Binding with custom object in MVVM textbox

Question

I have started learning MVVM with some basic applications and I just encountered below issue with binding.

I have 2 textboxes in my View say- Student_name and Student_year. I have a Student class implemented in my viewmodel with its properties. But, the actual Student class is in my Model layer.

<TextBox x:Name="StuName" 
Text="{Binding Path=MyStudent.Name, Mode=TwoWay,
UpdateSourceTrigger=PropertyChanged}" />   


<TextBox x:Name="StuYear" 
Text="{Binding Path=MyStudent.Year, Mode=TwoWay,
UpdateSourceTrigger=PropertyChanged}" />

ViewModel:

private Student _myStudent = new Student();

public Student MyStudent
 {
   get { return _myStudent ; }
   set
    {
     if (value != _myStudent )
      {
       _myStudent = value;
       OnPropertyChanged("MyStudent");                       
      }
    }
}

Model (Student Class):

public string Name
  {
   get { return _name; }
    set
    {
     if (_name!= value)
     {
      _name= value;      
      OnPropertyChanged("Name");
      }
    }
   }

I can see everything working fine on binding the values from VM to View. But, the other way is behaving little tricky here..

Wheneven I change Name/Year in the textbox, the control has to land on Viewmodel's Set property? Rather, it straight away goes to Model's Set property.

For Instance, When I modify txtbox 'StuName', SET method of Student class is invoked. But not SET method of Viewmodel(MyStudent object).

I am not sure why this behaves in such a way. Is it because I have directly bounded Student.Name to the textbox? What are the alternatives to handle this SET operation in Viewmodel class..

Thanks in advance.

PS: I have implemented INotifyPropertyChanged interface properly and rest other bindings(of primitive data type) are working fine with other controls.

Community · Accepted Answer · 2017-05-23 11:56:58Z

2

As Philip Stuyck correctly pointed out in his answer, the ViewModel only has a setter for the Student instance, which never changes. So the setter on the ViewModel is never invoked. The binding goes to the name property of that instance.

A different approach would be to wrap the name property in you ViewModel explicitly. This allows for a clearer separation of concerns between Model and ViewModel. I.e. right now your Model implements INotifyPropertyChanged which IMO belongs into the ViewModel, because in general it is only used for triggering View updates. Your ViewModel would look like this:

class StudentViewModel
{
    private Student _myStudent = new Student();

    public string Name
    {
        get { return _myStudent.Name ; }
        set
        {
            if (value != _myStudent.Name )
            {
                _myStudent.Name = value;
                OnPropertyChanged("Name");                       
            }
        }
    }
}

Your Model on the other hand becomes simpler, because it doesn't have to implement INotifyPropertyChanged anymore.

edited May 23, 2017 at 11:56

CommunityBot

11 silver badge

answered Feb 12, 2015 at 8:42

EagleBeak

7,4598 gold badges37 silver badges50 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Bharathi Over a year ago

Thanks for your response :) +1 for suggesting alternate workaround

Philip Stuyck · Accepted Answer · 2015-02-12 06:53:21Z

1

That is normal behavior because your binding is to MyStudent.Name. So the Mystudent setter is never called because the instance never changes. The setter of the name is called because in fact that is where your binding is going to.

answered Feb 12, 2015 at 6:53

Philip Stuyck

7,4673 gold badges31 silver badges40 bronze badges

Collectives™ on Stack Overflow

Binding with custom object in MVVM textbox

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related