I have to create a small WPF/MVVM based GUI that shows the user a list of objects that I get from an external library. The user cannot directly edit those objects, but only select them for further usage.
At first I though I could directly use the given objects in a regular collection as I did not see any need for an INotifyPropertyChanged implementation, but then I noticed that I would need an IsSelected property so that the view model would know which objects are selected by the user and furthermore there is also one case where I have to select specific objects from the view model. This means I have to somehow add the said IsSelected property to make this scenario work in MVVM.
What options do I have?
Do I have to write a wrapper class that inherits from the external class and only extends it by the said IsSelected Property? This would mean I would also have to convert the list of objects that I get from the external library before I can use them.
Or is there maybe a more convenient way to extend the external object so that I can handle the selection in an MVVM based way?
You can define a collection of the selected objects on your viewmodel, like:
public class YourViewModel
{
public List<Thing> SelectedThings { get; } = new List<Thing>();
}
Because the SelectedItems property of the built-in WPF ListBox is not a DependencyProperty so it cannot be bound, you can manage your collection with a simple event handler like
<ListBox SelectionChanged="ListBox_SelectionChanged" />
in codebehind:
private YourViewModel vm;
private void ListBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
foreach (var item in e.AddedItems)
{
vm.SelectedThings.Add(item);
}
foreach (var item in e.RemovedItems)
{
vm.SelectedThings.Remove(item);
}
}
If you want to set the selected items from the view model, too, I have found a good solution instead of event handler here: https://www.tyrrrz.me/Blog/WPF-ListBox-SelectedItems-TwoWay-binding
Related
I have the TreeView with objects. When I select item of these tree, than other control - ListView displays as its items properties of selected object. I want to save values of properties when in TreeView selection is change to other object.
So, is there a good way in WPF to gets values of "just before changing" items in ListView control? My idea for now is to override the PreviewMouseDown to check if user click tree node. By god way I mean better than mine. Maybe something in ListView template?
Indication that there is no need to change my idea with the PreviewMouseDown will be also good answer.
Could you please provide the relevant code snippets? I try to answer your question, but I'm not sure I understood it correctly.
If you bind the SelectedItem of you TreeView to a property (a.e. using MVVM pattern), you can save the values before actually setting the item.
Doing so in the setter is not so good though, because it becomes quite large then. I would have a setter like this:
private Foo bar;
public Foo Bar
{
get { return bar; }
set
{
OnPropertyChanging("Bar");
bar=value;
OnPropertyChanged("Bar");
}
}
Then you can listen to your own PropertyChanging events and do your stuff there:
private void this_PropertyChanging(object param, PropertyChangingEventArgs e)
{
switch(e.PropertyName)
{
case "Bar":
//Do you stuff
break,
}
}
I have a user control written in C# & WPF using the MVVM pattern.
All I want to do is have a property in the bound ViewModel exposed to outside of the control. I want to be able to bind to it and I want any changes to the property to be picked up by anything outside the control that is bound to the exposed value.
This sounds simple, but its making me pull out my hair (and there is not much of that left).
I have a dependency property in the user control. The ViewModel has the property implementing the INotifyPropertyChanged interface and is calling the PropertyChanged event correctly.
Some questions:
1) How do I pick up the changes to the ViewModel Property and tie it to the Dependency Property without breaking the MVVM separation? So far the only way I've managed to do this is to assign the ViewModels PropertyChanged Event in the Controls code behind, which is definitely not MVVM.
2) Using the above fudge, I can get the Dependency property to kick off its PropertyChangedCallback, but anything bound to it outside the control does not pick up the change.
There has to be a simple way to do all of this. Note that I've not posted any code here - I'm hoping not to influence the answers with my existing code. Also, you'd probably all laugh at it anyway...
Rob
OK, to clarify - code examples:
usercontrol code behind:
public static DependencyProperty NewRepositoryRunProperty = DependencyProperty.Register("NewRepositoryRun", typeof(int?), typeof(GroupTree),
new FrameworkPropertyMetadata( null, new PropertyChangedCallback(OnNewRepositoryRunChanged)));
public int? NewRepositoryRun
{
get { return (int?)GetValue(NewRepositoryRunProperty); }
set
{
SetValue(NewRepositoryRunProperty, value);
}
}
private static void OnNewRepositoryRunChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
if (e.OldValue != e.NewValue)
{
}
}
public GroupTree()
{
InitializeComponent();
GroupTreeVM vm = new GroupTreeVM();
this.DataContext = vm;
}
Viewmodel (GroupTreeVM.cs)
private int? _NewRepositoryRun;
public int? NewRepositoryRun
{
get
{
return _NewRepositoryRun;
}
set
{
_NewRepositoryRun = value;
NotifyPropertyChanged();
}
}
And now for my weekly "don't do that" answer...
