Develop Reference

Xamarin Forms RowDefinition height binding via converter set height autoD

Currently I would like to bind my Height property on a Rowdefinition in a Grid. I want to show the row if the property IsOnline on my ViewModel is set to true.
Binding a number as Height is no problem at all, I am just wondering how I could bind it to Auto.
My View:
<Grid.RowDefinitions>
<RowDefinition/>
<RowDefinition/>
<RowDefinition Height="{Binding IsOnline, Converter={StaticResource HeightConverter}}"/>
</Grid.RowDefinitions>
My Converter HeightConverter:
public class HeightConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (value is bool)
{
if ((bool)value)
{
return "Auto";
}
}
return 0;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}

Check GridLength Struct it has properties like Auto, Star and etc. You could use those as return values from the IValueConverter.

Is there an IValueConverter that does if-then

What I am trying to do:
<Grid>
<Grid.RowDefinitions>
...
<!--The next line is pseudo code for what I am trying to achieve-->
<RowDefintion Height="if(EditEnabled) { 10* } else { 0 }" />
...
</Grid.RowDefinition>
...
<DockPanel Visibility="{Binding EditEnabled, Converter={StaticResource InverseBooleanToVisibilityConverter}}" ...>
...
I am trying to change the visibility of the DockPanel depending on whether editing is enabled, while keeping he ability to resize and have fixed heights and relative heights.
The question:
Is there an IValueConverter (System.Windows.Data.IValueConverter) that can take a boolean, and two numbers and choose one of the GridLengths based on the boolean? From just inspecting the interface of IValueConverter it doesn't look like this is quite the right type to use.
Or is there a better way to inject the GridLength that I want?
What I have tried:
Looking through the inheritors of IValueConverter - nothing obvious to me
Moving the Height="10*" inside the DockPanel tag and changing the RowDefinition to be Auto - this created an conversion exception
Searching here

Unfortunately, there is no IValueConverter that does if-then.
(and to be more specific: you can not do if-then logic with the XAML)
But you can do the if-then logic in the C# code.
here is the solution
public class HeightConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
bool enableEdit = (bool)value;
double param = System.Convert.ToDouble(parameter);
if (enableEdit)
return new GridLength(param, GridUnitType.Star);
else
return new GridLength(0);
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
throw new NotImplementedException();
}
}
and the window like this.
<Window.Resources>
<local:HeightConverter x:Key="heightConverter"/>
<sys:Int32 x:Key="param">10</sys:Int32>
</Window.Resources>
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="{Binding Path=EditEnabled, Converter={StaticResource heightConverter}, ConverterParameter={StaticResource param}}" />
</Grid.RowDefinitions>
</Grid>
please consider also define the required namespace that you will use, like the following
xmlns:sys="clr-namespace:System;assembly=mscorlib"
xmlns:local="clr-namespace:[your namespace]"
Update the same result could be achieved by using IMutliValueConverter
public class HeightMultiConverter : IMultiValueConverter
{
public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
bool enableEdit = (bool)values[0];
double param = System.Convert.ToDouble(values[1]);
if (enableEdit)
return new GridLength(param, GridUnitType.Star);
else
return new GridLength(0);
}
public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
{
throw new NotImplementedException();
}
}
and the window like this
<Window.Resources>
<local:HeightMultiConverter x:Key="heightMutliConverter"/>
<sys:Int32 x:Key="param">10</sys:Int32>
</Window.Resources>
<Grid>
<Grid.RowDefinitions>
<RowDefinition >
<RowDefinition.Height>
<MultiBinding Converter="{StaticResource heightMutliConverter}">
<Binding Path="EditEnabled"/>
<Binding Source="{StaticResource param}"/>
</MultiBinding>
</RowDefinition.Height>
</RowDefinition>
</Grid.RowDefinitions>
</Grid>
NOTE: just do not forget, you have to take care of the Source by setting the DataContext property.

There is a built-in converter you may be able to use: AlternationConverter. You specify a list of values (of arbitrary type), bind to an integer, and the converter looks up the integer in the list of values (modulo the value count).
If you specify two values for this AlternationConverter, and you're able to provide your EditEnabled property as an integer 0 or 1, then you can map that 0 and 1 to any value you want.
If you feel it doesn't make sense to convert your bool to an integer first (something I can sympathise with), you could still use AlternationConverter as inspiration for a custom converter that doesn't require the model value to be of type int.

Create a BooleanConverter<T> base class as described in http://stackoverflow.com/a/5182660/469708
public class BooleanConverter<T> : IValueConverter
{
public BooleanConverter(T trueValue, T falseValue)
{
True = trueValue;
False = falseValue;
}
public T True { get; set; }
public T False { get; set; }
public virtual object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
return value is bool && ((bool) value) ? True : False;
}
public virtual object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
return value is T && EqualityComparer<T>.Default.Equals((T) value, True);
}
}
Then write
public class BooleanToGridLengthConverter : BooleanConverter<System.Windows.GridLength>
{
public BooleanToGridLengthConverter() : base(
new System.Windows.GridLength(1, System.Windows.GridUnitType.Star),
new System.Windows.GridLength(0))
{
}
}
The values for true and false can be set directly, no need for a MultiValueConverter (as long as only the boolean parameter needs to be bindable).
<convert:BooleanToGridLengthConverter x:Key="Converter" True="10*" False="0" />
Alternatively, you can derive the converter from MarkupExtension and directly use it like this:
<RowDefinition Height="{Binding EditEnabled, Converter={convert:BooleanToGridLengthConverter True=10*, False=0}" />

Why would I get a "BoolToRowHeightConverter is not supported in a Windows Presentation Foundation(WPF) project error in xaml?

