I am writing a MAUI app. I am trying to use a converter. But for some reason, the converter is called twice, and that makes the app crash. Here is the code:
xaml:
<Picker Title="Choose Tile Thickness..."
ItemsSource="{Binding TileThicknessesMetric}"
ItemDisplayBinding="{Binding Value}"
SelectedItem="{Binding SelectedTileThickness}"
IsVisible="{Binding SelectedUnit, Converter={converters:SelectedUnitToBoolShowMetricConverter}, Mode=TwoWay}"
Style="{StaticResource PickerStyle}"
Grid.Row="2" Grid.Column="1" />
Converter:
class SelectedUnitToBoolShowMetricConverter : IValueConverter, IMarkupExtension
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
return ((Unit)value).Code == UnitCode.Metric;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
return null;
}
public object ProvideValue(IServiceProvider serviceProvider)
{
return this;
}
}
As I mentioned, Convert() is called twice, and the second time value is null, which causes a null reference exception. I cannot figure out why it is called the second time.
The same code works as expected in a Xamarin.Forms app.
Related
I am using XAML to define a ListView, with multiple buttons for each cell.
I want to trigger visibility depending on whether a string value is empty or not.
My button inside the ListView is:
<Button Text="{Binding Phone}"
Clicked="OnPhoneClicked"
CommandParameter="{Binding Telefono}"
x:Name="btnPhone" />
Binding Phone is read from my model. It is correctly shown.
How can set a IsVisible property button if Phone's value is an empty string?
Try this code
<Button Text="{Binding Phone}"
Clicked="OnPhoneClicked"
CommandParameter="{Binding Telefono}"
x:Name="btnPhone"
IsVisible="{Binding Phone,Converter={StaticResource StringNullOrEmptyBoolConverter"} />
StringNullOrEmptyBoolConverter.cs file
public class StringNullOrEmptyBoolConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
var s = value as string;
return !string.IsNullOrWhiteSpace(s);
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
throw new NotImplementedException();
}
}
Finally add this in App.xaml file
<Application.Resources>
<ResourceDictionary>
<Converter:StringNullOrEmptyBoolConverter x:Key="StringNullOrEmptyBoolConverter" />
</ResourceDictionary>
</Application.Resources>
I have a TextBox bound to a Converter. Convert ist working fine, ConvertBack is never called.
Shouldn't ConvertBack be called after the TextBox.TextChanged Event ?
Following Code is a simplified example of my problem.
<Window.Resources>
<converter:Converter x:Key="MyConverter"/>
</Window.Resources>
<StackPanel>
<TextBox Name="TestTextBox" VerticalAlignment="Top" Margin="10"
Text="{Binding Path=.,
Converter={StaticResource MyConverter},
ConverterParameter=Name,
UpdateSourceTrigger=PropertyChanged,
Mode=TwoWay}" />
</StackPanel>
Simple ValueConverter
public class Converter : IValueConverter
{
public object Convert(object value, Type targetType, object
parameter, CultureInfo culture)
{
return "Yeahhhh";
}
public object ConvertBack(object value, Type targetType, object
parameter, CultureInfo culture)
{
return "F#*ยง";
}
}
How do I bind SourceObject and TargetObject to the TextBox-Element?
This works, but I want more than one textbox and that does not seem to be possible when they are named the same.
My goal is to have the TextBox change its background color when focused.
<TextBox xmlns="https://github.com/avaloniaui"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:i="clr-namespace:Avalonia.Xaml.Interactivity;assembly=Avalonia.Xaml.Interactivity"
xmlns:ia="clr-namespace:Avalonia.Xaml.Interactions.Core;assembly=Avalonia.Xaml.Interactions"
x:Class="Test.View.CustomTextBox"
Name="textBox">
<i:Interaction.Behaviors>
<ia:EventTriggerBehavior EventName="GotFocus" SourceObject="{Binding #textBox}">
<ia:ChangePropertyAction TargetObject="{Binding #textBox}" PropertyName="Background" Value="{StaticResource FocusedBackgroundColor}"/>
</ia:EventTriggerBehavior>
</i:Interaction.Behaviors>
</TextBox>
Thanks a lot!
