Implement a Tri-State CheckBox logic using MVVM

This tutorial will guide you through the process of implementing a 'tri-state' CheckBox functionality in the RadTreeView using MVVM.

The RadTreeView control supports check boxes/radio buttons elements next to each item out-of-the-box. However, their 'tri-state' logic implementation is designed to work when the RadTreeView.Items collection is populated with RadTreeViewItems. Basically it will work as expected when the RadTreeView is declaratively populated or its Items collection is populated in code-behind. However, the RadTreeView control is mostly used in databinding scenarios following the MVVM pattern. And if your applicaiton requirements include a 'tri-state' check box logic, then it's best to define a CheckBox control inside the RadTreeViewItem's DataTemplates and implement the 'tri-state' logic entirely in the view models.

Let's start with defining a sample view model for the RadTreeViewItems. It only contains a name, collection of children items and a checked property:

    using System; 
    using System.Collections.ObjectModel; 
    using Telerik.Windows.Controls; 
    using System.Linq; 
 
    namespace TreeViewMVVMCheckBoxSample.ViewModels 
    { 
        public class CategoryViewModel : ViewModelBase 
        { 
            private string _name; 
            private bool? _isChecked; 
 
            private ObservableCollection<CategoryViewModel> _subCategories = null; 
 
            public string Name 
            { 
                get 
                { 
                    return this._name; 
                } 
                set 
                { 
                    this._name = value; 
                } 
            } 
            public bool? IsChecked 
            { 
                get 
                { 
                    return this._isChecked; 
                } 
                set 
                { 
                    if (this._isChecked != value) 
                    { 
                        this._isChecked = value; 
                        OnPropertyChanged("IsChecked"); 
                    } 
                } 
            } 
 
            public ObservableCollection<CategoryViewModel> SubCategories 
            { 
                get 
                { 
                    if (this._subCategories == null) 
                    { 
                        this._subCategories = new ObservableCollection<CategoryViewModel>(); 
                    } 
                    return this._subCategories; 
                } 
            } 
        } 
    }

    Imports System.Collections.ObjectModel 
    Imports Telerik.Windows.Controls 
 
    Namespace TreeViewMVVMCheckBoxSample.ViewModels 
        Public Class CategoryViewModel 
            Inherits ViewModelBase 
            Private _name As String 
            Private _isChecked As Boolean? 
 
            Private _subCategories As ObservableCollection(Of CategoryViewModel) = Nothing 
 
            Public Property Name() As String 
                Get 
                    Return Me._name 
                End Get 
                Set(ByVal value As String) 
                    Me._name = value 
                End Set 
            End Property 
            Public Property IsChecked() As Boolean? 
                Get 
                    Return Me._isChecked 
                End Get 
                Set(ByVal value As Boolean?) 
                    If Not Me._isChecked.Equals(value) Then 
                        Me._isChecked = value 
                        OnPropertyChanged("IsChecked") 
                    End If 
                End Set 
            End Property 
 
            Public ReadOnly Property SubCategories() As ObservableCollection(Of CategoryViewModel) 
                Get 
                    If Me._subCategories Is Nothing Then 
                        Me._subCategories = New ObservableCollection(Of CategoryViewModel)() 
                    End If 
                    Return Me._subCategories 
                End Get 
            End Property 
        End Class 
    End Namespace

Please note that the CategoryViewModel class inherits from the Telerik.Windows.Controls.ViewModelBase class. It provides support for property change notifications and we need to notify the RadTreeViewItems when the IsChecked property is changed.

Now let's extend that sample model to include our 'tri-state' logic. Firstly, in order to update the checked state of the parent items, each item will have to keep a reference of its parent item.
```
    private CategoryViewModel parentItem; 
```
```
    Private parentItem As CategoryViewModel 
```

Then we need to implement the logic that determines the checked state of each item. For that purpose we have to traverse the children colleciton of a checked item as well as to find the checked state in which its parent item should be set.

