Windows Store App binding of CheckBox does not work (occassionally) - xaml

I have the following piece of XAML:
<GroupStyle.HeaderTemplate>
<DataTemplate>
<Grid Background="#286C9A" Width="336" Height="22">
<TextBlock Text="{Binding Checked}" Foreground="Black" HorizontalAlignment="Left"/>
<CheckBox IsChecked="{Binding Checked}" HorizontalAlignment="Right" />
</Grid>
</DataTemplate>
</GroupStyle.HeaderTemplate>
It's a template in a ListView. Checked is a bool property, and the problem is that the initial value of the property is transferred correctly to the view (both TextBlock and CheckBox). But following changes to the property is only reflected on the TextBlock, the Checkbox does not react.
Can someone tell me what happens?
EDIT:
The relevant part (I believe) of the ViewModel is this :
public class MenuGroup : ObservableCollection<MenuItem>
{
bool #checked;
public bool Checked
{
get { return #checked; }
set
{
if (#checked == value) return;
#checked = value;
OnPropertyChanged(new PropertyChangedEventArgs("Checked"));
}
}
}
EDIT: It is apparent that the binding stops working the first time I have clicked on the checkbox and thus manually changed it's state. And it works all the way if it's TwoWay binding. But why that is, I don't know.

Related

How to fire a command after clicking somewhere else?

So, I have a custom control, CusConA, that works basically like a textbox - you type amount of money that you need, and I have a button below, whom by getting clicked saves that amount(from CusConA) somewhere, and that is working fine.
But I want to try the same functionality basically by clicking anywhere on that page (something like OnBlur in asp.net), or to be precise, when my CusConA is not in focus anymore.
By doing what is shown with the --> in code, I achieved sort of a solution, this way when pressing anywhere, even if I never even tried to write an amount, the command is being executed.
So, to try to circle my question, I need this command to execute only after typing some amount, and clicking somewhere alse after. How can I do that?
<Frame
Margin="55,0"
Padding="0"
BorderColor="Blue"
CornerRadius="30">
<StackLayout Orientation="Horizontal">
<Label
Margin="10"
FontAttributes="Bold"
FontSize="20"
HorizontalTextAlignment="Center"
Text="RSD"
TextColor="Some text"
VerticalTextAlignment="Center" />
<customControls:CusConA
Margin="0,0,15,0"
HorizontalOptions="FillAndExpand"
Keyboard="Numeric"
Placeholder="0,00"
PlaceholderColor="Gray"
Text="Some text"
TextColor="Black" >
--> <customControls:CusConA.Behaviors>
<xct:EventToCommandBehavior EventName="Unfocused" Command="{Binding DoSomething}" ></xct:EventToCommandBehavior>
</customControls:CusConA.Behaviors>
</customControls:CusConA>
</StackLayout>
</Frame>
Can you change DoSomething to check whether the amount has been typed? Might involve adding a boolean property to your control:
bool CanExecute { get; set; }
Then have "amount" bound to a property whose setter sets CanExecute = true; or CanExecute = false;, depending on whether an amount has been typed. Something like:
string Amount
{
...
set {
_amount = value;
myControl.CanExecute = value.Count > 0;
}
}
Then change DoSomething body to
if (this.CanExecute) { ... }
Alternatively, other techniques can be used to have a change to Amount trigger a change to a property on myControl.
The essential points are:
Adding CanExecute property, so control can be told when it is valid to execute that command.
Using some technique to bind or trigger myControl.CanExecute change, from elsewhere.
I think you can use EventToCommandBehavior to achieve this function.
There is an example of an EventToCommandBehavior in the Xamarin.Forms samples (see here).
<ContentPage.BindingContext>
<focusapp:MyViewModel></focusapp:MyViewModel>
</ContentPage.BindingContext>
<StackLayout>
<Entry>
<Entry.Behaviors>
<Behaviors:EventToCommandBehavior
EventName="Unfocused"
Command="{Binding EntryUnfocused}" />
</Entry.Behaviors>
</Entry>
</StackLayout>
And define EntryUnfocused in your viewmodel.cs (e.g. MyViewModel) just as follows:
MyViewModel.cs
public class MyViewModel
{
public ICommand EntryUnfocused { get; protected set; }
public MyViewModel() {
EntryUnfocused = new Command(CompletedCommandExecutedAsync);
}
private void CompletedCommandExecutedAsync(object param)
{
System.Diagnostics.Debug.WriteLine("------------> come here....");
}
}

