I am getting this error after raising NotifyPropertyChange event from a view model property.
I added (as a test) an UI Dispatcher.Invoke call on the setter which seems to have fixed the problem temporarily.
public FeedTrackingSummary SelectedFeedTracking {
get { return _selectedFeedTracking; }
set {
Application.Current.Dispatcher.Invoke(DispatcherPriority.Normal, (Action)(() => {
_selectedFeedTracking = value; Notify("SelectedFeedTracking");
}));
}
}
SelectedFeedTracking below is set by choosing a dropdown value which is bound to this property:
<ComboBox... SelectedItem="{Binding SelectedFeedTracking}" />
the error happens after selecting a drop-down value. There is no other code setting this property. I guess my viewmodel is used in a background thread at the time this happens?
UPDATE
i tried removing the call to INotifyPropertyChanged, and set a totally different property, and the error still persists. so i guess this has to do with the accessibility of the whole viewmodel ?
set {
SelectedCalc = -1;
}
Some MVVM Frameworks (such as Caliburn.Micro, for example) have a base NotifyPropertyChanged class which automatically marshals property change notifications (by raising the PropertyChanged event) to the so-called "UI Thread".
So, instead of having to Application.Current.Dispatcher.Invoke(...) on every property setter, put that code in your Notify() method. Thus making sure every time you notify property changes in the ViewModel you do so in the UI thread.
Related
I saw this example - Binding.UpdateSourceTrigger Property
in the example the UpdateSourceTrigger set to Explicit and then in the view code he call to UpdateSource of the TextBox name.
But if i use the MVVM dp i dont want to have names to my controls and source properties are in the VM and not in the view so what is the right way to bind controls to VM properties and set the UpdateSourceTrigger to explicit?
I want to do this because in my case its ShowDialog window and I want that the source will update only if the user click "ok"
Thanks in advance!
If you are using MVVM truely then your OK button click must be handled by some Command. This command must be coming from your ViewModel. The Expliticly bound properties must be coming from your ViewModel again. So whats stopping you.
Do not use Explicit binding but use OneWay binding.
In you button, bind a command and bind a command parameter to the OneWay bound Dependency property.
In your Command's Execute handler (which must be some method from your ViewModel), change the ViewModel's property with the parameter coming.
Raise the NotifyPropertyChanged for that property from your ViewModel.
E.g.
Assume I need to update a TextBox's Text back into my model on OK button click.
So for that I have a EmployeeViewModel class that has EmployeeName property in it. The property is has a getter and a setter. The setter raises property changed notification. The view model also has another property of type ICommand named SaveNameCommand that return a command for me to execute.
EmployeeViewModel is the data context type of my view. Myview has a TextBox (named as x:Name="EmployeeNameTxBx") OneWay bound to the EmployeeName and a Button as OK. I bind Button.Command property to EmployeeViewModel.SaveNameCommand property and Button.CommandParameter is bound to EmployeeNameTxBx.Text property.
<StackPanel>
<TextBox x:Name="EmployeeNameTxBx"
Text="{Binding EmployeeName, Mode=OneWay}" />
<Button Content="OK"
Command="{Binding SaveNameCommand}"
CommandParameter="{Bidning Text, ElementName=EmployeeNameTxBx}" />
</StackPanel>
Inside my EmployeeViewModel I have OnSaveNameCommandExecute(object param) method to execute my SaveNameCommand.
In this perform this code...
var text = (string)param;
this.EmployeeName = text;
This way ONLY OK button click, updates the TextBox's text back into EmployeeName property of the model.
EDIT
Looking at your comments below, I see that you are trying to implement Validation on a UI. Now this changes things a little bit.
IDataErrorInfo and related validation works ONLY IF your input controls (such as TextBoxes) are TwoWay bound. Yes thats how it is intended. So now you may ask "Does this mean the whole concept of NOT ALLOWING invalid data to pass to model is futile in MVVM if we use IDataErrorInfo"?
Not actually!
See MVVM does not enforce a rule that ONLY valid data should come back. It accept invalid data and that is how IDataErrorInfo works and raises error notfications. The point is ViewModel is a mere softcopy of your View so it can be dirty. What it should make sure is that this dirtiness is not committed to your external interfaces such as services or data base.
Such invalid data flow should be restricted by the ViewModel by testing the invalid data. And that data will come if we have TwoWay binding enabled. So considering that you are implementing IDataErrorInfo then you need to have TwoWay bindings which is perfectly allowed in MVVM.
Approach 1:
What if I wan to explicitly validate certain items on the UI on button click?
For this use a delayed validation trick. In your ViewModel have a flag called isValidating. Set it false by default.
