在WPF中使用BindingExpression的最佳实践？

Question

I have some UI elements that do some UI-specific work in code behind then update a binding in the data context.

WPF Elements:

    <TextBox Grid.Row="1"
             Text="{Binding PartNumber, UpdateSourceTrigger=Explicit}"
             Name="ui_partNumber"
             FontSize="35"
             VerticalContentAlignment="Center" />
    <Button Grid.Column="1"
            Grid.Row="1"
            Content="OK"
            Click="PartOKClick"
            FontSize="20"
            Width="150" />

Code behind:

/// <summary>
/// Handle updating the view model with the part number
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void PartOKClick(object sender, RoutedEventArgs e) {

  //Get the textbox's binding expression
  BindingExpression be = ui_partNumber.GetBindingExpression(TextBox.TextProperty);

  if (Condition) {
    //Update part number binding
    be.UpdateSource();

    //Animate to next state
    InAnimate(ui_partGrid.Name);
    OutAnimate(ui_whichPartNumber.Name);
  }
  else {
    //Discard the text in the textbox 
    be.UpdateTarget();

    //Animate to notification state
    InAnimate(ui_invalidLocation.Name);
  }
}

The property in my ViewModel looks like:

public string PartNumber{
    get { return _partNumber; }
    set { _partNumber = value; OnPropertyChanged("PartNumber"); }
}

I'm using explicit binding and only updating the source if things check out, otherwise I'm just reverting to the original binding.

The question is, is this the best way to work with the binding explicitly? If I'm getting the BindingExpression of 100 elements of varying types, do I need to do it by hand each time? Would I be able to do it in a more reusable way?

Answer 1

If I understand correctly, you are willing to check the value entered in the TextBox and update the binding only if it is valid, right?

Well luckily, WPF has a built-in error handling process which is way cleaner that what you did there. You should read something about IDataErrorInfo

This article is pretty clear about how to use it

As an example in your case, you'd have something like this:

WPF Elements:

<TextBox Grid.Row="1"
         Text="{Binding PartNumber, ValidatesOnDataErrors=True}"
         Name="ui_partNumber"
         FontSize="35"
         VerticalContentAlignment="Center" />
<Button Grid.Column="1"
        Grid.Row="1"
        Content="OK"
        Click="PartOKClick"
        FontSize="20"
        Width="150" />

In your ViewModel , you should have this:

public string this[string columnName]
        {
            get
            {
                if (string.Equals(columnName, "PartNumber", StringComparison.OrdinalIgnoreCase) || columnName == string.Empty)
                {
                    // Here, IDataErrorInfo is checking the property "PartNumber" bound to your TextBox
                    if (this.IsPartNumberValid(ui_partNumber.Text))
                    {
                        // Not valid: return any error message (string.Empty = no error, otherwise it will be seen as not valid)
                        return "Not valid!";
                    }
                }
                return string.Empty;
            }
        }

This should do the trick for you: if the string "Not valid!" is returned, the TextBox will be displayed with a red border and the Binding won't be updated

Answer 2

Why are you forced to use explicit binding? Why not do the validation in your ViewModel and then fire off an OnPropertyChanged("BindingParameter") only if you need an update?

Something like this (in VB):

Property prop as Object
  Get
   return _prop
  End Get 
  Set(ByVal value As Object)
    If your_validation_check(value) then
      _prop = value
      OnPropertyChanged("prop") 'INotifyPropertyChanged
    End If
  End Set 
End Property