How to use x:Bind with different data type than data template

I'm working on a view (called 'Familify') which shows users a list of assets, and allows them to delete an asset from the list. The assets are stored in an ObservableCollection in the ViewModel, so the command to delete simply takes the asset object and removes it from collection. I'm having issues getting the 'delete' functionality working. Here is the XAML and codebehind:
Familify.xaml
<ListView
ItemsSource="{Binding Assets}">
<ListView.ItemTemplate>
<DataTemplate>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="80px" />
<ColumnDefinition Width="*" />
<ColumnDefinition Width="150px" />
<ColumnDefinition Width="*" />
<ColumnDefinition Width="60px" />
</Grid.ColumnDefinitions>
<TextBlock
Grid.Column="0"
Text="{Binding number}" FontFamily="Consolas"/>
<TextBlock
Grid.Column="1"
Text="{Binding type}"/>
<TextBlock
Grid.Column="2"
Text="add binding here"/>
<TextBlock
Grid.Column="3"
Text="add binding here"/>
<Button
Command="{x:Bind ViewModel.RemoveAssetCommand}"
CommandParameter="{Binding}"
Content=""
FontFamily="Segoe MDL2 Assets"
Grid.Column="4">
</Button>
</Grid>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
Familify.xaml.cs
namespace asset_manager.Views
{
public sealed partial class Familify : UserControl
{
FamilifyViewModel ViewModel { get; set; }
public Familify()
{
this.InitializeComponent();
DataContextChanged += (s, e) =>
{
ViewModel = DataContext as FamilifyViewModel;
};
}
}
}
The idea is that clicking the button removes the asset from the list. (Just to note, the normal binding showing number, type, etc. is working correctly.) My thinking so far:
Try to use binding to access the RemoveAssetCommand stored in the View Model for the page. However, I couldn't get ancestral binding to work (i.e. trying to find the data context of an element higher up in the XAML hierarchy didn't work because findAncestor isn't a thing in UWP.)
x:Bind looked like a good solution, because it uses an explicit path to the property. So, if I declared ViewModel in my code behind, I could use x:Bind ViewModel.property. All well and good. I did just that, and intellisense allowed me to access the ViewModel.RemoveAssetCommand when typing it out.
However, this did not work, because I get the error no DataType defined for DataTemplate. This makes sense, so I tried two things.
x:DataType="Models:Asset" (put in the DataTemplate tag above) is the model being shown in the data template, so I tried that first. Of course, the command is not declared in the model, it's declared in the View Model, so that didn't work.
I instead tried x:DataType="ViewModels:FamilifyViewModel", thinking I could just use x:Bind with that. However, I then got an error that it couldn't cast an object of type Asset to FamilifyViewModel. This makes sense, because the object getting passed to this data template is of the type Asset.
This is a pain, because the whole reason I thought x:Bind would work is that I could just access the property directly from the ViewModel in the codebehind.
Explicitly stated, 1) is it possible to use x:Bind within a data template to access a base level property (in this case, a Prism command) on the ViewModel? and 2) is there a better way to go about implementing this functionality?
Is it possible to use x:Bind within a data template to access a base level property (in this case, a Prism command) on the ViewModel?
Yes, if you want to access a base level, you can reassign DataContext of button like following:
<Button DataContext="{Binding ElementName=Familily, Path=DataContext}"/>
The Family is the name of UserControl.
is there a better way to go about implementing this functionality?
When you put commad in the ViewModel and bind the button as above. The the bind item of button will become Family DataContext. So you could not invoke delete action directly in the ViewModel.
The best practice to implement this functionality is that put the RemoveAssetCommand in the Asset class. And use the ItemsSource of ListView as Button CommandParameter.
<Button
Command="{Binding RemoveAssetCommand}"
CommandParameter="{Binding ElementName=MyListView, Path=ItemsSource}"
Content=""
FontFamily="Segoe MDL2 Assets"
Grid.Column="4">
</Button>
Asset.cs
public class Asset
{
public string number { get; set; }
public string type { get; set; }
public ICommand RemoveAssetCommand
{
get
{
return new CommandHandler<ObservableCollection<Asset>>((item) => this.RemoveAction(item));
}
}
private void RemoveAction(ObservableCollection<Asset> items)
{
items.Remove(this);
}
}
ViewModel.cs
public class FamilifyViewModel
{
public ObservableCollection<Asset> Assets = new ObservableCollection<Asset>();
public FamilifyViewModel()
{
Assets.Add(new Asset { number = "100001", type = "hello" });
Assets.Add(new Asset { number = "100001", type = "hello" });
Assets.Add(new Asset { number = "100001", type = "hello" });
Assets.Add(new Asset { number = "100001", type = "hello" });
}
}

