Two way function bindings Some troubles

Mauro Fantina 106

Hello,

I'm developing UWP application where I need to Bind muxc:NumberBox with ushort? property and I have some troubles:

I create simple code to test TwoWayBindings between two muxc:NumberBox sharing the same property.

    public sealed partial class MainPage : Page, INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        private ushort? _a = 16;

        public ushort? a
        {
            get => _a;

            set
            {
                _a = value;
                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(a)));
            }
        }

        public MainPage()
        {
            this.InitializeComponent();
        }

        public double MyFunc(ushort? x) =>(double) x.Value;

        public ushort? MyFunc2(double x) => (ushort?)x;
    }

xaml

        <muxc:NumberBox Value="{x:Bind MyFunc(a), BindBack=MyFunc2, Mode=TwoWay}" Minimum="0" Maximum="65535"/>
        <muxc:NumberBox Value="{x:Bind MyFunc(a), BindBack=MyFunc2, Mode=TwoWay}"  Minimum="0" Maximum="65535"/>

But it doesn't work : xaml calls MyFunc2 but the returned value is not pushed on property "a"

Ivanich 306 Reputation points

2020-08-27T11:17:52.773+00:00

But MyFunc2 does not set that property, how you expect it should change?
Mauro Fantina 106 Reputation points

2020-08-27T11:59:18.553+00:00
function-bindings+

says :

In a two-way binding scenario, a second function must be specified for the reverse direction of the binding. This is done using the BindBack binding property. In the below example, the function should take one argument which is the value that needs to be pushed back to the model.

<TextBlock Text="{x:Bind a.MyFunc(b), BindBack=a.MyFunc2, Mode=TwoWay}" />
Mauro Fantina 106 Reputation points

2020-08-27T12:02:15.347+00:00

Ah ... ok ... I have to write a routine , not a funcion that returns a value ... !!
Nico Zhu (Shanghai Wicresoft Co,.Ltd.) 12,866 Reputation points

2020-08-27T13:30:58.273+00:00

BindBack will be called when you edit NumberBox content, for this scenario, the better way is using converter to return back value.
Mauro Fantina 106 Reputation points

2020-08-27T13:33:25.707+00:00

I don't like Converter because I have to create new Class, and then create StaticResource ... I prefer to write specific functions

1 answer

Peter Fleischer (former MVP) 19,321

Hi Mauro,
I change code in your functions and it works fine. Instead of NumberBox I used Slider:

<Page  
    x:Class="App1.Page11"  
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"  
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"  
    xmlns:local="using:App1"  
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"  
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"  
    mc:Ignorable="d"  
    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">  
  <StackPanel>  
    <Slider Value="{x:Bind MyFunc(a), BindBack=MyFunc2, Mode=TwoWay}" Minimum="0" Maximum="65535"/>  
    <Slider Value="{x:Bind MyFunc(a), BindBack=MyFunc2, Mode=TwoWay}" Minimum="0" Maximum="65535"/>  
    <TextBlock Text="{Binding a}"/>  
  </StackPanel>  
</Page>

And CodeBehind:

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Two way function bindings Some troubles

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