I have an instance of ObservableCollection bound to a WPF listbox with two separate data templates (one for display, one for editing). The data template for editing has a one-way binding on the textbox, and a Save button.
What changes do I need to make so that when I press the Save button (after putting the list item in edit mode), the value I change the textbox to replaces the value in the ObservableCollection (and the display)?
Items in your collection should be of type that implements INotifyPropertyChanged interface. This way your list box will be notified that property value in your single item object has changed. ObservableCollection raises CollectionChanged event only when collection changes (items added, removed, etc.)
Quote from the MSDN library article on ObservableCollection
To fully support transferring data
values from binding source objects to
binding targets, each object in your
collection that supports bindable
properties must implement an
appropriate property changed
notification mechanism such as the
INotifyPropertyChanged interface.
For change notification to occur in a binding between a bound client and a data source, your bound type should either:
Implement the INotifyPropertyChanged
interface (preferred).
Provide a change event for each
property of the bound type.
Do not do both.
Source: MSDN: INotifyPropertyChanged Interface
I've solved similar problem using BindingList<T> class.
It has ListChanged event fired both on collection and individual item change.
Introduced in .Net 3.5
Related
Could someone explain the use of notify property changed and in which cases should I be using it?
For example:
I have in my silverlight application the domain data source which loads data and the event LoadedData where I set to some lists (List) the content of the entities from the domain context and bind the lists to the girds.
Do I need to use NotyfiPropertyChanged on the Lists?
Thanks,
I think you are getting the concepts slightly confused here...
There are two relevant notification interfaces for use with XAML Binding technologies.
INotifyCollectionChanged - notifies listeners when the collection of items changes (as in add/remove/replace/reorder actions).
INotifyPropertyChanged - notifies listeners that the content of an object has changed (as in values have been set and itself and other properties have changed).
In your case, if you want to notify that the contents of the List have changed, you need to use an INotifyCollectionChanged enabled collection to do that (i.e. not List - typically ObservableCollection).
If you wanted to notify that an item within the List has changed then the object type contained in the List should implement INotifyPropertyChanged.
Background:
I was trying to roll my own observable collection, by implementing IEnumerable, INotifyPropertyChanged and INotifyCollectionChanged. It works fine, but when I databind the CollectionChanged event is always null. The databound property does however updated, since I am sending a Items[] property changed event.
So this got me wondering what the point of INotifyCollectionChanged is in terms of databinding, since in my class it never gets triggered, but the databinding still works (it updates all the bindings to the collection).
I then decided to do some more digging, and decompiled ObservableCollection. When I databind to an ObservableCollection the CollectionChanged event isn't null like in my implementation.
So really I am wondering why ObservableCollection gets 'special' treatment, and what role INotifyCollectionChange plays in databinding (if any)
INotifyCollectionChanged can be implemented by collections so that when elements are added or removed from the collection, interested parties can be notified of those events. This is useful, for example, when you want a ListView or GridView or some other display control that displays the contents of collections to update its display when the contents of the collection have changed (through adding or removing elements). More generally, any object could data bind to the event to be notified when items are added/removed from the collection to do whatever the data bound component needs to do—it doesn't just have to be a GUI control. Any other operations on the collection, however, will result in no notifications being made to data bound controls/objects. In order for that to occur, you would also need to implement INotifyPropertyChanged on the collection, creating the other PropertyChanged events you also want to publish to notify data bound objects, and raise the event when the operation in question occurs.
Additionally, if you want each item within the collection to update its presentation in the UI when something about the item itself has changed, then the type representing the item should implement INotifyPropertyChanged.
It seems to me that you need to implement your own CollectionChanged event. The built-in System.Array and/or System.Collections.ArrayList classes do not have any events associated with them. So if you're using one of these classes as your backing store, then on each addition/removal of an item, you would need to be sure to raise the CollectionChanged event for your custom collection implementation.
However, I need to ask, why roll your own observable collection when Microsoft already provides the ObservableCollection<T> object, which you could possibly subclass and receive the functionality you're looking for for free?
