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
Related
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
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.
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.
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 a List collection where UserPropDef is a custom class with a few public properties. I want to display the contents of this list in a DataGrid. So I set the DataGrid.DataSource property of my grid to the list and it displays the contents just as I want.
Now if I add or remove items from the list I want the DataGrid to update. I can't use a BindingList because this list needs to be serialized. I've tried calling the Update() and Refresh() methods of both the DataGrid, and the form and nothing seems to cause the DataGrid to refresh based on the current contents of the collection.
The only thing that works seems to be setting the DataSource property again. Yet when debugging my code I can see that after the collection has changed the DataSource property of the DataGrid is in fact still referencing the correct and updated collection.
Is there a better way to cause the DataGrid to refresh based on it's current DataSource?
My suggestion would be take what is behind door number 3:
Create a custom Serializable List that implements IBindingList (Or something that inherits from BindingList and fixes the Serializable issues. Check out Fixing BindingList... for ideas).