PrepareContainerForItemOverride works different in Desktop than in Mobile UWP

I wanted to change the color of an item of ListView according the data value.
It would be easy doing:
<ListView.ItemContainerStyle>
<Style TargetType = "ListViewItem" >
< Setter Property="Background" Value="{Binding EventType, Converter={StaticResource EventTypeToBackColorConverter}}" />
</ListView.ItemContainerStyle>
But the thing is that UWP does not support binding in Setter Properties.
My second attempt was overriding PrepareContainerForItemOverride of the ListView:
public class EventListView : ListView
{
protected override void PrepareContainerForItemOverride(DependencyObject element, object item)
{
base.PrepareContainerForItemOverride(element, item);
var listViewItem = element as ListViewItem;
var ev = item as EventType;
if(ev.Warning)
listViewItem.Background = new SolidColorBrush(Color.Red);
}
}
The above code works fine running in a PC with Windows 10 and UWP. It colors in red some items according the underlying data. When I run the same app in Windows Mobile, at beginning it works fine, but when I scroll up and then I scroll down, returning to the original view that was ok at beginning, now other items are also in red color.
What I am missing?
I am not sure the reason, but the following code works for me:
public class EventListView : ListView
{
protected override void PrepareContainerForItemOverride(DependencyObject element, object item)
{
base.PrepareContainerForItemOverride(element, item);
var listViewItem = element as ListViewItem;
var ev = item as EventType;
if(ev.Warning)
listViewItem.Background = new SolidColorBrush(Color.Red);
else
listViewItem.Background = null;
}
}
I have added listViewItem.Background = null
This is because when there are a large number of Items, by default ListView has implement the function of data virtualization. It's not a good idea to disable this function since it can achieve a better performance.
But for your scenario, there is a much easier method to solve your problem. Since you're trying to modify the style of ListViewItem in the code behind, and we can't modify the existed one, we can set a new style of ListViewItem to ListView for example like this:
private void Button_Click(object sender, RoutedEventArgs e)
{
var dynamicStyle = new Style();
dynamicStyle.TargetType = typeof(ListViewItem);
dynamicStyle.Setters.Add(new Setter(BackgroundProperty, Colors.Red));
listView.ItemContainerStyle = dynamicStyle;
}
Only one problem is, if you are setting the Background property to all the ListViewItem, it makes no difference than binding data to the Background property of ListView or setting the Background to ListView like this:
listView.Background = new SolidColorBrush(Colors.Red);
So I just assume that you want to modify the root control in the DataTemplate for example like the Grid in the following xaml:
<ListView x:Name="listView" ItemsSource="{x:Bind collection}">
<ListView.ItemContainerStyle>
<Style TargetType="ListViewItem" x:Name="myListItemStyle">
<Setter Property="HorizontalContentAlignment" Value="Stretch" />
</Style>
</ListView.ItemContainerStyle>
<ListView.ItemTemplate>
<DataTemplate>
<Grid HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
<TextBlock Text="{Binding Testtext}" />
</Grid>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
Then in this scenario, you can use data binding probably like this:
<DataTemplate>
<Grid HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Background="{Binding EventType, Converter={StaticResource EventTypeToBackColorConverter}}">
<TextBlock Text="{Binding Testtext}" />
</Grid>
</DataTemplate>
Any way, if you insist to change some property of all ListViewItem in the ListView, you can use the first method I provided.

