How to Respond to a Collection Changed Event

Question

I have two ObservableCollections, ocOrganization and ocVendoers. The Organization object has a ReadOnly property to signal whether an Organization is also a Vendor:

Public ReadOnly Property IsVendor As Boolean
    Get
        Dim vdr As Vendor = ocVendors.FirstOrDefault(Function(v) v.OrganizationID = clsOrganizationID)
        Return vdr IsNot Nothing
    End Get
End Property

There is a button on the wpf window that displays each organization that allows the user to make an organization also a vendor. When clicked, I want that ReadOnly property to be reevaluated. To do that I have created this event handler:

  Public Sub VendorsCollectionChanged(sender As Object, e As NotifyCollectionChangedEventArgs)
        ????????
    End Sub

So I've got all the pieces in place but I don't know what to replace the "????????" with in the above Sub in order to wire them together. Or is there a different approach altogether that I should be using?

Answer 1

Public Class Organization
    Implements INotifyPropertyChanged

    Public Property OrganizationID As Integer
    <Many more....>

    Public ReadOnly Property IsVendor As Boolean
        Get
           Dim vdr As Vendor = ocVendors.FirstOrDefault(Function(v) v.OrganizationID = clsOrganizationID)
           Return vdr IsNot Nothing
        End Get
    End Property

    <Other methods>

End Class


Public Class Vendor
    Implements INotifyPropertyChanged

    Public Property VendorID() As Integer
    Public Property OrganizationID As Integer
    <Many more....>
    <Other methods>

End Class

ocOrganzatiions and ocVendors are the ObservableCollections of those classes.

On my wpf Window there is a button to make an existing organization a vendor:

Private Sub AddVendor(sender As Object, e As RoutedEventArgs) Handles btnVendor.Click
    cv = New Vendor With
        {
        .IsActive = True,
        .OrganizationID = co.OrganizationID
        }
    ocVendors.Add(cv)
    pnlVendor.DataContext = cv
End Sub

cv stands for CurrentVendor. pnlVendor is a stackpanel on the window that contains all the controls for the Vendor properties.

After AddVendor is invoked, since ocVendors has changed, I expected the ReadOnly Propety IsVendor to be re-evaluated automatically but apparently I have to do it in code and I don't know how to do that.

I should add that I know I could reove the ":ReadOnly" part and just set IsVendor = True but that seems quite inelegant and unnecessary. I will go that route if there is no other way.

Answer 2

Maybe I didn't ask clearly at the beginning, I want to confirm whether the change of your isVender property is caused by clsOrganizationID? Do you want to set isVender to true when OrganizationID and clsOrganizationID are equal? If so, you could add RaisePropertyChanged("IsVendor") to the get method of clsOrganizationIDproperty to trigger isVender property change. Like the demo below:
The code of xaml:

<StackPanel>  
        <DataGrid Width="400" Height="100" ItemsSource="{Binding OcOrganization, Mode=TwoWay ,UpdateSourceTrigger=PropertyChanged  }" AutoGenerateColumns="False">  
            <DataGrid.Columns>  
                <DataGridTextColumn Width="Auto" Header="OrganizationID" Binding="{Binding  OrganizationID}"/>  
                <DataGridTextColumn Width="*" Header="clsOrganizationID" Binding="{Binding clsOrganizationID}"/>  
                <DataGridTextColumn Width="*" Header="is render" Binding="{Binding IsVendor}"/>  
            </DataGrid.Columns>  
        </DataGrid>  
        <DataGrid ItemsSource="{Binding Orders, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Width="400" AutoGenerateColumns="False" >  
            <DataGrid.Columns>  
                <DataGridTextColumn Width="Auto" Header="Name" Binding="{Binding  Name}"/>  
               <DataGridTextColumn Width="Auto" Header="Price" Binding="{Binding Price}"/>  
                <DataGridTextColumn Width="Auto" Header="Quantity" Binding="{Binding Quantity}"/>  
                <DataGridTextColumn Width="Auto" Header="TotalPrice" Binding="{Binding TotalPrice}"/>  
            </DataGrid.Columns>  
        </DataGrid>  
    </StackPanel>  

