I'm kinda new to WPF. I'm making an app using WPF (all the UI controls are already fixed) and MVVM but most of the events are in the code-behind. I'm in the process of clearing the code-behind but I have codes like (the ones below) to switch through tabs, and to trigger visibility of controls depending on parameters:
tabItem1.Selected = true;
textBox1.Visibility = Visibility.Hidden;
lbxHusbandsWives.Items.Add(txtHusbandsWives.Text + '/' +
cbxHusbandsWivesCountry.Text + '/' +
dpHusbandsWives.SelectedDate.Value.ToShortDateString());
How can I do that in the viewmodel? Well, inside a Command? So I can clear the messy code-behind? Thanks for your help. :)
As I said many times before, MVVM doesn't mean "no code behind". There are things that you can or should do in code-behind, as long as they are strictly related to the view and are not necessary for the ViewModel to work properly.
That being said, in most cases you don't need to do anything in code-behind. You normally use bindings to control the view from the ViewModel. This allows the ViewModel to be completely ignorant of the view: it just exposes properties that the view can access, and sends notifications when the values of the properties change. The ViewModel should definitely not manipulate the view or its components.
Everything in the code you posted can be done with bindings in XAML:
textBox1.Visibility can be bound to a bool property of the ViewModel, using a BooleanToVisibilityConverter
lbxHusbandsWives.ItemsSource can be bound to an ObservableCollection in the ViewModel (an ObservableCollection notifies the view when items are added to or removed from it)
txtHusbandsWives.Text, cbxHusbandsWivesCountry.Text and dpHusbandsWives.SelectedDate can also be bound to properties of the appropriate type
Related
I'm working on a project that utilizes WPF, using the Prism library and Unity Container.
I have a mainwindow, which contains a mainviewmodel control which in turn is populated by a collection of user controls.
My goal is to set keyboard focus to the mainviewmodel where I have an EventTrigger InvokeCommandAction which reacts to keyeventsargs...
Currently the only way the command is fired if I use a textbox within the control (or child controls). My goal is to have the mainviewmodel control or grid get and preserve keyboard focus.
Any tips would be appreciated!
//Nathan
Either not understanding your question correctly or you should review the basic concepts of MVVM in a WPF implementation.
The View is the WPF controls.
WPF Window/UserControl files contain WPF markup which is the View.
Controls in a view leverage DataBindings to the DataContext property of either the control itself or the parent containing control (which it will inherit).
DataContext property is set to an instance of an object that is the ViewModel. It contains properties to hold values and commands to execute actions.
So conceptually there is no "mainviewmodel control", there is a MainView which contains controls and may in this case have its DataContext property set to an instance o MainViewModel. (hence my confusion)
Finally, while it is possible and some might even recommend writing UI rules/logic in a view model I haven't found much benefit in it.
You are much better off putting UI logic in the XAML or in the MinView code behind. You can still access the MainViewModel in the code behind by casting the MainView.DataContext property as a MainViewModel.
So for example:
MainView.KeyDown event can be wired up to call MainViewModel.CommandX.Execute();
I'm using MVVM Pattern (with MVVM Light) to build my XAML app (win8). I have a ListView, which is bound to a property of my ViewModel. I also have a button that triggers an operation on that ViewModel, which updates that property (which results in updating the ListView). The button uses commanding to execute the operation on the ViewModel. So far so good.
The problem is that after the list is refreshed I need to perform an operation that strictly belongs to my View, not to the ViewModel. It should scroll the list to a specific item. How to trigger that operation? Should I use a specific ListView event?
Using an EventHandler and the ScrollIntoView(Object) method you can achieve what you want without using references of the View inside the ViewMovel and respecting MVVM pattern.
Create an event in your ViewModel like this:
public event EventHandler ScrollListView;
In your View add a callback to scroll the ListView when the property is updated:
ViewModel vm;
vm.ScrollListView += (sender, e) =>
{
var specificItem = **some item**;
MyListView.SelectedItem = specificItem;
MyListView.UpdateLayout();
MyListView.ScrollIntoView(MyListView.SelectedItem);
};
Then in your ViewModel when you update that property and want to scroll the ListView:
if (this.ScrollListView != null)
{
this.ScrollListView(this, EventArgs.Empty);
}
This is how I usually do with some tweaks for each case of course.
The ViewModel is there to decouple the UI Code from the UI Design (E.g. XAML). [Separation of Concerns of Designer and Developer, Automated testing of UI Code, etc]
Ideally the code-behind file of the View will be empty (except the call to InitializeComponent) and all UI logic and state will be handled by the ViewModel. However, in practice there might be some specific UI manipulation that cannot be handled by data-binding alone and you will need to resort to code. Such code should be placed in the code-behind.
In your case, the logic for (a) when and (b) which item to scroll to must be in the ViewModel (not in the View). Only any additional logic required to perform the actual scrolling in the ListView will be in the View code-behind.
Yes, an event would be the ideal way to do this, to avoid having any references to the View inside the ViewModel. I would recommend however to create a custom event in the ViewModel (e.g. OnFirstItemInViewChanged with arguments the item to scroll to) and in the View code-behind register to this event and just call ListView.ScrollIntoView(item).
Note:
WinForms DataGridView had a property FirstDisplayedScrollingRowIndex. If there was something similar in WPF ListView, you could solve this by binding this property to a ViewModel property, therefore leaving the code-behind completely clean.
