I've set up a viewmodel to bind a listcontrol to an ObservableCollection in my program. A UI control on the page adds and deletes objects to the collection, which works fine as the list is automatically updated.
After App-Switching and returning to the app, the buttons adds the objects, but the bindings seem to be lost. Any idea how I can maintain this even after returning? I don't really see the need to rebind the object (after defining it in XAML). Is there any way to foolproof this pattern, and ensure the bindings aren't lost upon returning to the app?
The XAML looks like this, but it's inside a UserControl - forgot to mention that
ItemsControl x:Name="PartyCollection" ItemTemplate="{StaticResource PartyCollectiontemplate}" ItemsSource="{Binding RoomParty, Source={StaticResource FormControlVM}}"
the codebehind looks like this
public class FormControlVM : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public ObservableCollection<Party> RoomParty
{
get
{
return App.appData.currentChoices.roomParty;
}
set
{
App.appData.currentChoices.roomParty = value;
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs("RoomParty"));
}
}
}
Related
My application viewModel responds to a user clicking a button to see test results:
private void AddDetailRows(List<QuizHistory> quizHistoryList)
{
quizDetails.Children.Clear();
quizDetails.Children.Add(AddData(quizHistoryList));
quizDetails.Children.Add(new LineTemplate());
}
Where quizDetails is the name of an element in the view.
But this doesn't work for me as the view model doesn't know what the view looks like and does not have access to the names of elements.
In a MVVM application, how is this problem solved?
You are completely right, that is not something that ViewModel is responsible of.
So, whatever you want to do with UI is not responsibility of the ViewModel.
If this is really the only option, then you can think of creating boolean properties in your VM and binding them to your views and then changing that boolean from false to true or vice versa on button click command which is binded to your VM.
To simplify it:
MyView.xaml
<StackLayout>
<Button Command="{Binding ShowHideQuizHistoryCommand}" ... />
<StackLayout x:Name="QuizHistory"
IsVisible={Binding ShowQuizHistory }>
//
</StackLayout>
</StackLayout>
MyViewModel.cs
private bool _showQuizHistory ;
public bool ShowQuizHistory
{
get { return _showQuizHistory ; }
set
{
_showQuizHistory = value;
OnPropertyChanged();
}
}
public ICommand ShowHideQuizHistoryCommand => new Command(() =>
{
ShowQuizHistory = !ShowQuizHistory;
});
So, this is just an example based on what you provided in question.
You can also use visual states, converters, triggers and behaviors in order to achieve this, but in my opinion this is the easiest way.
I'm currently using telerik in UWP to create list of items, i want to be able to use a browse button and update a certain piece of data in the Telerik-RadDataForm. I have all the bindings setup using MVVM and it displays data fine if it isn't edited on the code side. My XAML is setup as so:
<Data:RadDataForm x:Name="dataform"
HorizontalAlignment="Left" Grid.Row="0"
Grid.RowSpan="2" Grid.Column="2"
VerticalAlignment="Center" Width="454"
Item="{Binding CurrentSceneViewModel, Mode=TwoWay,
UpdateSourceTrigger=PropertyChanged}" CommitMode="Immediate"
ValidationMode="Immediate" Height="664" Margin="0,28" />
The CurrentSceneViewModel is:
public SceneViewModel CurrentSceneViewModel
{
get => _currentSceneViewModel;
set=> _currentSceneViewModel= value;
}
And the data i wish to change is :
public string FileName
{
get => _fileName;
set
{
Scene.SceneFile = value;
_fileName = Path.GetFileName(value);
OnPropertyChanged(nameof(FileName));
}
}
The problem i have is pushing this information to the user interface the code-behind doesn't seem to update the UI, even using PropertyChanged. I'm not sure what else to try ? And if this is something the RadDataform simply doesn't support. It should be noted FileName is a property of CurrentScene ViewModel.
public abstract class BaseViewModel: INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged(string propertyName)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
protected IPageNavigationService navservice = new PageNavigationService();
}
I have a UserControl that has a CheckBox on it. When I consume the UserControl on my main XAML page, I'd like to TwoWay bind a property on the control to a property on my ViewModel e.g.
<myUserControl BtnIsBlacklisted="{Binding IsBlacklisted, Mode=TwoWay}" />
When IsBlacklisted changes, I'd like my checkbox to change too and vice-versa.
Here is what I have,
public static readonly DependencyProperty BtnIsBlacklistedProperty =
DependencyProperty.Register("BtnIsBlacklisted",
typeof(bool),
typeof(MyUserControl),
new PropertyMetadata(false, new
PropertyChangedCallback(BtnIsBlacklistedPropertyChanged))
);
private static void BtnIsBlacklistedPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
// ... do something here ...
}
public bool BtnIsBlacklisted
{
get { return (bool)GetValue(BtnIsBlacklistedProperty); }
set { SetValue(BtnIsBlacklistedProperty, value); }
}
My UserControl has this for the CheckBox,
<CheckBox x:Name="myCheckBox"
...