The code of xaml.vb:

Imports System.Collections.ObjectModel  
Imports System.ComponentModel  
Imports Microsoft.VisualBasic.CompilerServices  

Partial Public Class MainWindow  
    Inherits Window  

    Public Sub New()  
        InitializeComponent()  
        Me.DataContext = New ViewModel()  
    End Sub  
    Friend Class ViewModel  
        Implements INotifyPropertyChanged  

        Private ocVendorsField As ObservableCollection(Of Vendor)  

        Public Property OCVendors As ObservableCollection(Of Vendor)  
            Get  
                Return ocVendorsField  

            End Get  
            Set(ByVal value As ObservableCollection(Of Vendor))  
                ocVendorsField = value  
                RaisePropertyChanged("OCVendors")  
            End Set  
        End Property  

        Private ocOrganizationField As ObservableCollection(Of Organization)  

        Public Property OcOrganization As ObservableCollection(Of Organization)  
            Get  
                Return ocOrganizationField  
            End Get  
            Set(ByVal value As ObservableCollection(Of Organization))  
                ocOrganizationField = value  
                RaisePropertyChanged("OcOrganization")  
            End Set  
        End Property  

        Private ordersField As ObservableCollection(Of Order)  

        Public Property Orders As ObservableCollection(Of Order)  
            Get  
                Return ordersField  
            End Get  
            Set(ByVal value As ObservableCollection(Of Order))  
                ordersField = value  
                RaisePropertyChanged("OCVendors")  
            End Set  
        End Property  

        Public Sub New()  
            ocVendorsField = New ObservableCollection(Of Vendor) From {  
            New Vendor With {  
                .OrganizationID = 0,  
                .clsOrganizationID = 1  
            },  
            New Vendor With {  
                .OrganizationID = 1,  
                .clsOrganizationID = 1  
            }  
        }  
            ocOrganizationField = New ObservableCollection(Of Organization) From {  
            New Organization With {  
                .OrganizationID = 1,  
                .clsOrganizationID = 5,  
                .OCVendors = ocVendorsField  
            }  
        }  
            ordersField = New ObservableCollection(Of Order) From {  
            New Order() With {  
                .Name = "apple",  
                .Price = 1.3,  
                .Quantity = 4  
            },  
            New Order() With {  
                .Name = "orange",  
                .Price = 3.5,  
                .Quantity = 4  
            }  
        }  
        End Sub  

        Public Event PropertyChanged As PropertyChangedEventHandler _  
        Implements INotifyPropertyChanged.PropertyChanged  

        Public Sub RaisePropertyChanged(ByVal propertyName As String)  
            Dim handler As PropertyChangedEventHandler = PropertyChangedEvent  

            If handler IsNot Nothing Then  
                handler(Me, New PropertyChangedEventArgs(propertyName))  
            End If  
        End Sub  
    End Class  

    Public Class Vendor  
        Implements INotifyPropertyChanged  

        Private organizationIDField As Integer  

        Public Property OrganizationID As Integer  
            Get  
                Return organizationIDField  
            End Get  
            Set(ByVal value As Integer)  
                organizationIDField = value  
                RaisePropertyChanged("LoOrganizationIDg")  
            End Set  
        End Property  

        Private _clsOrganizationID As Integer  

        Public Property clsOrganizationID As Integer  
            Get  
                Return _clsOrganizationID  
            End Get  
            Set(ByVal value As Integer)  
                _clsOrganizationID = value  
                RaisePropertyChanged("clsOrganizationID")  

            End Set  
        End Property  

        Public Event PropertyChanged As PropertyChangedEventHandler _  
        Implements INotifyPropertyChanged.PropertyChanged  

