Should a ViewModel control focus in the View - wpf

I have an ItemsControl binding to a collection in the ViewModel. As a result of user input, a new item gets added to the collection and this gets displayed on the View.
The item is also a View-ViewModel pair, the View contains a TextBox that I would like to receive focus immediately after being added to the collection.
How do I set the focus to a TextBox without referencing the View from within the ViewModel? Are attached properties the way to go here?

well you can do this by creating a behaviour..
have a look here to get an idea of behaviours controlling focus

Related

Access Datagrid from ViewModel

I have a simple WPF window with a Datagrid and an "Add" button. The add button is wired to the ViewModel using a RelayCommand, and basically just adds a new object to the observable collection and then sets the selected item (also databound from the grid's SelectedItem property). The end result is a new row is added to the grid and is then set to the selected row. This is almost the result I'm trying to achieve, because the next step is to put the grid in edit mode by invoking BeginEdit. The problem is, from my understanding, the VM isn't supposed to access controls on the view directly.
So my question is, what would be the most "correct" way of achieving this using MVVM?

Setting Focus to a Control after the Loading of the DataGrid

I have a UserControl in WPF and it contains a variety of controls in it... The most important being the DataGrid Control. The DataGrid control is bound to an Observable Collection list. This list filled with different items based on couple of filters selected by the user. Now, once the DataGrid displays all the data. I want to set the Focus on the first (Filters) combobox on my usercontrol. Is there any way to know when the DataGrid has completely loaded??? Is there any DataTrigger I could set that i could trigger to inform me that Grid was refreshed with new list and I can set the ComboBox to focus. Basically i'm not able to set the focus to the first control on my UserControl when the data in the DataGrid has been repopulated... Please let me know if anyone knows how to resolve this issue!!
Thanks in advance.
I hate to work in code behind but sometimes that is only way. This can be done in two ways according to me:
Use EventAggregator to raise the event from your ViewModel once you are done with your filtering logic in it. Listen to this event in view code behind and in the handler do firstCombo.Focus()
Second is bit dirty, So there must be the button or the last control after which you apply filter on your listview. In the buttonClickHandler/or any event of the last control (like selectionchange) directly do firstCombo.Focus(). In this case the moment you will press your button focus will move to combo.
Thanks

WPF textbox not losing focus when selecting custom control

In a WPF/MVVM application I have a custom control on a particular view. This control extends WPF DataGrid and contains User names and ids.
On the same view I have some textboxes whose Text properties are bound to all different properties of User object exposed by the viewmodel and UpdateSourceTrigger for the Text properties are set to LostFocus.
Data gets updated as they should be whenever I leave a textbox (since the textbox looses focus). But problem is, this doesn't occur when I select any item in the custom control leaving any textbox, textbox data doesn't update. Can anyone explain what's happening?
Perhaps your custom control has its own focus scope defined, thus allowing logical focus to be in both the text box and your custom control? Try checking in snoop.

How can I prevent duplicates in a WPF Listbox control?

I have a WPF listbox control that is declaratively bound to a textbox. The listbox's ItemsSource is an ObservableCollection that is built from an XML file. I can easily prevent duplicate entries in the listbox when a new item is added because I can check for it in the "Add" button's Click event handler.
However, when an existing item's value is changed in the textbox (which obviously shows the listbox's selected item) to one that already exists in the list I want to prevent this, but I don't know how.
I'd appreciate help with this!
You can create your own validation rule by deriving from ValidationRule and apply it to your text box's binding. In the Validate method you can check for duplicates and return a ValidationResult of false to prevent the binding source from being updated.
Listen to the CollectionChanged event and check when the collection has been modified if there are any duplicates and remove them.
Also, you can take a look at this question and its' answer for an observable collection that also notifies you when its' items' properties change.
Edit:
If you don't want to use the collection I mentioned above, you can make sure your collection's items implement INotifyPropertyChanged and every time you add an item to the collection, listen to its PropertyChanged event. In the handler, you check if the property that changes is the one that is displayed in the ListBox and check if any other element has the same value of this property. If you find such an element, you either change the value of your property to its old value, or remove the element entirely, it depends on the logic of your application.

WPF listview databinding - How do I force update to underlying object without tabbing out?

I have a WPF ListView that is bound to a BindingList<T>. The binding works like a charm, but I have to tab out of the cell to get the bound property to update....this is an issue because most users don't tab out of the last column before clicking the save button.
How do I force the list view to "persist" the changes to the bound DataContext without doing something hackish.
Bindings in WPF have a property called "UpdateSourceTrigger" which tells the Binding when to update the thing the UI is bound to. By default, it's set to "LostFocus" for the Text property which is what you're most likely using.
Change the trigger to "PropertyChanged" in your binding like this:
Text="{Binding Foo,UpdateSourceTrigger=PropertyChanged}"
... and now the source "Foo" property will be updated as the Text changes in the UI.
There's also an "Explicit" setting for UpdateSourceTrigger which is handy if you need to hold off writing any changes to the source until, say, the user clicks the OK button.

Resources