IsChecked="{Binding Path=BtnIsBlacklisted,
ElementName=UserControl,
Converter={StaticResource BoolToNotBool},
Mode=TwoWay}" />
The property on my ViewModel object is as follows,
public bool IsBlacklisted
{
get
{
return App.VM.BlacklistedRetailers.Contains(this.Retailer);
}
set
{
if (value)
{
App.VM.BlacklistedRetailers.Add(this.Retailer);
}
else
{
while (App.VM.BlacklistedRetailers.Contains(this.Retailer))
{
App.VM.BlacklistedRetailers.Remove(this.Retailer);
}
}
this.NotifyPropertyChanged("IsBlacklisted");
}
}
The only way BlacklistedRetailers changes is through the set method above so there is no need to trigger a NotifyPropertyChanged from there ...
I have tried many of the suggestions in other questions i.e.
using a dependency property
including Mode=TwoWay
Binding on the UserControl using a self-referencing DataContext set on the containing grid (this does not work either).
however none of these have worked.
Some final notes:
This is for a Windows Phone 7.5 project
Edit: One way binding doe not work either, it seems it there is a problem binding to the UserControl's own properties
An ElementName Binding matches against x:Name values which are in the same name scope as the element on which the binding is being set. There's not enough of the code shown to tell but you're using "UserControl" which I'm guessing is not set as the name of the element, but is being used to try and match the type. The ElementName also might not be able to resolve if the CheckBox is declared inside a template.
In my xaml page i am using an image control
<Image x:Name="MyImage" Grid.Row="1" Stretch="UniformToFill" Source="{Binding SourceImage}"/>
like this. My question is. Is it possible to access this control's properties in my view model. "Like MyImage.Source =". If yes, how i can achieve similar implementation in windows phone.
Yes, you can, but you should not. The whole purpose of the ViewModel is to separate the logic into its own class rather than having it intermixed with view code.
Instead use the INotifyPropertyChanged interface to notify the UI that the image source has changed. If possible try to see if you can use a regular binding for the value you want to use, that will be the most robust and most easy way.
Another solution is to expose a interface on the view. Something like IView, you can probably come up with a more suitable name like IResetable.
interface IResetable
{
void Reset();
}
class MainWindow: Window, IResetable
{
publiv void Reset()
{
// Here you can access the view, but try to keep logic minimal.
}
}
class ViewModel
{
private readonly IResetable resetable;
public ViewModel(IResetable resetable)
{
_resetable = resetable;
}
void Foobar()
{
_resetable.Reset();
}
}
I have a command wired to the event such that it does fire, but what I get in the CommandParameter is the previously selected item, or maybe it's the selected item before the SelectionChanged completes.
Either way, not sure what to change to get the newly selected item from the event.
<i:Interaction.Triggers>
<i:EventTrigger EventName="SelectionChanged">
<cmd:EventToCommand
Command="{Binding Main.SelectedRecordCommand, Source={StaticResource Locator}}"
CommandParameter="{Binding SelectedItem, ElementName=listBillingRecords}"
/>
</i:EventTrigger>
</i:Interaction.Triggers>
Thanks
Is it worth using a trigger? If whatever your XAML element is for the collection (listbox, grid, etc) is bound to a property exposing a collection on your viewmodel, you can leverage both databinding and the built-in MVVM Light messenger to notify you of a property change with both old and new values in a more MVVM-friendly way. This example isn't necessarily WP7-specific, but I think it would work the same.
For example, this might be the databound collection:
public const string BillingRecordResultsPropertyName = "BillingRecordResults";
private ObservableCollection<BillingRecord> _billingRecordResults = null;
public ObservableCollection<BillingRecord> BillingRecordResults
{
get
{
return _billingRecordResults;
}
set
{
if (_billingRecordResults == value)
{
return;
}
var oldValue = _billingRecordResults;
_billingRecordResults = value;
// Update bindings and broadcast change using GalaSoft.MvvmLight.Messenging
RaisePropertyChanged(BillingRecordResultsPropertyName, oldValue, value, true);
}
}
I like to expose a property on my ViewModel that is a "selected item" of whatever collection I'm exposing. So, to the ViewModel, I would add this property using the MVVMINPC snippet:
public const string SelectedBillingRecordPropertyName = "SelectedBillingRecord";
private BillingRecord _selectedBillingRecord = null;
public BillingRecord SelectedBillingRecord
{
get
{
return _selectedBillingRecord;
}
set
{
if (_selectedBillingRecord == value)
{
return;
}
var oldValue = _selectedBillingRecord;
_selectedBillingRecord = value;
// Update bindings and broadcast change using GalaSoft.MvvmLight.Messenging
RaisePropertyChanged(SelectedBillingRecordPropertyName, oldValue, value, true);
}
}
Now, if you bind the SelectedItem of the XAML element to this exposed property, it will populate when selected in the View via databinding.
But even better, when you leverage the snippet MVVMINPC, you get to choose whether or not to broadcast the results to anyone listening. In this case, we want to know when the SelectedBillingRecord property changes. So, you can have this in the constructor for your ViewModel:
Messenger.Default.Register<PropertyChangedMessage<BillingRecord>>(this, br => SelectedRecordChanged(br.NewValue));
And elsewhere in your ViewModel, whatever action you want to have happen:
private void SelectedRecordChanged(BillingRecord br)
{
//Take some action here
}
Hope this helps...
I have seen the same issue and found that SelectedItem is the correct implementation.