        Public Sub RaisePropertyChanged(ByVal propertyName As String)  
            Dim handler As PropertyChangedEventHandler = PropertyChangedEvent  

            If handler IsNot Nothing Then  
                handler(Me, New PropertyChangedEventArgs(propertyName))  
            End If  
        End Sub  

        Public Overrides Function ToString() As String  
            Return OrganizationID & " " & clsOrganizationID  
        End Function  
    End Class  

    Public Class Organization  
        Implements INotifyPropertyChanged  

        Private organizationIDField As Integer  

        Public Property OrganizationID As Integer  
            Get  
                Return organizationIDField  
            End Get  
            Set(ByVal value As Integer)  
                organizationIDField = value  
                RaisePropertyChanged("OrganizationID")  
            End Set  
        End Property  

        Private ocVendorsField As ObservableCollection(Of Vendor)  

        Public Property OCVendors As ObservableCollection(Of Vendor)  
            Get  
                Return ocVendorsField  
            End Get  
            Set(ByVal value As ObservableCollection(Of Vendor))  
                ocVendorsField = value  
                RaisePropertyChanged("OCVendors")  
            End Set  
        End Property  

        Private _clsOrganizationID As Integer  

        Public Property clsOrganizationID As Integer  
            Get  
                Return _clsOrganizationID  
            End Get  
            Set(ByVal value As Integer)  
                _clsOrganizationID = value  
                RaisePropertyChanged("clsOrganizationID")  
                RaisePropertyChanged("IsVendor")  
            End Set  
        End Property  

        Public ReadOnly Property IsVendor As Boolean  
            Get  
                Dim vdr As Vendor = OCVendors.FirstOrDefault(Function(v) v.OrganizationID = clsOrganizationID)  

                Return vdr IsNot Nothing  

            End Get  
        End Property  

        Public Event PropertyChanged As PropertyChangedEventHandler _  
        Implements INotifyPropertyChanged.PropertyChanged  

        Public Sub RaisePropertyChanged(ByVal propertyName As String)  
            Dim handler As PropertyChangedEventHandler = PropertyChangedEvent  

            If handler IsNot Nothing Then  
                handler(Me, New PropertyChangedEventArgs(propertyName))  
            End If  
        End Sub  
    End Class  

    Public Class Order  
        Implements INotifyPropertyChanged  

        Public Event PropertyChanged As PropertyChangedEventHandler _  
        Implements INotifyPropertyChanged.PropertyChanged  

        Public Sub RaisePropertyChanged(ByVal propertyName As String)  
            Dim handler As PropertyChangedEventHandler = PropertyChangedEvent  

            If handler IsNot Nothing Then  
                handler(Me, New PropertyChangedEventArgs(propertyName))  
            End If  
        End Sub  

        Private nameField As String  
        Private priceField As Double  
        Private quantityField As Integer  

        Public Property Name As String  
            Get  
                Return nameField  
            End Get  
            Set(ByVal value As String)  

                If value IsNot nameField Then  
                    nameField = value  
                    RaisePropertyChanged("Name")  
                End If  
            End Set  
        End Property  

        Public Property Price As Double  
            Get  
                Return priceField  
            End Get  
            Set(ByVal value As Double)  

                If value <> priceField Then  
                    priceField = value  
                    RaisePropertyChanged("Price")  
                    RaisePropertyChanged("TotalPrice")  
                End If  
            End Set  
        End Property  

        Public Property Quantity As Integer  
            Get  
                Return quantityField  
            End Get  
            Set(ByVal value As Integer)  

                If value <> quantityField Then  
                    quantityField = value  
                    RaisePropertyChanged("Quantity")  
                End If  
            End Set  
        End Property  

        Public ReadOnly Property TotalPrice As Double  
            Get  
                Return Price * Quantity  
            End Get  
        End Property  
    End Class  
End Class  

Demo Result:

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