As explained on http://www.lhotka.net/weblog/DataRefreshInWPF.aspx
A WPF control sometimes does not rebind to the new datacontext. Specifically when using viewmodels that implement Object.Equals to see if viewmodels are logically equal. The problem here is that the control does not rebind to the new datacontext if it is logically equal to the previous. This causes the controls to become limp, they are not bound to the new viewmodel.
I circumvented this by first setting the DataContext to a new object(). This makes sure the control rebinds to the new datacontext. This does not work however when a viewmodel contains another viewmodel that implements Object.Equals. A view bound to that viewmodel using a DataTemplate will not rebind. How can I work around this?
Obviously WPF databinding should use ReferenceEquals instead of Equals..
UPDATE: I solved it for now by having the sub ViewModels implement IEquatable<T> instead of overriding Object.Equals.
I solved it for now by having the sub ViewModels implement IEquatable instead of overriding Object.Equals.
I'm working on PRISM application with modules, MVVM and so on. I understand PRISM pretty good now and I understand value of MVVM. All those things good to deliver business value which comes from testability, "uniformity" and so on.
But now I'm stuck with certain interaction issues. I already spent hours and hours trying to see how I set focus in Silverlight via MVVM. All this additional behaviors, attached properties, triggers. It just seems like bunch of junk code with MVVM being root cause.
For example, I need to create Lookup control which is basically textbox with button and popup window. This control itself needs lot of focus control, it needs to overlay view over parent (popups) and so on. It seems to be pretty easy to create it with code-behind, stick it into separate library and move on. My business forms will use this control inside my nice MVVM PRISM.
So, question is.. Is it justified to use code-behind in isolated islands like controls and keep MVVM and TDD for actual code that brings business value?
Is there line where you say "MVVM is not going to be used here" ?
I see absolutely nothing wrong with using Code Behind providing that the code is related to view-specific properties, such as setting Focus. Your ViewModel should never need to know about or care who or what has focus, since that is a View-Specific concept.
Usually I build UserControls in two ways: they are either built for a specific Model or ViewModel, or they are meant to be generic and have their values provided by whoever calls them.
In the case of the former, such as if I wanted a SearchResultsPopup, I would build the UserControl expecting to have something like a SearchResultsViewModel as the DataContext.
For example, my UserControl would expect to find the following properties on it's DataContext, and would use them in bindings to build the View.
ObservableCollection<SearchResult> Results
SearchResult SelectedResult
bool IsOpen
ICommand OkCommand
ICommand CancelCommand
I could then use the UserControl like this:
<local:SearchResultsPopup DataContext="{Binding MySearchResultsVM}" />
In the later situation, where I am creating something generic which can be used by any Model or ViewModel, I would use custom Dependency Properties to provide my UserControl with the values it needs to bind to.
So in this example, I would have DependencyProperties for
bool IsOpen
ICommand OkCommand
ICommand CancelCommand
And my XAML would look something like this:
<local:GenericPopup local:GenericPopup.IsOpen="{Binding IsPopupOpen}"
local:GenericPopup.SaveCommand="{Binding SavePopupCommand}"
local:GenericPopup.CancelCommand="{Binding HidePopupCommand}">
<local:MySearchResultsView ... />
</local:GenericPopup>
In summary, your UserControl is either a reflection of your ViewModel (meaning it becomes a View), or it is provided values by the View. The ViewModel doesn't care either way.
What's the best method of dynamically specifying DataGrid columns in the Silverlight DataGrid control at runtime following the MVVM pattern?
What I'd like to do would be bind the "DataGrid.Columns" property to a property in my ViewModel so that, if the user adds/removes columns, I simply update the ViewModel property and the DataGrid would change. Problem is, the "DataGrid.Columns" property can't be bound to (I don't think).
Because this property isn't available nor is the DataGrid control itself available at the ViewModel level, my current approach is to step outside of the MVVM pattern for this particular implementation and capture certain events in View's code-behind using MVVM Light's Messenger class and then talk directly with the DataGrid control to achieve this capability. I know this is a general statement to this approach without details but is there an easier way... or maybe not so much easier, but a better way that adheres to the MVVM pattern a little better?
It's driving me crazy that the DataGrid control's Columns property can't be bound... seems like such a simple thing.
FYI - Before it's suggested to use AutoGenerateColumns = True, the class being bound for each item in the collection that's bound to DataGrid.ItemsSource does not have individual properties to identify what is bound to the columns... it's a collection property that contains the columns in order to keep them completely dynamic so that particular path is out. Also, handling the AutoGeneratingColumns and using e.Cancel to show/hide columns is also iffy for this same reason (I think).
I agree that it is a pain that DataGrid.Columns cannot be bound to. My recommendation here would be to define your columns in the ViewModel in an ObservableCollection. In the View (code behind), handle the CollectionChanged event of this ObservableCollection, and modify the DataGrid.Columns in code.
While this solution is less elegant, it is straightforward. For your ViewModel, you can unit test that the CollectionChanged event is raised properly when columns are added, removed or moved. The View code cannot be tested, so I guess this is something you need to live with. The advantage is that, if some day the DataGrid.Columns property can be databound, it will be easy to refactor this to remove the code behind.
Another way (I think) would be to create an attached behavior or a Blend behavior to take care of this. Attach it to the DataGrid; instead of binding to the DataGrid.Columns directly, bind to a property on the behavior, and have the behavior modify the DataGrid (the AssociatedObect) directly.
Does that make sense?
Cheers,
Laurent