Let's create a method traversing the children collection of an item:

        private void UpdateChildrenCheckState() 
        { 
            foreach (var item in this.SubCategories) 
            { 
                if (this.IsChecked != null) 
                { 
                    item.IsChecked = this.IsChecked; 
                } 
            } 
        }

        Private Sub UpdateChildrenCheckState() 
            For Each item In Me.SubCategories 
                If Me.IsChecked IsNot Nothing Then 
                    item.IsChecked = Me.IsChecked 
                End If 
            Next item 
        End Sub

We can also create a method that updates the checked state of the parent item. In order to simplify the code, we can use a lambda function to count the number of the checked children of the parent item. If this number indicates that all its children are checked, we can set the parent item checked state to checked, if the count of its checked children is 0, then we need to uncheck it. In all other cases, its state should stay indeterminate.

        private bool? DetermineCheckState() 
        { 
            bool allChildrenChecked = this.SubCategories.Count(x => x.IsChecked == true) == this.SubCategories.Count; 
            if (allChildrenChecked) 
            { 
                return true; 
            } 
 
            bool allChildrenUnchecked = this.SubCategories.Count(x => x.IsChecked == false) == this.SubCategories.Count; 
            if (allChildrenUnchecked) 
            { 
                return false; 
            } 
 
            return null; 
        }

        Private Function DetermineCheckState() As Boolean? 
            Dim allChildrenChecked As Boolean = Me.SubCategories.LongCount(Function(x) x.IsChecked.Equals(True)) = Me.SubCategories.Count 
            If allChildrenChecked Then 
                Return True 
            End If 
 
            Dim allChildrenUnchecked As Boolean = Me.SubCategories.LongCount(Function(x) x.IsChecked.Equals(False)) = Me.SubCategories.Count 
            If allChildrenUnchecked Then 
                Return False 
            End If 
 
            Return Nothing 
        End Function

We need to call both methods when the checked state of each item is changed. That basically means that we need to call them when the IsChecked property value is changed:

    public bool? IsChecked 
    { 
        get 
        { 
            return this._isChecked; 
        } 
        set 
        { 
            if (this._isChecked != value) 
            { 
                this._isChecked = value; 
                this.UpdateCheckState(); 
                OnPropertyChanged("IsChecked"); 
            } 
        } 
    } 
 
    private void UpdateCheckState() 
    { 
        // update all children: 
        if (this.SubCategories.Count != 0) 
        { 
            this.UpdateChildrenCheckState(); 
        } 
        //update parent item 
        if (this.parentItem != null) 
        { 
            bool? parentIsChecked = this.parentItem.DetermineCheckState(); 
            this.parentItem.IsChecked = parentIsChecked; 
        } 
    }

    Public Property IsChecked() As Boolean? 
        Get 
            Return Me._isChecked 
        End Get 
        Set(ByVal value As Boolean?) 
            If Not Me._isChecked.Equals(value) Then 
                Me._isChecked = value 
                Me.UpdateCheckState() 
                OnPropertyChanged("IsChecked") 
            End If 
        End Set 
    End Property 
    Private Sub UpdateCheckState() 
        ' update all children: ' 
        If Me.SubCategories.Count <> 0 Then 
            Me.UpdateChildrenCheckState() 
        End If 
        'update parent item ' 
        If Me.parentItem IsNot Nothing Then 
            Dim parentIsChecked? As Boolean = Me.parentItem.DetermineCheckState() 
            Me.parentItem.IsChecked = parentIsChecked 
 
        End If 
    End Sub

Now our CategoryViewModel logic is almost complete. However, if you take a closer look at the IsChecked property setter implementation, you will notice that the UpdateCheckState() method will cause the setter to be executed multiple times for the same item. This is why we'll have to implement a reentrancy check:

    private bool reentrancyCheck = false; 
    public bool? IsChecked 
    { 
        get 
        { 
            return this._isChecked; 
        } 
        set 
        { 
            if (this._isChecked != value) 
            { 
                if (reentrancyCheck) 
                    return; 
                this.reentrancyCheck = true; 
                this._isChecked = value; 
                this.UpdateCheckState(); 
                OnPropertyChanged("IsChecked"); 
                this.reentrancyCheck = false; 
            } 
        } 
    }

