Why isn't my ValueConverter being triggered?

Question

Why isn't my ValueConverter being triggered?

My value-converter is triggered when the constructor of my view-model is executed. However, it doesn't get triggered when I assign a new value to the Cells property.

I expect this line to trigger my value-converter update:

this.Cells <- grid |> cycleThroughCells
                   |> Map.toSeq
                   |> Seq.map snd
                   |> Seq.toList

But it doesn't.

I have the following ViewModel:

type ViewModel() as this =
    inherit ViewModelBase()

    let rowCount = 6
    let mutable grid = rowCount |> createGrid
                                |> setCell { X=3; Y=1; State=Alive }
                                |> setCell { X=3; Y=0; State=Alive }
                                |> setCell { X=4; Y=1; State=Alive }

    let mutable _cells = grid |> Map.toSeq
                              |> Seq.map snd
                              |> Seq.toList
    let cycleHandler _ = 

        this.Cells <- grid |> cycleThroughCells
                           |> Map.toSeq
                           |> Seq.map snd
                           |> Seq.toList
    member this.Play =
        DelegateCommand ((fun _ -> let timer = createTimer 500 cycleHandler
                                   do while true do
                                      do Async.RunSynchronously timer), fun _ -> true) :> ICommand
    member this.Cells
        with get() = _cells 
        and set(value) =
            _cells <- value
            base.NotifyPropertyChanged(<@ this.Cells @>)

My ViewModelBase is the following:

open System.ComponentModel
open Microsoft.FSharp.Quotations.Patterns

type ViewModelBase () =
    let propertyChanged = 
        Event()

    let getPropertyName = function 
        | PropertyGet(_,pi,_) -> pi.Name
        | _ -> invalidOp "Expecting property getter expression"

    interface INotifyPropertyChanged with
        []
        member this.PropertyChanged = propertyChanged.Publish

    member private this.NotifyPropertyChanged propertyName = 
        propertyChanged.Trigger(this,PropertyChangedEventArgs(propertyName))

    member this.NotifyPropertyChanged quotation = 
        quotation |> getPropertyName |> this.NotifyPropertyChanged

My XAML is the following:

My ValueConverter is the following:

type StateToBrushConverter() =
    interface IValueConverter with
        member x.Convert(value, targetType, parameter, culture) = 
            let cell = value :?> Cell
            match cell.State with 
            | Alive -> SolidColorBrush(Colors.LightGreen) :> obj
            | Dead  -> SolidColorBrush(Colors.Black)      :> obj

        member x.ConvertBack(value, targetType, parameter, culture) = failwith "Not implemented yet"

FoggyFinder · Accepted Answer

When I try to run this code I got the exception. Not sure, this may be due, but this code works for me without errors:

 type MainViewModel() as self = 
    inherit ViewModelBase()   
     
    let rowCount = 6
    let mutable grid = rowCount |> createGrid
                                |> setCell { Y=2; X=1; State=Alive }
                                |> setCell { Y=2; X=2; State=Alive }
                                |> setCell { Y=3; X=2; State=Alive }
                                |> setCell { Y=3; X=3; State=Alive }

    let mutable _cells = grid |> Map.toSeq
                              |> Seq.map snd
                              |> Seq.toList
    
    let mutable count = 0
    let mutable isEnabled = true

    let cycleHandler _ = 
        _cells <- grid |> cycleThroughCells
                       |> Map.toSeq
                       |> Seq.map snd
                       |> Seq.toList
        count <- count + 1
        self.NotifyPropertyChanged(<@ self.Cells @>)
        self.NotifyPropertyChanged(<@ self.Count @>)
       
    let change _ = 
        isEnabled <- false
        self.NotifyPropertyChanged(<@ self.IsEnabled @>)
        
        async
            {
                while true do
                    do! Async.Sleep 2000
                    cycleHandler()
            }
        |> Async.Start

    member __.Play = DelegateCommand (change, fun _ -> isEnabled) :> ICommand
    member __.N = rowCount
    member __.Cells = _cells
    member __.Count = count
    member __.IsEnabled = isEnabled

I changed your Xaml, because I don't think to describe 36 rectangles is a good idea. One option is to use a ListBox:

Thus you can create grid of different dimensions depending on the value of rowCount.

I added a variable "count" to show that the function is called with the specified interval, even though the picture does not change.

Result:

Why isn't my ValueConverter being triggered?

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