Handling Attributes on Custom XAML Controls in Xamarin

Question

I have a custom ContentView (FolioReceiptView) that exposes a property called "ReceiptMarker" (Code condensed for clarity):

public partial class FolioReceiptView : ContentView {
  public FolioReceiptView() {
    InitializeComponent ();
    if (!String.IsNullOrWhiteSpace (ReceiptMarker)) {
      var vm = BindingContext as FolioReceiptViewModel;
      if (vm != null) {
        vm.Marker = ReceiptMarker;
      }
    }
  }

  public static readonly BindableProperty ReceiptMarkerProperty = BindableProperty.Create ("ReceiptMarker", typeof(string), typeof(FolioReceiptView), null);

  public string ReceiptMarker { 
    get {
      return (string)GetValue (ReceiptMarkerProperty);
    } 
    set {
      SetValue (ReceiptMarkerProperty, value);
    }
  }
}

I use the control in a Page like so:

<StackLayout Orientation="Horizontal" HorizontalOptions="FillAndExpand" Spacing="10" VerticalOptions="FillAndExpand">
  <view:FolioReceiptView x:Name="ReceiptA" ReceiptMarker="A" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand" />
  <view:FolioReceiptView x:Name="ReceiptB" ReceiptMarker="B" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand" />
  <view:FolioReceiptView x:Name="ReceiptC" ReceiptMarker="C" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand" />
</StackLayout>

However, in the class's constructor, the ReceiptMarker value is null. Why isn't the value picked up from the XAML?

Answer 1

"An Object Initializer lets you set properties or fields on your object after it's been constructed" - that means you assign ReceiptMarker="A" after constructor execution ends.

If you want ReceiptMarker to be available in constructor, pass it as argument.

Answer 2

As it turns out, I was trying to set the ReceiptMarker on the ViewModel before the view model was set. All is fixed now. :)