Explicit UpdateSource on element with LostFocus trigger

Question

My Scenario is, upon opening application, I read lots of data from file (using XMLSerializer) and put it into ObservableCollection<MyClass>. Using Bindings, I present data to users. When they change data in a field, proper data in collection is updated, but I don't want that data to be saved to file upon LostFocus. I have a button 'SAVE'.

I don't want to use UpdateSOurceTrigger = PropertyChanged, I want to keep LostFocus. The problem is, when user enters data to a TextBox and presses Save button, TextBox does not loses focus, meaning data is not propagated to collection and is not saved. Below is my solution that works, yet, my question is, Is it a proper way of doing, or is there other, better way?

The code I added to Save button, before saving collection:

IInputElement focusedElement = Keyboard.FocusedElement;

if (focusedElement is TextBox)
{
    BindingExpression myBinding = BindingOperations.GetBindingExpression((TextBox)focusedElement, TextBox.TextProperty);
    myBinding.UpdateSource();
}

Answer 1

Here is a small example of your scenario:

<Window x:Class="MementoAndLostFocus.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:MementoAndLostFocus"
    mc:Ignorable="d"
    Title="MainWindow" Height="350" Width="525">
<Window.DataContext>
    <local:MainViewModel/>
</Window.DataContext>
<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="10*"/>
        <RowDefinition />
    </Grid.RowDefinitions>
    <TextBox Text="{Binding ExampleInstance.ExampleName, UpdateSourceTrigger=LostFocus}"
             BorderThickness="1"
             BorderBrush="Black"
             VerticalAlignment="Top"
             Margin="10"/>
    <Button Grid.Row="1"
            HorizontalAlignment="Left" 
            VerticalAlignment="Center" 
            Content="Save"
            Command="{Binding SaveCommand}"/>
</Grid>

public class MainViewModel
{
    public ExampleClass ExampleInstance { get; set; }
    public ICommand SaveCommand { get; set; }

    public MainViewModel()
    {
        ExampleInstance = new ExampleClass() { ExampleName = "Example Name" };
        SaveCommand = new SaveCommand(this);
    }

    internal void Save()
    {
        //TO DO  - Save item to database
        Console.WriteLine(ExampleInstance.ExampleName);
    }
}

public class ExampleClass : INotifyPropertyChanged
{
    private string _exampleName;

    public string ExampleName
    {
        get { return _exampleName; }
        set
        {
            PropertyChanged(this, new PropertyChangedEventArgs(nameof(ExampleName)));
            _exampleName = value;
        }
    }

    public event PropertyChangedEventHandler PropertyChanged = delegate { };
}

public class SaveCommand : ICommand
{
    private MainViewModel _vm;

    public event EventHandler CanExecuteChanged;

    public SaveCommand(MainViewModel vm)
    {
        _vm = vm;
    }

    public bool CanExecute(object parameter)
    {
        return true;
    }

    public void Execute(object parameter)
    {
        _vm.Save();
    }
}

So what exactly is going on here:

As you see, the TextBox is there and its UpdateSourceTrigger is set to LostFocus.

When I modify the value there and click Save button, I am able to see what happens, with a break point in the Save method and the value is updated, because when you click the button, the TextBox loses the focus.

Anyway i wanted to write this to you because there could be something more.

Have a look at IEditableObject interface :

Provides functionality to commit or rollback changes to an object that is used as a data source.

That's about it.