we’ve seen earlier, allKeyDowns is an Observable, of type IObservable<Event<KeyEventArgs>> .

It is a good practice to wrap the creation of an observable from an event in an extension method. Let us create a handy GetKeyDown() extension method to create an Observable from the key down event, for any UI Element.

public static class UIElementExtensions 
 {							
      public static IObservable<Event<KeyEventArgs>> GetKeyDown 
                                                      (this UIElement el)
        {                        
            var allKeyDowns = Observable.FromEvent<KeyEventHandler, KeyEventArgs>
                (   h => new KeyEventHandler(h), 
                    h => el.KeyDown += h, 
                    h=> el.KeyDown -= h
                 );

            return  allKeyDowns;            
        }
}				

Also, you may easily abstract this out. For example, assume that you are building a racing game, and you might need to subscribe only to Arrow Key downs. You may create an extension method GetArrowKeyDown the arrow key down events. 

      //Get the arrow keys down only
       public static IObservable<Event<KeyEventArgs>> GetArrowKeyDown
				(this UIElement el)
       {
	   var arrows = new List<Key> 
                               { Key.Left, Key.Right, Key.Up, Key.Down };
	   var arrowsPressed = from kd in el.GetKeyDown()
				 where arrows.Contains(kd.EventArgs.Key)
				 select kd;
	   return arrowsPressed;

       }	

    //---- 
  
    //Some where in your main application

    var arrowPressed=this.GetArrowKeyDown();
    arrowPressed.Subscribe(arrow=>MessageBox.Show
                    (arrow.EventArgs.Key.ToString() + " Pressed!!")); 

 

Sequencing the events

Let us create a simple drawing application, so that when the user drags, we draw a red line from the initial mouse down position to the current location, and also a blue spot at the current location. Here is what I mean :)

image

And we’ll do the same in a declarative manner, instead of doing it the classical way. Assuming that you have the extension methods GetMouseUp, GetMouseDown, and GetMouseMove for the corresponding Mouse Events, let us see how to handle a drag operation, in a declarative manner. You may see how simple this approach is - instead of having event handlers sprinkled every where

//A draw on drag method to perform the draw
void DrawOnDrag(Canvas e)
        {

            //Get the initial position and dragged points using LINQ to Events
            var mouseDragPoints = from md in e.GetMouseDown()
                                  let startpos=md.EventArgs.GetPosition(e)
                                  from mm in e.GetMouseMove().Until(e.GetMouseUp())
                                  select new
                                  {
                                      StartPos = startpos,
                                      CurrentPos = mm.EventArgs.GetPosition(e),
                                  };


            //Subscribe and draw a line from start position to current position
            mouseDragPoints.Subscribe
                (item =>
                {
                    e.Children.Add(new Line()
                    {
                        Stroke = Brushes.Red,
                        X1 = item.StartPos.X,
                        X2 = item.CurrentPos.X,
                        Y1 = item.StartPos.Y,
                        Y2 = item.CurrentPos.Y
                    });

                    var ellipse = new Ellipse()
                    {
                        Stroke = Brushes.Blue,
                        StrokeThickness = 10,
                        Fill = Brushes.Blue
                    };
                    Canvas.SetLeft(ellipse, item.CurrentPos.X);
                    Canvas.SetTop(ellipse, item.CurrentPos.Y);
                    e.Children.Add(ellipse);
                }
                );
        }

 image

One very interesting point there - we are running a LINQ query against the observables, and creating a new observable of an anonymous type that has two properties, StartPos and CurrentPos. If you examine the type of mouseDragPoints, you'll see this >.

And here are the related GetEventName wrapper methods, for MouseUp, MoveMove and MouseDown. You may read more about Observable.FromEvent method we use below in my previous post about .NET Rx, if you havn't yet read that.

 public static class UIElementExtensions 
{
 
  public static IObservable<Event<MouseButtonEventArgs>> 
                                      GetMouseUp (this UIElement el)
        {                        
            var allevents = Observable.FromEvent<MouseButtonEventHandler, MouseButtonEventArgs>
                (   h => new MouseButtonEventHandler(h), 
                    h => el.MouseUp += h, 
                    h=> el.MouseUp -= h
                 );

            return allevents;            
        }
		
						
   public static IObservable<Event<MouseButtonEventArgs>> 
                                      GetMouseDown (this UIElement el)
	{                        
		var allevents = Observable.FromEvent<MouseButtonEventHandler, MouseButtonEventArgs>
			(   h => new MouseButtonEventHandler(h), 
				h => el.MouseDown += h, 
				h=> el.MouseDown -= h
			 );

		return allevents;            
	}		
	
 public static IObservable<Event<MouseEventArgs>> 
                               GetMouseMove (this UIElement el)
        {                        
            var allevents = Observable.FromEvent<MouseEventHandler, MouseEventArgs>
                (   h => new MouseEventHandler(h), 
                    h => el.MouseMove += h, 
                    h=> el.MouseMove -= h
                 );

            return allevents;            
        }	
		
}

If you havn’t yet done, read my other posts on .NET Rx as well. Also, have a look at this T4 template to generate all those GetEventName wrapper methods for a given type.

 

Enjoy coding!!

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