Why would I get a "BoolToRowHeightConverter is not supported in a Windows Presentation Foundation(WPF) project error in xaml?
I was using a converter to convert rowheight to * and Auto in a grid based on the expander's IsExpanded property.
Code in xaml:
<RowDefinition Height="{Binding IsExpanded, ElementName=Expander5, Converter={x:Static BoolToRowHeightConverter.Instance}}"/>
Code in xaml.cs:
public class BoolToRowHeightConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
if ((bool)value) return new GridLength(1, GridUnitType.Star);
else
return GridLength.Auto;
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
throw new NotImplementedException();
}
}

Typically, IValueConverters are used like this:
a) Add a namespace in your XAML page that references your converter class... usually it looks something like this:
xmlns:Converters="clr-namespace:WpfApplication1.Converters"
b) Add an instance of your converter class into the Resources section of your page (or of App.xaml:
<Window.Resources>
<Converters:BoolToRowHeightConverter x:Key="BoolToRowHeightConverter" />
...
</Window.Resources>
c) Reference your converter instance by the x:Key value that you gave it:
<RowDefinition Height="{Binding IsExpanded, ElementName=Expander5,
Converter={StaticResource BoolToRowHeightConverter}}" />

You have decided to reference the value converter by using the x:Static markup extension ({x:Static BoolToRowHeightConverter.Instance}) but then you also need to provide the actual field or property that you reference (Instance). To do that you need to add it to the BoolToRowHeightConverter class:
public class BoolToRowHeightConverter : IValueConverter
{
// Convert and ConvertBack methods ...
public static readonly BoolToRowHeightConverter Instance = new BoolToRowHeightConverter();
}

Display image if present in datagrid

I am writing a program to display text and image associated with the text. The nature of data is such that I may or may not have image for every text. This is information is in collection of objects. Each object has text and path of image. If image is not there then path is empty. The class for the object is
public class MyInfo
{
public DateTime EntryDate { set; get; }
public string NoteText { set; get; }
public string ImagePath { set; get; }
}
I used DataGrid to display the information. 1st column shows text and the second shows image. If there is no image then the 2nd column is empty. This does not look nice and customer is asking to change the UI so that it should take the full row if there is no image. Also he wants to have clear separator between the rows. I already have alternating colors but does not go well with the both text and image in place.
Please suggest how to enhance the grid. If DataGrid is not the right control then what is the other control/approach to resolve it.
Thanks

Personally I would a ListBox and use the ListBox.ItemTemplate to define how the row will look. This will give greater flexibility and better achieve what you want. As Ashok said you will want to use a value converter to convert a empty string into a "Collapsed" visibility option.
Converter example:
public class EmptyStringToCollapsedConverter : IValueConverter
{
public override object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
var s = (value as string);
return String.IsNullOrWhiteSpace(s)? Visibility.Collapsed : Visibility.Visible;
}
public override object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotSupportedException();
}
}

Instead of using DataGrid, I suggest to use the ListBox with a DataTemplate, in a way similar to this:
<ListBox ItemsSource="{Binding Path=MyInfoCollection}">
<ListBox.ItemTemplate>
<DataTemplate>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<TextBlock Grid.Column="0" Text="{Binding Path=EntryDate}" />
<TextBlock Grid.Column="1" Text="{Binding Path=NoteText}" />
<Image Grid.Column="2" Source="{Binding Path=ImagePath}" />
</Grid>
</DataTemplate>
</ListBox .ItemTemplate>
</ListBox >
Where MyInfoCollection is an ObservableCollection of MyInfo objects.

Value Converter not Being Called XAML

I have put breakpoints inside the Value Converter and they are never triggered, but the page renders with no image being shown.
The XAML:
xmlns:datatypes="clr-namespace:DataTypes_Portable;assembly=DataTypes_WinPhone8"
...
<phone:PhoneApplicationPage.Resources>
<datatypes:WinPhone8ImageConverter x:Key="ImageConverter" />
</phone:PhoneApplicationPage.Resources>
...
<Image x:Name="LevelImage" HorizontalAlignment="Left" Height="Auto" VerticalAlignment="Top" Width="Auto" Margin="0" Grid.ColumnSpan="5" Source="{Binding Converter={StaticResource ImageConverter}, Path=App.Main.Game.CurrentLevel.CurrentPart.Image}"/>
The CS:
public class WinPhone8ImageConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
var imageProvider = value as WinPhone8ImageProvider;
return imageProvider.Image;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
From what I can understand (by process of trial and elimination and looking at what exceptions are thrown) the problem is coming with part of the XAML where I am trying to bind to the value.
At a breakpoint the value at App.Main.Game.CurrentLevel.CurrentPart.Image is being set correctly (ie is an instance of WinPhone8ImageProvider).

It turns out this was nothing to do with the converter at all. The value was binding before the image had loaded (so the source was empty). For future reference check to make sure you are implementing theINotifyPropertyChanged correctly in your view models.