I have an MvxListView that binds to a property on the ViewModel which is a List<MyClass>.
MyClass is a plain old object that has a boolean property named Completed that I've bound to the Checked property of a CheckedTextView in my list view item template.
When I click on the list view item, it invokes a command which calls a DoSomething(MyClass item) method. In the DoSomething method, I set the Completed property to its new value. However, because MyClass is not a ViewModel with RaisePropertyChanged properties, the checked property doesn't get updated in the user interface.
How would I accomplish data binding on this basic POCO to get the user interface to update when the Completed property changes?
How would I accomplish data binding on this basic POCO to get the user interface to update when the Completed property changes?
Xaml/C# style Data-Binding relies on INotifyPropertyChanged - without this the UI has no way to know that it needs to update.
So to get data-binding to work, your MyClass objects can't just be a POCO - it needs to implement INotifyPropertyChanged somehow - e.g. by inheriting from MvxNotifyPropertyChanged or by implementing INotifyPropertyChanged directly (e.g. see http://msdn.microsoft.com/en-us/library/vstudio/ms229614%28v=vs.100%29.aspx)
Aside: MvvmCross does also make other binding patterns possible beyond INotifyPropertyChanged - e.g. see INotifyChanged in FieldBinding in https://github.com/MvvmCross/MvvmCross/wiki/Databinding#rio - but this still requires something more than just POCOs for dynamically updating bindings.
I am binding a BindingList two way to a listbox. The Binding list contains a number of images which apparently only update the listbox if items are added or removed from the binding list. How can I make it so that the bindinglist also raises the listchanged event when an item is modified?
EDIT: I find the problem I am having is that a property of an object is not being changed, rather the base object.
BindingList<ImageSource>();
This wont work however if I did this:
BindingList<Image>();
And then set the binding path to Image.Source, it would update correctly and this is because a property of the Image has changed but in the case of the first example, only a direct item in the list has changed. So how may I get the same behaviour as the second example?
FINAL EDIT : It seems that using ObservableCollection instead of BindingList fixes this issue. I was under the impression that they were identical in notifying of changes in the collection. Full answer below
The list does raise that event but only if the underlying items provides the proper notifications via INotifyPropertyChanged.
The BindingList differs from ObservableCollection in that BindingList does not notify that its direct items are changed (except when items are added or removed from the collection). ObservableCollection however implements INotifyCollectionChanged and INotifyPropertyChanged interfaces. This means that any change to direct items of an ObservableCollection are reported to the UI.
If you are using bindings to direct items and need to update items and not properties of those items, it seems that you have to use ObservableCollection. Another solution would be to derive from BindingList and implement INotifyCollectionChanged.
I am not an expert but this is what i have gathered during the last hour, if anyone has anything to add or correct please let me know.
I am using an ObservableCollection for databinding as ItemsSource for DataGrid. Collection contains complex type objects. One of this type properties is a List of strings.
Just for now I see that when I update this List property from code nothing changes in the UI (the primary binding works fine). So, my question is: is it an expected behaviour? Maybe I should not use List as part of the type, but also use an ObservableCollection?
Update
Mode is set to OneWay.
Use a collection, instead of List, that implementes the interface INotifyCollectionChanged (like ObservableCollection). Then changes to the collection get populated to the ui.
Yes it is expected behaviour. The observable collection only notifies of changes to its own contents - that is add, delete, reorder.
What you are looking at is a change to an element in the observablecollection - if you want to see your changes to the class you put in, your element has to implement INotifyPropertyChanged.
So currently: If your list property on you complex object changes you won't see it, however if you change that too to be an observablecollection you could see changes to that collection in a sub-itemscontrol like a combobox - but not if you change the collection object to another one - so if you do not implement INotifyPropertyChanged you should set the collectionproperty before the binding is applied.
When you are updateding your list u have to call INotifyPropertyChange other wise UI wont get update the list result..
INotifyPropertyChange is the indication that here some changes occurred in the items source so update it.
This might help as well:
ObservableCollection that also monitors changes on the elements in collection