    Private reentrancyCheck As Boolean = False 
    Public Property IsChecked() As Boolean? 
        Get 
            Return Me._isChecked 
        End Get 
        Set(ByVal value As Boolean?) 
            If Not Me._isChecked.Equals(value) Then 
                If reentrancyCheck Then 
                    Return 
                End If 
                Me.reentrancyCheck = True 
                Me._isChecked = value 
                Me.UpdateCheckState() 
                OnPropertyChanged("IsChecked") 
                Me.reentrancyCheck = False 
            End If 
        End Set 
    End Property

So finally the CategoryViewModel looks like that:

    using System; 
    using System.Collections.ObjectModel; 
    using Telerik.Windows.Controls; 
    using System.Linq; 
 
    namespace TreeViewMVVMCheckBoxSample.ViewModels 
    { 
        public class CategoryViewModel : ViewModelBase 
        { 
            private string _name; 
            private bool? _isChecked; 
            private bool reentrancyCheck = false; 
            private CategoryViewModel parentItem; 
 
            private ObservableCollection<CategoryViewModel> _subCategories = null; 
 
            public string Name 
            { 
                get 
                { 
                    return this._name; 
                } 
                set 
                { 
                    this._name = value; 
                } 
            } 
            public bool? IsChecked 
            { 
                get 
                { 
                    return this._isChecked; 
                } 
                set 
                { 
                    if (this._isChecked != value) 
                    { 
                        if (reentrancyCheck) 
                            return; 
                        this.reentrancyCheck = true; 
                        this._isChecked = value; 
                        this.UpdateCheckState(); 
                        OnPropertyChanged("IsChecked"); 
                        this.reentrancyCheck = false; 
                    } 
                } 
            } 
 
            public ObservableCollection<CategoryViewModel> SubCategories 
            { 
                get 
                { 
                    if (this._subCategories == null) 
                    { 
                        this._subCategories = new ObservableCollection<CategoryViewModel>(); 
                    } 
                    return this._subCategories; 
                } 
            } 
 
            public CategoryViewModel(CategoryViewModel parent) 
            { 
                this.parentItem = parent; 
            } 
 
            private void UpdateCheckState() 
            { 
                // update all children: 
                if (this.SubCategories.Count != 0) 
                { 
                    this.UpdateChildrenCheckState(); 
                } 
                //update parent item 
                if (this.parentItem != null) 
                { 
                    bool? parentIsChecked = this.parentItem.DetermineCheckState(); 
                    this.parentItem.IsChecked = parentIsChecked; 
 
                } 
            } 
 
            private void UpdateChildrenCheckState() 
            { 
                foreach (var item in this.SubCategories) 
                { 
                    if (this.IsChecked != null) 
                    { 
                        item.IsChecked = this.IsChecked; 
                    } 
                } 
            } 
 
            private bool? DetermineCheckState() 
            { 
                bool allChildrenChecked = this.SubCategories.Count(x => x.IsChecked == true) == this.SubCategories.Count; 
                if (allChildrenChecked) 
                { 
                    return true; 
                } 
 
                bool allChildrenUnchecked = this.SubCategories.Count(x => x.IsChecked == false) == this.SubCategories.Count; 
                if (allChildrenUnchecked) 
                { 
                    return false; 
                } 
 
                return null; 
            } 
        } 
    }

    Imports System.Collections.ObjectModel 
    Imports Telerik.Windows.Controls 
 
    Namespace TreeViewMVVMCheckBoxSample.ViewModels 
        Public Class CategoryViewModel 
            Inherits ViewModelBase 
            Private _name As String 
            Private _isChecked As Boolean? 
            Private reentrancyCheck As Boolean = False 
            Private parentItem As CategoryViewModel 
 
