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. — H H, Mar 04 '17 at 10:34
@HenkHolterman Nice, thanks for mentioning that - I've somehow [missed this](https://social.technet.microsoft.com/wiki/contents/articles/34846.uwp-compiled-binding-windows-10-anniversary-update.aspx). — Romasz, Mar 04 '17 at 10:45
In _14393_, `Visibility="{x:Bind ViewModel.Discount.HasValue, Mode=OneWay}"`. — Justin XL, Mar 04 '17 at 22:23
Thanks for the suggestion @Justin XL but I couldn't get this to work in 14393 — Vague, Mar 04 '17 at 23:15
@Vague ahh I think I was wrong as HasValue might only work for the first time. So might still need to create a new bool property? — Justin XL, Mar 04 '17 at 23:28
@Justin XL the answer accepted is probably the best as it does not require creation and setting of another property. — Vague, Mar 04 '17 at 23:32
Actually if you are using 14393 you should avoid converters as they are slow. Instead you can bind a function directly. — Justin XL, Mar 04 '17 at 23:40
Yes @Justin XL that's what we have found ... converters are terribly slow. Thanks for the tip! — Vague, Mar 04 '17 at 23:55
"creation and setting of another property" is not a problem when it's in the right place (and the VM usually is the right place). — H H, Mar 05 '17 at 22:44
@JustinXL - `Discount.HasValue` will not have INPC implemented. — H H, Mar 06 '17 at 09:58
@HenkHolterman your right. I realized that after I posted the comment. :P — Justin XL, Mar 06 '17 at 10:01

score 1 · Accepted Answer · answered Mar 04 '17 at 10:02

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 Answers1