In your IDataErrorInfo.this property skip the validation by checking isValidating flag...
string IDataErrorInfo.this[string columnName]
{
get
{
if (!isValidating) return string.Empty;
string result = string.Empty;
bool value = false;
if (columnName == "EmployeeName")
{
if (string.IsNullOrEmpty(AccountType))
{
result = "EmployeeName cannot be empty!";
value = true;
}
}
return result;
}
}
Then in your OK command executed handler, check employee name and then raise property change notification events for the same property ...
private void OnSaveNameCommandExecute(object param)
{
isValidating = true;
this.NotifyPropertyChanged("EmployeeName");
isValidating = false;
}
This triggers the validation ONLY when you click OK. Remember that EmployeeName will HAVE to contain invalid data for the validation to work.
Approach 2:
What if I want to explicitly update bindings without TwoWay mode in MVVM?
Then you will have to use Attached Behavior. The behavior will attach to the OK button and will accept list of all items that need their bindings refreshed.
<Button Content="OK">
<local:SpecialBindingBehavior.DependentControls>
<MultiBinding Converter="{StaticResource ListMaker}">
<Binding ElementName="EmployeeNameTxBx" />
<Binding ElementName="EmployeeSalaryTxBx" />
....
<MultiBinding>
</local:SpecialBindingBehavior.DependentControls>
</Button>
The ListMaker is a IMultiValueConverter that simply converts values into a list...
Convert(object[] values, ...)
{
return values.ToList();
}
In your SpecialBindingBehavior have a DependentControls property changed handler...
private static void OnDependentControlsChanged(
DependencyObject depObj,
DependencyPropertyChangedEventArgs e)
{
var button = sender as Button;
if (button != null && e.NewValue is IList)
{
button.Click
+= new RoutedEventHandler(
(object s, RoutedEventArgs args) =>
{
foreach(var element in (IList)e.NewValue)
{
var bndExp
= ((TextBox)element).GetBindingExpression(
((TextBox)element).Textproperty);
bndExp.UpdateSource();
}
});
}
}
But I will still suggest you use my previous pure MVVM based **Approach 1.
This is an old question but still I want to provide an alternative approach for other users who stumble upon this question...
In my viewmodels, I do not expose the model properties directly in the get/set Property methods. I use internal variables for all the properties. Then I bind all the properties two-way. So I can do all the validation as "usual" because only the internal variables are changed. In the view model constructor, I have the model object as parameter and I set the internal variables to the values of my model. Now when I click on the "Save" Button (-> Save Command fires in my view model fires) and there are no errors, I set all the properties of my model to the values of the correspondng internal variable. If I click on the "Canel/Undo"-Button (-> Cancel-Command in my view model fires), I set the internal variables to the values of my untouched model (using the setters of the view model properties so that NotifyPropertyChanged is called and the view shows the changes=old values).
Yet another approach would be to implement Memento-Support in the model, so before you start editing you call a function in the model to save the current values, and if you cancel editing you call a function to restore those values...that way you would have the undo/cancel support everywhere an not just in one view model...
I've implemented both methods in different projects and both work fine, it depends on the requirements of the project...
I have a XAML page whose DataContext is set to my ViewModel. A switch control on the page is bound to the following code in the ViewModel:
public bool TeamLiveTileEnabled
{
get
{
return Data.Subscriptions.Any(s => s.TeamName == this.Team.Name);
}
}
When this page is initialized, Data.Subscriptions is an empty list. I retrieve the list of subscriptions through an async web service call, so it comes back after the getter above is called.
When the web service call comes back, Data.Subscriptions has items added to it, and I'd like the UI to update based on the new result of the LINQ expression. Right now nothing happens, and I confirmed that Data.Subscriptions contains items that satisfy the condition above.
Data.Subscriptions is an ObservableCollection of Subscription items.
Can someone point me to what to do? Thanks!
The problem is that your ViewModel is not aware of any changes to the ObservableCollection. Within the ViewModel, subscribe to the CollectionChanged event of Data.Subscriptions.
Data.Subscriptions.CollectionChanged += SubscriptionsChangedHandler;
Within the event handler notify listeners of TeamLiveTileEnabled by sending a PropertyChanged notification
NotifyPropertyChanged( "TeamLiveTileEnabled" );
My Model implements INotifyPropertyChanged, and I have a WPF window bound to it (two way bindings).
I need to know when the model is being changed through the bound UI, so I could call an Update method from another module (which then copies My model to it's internal structures).
Model can also be changed by another module.
How to tell (in my PropertyChanged event handler) if the change originated in my UI, and not in that other module?
I do not want to call Update method if it was the other module that triggered the PropertyChanged event.
I'm fairly new to WPF myself, but the only immediately obvious way I can think of to do this would be to add extra set methods to the model, that modify the backing store without directly changing the property and thus firing the PropertyChanged event. To remove duplication, the property setter should probably call those methods as well and there should be a boolean argument fireChangedEvent. Something like this:
public string SomeThing
{
get { return _someThing; }
set { SetSomeThing(value, true); }
}
public void SetSomeThing(string value, bool fireChangedEvent)
{
_someThing = value;
if(fireChangedEvent)
{
NotifyPropertyChanged("SomeThing");
}
}
Then, in the other module, it would be
public void DoStuff
{
// ...
model.SetSomeThing("foo",false);
// ...