            Private _subCategories As ObservableCollection(Of CategoryViewModel) = Nothing 
 
            Public Property Name() As String 
                Get 
                    Return Me._name 
                End Get 
                Set(ByVal value As String) 
                    Me._name = value 
                End Set 
            End Property 
            Public Property IsChecked() As Boolean? 
                Get 
                    Return Me._isChecked 
                End Get 
                Set(ByVal value As Boolean?) 
                    If Not Me._isChecked.Equals(value) Then 
                        If reentrancyCheck Then 
                            Return 
                        End If 
                        Me.reentrancyCheck = True 
                        Me._isChecked = value 
                        Me.UpdateCheckState() 
                        OnPropertyChanged("IsChecked") 
                        Me.reentrancyCheck = False 
                    End If 
                End Set 
            End Property 
 
            Public ReadOnly Property SubCategories() As ObservableCollection(Of CategoryViewModel) 
                Get 
                    If Me._subCategories Is Nothing Then 
                        Me._subCategories = New ObservableCollection(Of CategoryViewModel)() 
                    End If 
                    Return Me._subCategories 
                End Get 
            End Property 
 
            Public Sub New(ByVal parent As CategoryViewModel) 
                Me.parentItem = parent 
            End Sub 
 
            Private Sub UpdateCheckState() 
                ' update all children: ' 
                If Me.SubCategories.Count <> 0 Then 
                    Me.UpdateChildrenCheckState() 
                End If 
                'update parent item ' 
                If Me.parentItem IsNot Nothing Then 
                    Dim parentIsChecked? As Boolean = Me.parentItem.DetermineCheckState() 
                    Me.parentItem.IsChecked = parentIsChecked 
 
                End If 
            End Sub 
 
            Private Sub UpdateChildrenCheckState() 
                For Each item In Me.SubCategories 
                    If Me.IsChecked IsNot Nothing Then 
                        item.IsChecked = Me.IsChecked 
                    End If 
                Next item 
            End Sub 
 
            Private Function DetermineCheckState() As Boolean? 
                Dim allChildrenChecked As Boolean = Me.SubCategories.LongCount(Function(x) x.IsChecked.Equals(True)) = Me.SubCategories.Count 
                If allChildrenChecked Then 
                    Return True 
                End If 
 
                Dim allChildrenUnchecked As Boolean = Me.SubCategories.LongCount(Function(x) x.IsChecked.Equals(False)) = Me.SubCategories.Count 
                If allChildrenUnchecked Then 
                    Return False 
                End If 
 
                Return Nothing 
            End Function 
        End Class 
    End Namespace

As the items ViewModel is ready, we can create a MainViewModel to define a collection of CategoryViewModel objects that will be used as the RadTreeView.ItemsSource.

    using System; 
    using System.Collections.ObjectModel; 
 
    namespace TreeViewMVVMCheckBoxSample.ViewModels 
    { 
        public class MainViewModel 
        { 
            public ObservableCollection<CategoryViewModel> Categories { get; set; } 
 
            public MainViewModel() 
            { 
                Categories = new ObservableCollection<CategoryViewModel>(); 
 
                CategoryViewModel beverages = new CategoryViewModel(null); 
                beverages.Name = "Bevereges"; 
 
                for (int i = 0; i < 5; i++) 
                { 
                    CategoryViewModel prod = new CategoryViewModel(beverages) 
                    { 
                        Name = String.Format("Beverage {0}", i), 
                        IsChecked = false 
                    }; 
 
                    for (int j = 0; j < 3; j++) 
                    { 
                        prod.SubCategories.Add(new CategoryViewModel(prod) 
                    { 
                        Name = String.Format("SubBeverage {0}.{1}", i, j), 
                        IsChecked = false 
                    }); 
                    } 
                    beverages.SubCategories.Add(prod); 
                } 
                Categories.Add(beverages); 
 
