Implementing the viewmodel to reflect the base model?

Question

Something that has been confusing me for a while now with WPF MVVM is for example, when I have a base model containing nothing but a few properties and some validation code and I then build a view model around this base model, how should the view model be structured.

For Example:

Base Model ->

Imports ModellingHelper
Imports FTNHelper
Imports System.ComponentModel
Imports System.ComponentModel.DataAnnotations

Public Class Parser
    Inherits BaseModel

    <Required(ErrorMessage:="Name is required.")>
    Public Property Name As String
        Get
            Return GetValue(Function() Name)
        End Get
        Set(value As String)
            SetValue(Function() Name, value)
        End Set
    End Property

    <Required(ErrorMessage:="Description is required.")>
    Public Property Description As String
        Get
            Return GetValue(Function() Description)
        End Get
        Set(value As String)
            SetValue(Function() Description, value)
        End Set
    End Property

    Public Property InputHeaderInfo As InputHeader
        Get
            Return GetValue(Function() InputHeaderInfo)
        End Get
        Set(value As InputHeader)
            SetValue(Function() InputHeaderInfo, value)
        End Set
    End Property

    Public Property InputVariables As ObservableList(Of Variable)
        Get
            Return GetValue(Function() InputVariables)
        End Get
        Set(value As ObservableList(Of Variable))
            SetValue(Function() InputVariables, value)
        End Set
    End Property

    Public Property OutputVariables As ObservableList(Of Variable)
        Get
            Return GetValue(Function() OutputVariables)
        End Get
        Set(value As ObservableList(Of Variable))
            SetValue(Function() OutputVariables, value)
        End Set
    End Property

    Public Sub New()
        Name = "New Parser"
        Description = "This is a new parser."
        InputHeaderInfo = New InputHeader()
        InputVariables = New ObservableList(Of Variable)
        OutputVariables = New ObservableList(Of Variable)
    End Sub
End Class

ViewModel ->

Imports WinTransform.DataModel
Imports System.IO
Imports WPFHelper
Imports System.ComponentModel
Imports System.ComponentModel.DataAnnotations
Imports ModellingHelper
Imports Omu.ValueInjecter

Namespace ViewModels
    Public Class ParserViewModel
        Inherits ViewBase

#Region "Properties"
        Public Property Source As Parser
            Get
                Return GetValue(Function() Source)
            End Get
            Set(value As Parser)
                SetValue(Function() Source, value)
            End Set
        End Property

        Public Property InputFile As FileInfo
            Get
                Return GetValue(Function() InputFile)
            End Get
            Set(value As FileInfo)
                SetValue(Function() InputFile, value)
                NotifyPropertyChanged(Function() InputFileContents)
                NotifyPropertyChanged(Function() InputFileParseLine)
                NotifyPropertyChanged(Function() TabVisability)
            End Set
        End Property

        Public ReadOnly Property InputFileContents As String
            Get
                If Not InputFile Is Nothing Then
                    Dim mReader = InputFile.OpenText()
                    Try
                        Return mReader.ReadToEnd()
                    Catch ex As Exception
                        MessageBox.Show(String.Format("Failed to load transform file contents: {0}", ex.Message))
                    Finally
                        mReader.Close()
                    End Try
                End If
                Return String.Empty
            End Get
        End Property

        Public ReadOnly Property InputFileParseLine As String
            Get
                If Not InputFile Is Nothing Then
                    Dim mReader = InputFile.OpenText()
                    Try
                        Dim mLines = mReader.ReadToEnd().Split(vbNewLine).Select(Function(l As String) l.Trim())
                        Dim mLineNo = Source.InputHeaderInfo.TitleLinesFixed + Source.InputHeaderInfo.TitleLinesSkipped + Source.InputHeaderInfo.ColumnHeaderLines + Source.InputHeaderInfo.LinesFixed + Source.InputHeaderInfo.LinesSkipped
                        If mLineNo >= 0 And mLineNo < mLines.Count() Then
                            Return mLines(mLineNo)
                        End If
                    Catch ex As Exception
                        MessageBox.Show(String.Format("Failed to load transform file contents: {0}", ex.Message))
                    Finally
                        mReader.Close()
                    End Try
                End If
                Return String.Empty
            End Get
        End Property

        Public ReadOnly Property TabVisability As Visibility
            Get
                If Not InputFile Is Nothing Then
                    Return Visibility.Visible
                End If
                Return Visibility.Hidden
            End Get
        End Property

        Public ReadOnly Property InputVariablesViews As ObservableList(Of VariableViewModel)
            Get
                Dim mVars As New ObservableList(Of VariableViewModel)
                For Each mVar In Source.InputVariables
                    mVars.Add(New VariableViewModel(mVar))
                Next
                AddHandler mVars.CollectionChanged, Sub() Source.InputVariables.RefreshList(mVars.Select(Function(v As VariableViewModel) v.Source))
                Return mVars
            End Get
        End Property