Creating a ViewModel for your UserControl is a code smell.
You're experiencing this issue because of that smell, and it should be an indication that you're doing something wrong.
The solution is to ditch the VM built for the UserControl. If it contains business logic, it should be moved to an appropriate location in another ViewModel.
You should think of a UserControl as nothing more than a more complex control. Does the TextBox have its own ViewModel? No. You bind your VM's property to the Text property of the control, and the control shows your text in its UI.
Think of UserControls in MVVM like this--For each model, you have a UserControl, and it is designed to present the data in that model to the user. You can use it anywhere you want to show the user that model. Does it need a button? Expose an ICommand property on your UserControl and let your business logic bind to it. Does your business logic need to know something going on inside? Add a routed event.
Normally, in WPF, if you find yourself asking why it hurts to do something, it's because you shouldn't do it.
Perhaps I've misunderstood, but it seems like you're trying to use binding in the code behind?
public MyUserControl()
{
InitializeComponent();
// Set your datacontext.
var binding = new Binding("SomeVMProperty");
binding.Source = this.DataContext;
SetBinding(MyDependencyProperty, binding);
}
I am working on a user control which contains a SpinEdit (numeric up-down) control and a ComboBoxEdit. The option selected in the combo-box provides a factor by which number in the SpinEdit is multiplied. At the moment, I have something like this:
public class MyUserControl : DevExpress.XtraEditors.XtraUserControl
{
private static List<String> listItems;
static MyUserControl() // Populate the list of options with the default options
{
listItems = new List<String?();
listItems.Add("Option1");
listItems.Add("Option2");
listItems.Add("Option3");
}
public MyUserControl()
{
InitializeComponent();
// Add default options to the combo box
foreach (String item in listItems)
{
this.cboBox.Properties.Items.Add(item);
}
}
}
That works fine (note that the above is simplified, in reality the static list is a static Dictionary which maps the strings to the multiplication factor) except that I want to allow the user to add custom options to listItems and have these appear on every instance of this user control in my application. That is why listItems is static, as my hope was to do this.cboBox.Properties.Items = listItems; so that any additions to listItems would appear on every control. However, the Items property is read-only so I cannot do that.
How can I go about ensuring every instance of my user control has the same set of options, even if these are changed? Having the static members fire an event when the user changes the option list might do the trick, but that seems a bit overkill for something that looks as simple as this. Does anyone have any other ideas?
In your case is better to use LookUpEdit control instead of ComboBoxEdit. Just make some adjustments to it:
lookUpEdit1.Properties.ShowHeader = false;
You can use your listItems in that way:
lookUpEdit1.Properties.DataSource = listItems;
But there is some problem with Dictionary as DataSource. For DataSource you must use an collection that implements IList, ITypedList or IBindingList interface. So, you can convert your Dictionary to List or you can use this trick:
private static Dictionary<int, string> items;
//...
lookUpEdit1.QueryPopUp += lookUpEdit_QueryPopUp;
lookUpEdit2.QueryPopUp += lookUpEdit_QueryPopUp;
//...
private void lookUpEdit_QueryPopUp(object sender, CancelEventArgs e)
{
var lookUpEdit = (LookUpEdit)sender;
lookUpEdit.Properties.DataSource = null;
lookUpEdit.Properties.DataSource = items;
}
But I think is better to use List instead of Dictionary.
I don't know about the DevExpress controls (you might want to ask on their support forums), but in classic Windows comboboxes, the list is maintained inside the control. The Items property is a .Net wrapper that makes the internal structure easier to work with. So, it's not possible to assign the same data structure to each combobox. They have their own copy.
Your idea to have a publish/subscribe mechanism to what are now static lists seems like a reasonable solution.
If it were me, instead of having static stuff on the control class, I'd make that master list container a separate class and have a property on the control that takes an instance of that class. It'd make your overall architecture a bit more flexible and testable.
I'm a newbie to WPF and C# and am building my first app mostly by using code examples. I'm sure there might be some better ways to do this, that I'm not understanding yet, so I'm coming to you guys for some advice.
I have a treeview control with of a bunch of nested objects that is downloaded into an ObservableCollection viewmodel from a WCF Service that I also built. I have the viewmodel declared in the Windows.Resources of the XAML.
My treeview then binds to that StaticResource by its key name.
Items="{Binding Source={StaticResource MyCatalogModel},Path=Items, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
The data in the tree is saved locally to a file. When the viewmodel is instantiated it reads the file, or it creates it if it doesn't exist.
I have 2 related problems right now.
