Drag and Drop in Silverlight with F# and Asynchronous Workflows
- by knotig
Hello everyone!
I'm trying to implement drag and drop in Silverlight using F# and asynchronous workflows.
I'm simply trying to drag around a rectangle on the canvas, using two loops for the the two states (waiting and dragging), an idea I got from Tomas Petricek's book "Real-world Functional Programming", but I ran into a problem:
Unlike WPF or WinForms, Silverlight's MouseEventArgs do not carry information about the button state, so I can't return from the drag-loop by checking if the left mouse button is no longer pressed. I only managed to solve this by introducing a mutable flag.
Would anyone have a solution for this, that does not involve mutable state?
Here's the relevant code part (please excuse the sloppy dragging code, which snaps the rectangle to the mouse pointer):
type MainPage() as this =
inherit UserControl()
do
Application.LoadComponent(this, new System.Uri("/SilverlightApplication1;component/Page.xaml", System.UriKind.Relative))
let layoutRoot : Canvas = downcast this.FindName("LayoutRoot")
let rectangle1 : Rectangle = downcast this.FindName("Rectangle1")
let mutable isDragged = false
do
rectangle1.MouseLeftButtonUp.Add(fun _ -> isDragged <- false)
let rec drag() = async {
let! args = layoutRoot.MouseMove |> Async.AwaitEvent
if (isDragged) then
Canvas.SetLeft(rectangle1, args.GetPosition(layoutRoot).X)
Canvas.SetTop(rectangle1, args.GetPosition(layoutRoot).Y)
return! drag()
else
return()
}
let wait() = async {
while true do
let! args = Async.AwaitEvent rectangle1.MouseLeftButtonDown
isDragged <- true
do! drag()
}
Async.StartImmediate(wait())
()
Thank you very much for your time!
