I have heard a lot about two-way bindings in WPF, but I'm not entirely clear on how to accomplish it or what it actually means.
I have a ListView with a bunch of items in it. When the user selects a new item, a TextBox in the application will change its text to display some property of the selected item.
But when the user changes the text in the text box I want the ListView item to be updated immediately as well. Is there any "two-way binding" magical WPF way of accomplishing this?
Mark's answer shows how to accomplish what you want, but you also asked more generally about "how to accomplish [two-way binding] and what it actually means."
One-way binding means that the binding target (e.g. control) will display data from the binding source (e.g. business object), and will update itself as the business object changes, but that changes to the control will not be propagated back to the business object. E.g. if the Person.Name changes from "bob" to "kate", the TextBlock.Text bound to the Name will change from "bob" to "kate" too.
Two-way binding simply means that not only are changes in the business object reflected in the UI, but changes made by the user in the UI are propagated back to the business object too. So now when the user edits the TextBox.Text bound to the Name, say changing "kate" to "edmund", WPF will set the Person.Name property to "edmund" as well.
To accomplish this, just set Mode=TwoWay on the Binding declaration. Some properties bind two-way by default: TextBox.Text, for example, binds TwoWay by default, which is why Mark's code doesn't need the Mode declaration. In addition, as Mark notes, by default WPF only propagates changes back to the business object when the control loses focus. If you have two UI elements bound to the same property, this can mean they appear out of sync, in which case you can use the UpdateSourceTrigger to force WPF to propagate whenever the property changes.
MSDN covers this in detail with some good clear diagrams: see Data Binding Overview in the WPF SDK.
If you haven't you'll need to implement INotifyPropertyChanged for your class that you're binding to.
Also, when you say you want the ListBox item to be updated immediately, you mean that you want it to change as you type in the TextBox. By default the TextBox.Text property updates its source when it loses focus, but you can change this by setting the binding UpdateSourceTrigger to PropertyChanged:
{Binding Source={...}, Path=Whatever, UpdateSourceTrigger=PropertyChanged}
What is the Type of the items in the ListView? To get the two way binding going the need you implement INotifyPropertyChanged...
This might help WPF event property changed?
Related
As far as I know, the default way to use a ObservableCollection that is bound to a listview is with model classes as elements (ObservableCollection<MyModel>). So when a listview element is selected, we use NavigateAsync and pass the model, which then can be used by the ViewModel to "fill itself".
The problem with this approach is, that it's not possible to use ViewModel properties for binding in the listview.
For example:
I have a View, ViewModel and Model "PickList", which contains a collection of "PickLine" objects - each having a View, ViewModel and Model themselves. The PickLine object contains a property "PickedQuantity" and a property "OpenQuantity". Now in my PickList view, I don't want to bind these two to separate items (e.g. two labels), but I want to have one label to display both I a format like for example "PickedQuantity / OpenQuantity". I know this example can be solved by using multi binding or something like this. But that's not the meaning of it all.
My PickLine ViewModel already has a property "QuantityString", that I want to bind to the label of a listview element via DataTemplate. But how can I do this. Is it even possible?
Make a property that combines the two other properties and bind to that. E.g.:
public string FullQuantity {get {return $"{PickedQuantity} / {OpenQuantity}";}}
Then in the setter for PickedQuantity and OpenQuantity, you will want to call whatever PropertyChanged method you have set up to notify the bindings of a property change and pass in the FullQuantity property name so elements that are bound to FullQuantity get updated when either PickedQuantity or OpenQuantity are changed.
This way, you are only binding one label's text to one property and that label would get updated when either of the two quantity properties are changed.
Note: I am unfamiliar with Prism, but this approach should work regardless of the Mvvm framework in use.
Your PickListViewModel should expose a collection property whose items are of type PickLineViewModel (not PickLine).
Whether you need an ObservableCollection<PickLineViewModel> depends on where changes can happen - in service / model that initially created the PickLines or in the GUI or both. In any way, you have to make sure the changes are propagated from one side (the collection of view models) to the other (the collection of models). Google wrapping observable collection as a starter (hint: avoid two-way sync if possible). These blog posts are old but still relevant and make a good reading. A trivial wrapping is described in this answer.
A while back i had read a tutorial on data binding (maybe it was MVVM?) in Windows Forms. I've sense forgot everything and forgot the name of the tutorial.
What i would like to do is bind the enabled property of a button to a combox's selected item.
Logic: If combobox has selected item- enable button.
else disable button.
I'm aware of the combobox_textchanged and combobox_selecteditemchanged event and i would like to avoid using it if possible.