 
                CategoryViewModel confections = new CategoryViewModel(null); 
                confections.Name = "Confections"; 
                for (int i = 0; i < 7; i++) 
                { 
                    confections.SubCategories.Add(new CategoryViewModel(confections) 
                    { 
                        Name = String.Format("Confection {0}", i), 
                        IsChecked = false 
                    }); 
                } 
                Categories.Add(confections); 
 
                CategoryViewModel condiments = new CategoryViewModel(null); 
                condiments.Name = "Condiments"; 
                for (int i = 0; i < 3; i++) 
                { 
                    condiments.SubCategories.Add(new CategoryViewModel(condiments) 
                    { 
                        Name = String.Format("Condiment {0}", i), 
                        IsChecked = false 
                    }); 
                } 
                Categories.Add(condiments); 
            } 
        } 
    }

    Imports System.Collections.ObjectModel 
 
    Namespace TreeViewMVVMCheckBoxSample.ViewModels 
        Public Class MainViewModel 
            Public Property Categories() As ObservableCollection(Of CategoryViewModel) 
 
            Public Sub New() 
                Categories = New ObservableCollection(Of CategoryViewModel)() 
 
                Dim beverages As New CategoryViewModel(Nothing) 
                beverages.Name = "Bevereges" 
 
                For i As Integer = 0 To 4 
                    Dim prod As New CategoryViewModel(beverages) With {.Name = String.Format("Beverage {0}", i), .IsChecked = False} 
 
                    For j As Integer = 0 To 2 
                        prod.SubCategories.Add(New CategoryViewModel(prod) With {.Name = String.Format("SubBeverage {0}.{1}", i, j), .IsChecked = False}) 
                    Next j 
                    beverages.SubCategories.Add(prod) 
                Next i 
                Categories.Add(beverages) 
 
 
                Dim confections As New CategoryViewModel(Nothing) 
                confections.Name = "Confections" 
                For i As Integer = 0 To 6 
                    confections.SubCategories.Add(New CategoryViewModel(confections) With {.Name = String.Format("Confection {0}", i), .IsChecked = False}) 
                Next i 
                Categories.Add(confections) 
 
                Dim condiments As New CategoryViewModel(Nothing) 
                condiments.Name = "Condiments" 
                For i As Integer = 0 To 2 
                    condiments.SubCategories.Add(New CategoryViewModel(condiments) With {.Name = String.Format("Condiment {0}", i), .IsChecked = False}) 
                Next i 
                Categories.Add(condiments) 
 
            End Sub 
        End Class 
    End Namespace

Finally we need to set up the RadTreeView control and its ItemTemplate. Please note that we won't use the RadTreeView check-box support, but instead we will define a CheckBox in the ItemTemplate of the control.

    <Window.DataContext> 
        <vm:MainViewModel /> 
    </Window.DataContext> 
    <Grid> 
        <telerik:RadTreeView Margin="5" ItemsSource="{Binding Categories}" Padding="5"> 
            <telerik:RadTreeView.ItemTemplate> 
                <HierarchicalDataTemplate ItemsSource="{Binding SubCategories}"> 
                    <StackPanel Orientation="Horizontal"> 
                        <CheckBox IsChecked="{Binding IsChecked, Mode=TwoWay}" telerik:StyleManager.Theme="Office_Black" /> 
                        <TextBlock VerticalAlignment="Center" Text="{Binding Name}" /> 
                    </StackPanel> 
                </HierarchicalDataTemplate> 
            </telerik:RadTreeView.ItemTemplate> 
        </telerik:RadTreeView> 
    </Grid>

The telerik alias represents the telerik namespace: xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation" The vm alias represents the viewmodels local namespace. For example: xmlns:vm="clr-namespace:TreeViewMVVMCheckBoxSample.ViewModels"

When you run this project, you should see the following output:

You can find the sample solution in our CodeLibrary.

Implement a Tri-State CheckBox logic using MVVM

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