You can use RelativeSource and converter, something like that:
public class BoolColorBrushConverter : IValueConverter
{
public Brush TrueBrush {get;set;}
public Brush FalseBrush {get;set;}
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if(value is bool b && b)
return TrueBrush;
else
return FalseBrush;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) => throw new NotSupportedException();
}
xaml:
<MyControl>
<MyControl.Resources>
<BoolBrushConverter TrueColor="Red" FalseColor="Blue" x:Key="TextBoxFocusedBackgroundConverter"/>
</MyControl.Resources>
<TextBox Background="{Binding IsFocused, RelativeSource={RelativeSource Self}, Converter={StaticResource TextBoxFocusedBackgroundConverter}}}"/>;
</MyControl>
Currently I am working on Xamarin.Forms and wondering about any possibility to add show/hide option to an entry field?
I have solved a similar issue by using an expand/collapse icon above a number of entry fields.
The show/hide element in XAML
Add a clickable image with fixed size(20x20) referring to embedded resources in the PCL:
<Image Source="{Binding ShowHideIcon, Converter={StaticResource StringToResImageSourceConverter}}" WidthRequest="20" HeightRequest="20"">
<Image.GestureRecognizers>
<TapGestureRecognizer Command="{Binding ShowHideCommand}" />
</Image.GestureRecognizers>
</Image>
The ViewModel processes the command:
Switch the boolean every time the image is touched.
public bool EntryVisible { get; set; }
public Command ShowHideCommand{
get {
return new Command((object o) => {
EntryVisible = !EntryVisible;
if (EntryVisible) {
ShowHideIcon = "ic_collapse";
} else {
ShowHideIcon = "ic_expand";
}
}
}
}
The label and Entry in XAML
Bind the IsVisible attribute of the Label and Entry to the boolean in the ViewModel.
<Label Text="Quantity" IsVisible="{Binding EntryVisible}" />
<Entry Text="{Binding Quantity}" IsVisible="{Binding EntryVisible}" />
For completeness sake, I have used https://developer.xamarin.com/guides/xamarin-forms/working-with/images/#Embedded_Images to store images ic_expand.png and ic_collapse.png in the PCL Resources folder.
A Converter is required to turn a string e.g. "ic_expand" into an image reference that XAML can use.
public class StringToResImageSourceConverter : IValueConverter {
public object Convert(object value, Type targetType, object parameter, CultureInfo culture) {
var resString = (string)value;
if (!string.IsNullOrEmpty(resString)) {
return ImageSource.FromResource("ProjectName.Resources." + resString + ".png");
}
return null;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) {
throw new NotImplementedException();
}
}
Entry entry = new Entry();
// Hide it
entry.IsVisible = false;
I have a collection of items in my app, and I want to set the Content of a ContentPresenter to one of these items. The item will be randomly defined by an int index. I can bind an item like this:
<ContentPresenter Content={Binding Items[0]}/>
but not like this:
<ContentPresenter Content={Binding Items[{Binding Index}]}/>
I've seen a number of answers suggesting using MultiBinding in WPF, but this isn't available in UWP. Is there an alternative?
You could create a view model property, returning Items[Index]:
public string RandomItem => Items[Index];
For the PropertyChanged notifications to work, you will need to raise the event whenever Index or Items changes, e.g.:
public int Index
{
get { return _index; }
set
{
_index = value;
RaisePropertyChanged();
RaisePropertyChanged(() => RandomItem);
}
}
If you prefer to have the logic in the view and go the multi-binding way, you can use the Cimbalino toolkit. For that to work, first add 2 NuGet packages:
Cimbalino.Toolkit
Microsoft.Xaml.Behaviors.Uwp.Managed
Now you can create a converter:
public class CollectionIndexConverter : MultiValueConverterBase
{
public override object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
var collection = (IList) values[0];
var index = (int?) values[1];
return index.HasValue ? collection[index.Value] : null;
}
public override object[] ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new System.NotImplementedException();
}
}
And use it from XAML:
<ContentPresenter>
<interactivity:Interaction.Behaviors>
<behaviors:MultiBindingBehavior PropertyName="Content" Converter="{StaticResource CollectionIndexConverter}">
<behaviors:MultiBindingItem Value="{Binding Items}" />
<behaviors:MultiBindingItem Value="{Binding Index}" />
</behaviors:MultiBindingBehavior>
</interactivity:Interaction.Behaviors>
</ContentPresenter>