Should one bind DataGrid to the
ICollectionView = CollectionViewSource.GetDefaultView(collection)
or to the
ObservableCollection<T> collection; ???
What is the best practice for MVVM and why?
You always bind to an ICollectionView, whether you make it explicit or not.
Assume that we have
var collection = new ObservableCollection<string>();
var collectionView = CollectionViewSource.GetDefaultView(collection);
In this case, binding to collection or to collectionView is one and the same: the binding engine will bind to the default collection view (which is reference equal to collectionView) if you tell it to bind to collection.
This means that the answer to your question is "it makes absolutely no difference".
Just to be totally clear: even if you bind to the collection directly, the binding engine will bind to the default view. Modifying properties of the view such as sort criteria will affect the binding that appears to bind directly to the collection, since behind the covers it's a binding to the default view instead.
However, there is another interesting and related question: should one bind to the default collection view (i.e., to the collection itself, because there's no reason to explicitly bind to the default view) or to another view of the same collection?
Considering that each view has its own notion of current item, sort criteria, etc, it follows that if you intend to have multiple bindings to the same collection, and the bound controls need to have distinct notions of current item, filters and company, then what you want is to explicitly bind to multiple views of the same underlying collection.
ObservableCollection<T> implements INotifyCollectionChanged and will notify the UI when the items in the collection have been changed.
ICollectionView will give you the ability to filter, sort, or group the collection in addition to propogating INotifyCollectionChanged events if the underlying collection implements it.
Either type works well with MVVM as long as you bind to it. Use ICollectionView when you need sorting, filtering, or grouping. Use ObservableCollection<T> directly when you don't.
Just to add to what Jon said. The main difference is, that by using CollectionViewSource.GetDefaultView(collection), you are making you ViewModel dependent on WPF. Many MVVM purists don't like this and this would leave ObservableCollection only valid option.
Other option would be to use ICollectionView and use a class, that implement it, but is not part of WPF itself.
I don't think so it has to do anything with MVVM itself. ICollectionView provides additional features like soring grouping and etc if you need those use IColectionView otherwise simply use ObservableCollection
You would bind to the view if you wish your grid to display the settings applied to the view, e.g. filtering, otherwise the view is redundant.
Related
From what I can gather from Intellisense, the difference is the return types and the ItemsSource has a setter whereas Items simply has a getter. Practically speaking though, I do not understand the need for these two properties being separate from each other instead of just being one property.
Could someone explain to me why these are separate properties instead of just one property?
And also, if I'm missing something, could someone please explain to me when I'd want to use one over the other (besides the obvious need of a setter)? E.g., when specifically would I want to use Items over ItemsSource?
What's the difference between a WPF DataGrid's Items and ItemsSource properties?
A DataGrid is an ItemsControl so this applies to all other ItemsControl classes as well.
The Items property is an ItemCollection and is filled in through XAML. It holds objects but is intended for FrameworkElements.
The ItemsSource is bindable to a simple IEnumerable, with the ability to support INotifyCollectionChanged when available. It also supports DataTemplates.
when specifically would I want to use Items over ItemsSource?
ItemsSource is for databinding to a ViewModel.
You would use Items only in a few situations where you have a fixed number of XAML items. Unlikely for a Grid, more usable for a ComboBox.
You never use both at the same time.
This MSDN Page shows the typical usage for both.
I have a WPF application with a ListBox (called listMyItems) which is successfully bound to a class of MyItems that I created. I have a List of MyItems called currentMyItems which is then assigned as ItemSource to the ListBox. It all works fine, if I add an item to the currentMyItems it pops up on the list, etc.
The problem occurs when I try to remove the selected item in the ListBox. This is the code that I use:
currentMyItems.Remove((MyItem)listMyItems.SelectedItem);
The item disappears from the ListBox but the next time I update it, it pops back up as it was never deleted. Any tips?
I think you may be confused about how data binding works. When you bind a property, you are telling WPF to go look somewhere else for the value of that property.
When you bind the ListBox.ItemsSource property to currentMyItems, you are telling WPF to go look at the currentMyItems list to find its list of items. If currentMyItems is an ObservableCollection instead of a List<T>, then the UI will automatically receive a notification to update the bound value when you add or remove an item from the collection.
Based on what you say in the question, it sounds like you you have two collections, one of which is bound, and the other which is used to recreate the first collection anytime a change occurs. All that is not needed.
Just create one ObservableCollection<MyItem>, bind it to the ListBox.ItemsSource property, and then add or remove items from that single collection. It should work as you would expect.
<ListBox x:Name="listMyItems" ItemsSource="{Binding MyItems}" />
and
MyItems.Add((MyItem)listMyItems.SelectedItem)
MyItems.Remove((MyItem)listMyItems.SelectedItem)
If you're interested, I also have some beginner articles on my blog for WPF users who are struggling to understand the DataContext. You may want to check out Understanding the change in mindset when switching from WinForms to WPF and What is this “DataContext” you speak of?
If you bound it correctly to an ObservableCollection and currentMyItems is that collection. Than it means that you must have reloaded currentMyItems in meantime.
Also consider binding the SelectedItem property of your ListView - your view model doesn't have to know about the view at all.
Your source collection must be modufy (inherit from IList or ICollection). If your source collection does not support this method of your interface Remove, you can't remove item from source.
So, when you want to remove item you must cast ItemsSource to IList or ICollection:
var source = listbox.ItemsSource as IList ?? listbox.ItemsSource as ICollection;
and then check:
if (source == null) return;
then:
listbox.SelectedItems.ForEach(source.Remove);
listbox.Items.Refresh();
Make the currentMyItems<MyItem> an ObservableColection<MyItem>. This way it will raise a property change whenever modified and the UI gets updated accordingly.
