by on February 4, 2012
Lately I wanted to use a WPF ListBox with selectable items (with a DataTemplate and a simple CheckBox attached). In order to properly use such a thing you should implement the INotifyPropertyChanged interface 1. I hadn’t used events in F# before so had to experiment a few things to get it right.
The solution was to implement a CheckItem class that wraps the selection functionality and the implementation of the INotifyPropertyChanged interface. This is what it looks like:
open System.ComponentModel
open Microsoft.FSharp.Quotations.Patterns
/// Observable object class implementing the INotifyPropertyChanged
/// interface
type ObservableItem() =
let propertyChanged = Event<_,_>()
let getPropertyName = function
| PropertyGet(_, p, _) -> p.Name
| _ -> invalidOp "Invalid expression argument: expecting property getter"
interface INotifyPropertyChanged with
[<CLIEvent>]
member this.PropertyChanged = propertyChanged.Publish
member this.NotifyPropertyChanged name =
propertyChanged.Trigger(this, PropertyChangedEventArgs(name))
member this.NotifyPropertyChanged expr =
expr |> getPropertyName |> this.NotifyPropertyChanged
/// Simple wrapper class for a selectable item
type CheckItem<'a>(item : 'a) =
inherit ObservableItem()
let mutable _isSelected = true
let _item = item
member this.Item
with get () = _item
member this.IsSelected
with get () = _isSelected
and set value =
_isSelected <- value
this.NotifyPropertyChanged <@ this.IsSelected @>There are a few things worth noting here: The first interesting thing is the attribute CLIEventAttribute 2 that allows us the use a more consise way of implementing events. Basically it adds the necessary CLI metadata to the event and implements the add_EventName and remove_EventName methods.
The second interesting fact is the usage of F# quotations to attach the right property name to the PropertyChangedEventArgs structure. Instead of specifying a string constant it is possible to use a property expression.