[Proposal] IsNull / IsNotNull Converters

[IeuanWalker]

IsNull IsNotNull Converters

• Proposed
• Prototype
• Implementation
  • iOS Support
  • Android Support
  • macOS Support
  • Windows Support
• Unit Tests
• Sample
• Documentation: Done

Link to Discussion

Discussion

Summary

The IsNullConverter is a converter that allows users to convert an incoming binding to a bool value. This value represents if the incoming binding value is null.

The IsNotNullConverter is a converter that allows users to convert an incoming binding to a bool value. This value represents if the incoming binding value is not null.

Motivation

As mentioned in the discussion this was the behaviour of another converter, so these converters would just reimplement the old behaviour. It's been quite common in the projects I've worked on to be able to check any object type if it's null or not.

Detailed Design

IsNotNullConverter.shared.cs

/// <summary>
/// Converts the incoming value to a <see cref="bool"/> indicating whether or not the value is not null.
/// </summary>
public class IsNotNullConverter : BaseConverter<object?, bool>
{
	/// <summary>
	/// Converts the incoming object to a <see cref="bool"/> indicating whether or not the value is not null.
	/// </summary>
	/// <param name="value">The value to convert.</param>
	/// <returns>A <see cref="bool"/> indicating if the incoming value is not null</returns>
	public bool ConvertFrom(object? value) =>  value is not null;
}

IsNullConverter.shared.cs

/// <summary>
/// Converts the incoming value to a <see cref="bool"/> indicating whether or not the value is null.
/// </summary>
public class IsNullConverter : BaseConverter<object?, bool>
{
	/// <summary>
	/// Converts the incoming object to a <see cref="bool"/> indicating whether or not the value is not null.
	/// </summary>
	/// <param name="value">The value to convert.</param>
	/// <returns>A <see cref="bool"/> indicating if the incoming value is not null</returns>
	public bool ConvertFrom(object? value) =>  value is null;
}

Usage Syntax

XAML Usage

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:xct="http://xamarin.com/schemas/2020/toolkit"
             x:Class="MyLittleApp.MainPage">

    <ContentPage.Resources>
        <ResourceDictionary>
            <xct: IsNullConverter x:Key="IsNullConverter" />
        </ResourceDictionary>
    </ContentPage.Resources>

    <StackLayout IsVisible="{Binding MyObject, Converter={StaticResource IsNullConverter}}">

    </StackLayout>
</ContentPage>

C# Usage

class MyPage : ContentPage
{
  public MyPage()
  {
    Content = new StackLayout().Bind(StackLayout.IsVisibleProperty, nameof(ViewModel.MyObject), converter: new IsNullConverter())
  }
}

Drawbacks

None

Alternatives

None

[IeuanWalker]

If approved, I'm happy to put a PR together for it

[brminnick]

Thanks @IeuanWalker! This all looks good!

The only thing I'd tweak is to use is null and is not null just to match our Contribution Guidelines.

I'll raise this for a vote/approval at our next standup, 03 March.

[brminnick]

I approve this proposal ✅

[VladislavAntonyuk]

can we use BaseConverter<object?, bool>? example: https://github.com/CommunityToolkit/Maui/pull/327/files
I approve

[brminnick]

Good catch! Yes, this is a perfect use case for BaseConverter<object?, bool>.

I'll edit the description now 👍

[bijington]

I vote to approve

[brminnick]

Congrats @IeuanWalker! This Proposal has officially been approved and can be implemented!

Would you like to also do the implementation and open a Pull Request?

[IeuanWalker]

@brminnick awesome, ye will do

[ghost]

Reopening Proposal.

Only Proposals moved to the Closed Project Column and Completed Project Column can be closed.

[ghost]

Reopening Proposal.

Only Proposals moved to the Closed Project Column and Completed Project Column can be closed.