Binding ListPicker.SelectedIndex problem

I'm trying to do a two way binding of the SelectedIndex attribute of a ListPicker in a Windows Phone 7 UserControl.
It raises the following exception when I set the DataContext:
SelectedIndex must always be set to a valid value.
This is the XAML code
<Grid x:Name="LayoutRoot">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<toolkit:ListPicker
Grid.Row="0"
x:Name="List1"
SelectionChanged="Picker_SelectionChanged"
SelectedIndex="{Binding PickerSelectedIndex, Mode=TwoWay}"
ItemTemplate="{StaticResource PickerTemplate}"
ItemsSource="{Binding MyList}"/>
</Grid>
And the code behind in DataContext
private ObservableCollection<MyClass> myList = null;
public ObservableCollection<MyClass> MyList
{
get { return this.myList; }
set
{
if (value != this.myList)
{
this.myList= value;
NotifyPropertyChanged("MyList");
this.PickerSelectedIndex = 0;
}
}
}
private int pickerSelectedIndex = 0;
public int PickerSelectedIndex
{
get
{
return this.pickerSelectedIndex;
}
set
{
this.pickerSelectedIndex= value;
}
}
Putting a breakpoint in PickerSelectedIndex.get I can see that it is returned correctly (0).
I am sure that the problem is SelectedIndex="{Binding PickerSelectedIndex, Mode=TwoWay}" because deleting this line solves the problem, and I can see the ListPicker correctly loaded with the data from MyList.
I can't see where is the problem...
Moving SelectedIndex after ItemsSource solved the problem.
This is the working snippet
<toolkit:ListPicker
Grid.Row="0"
x:Name="List1"
SelectionChanged="Picker_SelectionChanged"
ItemTemplate="{StaticResource PickerTemplate}"
ItemsSource="{Binding MyList}"
SelectedIndex="{Binding PickerSelectedIndex, Mode=TwoWay}"/>
Anyone have an explanation for this?
My guess would be that the binding is being applied with a default value of zero when created and before the items exist. It's therefore trying to select the first item (with a zero index) before that item is created.
Try making sure that the ViewModel property of PickerSelectedIndex defaults to -1.
You may also want to delay setting the binding until the items are created.
Matt Lacey is right; binding happens before the data items get populated & hence the error. If you have an event handler for SelectionChanged, you will notice that a breakpoint in it will be hit as the page/listpicker loads. Here is one way to get around this initialization issue:
private void SomeListPicker_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
// Make sure we don't handle the event during initiation.
if (e.RemovedItems != null && e.RemovedItems.Count > 0)
{
if (this.SomeListPicker.SelectedItem != null)
{
// Do actual stuff.
}
}
}
I had the same issue and the ordering of the properties in XAML didn't help. In my case, I was binding ItemsSource to a property on a StaticResource, but I was binding SelectedIndex with a property of my page's ViewModel. The moment I changed the binding of ItemsSource to bind to a property on the ViewModel itself (i.e. duplicated a property of the StaticResource), my issue went away and I was able to perform 2-way binding on SelectedIndex as well.
I've found the same problem with my app.
But I noticed that it happens when I delete all the elements of the list bounded to the ListPicker in the ViewModel.
So it isn't necessary to Bind SelectedIndex with another property since the problem depends only on the list bounded.
Here is my code which work fine for me:
<toolkit:ListPicker x:Name="ListaCorsi"
SelectionChanged="ListaCorsi_SelectionChanged"
ItemsSource="{Binding ListaCorsiUser}"
SelectionMode="Single"
ItemTemplate="{StaticResource CorsiDataTemplate}"
ItemsPanel="{StaticResource ItemsPanelTemplateListaCorsi}"/>
The list in view model:
private ObservableCollection<Corsi> _listaCorsiUser;
public ObservableCollection<Corsi> ListaCorsiUser
{
get { return _listaCorsiUser; }
set
{
_listaCorsiUser = value;
OnPropertyChanged("ListaCorsiUser");
}
}
The handler for SelectionChanged:
void ListaCorsi_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
if (ListaCorsi.SelectedItem != null)
{
---
this.CorsoSelected = ListaCorsi.SelectedItem as Corsi;
}
}
Where Corsi is the class type of the list.
Here ListPicker template:
<DataTemplate x:Key="CorsiDataTemplate">
<Grid>
<Grid.Background>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="Black" Offset="0"/>
<GradientStop Color="#FF3523FF" Offset="0.25"/>
<GradientStop Color="Black" Offset="1"/>
<GradientStop Color="#FF3523FF" Offset="0.75"/>
</LinearGradientBrush>
</Grid.Background>
<TextBlock TextWrapping="Wrap" Text="{Binding NomeCorso}" FontSize="24" FontFamily="Freestyle Script" TextAlignment="Center"/>
</Grid>
</DataTemplate>
And, in the end, the method delete that checks if the list returned by IsolatedStorage is empty, if so, I put a fake empty element in the list bounded to the ListPicker, in order to not receive the error mentioned in this post.
if (this.CorsoSelected != null)
{
---
List<Corsi> corsi = new List<Corsi>(DBHelper.GetCorsiByUserId(PassaggioValori.UserId));
if (corsi.Count > 0)
{
this.ListaCorsiUser = new ObservableCollection<Corsi>(corsi);
}
else
{
this.ListaCorsiUser = new ObservableCollection<Corsi>(new List<Corsi>() { new Corsi()});
}
----
}
The strange thing was that, if the list was empty when the page has been loaded, nothing happens, instead, when I removed the last element from the list, the application raised the exception "SelectedItem must always be set to a valid value".
Now the problem is solved.

