Saving New or Edited Cars

In this step you will add the functionality to save a new or edited car to the database and expose commands for the view to bind to. You will need to implement the required methods and expose command objects based on them.

Implementing the required methods
Exposing commands to the the view

Implementing the required methods

In the constructor add a call to a method which will be defined shortly - InitializeCurrentCarAndTitle.
```
this.InitializeCurrentCarAndTitle();
```
```
Me.InitializeCurrentCarAndTitle()
```

Create a new method called InitializeCurrentCarAndTitle. This model will initialize currentCar and title depending on wheter a car Id has been passed to the view model`s constructor. If a car Id has been passed, the view model is in edit mode, if there is no car Id, the view model is in add mode.

private void InitializeCurrentCarAndTitle()
{
    if (this.currentCarId.HasValue)
    {
        //The view model is in Edit mode
        this.CurrentCar = this.context.Cars.FirstOrDefault(car => car.CarID == this.currentCarId.Value);
        this.Title = EDIT_MODE_TITLE;
    }
    else
    {
        //The view model is in Add mode
        this.CurrentCar = new Car()
        {
            Latitude = 0.000000d,
            Longitude = 0.000000d
        };
        this.Title = ADD_MODE_TITLE;
    }
    if (this.CurrentCar.CarYear.HasValue == false)
    {
        this.CurrentCar.CarYear = (short)DateTime.Now.Year;
    }
}

Private Sub InitializeCurrentCarAndTitle()
    If Me._currentCarId.HasValue Then
        'The view model is in Edit mode
        Me.CurrentCar = Me._context.Cars.FirstOrDefault(Function(car) car.CarID = Me._currentCarId)
        Me.Title = EDIT_MODE_TITLE
    Else
        'The view model is in Add mode
        Me.CurrentCar = New Car() With {.Latitude = 0D, .Longitude = 0D}
        Me.Title = ADD_MODE_TITLE
    End If
    If Me.CurrentCar.CarYear.HasValue = False Then
        Me.CurrentCar.CarYear = DateTime.Now.Year
    End If
End Sub

Add a new method called Save. This method will add the CurrentCar object in the database if the view model is in add mode or save the changes made to CurrentCar if the view model is in edit mode.

private void Save()
{
    if (this.currentCarId.HasValue == false)
    {
        //Add the new car to the context
        this.context.Add(this.CurrentCar);
    }
    try
    {
        if (this.context.HasChanges)
        {
            this.context.SaveChanges();
        }
        this.OperationIsSuccessful = true;
    }
    catch (DataStoreException ex)
    {
        this.HandleException(ex.BackendError.Description);
        return;
    }
    catch (DuplicateKeyException ex)
    {
        this.HandleException(ex.BackendError.Description);
        return;
    }
    catch (Exception ex)
    {
        this.HandleException(ex.Message);
        return;
    }
}

Private Sub Save()
    If Me._currentCarId.HasValue = False Then
        'Add the new car to the context
        Me._context.Add(Me.CurrentCar)
    End If
    Try
        If Me._context.HasChanges Then
            Me._context.SaveChanges()
        End If
        Me.OperationIsSuccessful = True
    Catch ex As DataStoreException
        Me.HandleException(ex.BackendError.Description)
        Exit Sub
    Catch ex As DuplicateKeyException
        Me.HandleException(ex.BackendError.Description)
        Exit Sub
    Catch ex As Exception
        Me.HandleException(ex.Message)
        Exit Sub
    End Try
End Sub

Create the HandleException method. It is called from Save() and takes as an argument the exception message which should be displayed. Displays a message box with information about the occurred error, and causes the view`s controls displaying information about the car to rebind.

private void HandleException(string exceptionMessage)
{
    MessageBox.Show(SAVE_ERROR_MESSAGE + exceptionMessage, SAVE_ERROR_TITLE, MessageBoxButton.OK, MessageBoxImage.Error);
    this.OperationIsSuccessful = null;
    //Cause binding refresh
    this.RaisePropertyChanged("CurrentCar");
}

Private Sub HandleException(exceptionMessage As String)
    MessageBox.Show(SAVE_ERROR_MESSAGE + exceptionMessage, SAVE_ERROR_TITLE, MessageBoxButton.OK, MessageBoxImage.Error)
    Me.OperationIsSuccessful = Nothing
    'Cause binding refresh
    Me.RaisePropertyChanged("CurrentCar")
End Sub

Create a method called Cancel. Here you should call the Close method of the related view.

private void Cancel()
{
    this.View.Close();
}

Private Sub Cancel()
    Me.View.Close()
End Sub

Exposing commands to the the view

Now you need to expose commands based on the methods Save and Cancel to the view.

Create a property called SaveCommand. In its getter, initialize a RelayCommand object based on the Save method and return it.

public RelayCommand SaveCommand
{
    get
    {
        this.command = new RelayCommand(this.Save);
        return this.command;
    }
    set
    {
        this.command = value;
    }
}

Public Property SaveCommand As RelayCommand
    Get
        Me._command = New RelayCommand(AddressOf Me.Save)
        Return Me._command
    End Get
    Set(value As RelayCommand)
        Me._command = value
    End Set
End Property

Create a property called CancelCommand. In its getter, initialize a RelayCommand object based on the Cancel method and return it.

public RelayCommand CancelCommand
{
    get
    {
        this.command = new RelayCommand(this.Cancel);
        return this.command;
    }
    set
    {
        this.command = value;
    }
}

Public Property CancelCommand As RelayCommand
    Get
        Me._command = New RelayCommand(AddressOf Me.Cancel)
        Return Me._command
    End Get
    Set(value As RelayCommand)
        Me._command = value
    End Set
End Property

Disposing the context

In AddEditViewModel implement the IDisposable interface to make it disposable. In the Dispose method, dispose the OpenAccessContext object. This way when the view model is no longer needed and thus disposed, the context will be disposed as with it.

public void Dispose()
{
    //The context is local to the ViewModel and must be disposed together with it
    if (this.context != null)
    {
        this.context.Dispose();
    }
}

Public Sub Dispose() Implements IDisposable.Dispose
    'The context is local to the ViewModel and must be disposed together with it
    If IsNothing(Me._context) = False Then
        _context.Dispose()
    End If
End Sub

Next step: Initializing AddEditModel