In WPF / MVVM, this is a UI issue that can be handled in a ViewModel class. In Windows Forms, you might also want to create a ViewModel class separate from your model class to keep UI concerns away from the Model classes. Either way, you can create a Boolean property like "IsActiveCustomer", or whatever it is in your case, in the object class you are binding to. Your property could either have a getter that returns a value based on the property that is bound to the combo box -- or you could use the combo box selected index changed or selected value changed events and set the Boolean property accordingly. Then, of course, bind the Enabled property of the button to the Boolean property. Probably need to know what you are data binding to in order to provide specifics (binding to object vs. BindingSource/table adapter, etc.)
I have a C# Windows Phone 7.1 project that uses the MVVM Light toolkit. On one application page in the app I have a Telerik data bound list box that is bound to a property in my view model. At first that property had a return type of:
List<string>
With that return type the property did not show up in the list of candidate elements in the Path list box, when I activated the Create Data Binding dialog box in order to assign the list box's ItemsSource property. However, when I changed that property's return type to:
ObservableCollection<string>
it showed up immediately. In the future, how can I determine the correct return type for a particular control's ItemsSource or other data bindable property?
Your question is little confusing. Both are completely okay and work well for different scenarios. You use list<string> when you don't plan to change collection, and ObservableCollection otherwise.
Now, you can use MSDN(see ItemsSource property to see what property needs to be what.
As you can see from documentation, ItemsSorce has to be IEnumerable, meaning all types that derive from it, can be used.
Now as for the real question, why didn't your property show up in the IDE, I can only guess that Microsoft wants you to use ObservableCollection always, but I don't agree with this. So that's why it's best to manually do the bindings to properties through Xaml.
I have a WPF application , and I would like to modify the background color of the UI based on the user's selection at run time .
Now , I want to create a button that changes the color each time its clicked , so this button must change a value from the XAML tags
Please , help me doing it , I need it badly
Thanks
To make a control red from code:
yourControl.Background = Brushes.Red;
Thomas has given one example of what you could do.
Another option would be to bind the background colour to some property of your data context (probably the ViewModel if you're using MVVM) and make the button click change that property - possibly indirectly via commanding.
They're just different approaches - directly setting the background colour is certainly simpler than going via binding, but it may be less easily testable.
Another option would be to bind the background colour to a property in your DataContext, and update only the property's value when you click the button. This way, you also get to keep your logic and display responsibilities separate.
To say it simply. There is no difference between XAML and C#. In the end it both produces same executable code.
To change property on GUI you should either name your control through x:Name property and then set your property in the backend code file. Or you can DataBind your property to some backing field, preferably by using MVVM pattern.
But you should first understant how WPF works (ESPECIALY DataBinding) before moving to more advanced topics like MVVM.
I have ObservableCollection<Foo> that is bound to an ItemsControl (basically displaying a list).
Foo mostly looks like this (there are other members but it doesn't implement any interfaces or events):
class Foo
{
public string Name { get; set; }
//...
}
When the user clicks on an item I open a dialog where the user can edit Foo's properties (bound to a small viewmodel with a Foo property for the selected item), the Xaml looks like this:
<TextBox Text="{Binding Foo.Name,Mode=TwoWay}"
Grid.Column="1" Grid.Row="0" Margin="2" />
The really strange thing is, when the user edits the name, the value in the list changes! (not while typing but after the focus leaves the field)
How does it do that? I haven't implemented the INotifyPropertyChanged interface on the Foo object!
So far I checked that it doesn't just refresh the whole list - only the selected item. But I don't know where I could set a breakpoint to check who's calling.
Update: thanks to casperOne for the link to the solution! I'll add a summary here in case it goes 404:
[..] actually you are encountering a another hidden aspect of WPF, that's it WPF's data binding engine will data bind to PropertyDescriptor instance which wraps the source property if the source object is a plain CLR object and doesn't implement INotifyPropertyChanged interface. And the data binding engine will try to subscribe to the property changed event through PropertyDescriptor.AddValueChanged() method. And when the target data bound element change the property values, data binding engine will call PropertyDescriptor.SetValue() method to transfer the changed value back to the source property, and it will simultaneously raise ValueChanged event to notify other subscribers (in this instance, the other subscribers will be the TextBlocks within the ListBox.
And if you are implementing INotifyPropertyChanged, you are fully responsible to implement the change notification in every setter of the properties which needs to be data bound to the UI. Otherwise, the change will be not synchronized as you'd expect.
This is a total guess, but I'm thinking that because you have two-way binding enabled, WPF is now aware of when it makes changes, and will update other items it knows is bound to the same instance.
So because you have changed the value of the Name property through the textbox, and WPF knows when you have changed that value, it does its best to update whatever else it knows is bound to it.
It uses reflection to set the value of that property. INotifyPropertyChanged is only needed if the TextBox needs to be informed of a change in the Name property of the Foo class.
Because they're databound to the same object. If you change the binding to
{Binding Foo.Name, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}
then they'll be in synch when the user types in the textbox.