By using ObservableCollection you will automatically get updates on the UI.
You should use an ObservableCollection instead of List.
A good thing is to always use ObservableCollection instead of List when something to do with UI
As I am quite new to WPF and MVVM, this might be something quite obvious and trivial, so bear with me here.
Anyway, I have a view model with these properties:
class ViewModel : INotifyPropertyChanged {
ICollectionView Items; // a list of Items object, wraps _items field, each Item has a Date property
string Filter; // a filter key, wraps _filter field and calls ApplyFilter() as it is changed
void ApplyFilter(); // based on the filter key, _items.Filter gets set to some predicate
}
The properties raise the PropertyChanged event when set and all that common MVVM stuff.
In the view i have a simple ItemsControl which binds to the Items property, and some fancy data template to display each Item.
A request has been made to show the items grouped by day so that you see a date header for each day and a list of Items whose Date property corresponds to the date in the header.
Since this is strictly a display issue, I decided to leave the view model as it is, but use a converter to convert the ICollectionView Items into a Dictionary where the key is the date, and the collection is a subset of Items with that date.
The ItemsControl now has a StackPanel with a TextBlock to show the date header (the dictionary key) and another ItemsControl which is basically a copy of the old one, which just listed the items (the dictionary value).
The view renders nicely, but the filter no longer works. As the control is bound to Items, and the ICollectionView implements INotifyCollectionChanged, I was expecting the filter to work as it is changing the Items list, and that the converter would be re-run to rebuild the dictionary. Well, it is not. Changing the filter does call the ApplyFilter(), and the _items.Filter gets set to the required predicate, but the view never changes. I have also tried calling the PropertyChanged for Items from the ApplyFilter, but that does not work either.
Obviously, my contrived scenario of how this should work is wrong, and to be honest I am out of ideas, apart from creating new objects which will hold the date and list of items as properties and then using a list of those in the VM. But, as I said, in my mind, this is strictly a view issue, so the model just needs to provide a list of items, and it is the view's responsibility to decide how to render them.
Any help is greatly appreciated and thanks in advance.
EDIT:
Now I'm thinking that if I'm changing the _filter.Filter, then the PropertyChanged event for the Items is actually never raised, as it is in fact not changed (the internals have changed, but Items themselves are still the same ICollectionView).
Hence, the converter is never again triggered.
If this is the case, how can I trigger the converter? Raising the PropertyChanged for Items after doing ApplyFilter() also did nothing.
Perhaps using ListView instead of simple ItemsControl+converter is better idea? ListView has many good functions. Such as virtualizing, grouping, etc.
All you have to do, is modify your ICollectionView grouping property and apply templates(GroupStyle)
As for your problem, your current behaviour makes sense to me. If you want to converter re-run, you're supposed to create new method RefreshBinding() and do something like this;
var referenceCopy = Items;
Items = null;//make sure INotifyPropertyCHanged is fired.
Items = referenceCopy; //converter should be called again.
you will call it after you need your converter to be re-run. But to be completely honest, just use ICollectionView+Grouping property and ListView. Or you can implement ListView functionality yourself. Using converter does not seem good solution.
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
I have an IEnumerable<> which lazy loads it's data. I want to just set a Combobox's ItemsSource to the IEnumerable, but when I do it goes and loads all the data anyway (which removes the point of lazy loading).
I've tried it with Linq-To-Sql as well since it seems to be a similar theory and it also loads all the data.
Is there an easy way to do this?
Try setting the IsAsync-Property in the ItemsSource-Binding of the ComboBox to True:
<ComboBox ItemsSource={Binding YourItemsSourceProperty, IsAsync=True}
SelectedItem={Binding YourSelectionProperty} />
If that does not change anything, have a look at this one:
http://social.msdn.microsoft.com/Forums/en-US/wpf/thread/3d343489-90c4-4bdc-8bd9-1046ec9daf76
Maybe you will need to use IList instead.
Alternatively, you could use PriorityBinding, to fill the list with some temporary data until the final list is completely loaded.
Don't bind the control to the IEnumerable directly. Instead, bind it to a ObservableCollection (which is empty at the beginning.) Meanwhile, still do your lazy loading on the IEnumerable as usual (either triggered by drop down combobox or something else.) While the data is loaded or when you have enough data, add the items to that ObservableCollection to populate the comboBox.
I don't think the WPF ComboBox supports lazily loading the items from the ItemsSource. Why do you need to lazy load anyway, and when would you expect it to trigger the lazy load?
Bind your ComboBox's ItemsSource to an ObservableCollection.
Now whenever your IEnumerable lazy loads the data, add it to the ObservableCollection instantly
foreach(Item i in myIEnumerable)
{
myObsCol.Add(i);
}
This would update the UI once each item is added.
I am trying to do same thing. But as I investigated, if you want to use standard bindings on combobox (collection to ItemsSource and dataItem to SelectedValue/SelectedItem), it is necessary to write your own control.
Combobox is inherited from Selector and when you have bounded collection to ItemsSource property and you change your value of property that is bounded to SelectedValue/SelectedItem then the Selector call it's own private method FindItemWithValue(object value). This method walks through items in bounded collection from first until it finds equal value. That of course will make you collection to load all items before the selected one.
If you are willing to do your own custom class that will have a list, you can use INotifyPropertyChanged interface to tell that your collection has been modified. Or as use ObservableCollection as it has been already suggested