}
It's not an elegant method I know, and I hope someone else can think of something smarter, but I can't think of a good way of finding out from inside a property setter what exactly is setting that property.
Hopefully this is at least a workaround suggestion.
There is another way: using Binding.SourceUpdated
Every binding on the window would have to be set NotifyOnSourceUpdated=true, and the common handler for SourceUpdated event would do the rest (raise the Window.ModelEdited event that would trigger Update on another module).
We've using Caliburn.Micro on a new Silverlight project and everythings working great. The inbuilt conventions bind buttons click events to the viewModel, but I'm not sure what the best way to handle the selectionChanged event on datagrids and comboboxes is.
At the moment, I'm binding to the selected item and calling custom logic, but I feel like this is a bit of a code smell and that I should seperate the setting of the property and the selectedChange event. But if I seperate these, how do I bind the selection changed event to my viewModel, by commands? or an EventTrigger? Or is the code below acceptable? Its a small change but I do this logic everywhere.
private Foo _selectedFoo;
public Foo SelectedFoo
{
get
{
return _Foo;
}
set
{
if (_Foo != null && _Foo.Equals(value)) return;
_Foo = value;
NotifyOfPropertyChange("SelectedFoo");
NotifyOfPropertyChange("CanRemove");
LoadRelatedBars();
}
}
I use this technique regularly and I feel very comfortable with it.
I find perfectly fine that the VM reacts to its own state change, without the need for the external actor (which incidentally is the View, but could be another component, too) to set the new state, THEN signal the VM that the state is changed.
If you really want to, however, you can use the Message.Attach attached property to hook an event in the View to an action in the VM:
cal:Message.Attach="[Event SelectionChanged] = [OnSelectionChangedAction]"
(see also https://caliburnmicro.com/documentation/actions)
Here is a sample for MVVM and Caliburn.Micro using. Some actions like SelectionChanged should get an explicit a event arguments, so you should set it in caliburn event action part. Freqently first argument is passing $this (The actual ui element to which the action is attached.) and you gets in handler a datacontext for the row but to get to the Grid you should pass $source, as the first argument ($source - is the actual FrameworkElement that triggered the ActionMessage to be sent). According to the manual Caliburn manual.
XAML
cal:Message.Attach="[Event SelectionChanged]=[Action DataGrid_JobTypesSelectionChanged($source,$eventArgs)];"
Code:
public void DataGrid_JobTypesSelectionChanged(object sender, SelectionChangedEventArgs e)
{
var grid = sender as DataGrid;
JobTypesSelectedCollection = grid.SelectedItems.Cast<JobComplexModel>().ToList();
}
In my WPF application I have a CheckBox whose IsChecked value is bound to a property in my viewmodel. Notice that I have commented out the actual line which sets the value in my viewmodel. It's the standard pattern:
View.xaml
<CheckBox IsChecked="{Binding Path=SomeProperty}" />
ViewModel.cs
public bool SomeProperty
{
get { return this.mSomeProperty; }
set
{
if (value != this.mSomeProperty)
{
//this.mSomeProperty = value;
NotifyPropertyChanged(new PropertyChangedEventArgs("SomeProperty"));
}
}
}
When I click the CheckBox I expect nothing to happen, since the value of this.mSomeProperty does not get set. However the observed behavior is that the CheckBox is being checked and unchecked regardless of the value of this.mSomeProperty.
What is going on? Why isn't my binding forcing the CheckBox to show what the underlying data model is set to?
Because WPF does not automatically reload from the binding source after updating the source. This is probably partly for performance reasons, but mostly to handle binding failures. For example, consider a TextBox bound to an integer property. Suppose the user types 123A. WPF wants to continue showing what the user typed so that they can correct it, rather than suddenly resetting the TextBox contents to the old value of the property.
So when you click the CheckBox, WPF assumes that it should continue to display the control state, not to re-check the bound property.
The only way I've found around this, which is not very elegant, is to raise PropertyChanged after WPF has returned from calling the property setter. This can be done using Dispatcher.BeginInvoke:
set
{
// ...actual real setter logic...
Action notify = () => NotifyPropertyChanged(...);
Dispatcher.CurrentDispatcher.BeginInvoke(DispatcherPriority.ApplicationIdle, notify);
}
This could be made a bit less horrible by consolidating it into the NotifyPropertyChanged implementation so that you wouldn't have to pollute individual properties with this implementation concern. You might also be able to use NotifyOnSourceUpdated and the SourceUpdated attached event, but I haven't explored this possibility.