Binding to a method with parameter in XAML using polymorphism in ViewModel

Question

I have a TabControl with six tabs in my ResultView. The ViewModel that sits behind this View can be either a ResultTypeOneViewModel or ResultTypeTwoViewModel, each of which derives from ResultViewModel but you can interchangeably use the result viewer with either result type.

The difference is that in ResultTypeOneViewModel, tabs 1 & 3 need to be visible and the rest hidden. In ResultTypeTwoViewModel, tabs 2, 3, 4, 5, 6 need to be visible and tab 1 hidden.

I wanted to do this via something like

<TabItem Name="1" Visibility={Binding IsTabVisible(0)}>
<TabItem Name="2" Visibility={Binding IsTabVisible(1)}>
<TabItem Name="3" Visibility={Binding IsTabVisible(2)}>
etc...

And have an abstract method declaration in ResultsViewModel such as

public abstract Visibility IsTabVisible(int index);

And in ResultsOneViewModel have

public override Visibility IsTabVisible(int index)
{
    if (index == 0 || index == 2) return Visibility.Visible;
    return Visibility.Hidden;
}

And in ResultsTwoViewModel have

public override Visibility IsTabVisible(int index)
{
    if (index == 0) return Visibility.Hidden;
    return Visibility.Visible;
}

But I cannot figure out how to call a method like this with a parameter through bindings iN WPF XAML.

Can anyone suggest how I can do this or if it's not possible via this method, another way I could solve this problem?

Answer 1

You can use an indexer property to be able to pass a single parameter to a property. Although it's probably not very intuitive to return a boolean value from an indexer property it works fine for me. Also keep in mind the indexer property looses it's expected functionality.

    class MyClass
    {
        public bool this[int tabNumber]
        {
            get
            {
                // Do logic to determine visibility here or in a separate method
                return IsTabVisible(tabNumber);
            }
        }

        bool IsTabVisible(int tabNumber)
        {
            bool result;

            // Method logic...

            return result;
        }
    }

So in XAML you can use the class name and provide a parameter between square brackets.

<TabItem Visibility="{Binding MyClass[0]}}"/>

If you need to raise a NotifyPropertyChanged for the indexer property use:

    NotifyPropertyChanged("Item[]");

I don't know if this fits into the MVVM pattern but it could be useful for anyone who wants to bind to a method with a single parameter.

Answer 2

To answer your question directly, you can use the ObjectDataProvider to call a method for you so you can work with the results:

xmlns:System="clr-namespace:System;assembly=mscorlib"
xmlns:Windows="clr-namespace:System.Windows;assembly=PresentationCore"

...

<Window.Resources>
    <ObjectDataProvider x:Key="IsTab1VisibleMethod" 
        ObjectType="{x:Type Windows:Visibility}" IsAsynchronous="True" 
        MethodName="IsTabVisible">
        <ObjectDataProvider.MethodParameters>
            <System:Int32>0</System:Int32>
        </ObjectDataProvider.MethodParameters>
    </ObjectDataProvider>
</Window.Resources>

You should then be able to access the result like this (but you'd need one of these for each TabItem):

<TabItem Visibility="{Binding Source={StaticResource IsTab1VisibleMethod}}" />

However, it's generally not good practice to manipulate UI properties such as Visibility in code, so a better idea would be to use a BooleanToVisibilityConverter to Bind the Visibility properties to bool values as @Highcore suggested. You can see an example of this in the Binding a Button Visibility to bool value in ViewModel post here on StackOverflow.

In my opinion, an even better solution would be to simply provide one view for every view model.

Answer 3

A better suggestion to this question is to use style.DataTrigger in style of TabItem like this:

<TabItem>
    <TabItem.Style>
        <Style target="{x:Type TabItem}">
            <Style.DataTriggers>
                <DataTrigger Binding="{Binding IsTabVisible}" Value="False">
                    <Setter Property="Visibility" Value = "Collapsed"/>
                </DataTrigger>
            </Style.DataTrigers>
        </Style>
    <TabItem.Style>
</TabItem>