Why can't get validation error display in validationSummary?

I have a form with some validations set in entity metadata class. and then binding entity instance to UI by VM. Something as below:
Xaml like:
<Grid x:Name="LayoutRoot">
<StackPanel VerticalAlignment="Top">
<input:ValidationSummary />
</StackPanel>
<TextBox Text="{Binding Name, Mode=TwoWay}" />
<ComboBox x:Name="xTest" ItemsSource="{Binding MyList}"
SelectedItem="{Binding MyItem,Mode=TwoWay,
DisplayMemberPath="MyName"
ValidatesOnDataErrors=True,
ValidatesOnNotifyDataErrors=True,
ValidatesOnExceptions=True,
NotifyOnValidationError=True,UpdateSourceTrigger=Explicit}" />
</Grid>
Code-behind like:
public MyForm()
{
InitializeComponent();
this.xTest.BindingValidationError +=new EventHandler<ValidationErrorEventArgs>((s,e)=>{
BindingExpression be = this.xTest.GetBindingExpression(ComboBox.SelectedItemProperty);
be.UpdateSource();
if (e.Action == ValidationErrorEventAction.Added)
((ComboBox)s).Foreground = new SolidColorBrush(Colors.Red);
});
}
Metadata like:
[Required]
public string Name { get; set; }
[RequiredAttribute]
public int MyItemID { get; set; }
But when running the app, I got nothing display in valudationSummary.
For CombBox, even there is error, looks like BindingValidationError event is never fired.
How to resolve it?
Why are you using an Explicit UpdateSourceTrigger?
Silverlight validation happens inside the binding framework, when the binding is updating the source object. The way you have this, there won't be a binding validation error because you never tell the binding to update the source object. Well, actually you do, but it happens inside the validation error event handler. You've written chicken-and-egg code.
Remove your UpdateSourceTrigger on your binding or set it to Default.
Remove the explicit call to BindingExpression.UpdateSource.
Remove setting the ComboBox foreground to red - you are using NotifyOnValidationError=True, which eliminates any need to manually color the control.
Remove the DisplayMemberPath from the binding
So your XAML:
<Grid x:Name="LayoutRoot">
<StackPanel VerticalAlignment="Top">
<input:ValidationSummary />
<ComboBox x:Name="xTest" ItemsSource="{Binding MyList}"
SelectedItem="{Binding MyItem,
Mode=TwoWay,
ValidatesOnDataErrors=True,
ValidatesOnNotifyDataErrors=True,
ValidatesOnExceptions=True,
NotifyOnValidationError=True}" />
</StackPanel>
</Grid>
And your code:
public MyForm()
{
InitializeComponent();
// you don't need anything here to have the validations work
}