Sometimes the data object that is imported is rather large with lots of nested objects (children). This is taking a long time to update the tree. How can I speed this up? Can I "turn off" the Notify changed stuff of the ObservableCollection, and just reload (rebind?) the viewmodel when it's finished?
I'd like to give the user the ability to basically clear out all the items from the tree and start from scratch. I have code that dumps the underlying file and as I said, it will be recreated when a new viewmodel is instantiated, but I don't know how to "reset" the binding of the resource and the tree. How do I do this?
Thanks to all who respond and any code snippets will be greatly appreciated!!
I had a similar problem, where I had a very large amount of data in a collection - and the event for OnPropertyChanged was firing for each item in the collection. I added an extension with a method to add a range to an ObservableCollection. Here is the code for the extension.
public class SmartCollection<T> : ObservableCollection<T>
{
public SmartCollection()
: base()
{
_suspendCollectionChangeNotification = false;
}
bool _suspendCollectionChangeNotification;
protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
{
if (!_suspendCollectionChangeNotification)
{
base.OnCollectionChanged(e);
}
}
public void SuspendCollectionChangeNotification()
{
_suspendCollectionChangeNotification = true;
}
public void ResumeCollectionChangeNotification()
{
_suspendCollectionChangeNotification = false;
}
public void AddRange(IEnumerable<T> items)
{
this.SuspendCollectionChangeNotification();
int index = base.Count;
try
{
foreach (var i in items)
{
base.InsertItem(base.Count, i);
}
}
finally
{
this.ResumeCollectionChangeNotification();
var arg = new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset);
this.OnCollectionChanged(arg);
}
}
}
So instead of an ObservableCollection its a SmartCollection. What I did was build my collection of objects into a List then you call the AddRange method and pass in your List of objects. This greatly improved the performance.
As far as recreating the tree - if its based on the viewmodel. Just new up the viewmodel that its bound to.
It's amazing how many times the same old questions come up again and again. Right, I have no idea why you are Binding to a StaticResource, but that is a bad idea.
Just create a public property of type ObservableCollection<T> in your view model, set an instance of it as the DataContext of your view in whichever way you prefer or know. Make sure you implement the INotifyPropertyChanged Interface correctly in the code behind, or declare a DependencyProperty instead. Then you can Bind directly to this property, let's call it Items:
<TreeView ItemsSource="{Binding Items}" ... />
When you have it set up this way, then all you need to do to empty or reset the TreeView is this (in the view model):
Items = new ObservableCollection<YourItemDataType>();
As for speed, it's hard to know what you're doing, but WPF is known for being slow when rendering large collections. I can't help with that, sorry.
I am simplifying a little here:
I have a tab control, and should like for individual tabs to have the power to create further tabs; siblings if you will. So I am calling the tab control the parent, and its tab pages the children.
Using MVVM, my tab control view model is something like this:
class ParentViewModel
{
ObservableCollection<ChildViewModel> _pages;
public ObservableCollection<ChildViewModel> Pages
{
get
{
if (_pages == null)
_pages = new ObservableCollection<ChildViewModel>();
return _pages;
}
}
public ParentViewModel()
{
Pages.Add(new ChildViewModel());
}
}
So I have a collection of ChildViewModel objects on my ParentViewModel.
This works a treat, and from inside the ParentViewModel I can of course easily add all the extra ChildViewModel objects I want to my collection and have it all nicely reflected in my Views.
What I want to do is allow a button press (for example) on a ChildView to result in the addition of another ChildViewModel to the collection on the ParentViewModel object.
I have read a lot about relay commands, routed commands, relativesource bindings, the dependancy injection pattern and so forth, but could someone tell me please the 'proper' (in an MVVM sense) way to achieve this, and exactly how it is best done. Thank you!
One of the ways I like to handle a situation like this is with Event Aggregating.
It is an ability added with Unity (if you aren't already using it)
Basically you add the Event Aggregator to your Dependancy Injections and then your Parent would subscribe as a listener to the event and your children would publish the event.
The nice part about this is that the children have no concept of who is listening and the parent just knows it has an event request to handle. For more information you can go HERE!
You can use MVVM Lite Messenger class (or write you own):
And send a Message from a childe class to Parent class. See an example here.
create a message class:
public class AddNewChildMessage
{
public string Data {get;set;} //any data you need to pass
}
In a ParrentViewModel's constructor:
Messenger.Default.Register<AddNewChildMessage>
(
this,
( message ) => AddNewChild(message )
);
private void AddNewChild(AddNewChildMessage message)
{
//do staf with message.Data if any
Pages.Add(new ChildViewModel());
}
In a child view model:
Messenger.Default.Send<AddNewChildMessage>( new AddNewChildMessage() );
Adding childs is an example - you can add any logic you want.