        Public ReadOnly Property OutputVariablesViews As ObservableList(Of VariableViewModel)
            Get
                Dim mVars As New ObservableList(Of VariableViewModel)
                For Each mVar In Source.OutputVariables
                    mVars.Add(New VariableViewModel(mVar))
                Next
                AddHandler mVars.CollectionChanged, Sub() Source.OutputVariables.RefreshList(mVars.Select(Function(v As VariableViewModel) v.Source))
                Return mVars
            End Get
        End Property

        Public Property IsSaved As Boolean
            Get
                If String.IsNullOrEmpty(SaveFile) Then
                    Return False
                End If

                If Not IsValid Then
                    Return False
                End If

                Return GetValue(Function() IsSaved)
            End Get
            Set(value As Boolean)
                SetValue(Function() IsSaved, value)
            End Set
        End Property

        Public Property SaveFile As String
            Get
                Return GetValue(Function() SaveFile)
            End Get
            Set(value As String)
                SetValue(Function() SaveFile, value)
            End Set
        End Property
#End Region

#Region "Commands"
        Public ReadOnly Property SelectInputFile As ICommand
            Get
                Return New RelayCommand(Sub() SelectInputFileExecute())
            End Get
        End Property

        Private Sub SelectInputFileExecute()
            Dim mOpenDialog = OpenDialog
            If mOpenDialog.ShowDialog() Then
                InputFile = New FileInfo(mOpenDialog.FileName)
            End If
        End Sub
#End Region

        Public Sub New()
            Source = New Parser()
            Init()
        End Sub

        Public Sub New(ByVal mFileInfo As FileInfo)
            Source = LoadParser(mFileInfo)
            SaveFile = mFileInfo.FullName
            Init()
        End Sub

        Public Sub Init()
            AddHandler PropertyChanged, Sub() IsSaved = False
            AddHandler Source.InputHeaderInfo.PropertyChanged, Sub() NotifyPropertyChanged(Function() InputFileParseLine)
        End Sub

        Public Shared Function LoadParser(ByVal mFileInfo As FileInfo) As Parser
            Try
                Dim xmlParser As New XmlDataModel.Parser()
                xmlParser.FromXmlFile(mFileInfo.FullName)
                Dim baseParser As New Parser()
                baseParser.InjectFrom(New ParserInjectionXml(baseParser, xmlParser), xmlParser)
                Return baseParser
            Catch ex As Exception
                MessageBox.Show(String.Format("Could not open parser: {0}", ex.Message))
                Return New Parser()
            End Try
        End Function

        Public Sub Save()
            If String.IsNullOrEmpty(SaveFile) Then
                Dim mSaveDialog = SaveDialog
                If mSaveDialog.ShowDialog() Then
                    SaveFile = mSaveDialog.FileName
                Else
                    Return
                End If
            End If
            IsSaved = Save(SaveFile)
        End Sub

        Public Function Save(ByVal mFilePath As String) As Boolean
            SaveFile = mFilePath
            Return SaveParser(mFilePath, Source)
        End Function

        Public Shared Function SaveParser(ByVal mFilePath As String, ByVal mParser As Parser) As Boolean
            If Not mParser.IsValid Then
                Return False
            End If

            Try
                Dim xmlParser As New XmlDataModel.Parser()
                xmlParser.InjectFrom(New ParserInjectionXml(mParser, xmlParser), mParser)
                xmlParser.ToXmlFile(mFilePath)
                Return True
            Catch ex As Exception
                MessageBox.Show(String.Format("Could not save parser: {0}", ex.Message))
                Return False
            End Try
        End Function
    End Class
End Namespace

What I am wondering is, if there is a better way to structure the view model to improve data binding, so I don't have to bind to Source.Name etc. How should I handle the base model in the view model?

Thanks, Alex.

Answer 1

It really depends.

If your Model already implements INotifyPropertyChanged and uses collection types that implement INotifyCollectionChanged, I personally feel that directly encapsulating and binding to "Source.Name" in XAML has some real advantages - mainly, it dramatically reduces the amount of code, and it (more importantly) reduces the amount of unnecessary code duplication.

However, Model classes often aren't designed specifically with WPF or Silverlight support in mind, and require wrapping. As soon as you have to wrap parts of the Model into the ViewModel for handling specific notifications, wrapping the entire model leads to more consistency.

This really has a cost-benefit trade-off to consider. If you're working with a different designer, wrapping everything leads to consistency in the API (that the designer uses), which can help reduce bugs (at the cost of some extra duplication on your part). If you're doing everything, then it's really up to you what makes the most sense.