Drag and drop in WinForms with evReact

open EvReact
open EvReact.Expr
open EvReact.Orchestrator

open System.Windows.Forms
open System.Drawing

let E (e:IEvent<'c,'a>) = 
  let evt = new Control.Event<'a>()
  e.Add(fun e -> evt.Trigger(e))
  evt.Publish

type MyForm() as this =
  inherit Form()

  do this.SetStyle(ControlStyles.AllPaintingInWmPaint ||| ControlStyles.OptimizedDoubleBuffer, true)

let f = new MyForm(Text="Drag&Drop test with evReact", Width=640, Height=600)

let rect = ref(new Rectangle(0, 0, 50, 50))
let target = ref(new Rectangle(300, 300, 200, 200))
let bgcol = ref(Brushes.Red)
let off = ref(new Point())

f.Paint.Add(fun e ->
  let g = e.Graphics
  g.DrawRectangle(Pens.Gray, !target)

  g.FillRectangle(!bgcol, !rect)
  g.DrawRectangle(Pens.Black, !rect)
)

let md = E f.MouseDown
let mm = E f.MouseMove
let mu = E f.MouseUp

// Utilities
let uc col = bgcol := col; f.Invalidate()
let dc col = !bgcol <> col
let inR (p:Point) = (!rect).Contains(p)
let placeR (p:Point) = let r = !rect in rect := new Rectangle(p.X, p.Y, r.Width, r.Height);f.Invalidate()
let inT () = (!target).Contains(!rect)

// Net recognizing hovering
let highlight =
  +(
        ((mm %- fun e -> dc Brushes.Yellow && inR(e.Location)) |-> fun _ -> uc Brushes.Yellow)
    ||| ((mm %- fun e -> dc Brushes.Red && not(inR(e.Location))) |-> fun _ -> uc Brushes.Red)
   )

// Net performing the drag
let drag = 
  +(
     ((md %- fun e -> (!rect).Contains(e.Location)) 
         |-> fun e -> let r = !rect in off := new Point(e.X - r.Left, e.Y - r.Top))
     - ((+(!!mm) |-> fun e -> placeR(new Point(e.X - (!off).X, e.Y - (!off).Y))) / [| mm; mu |])
     - (!!mu |-> fun e -> if not(inT()) then (placeR(new Point()); uc Brushes.Red))
   )

let orch = Orchestrator.create()
Expr.start null orch highlight
Expr.start null orch drag

f.Show()