如何从Convert获取ConvertBack参数？

Question

The problem:

I am using MultiBinding with converter to pass (x,y) coordinates into method.

And I can't make it working in back direction:

public class MyConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        var x = (int)values[0];
        var y = (int)values[1];
        return Model.Get(x, y);
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        Model.Set(x, y, value); // how to get x, y here?
        return new object[] { Binding.DoNothing, Binding.DoNothing };
    }
}

Additional info:

The data will be visualized in a form of table. Here is cell template:

<TextBox>
    <TextBox.Text>
        <MultiBinding Converter="{StaticResource converter}" Mode="TwoWay">
            <Binding Path="X" Mode="OneWay" />
            <Binding Path="Y" Mode="OneWay" RelativeSource="..." />
        </MultiBinding>
    </TextBox.Text>
</TextBox>

The idea is to use converter, which receive x (from cell view model) and y (from parent column view model, notice RelativeSource ) and calls Get(x,y) to display value.

However, when user entered something, ConvertBack is called and I need to call Set(x, y, value) method.

How do I pass x and y into ConvertBack ?

Answer 1

There might be more-or-less dirty workarounds to get such a multivalue converter working. But I'd suggest you keep your multivalue converter one-way, but return a container object that wraps the actual text property.

Instead of directly binding to the TextBox.Text property, bind to some other property (eg. DataContext or Tag ) and then bind the text to the container value.

Small example:

<TextBox Text="{Binding Value}">
    <TextBox.DataContext>
        <MultiBinding Converter="{StaticResource cMyConverter}">
            <Binding Path="X"/>
            <Binding Path="Y"/>
        </MultiBinding>
    </TextBox.DataContext>
</TextBox>

With container and converter:

public class ValueProxy
{
    public int X { get; set; }
    public int Y { get; set; }

    public string Value
    {
        get { return Model.Get(X, Y); }
        set { Model.Set(X, Y, value); }
    }
}
public class MyConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        var x = (int)values[0];
        var y = (int)values[1];
        return new ValueProxy { X = x, Y = y };
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        return new object[] { Binding.DoNothing, Binding.DoNothing };
    }
}

Answer 2

The short answer is that you can't directly get the values of x and y inside your ConvertBack method. The IMultiValueConverter Convert multiple values into a single value. So, the ConvertBack method will do the opposite: convert a single value into multiple values.

It all depends on what your Model.Get(x, y) method returns. It needs to return a value that is unique enough for you to get the separate values of x and y from it.

Example: create unique strings for each pair of (x,y).

Answer 3

It seems hard to pass parameters into ConvertBack . It might be possible, but there is a workaround, which makes ConvertBack unnecessary. Thanks to @XAMlMAX for an idea.

One possibility to achieve it (there could be a better way) is to use data templates. Instead of multi-binding TextBlock.Text with string we can bind ContentControl.Content with some viewmodel, and this viewmodel should do the rest, including Set(x, y, value) call.

Here is code:

public class ViewModel
{
    public int X { get; set; }
    public int Y { get; set; }

    string _text;
    public string Text
    {
        get { return _text; }
        set
        {
            // this should be only called by the view
            _text = value;
            Model.Set(X, Y, value);
        }
    }

    public ViewModel(string text)
    {
        _text = text;
    }
}

public class MyConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        var x = (int)values[0];
        var y = (int)values[1];
        return new ViewModel(Model.Get(x, y)) { X = x, Y = y };
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

and the xaml will become

<ContentControl Focusable="False">
    <ContentControl.Content>
        <MultiBinding Converter="{StaticResource converter}">
            <Binding Path="X" />
            <Binding Path="Y" RelativeSource="..."/>
        </MultiBinding>
    </ContentControl.Content>
</ContentControl>

where data template is

<DataTemplate DataType="{x:Type local:ViewModel}">
    <TextBox Text="{Binding Text}" />
</DataTemplate>