Hide xaml element when null

Question

We have a UWP app using Template10. There is a textblock and textbox which show a discount. We wish to hide the textblock when ViewModel.Discount is null.

In App.xaml we have defined a converter

<T10Converters:ValueWhenConverter x:Key="HideWhenNullConverter" When="{x:Null}">
    <T10Converters:ValueWhenConverter.Value>
        <Visibility>Collapsed</Visibility>
    </T10Converters:ValueWhenConverter.Value>
    <T10Converters:ValueWhenConverter.Otherwise>
        <Visibility>Visible</Visibility>
    </T10Converters:ValueWhenConverter.Otherwise>
</T10Converters:ValueWhenConverter>

In the View we set the visibility of the TextBlock

Visibility="{x:Bind ViewModel.Discount, Converter={StaticResource HideWhenNullConverter}}"

In the ViewModel:

public class ViewModel : ViewModelBase
{
    decimal? _Discount = default(decimal?);
    public decimal? Discount
    {
        get
        {
            return _Discount;
        }
        set
        {
            if (value == 0) value = null;
            Set(ref _Discount, value);
        }
    }

However the textblock is always visible even if the value of ViewModel.Discount is null. How do we hide the textblock when ViewModel.Discount is null

You could also state that the Visibility logic is the responsibility of the ViewModel. The latest UWP can bind Visibility to a bool directly. Imagine you later have additional business rules, like a max price or so. — Henk Holterman
@HenkHolterman Nice, thanks for mentioning that - I've somehow missed this. — Romasz
In 14393, Visibility="{x:Bind ViewModel.Discount.HasValue, Mode=OneWay}". — Justin XL
Actually if you are using 14393 you should avoid converters as they are slow. Instead you can bind a function directly. — Justin XL
Yes @Justin XL that's what we have found ... converters are terribly slow. Thanks for the tip! — Vague

Romasz Romasz · Accepted Answer · 2017-03-04T10:02:19

As I've tried with Template10's source it should work. I suspect that you are just missing redefinition of Mode with x:Bind, which as default is OneTime. Try like this:

Visibility="{x:Bind ViewModel.Discount, Mode=OneWay, Converter={StaticResource HideWhenNullConverter}}"

Hide xaml element when null

1 Answers