Working with F# - DependencyProperty and static readonly field workaround

In my last blog entry, I talked about how I had problems implementing the DependencyProperty in both SpaceButton and SpaceWindow class. After sleeping on it, I figured out a way to workaround that. I was about to post my workaround to my blog only to find that Dr. Don Syme has already posted a comment with the workaround to my blog already! Thanks! I am honored that he's even taken the time to read my blog.

By the way, I recently purchased a copy of Expert F# (I'm kind of surprised that Expert F# is published by Apress instead of Microsoft Press) and slowly reading through the book. While I only finished the first couple chapters, I heartily recommend this book for anyone who wants to learn F#. It certainly made clearer many fuzzy notions that I have of the F# language and introduce me to features that I did not know exist such as option values.

I'm reposting Don Syme's solutions below adding color syntax highlighting...

#light
#I @"C:\Program Files\Reference Assemblies\Microsoft\Framework\v3.0"
#r @"WindowsBase.dll"
#r @"PresentationCore.dll"
#r @"PresentationFramework.dll"

open System
open System.Text
open System.Windows
open System.Windows.Controls
open System.Windows.Input
open System.Windows.Media
//  Workaround solution from Don Syme!
(* From Chap 8 - SetSpaceProperty example with DependencyProperty *)
//
(* From Chap 8 - SpaceButton.cs *)
// Workaround solution 
let mutable private initSpaceButtonSpaceProperty : DependencyProperty = null
let mutable private initSpaceWindowSpaceProperty : DependencyProperty = null
//
// Switched to implicit construction for classes
type SpaceButton() =
  inherit Button() as base

  let mutable txt = null

  static member SpaceProperty =
    if initSpaceButtonSpaceProperty = null then
       let metadata = new FrameworkPropertyMetadata(DefaultValue=1, 
                                                    AffectsMeasure=true, 
                                                    Inherits=true)
       metadata.PropertyChangedCallback <- new PropertyChangedCallback
         (SpaceButton.OnSpacePropertyChanged)
         
       initSpaceButtonSpaceProperty <-
          DependencyProperty.Register
            ("Space",
             typeof<int>,
             typeof<SpaceButton>,
             metadata,
             // callback method for value validation
             (fun obj -> let i = (obj :?> int) in (i >= 0)))

    initSpaceButtonSpaceProperty

  static member OnSpacePropertyChanged (obj:DependencyObject) (args:DependencyPropertyChangedEventArgs) =
    let btn = obj :?> SpaceButton
    btn.Content <- btn.SpaceOutText btn.Text

  member this.Text
    with get() = txt
    and set value =
      txt <- value
      this.Content <- this.SpaceOutText(txt)

  member this.Space
    with get() =
      let value = this.GetValue(SpaceButton.SpaceProperty)
      (value :?> int)
    and set (value:int) = this.SetValue(SpaceButton.SpaceProperty,value)

  member this.SpaceOutText (str:string) =
    if (str <> null) then
      let appendSpace c = String.of_char(c) + new string(' ',this.Space)
      let build = String.map_concat appendSpace str
      build
    else
      null


(* From Chap 8 - SpaceWindow.cs *)
// Switched to implicit construction for classes
type SpaceWindow() =
  inherit Window() as base

  // A static DependencyProperty
  static member SpaceProperty =
    if initSpaceWindowSpaceProperty = null then
      let metadata = new FrameworkPropertyMetadata()
      metadata.Inherits <- true

      // Add owner to SpaceProperty & override metadata
      initSpaceWindowSpaceProperty <- SpaceButton.SpaceProperty.AddOwner(typeof<SpaceWindow>)
      initSpaceWindowSpaceProperty.OverrideMetadata(typeof<SpaceWindow>,metadata)
    // returns the SpaceProperty value  
    initSpaceWindowSpaceProperty

  member this.Space
    with get() = (this.GetValue(SpaceWindow.SpaceProperty) :?> int)
    and set (value:int) = this.SetValue(SpaceWindow.SpaceProperty,value)


(* From Chap 8 - SetSpaceProperty.cs *)
// Switched to implicit construction for classes
type SetSpaceProperty() as this =
  inherit SpaceWindow() as base

  // I did not know you can do this until Don Syme showed me this...
  do this.Title <- "Set Space Property"
     this.SizeToContent <- SizeToContent.WidthAndHeight
     this.ResizeMode <- ResizeMode.CanMinimize
     let iSpaces = [|0;1;2|]

     let grid = new Grid()
     this.Content <- grid

     for i in [0..2] do
       let row = new RowDefinition()
       row.Height <- GridLength.Auto
       grid.RowDefinitions.Add(row)

     for i in [0..(iSpaces.Length-1)] do
       let col = new ColumnDefinition()
       col.Width <- GridLength.Auto
       grid.ColumnDefinitions.Add(col)

     for i in [0..(iSpaces.Length-1)] do 
       let btn = new SpaceButton(Text="Set window Space to " + Int32.to_string(iSpaces.[i]),
                                 Tag=iSpaces.[i],
                                 HorizontalAlignment=HorizontalAlignment.Center,
                                 VerticalAlignment=VerticalAlignment.Center)

       //btn.Click += WindowPropertyOnClick;
       btn.Click.Add(fun _ -> this.Space <- (btn.Tag :?> int))
       grid.Children.Add(btn)|>ignore
       Grid.SetRow(btn, 0)
       Grid.SetColumn(btn, i)


       let btn = new SpaceButton(Text= "Set button Space to " + Int32.to_string(iSpaces.[i]),
                                 Tag=iSpaces.[i],
                                 HorizontalAlignment=HorizontalAlignment.Center,
                                 VerticalAlignment=VerticalAlignment.Center)

       //btn.Click += ButtonPropertyOnClick
       btn.Click.Add(fun _ -> btn.Space <- (btn.Tag :?> int))
       grid.Children.Add(btn) |> ignore
       Grid.SetRow(btn, 1)
       Grid.SetColumn(btn, i)


#if COMPILED
[<STAThread()>]
do
let app = new Application() in
app.Run(new SetSpaceProperty()) |> ignore
#endif