Circular referenced IValueConverter in Application.Resources - xaml

Question

I'm getting the error :
StaticResource not found for key maxLength
the setup is as follows:
Converter setup in app.xaml, which also contains a datatemplate
<Application.Resources>
<ResourceDictionary>
<ext:MaxLengthStringConverter x:Key="maxLength"/>
....
<DataTemplate x:Key="HotelViewModel">
<tripSegmentPartViews:HotelView
Padding="0"
HeightRequest="60"
BorderWidth="1"
BorderColor="{ext:ColourResource Divider}"
BordersToDraw="{x:Static controls:Borders.Top}"
BackgroundColor="Transparent"/>
</DataTemplate>
....
view in the HotelView.xaml which is in the datatemplate, uses the converter
....
<Label Text="{Binding HotelName, Converter={StaticResource maxLength}, ConverterParameter=10}"
AbsoluteLayout.LayoutFlags="All"
AbsoluteLayout.LayoutBounds="1,0.2,0.62,0.5"
VerticalOptions="End"
HorizontalOptions="Start"
FontSize="20"
/>
....
If I move the converter to HotelView.xaml resource dictionary it works
If I change the reference to a DynamicResource it is not used
Obviously with something as basic as max length (which shortens the string and adds '...' if its over the required length) I want to be able to use it through out the application, and not have to reference it in multiple resource dictionaries.
Is this a bug?
----------------- edit ------------------
OK I have reproduced this errror with a minimum app consisting of:
App1.xaml
<?xml version="1.0" encoding="utf-8" ?>
<Application xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="App1.App"
xmlns:ext="clr-namespace:App1.Extensions;assembly=App1"
xmlns:local="clr-namespace:App1;assembly=App1">
<Application.Resources>
<ResourceDictionary>
<ext:MyConverter x:Key="conv"></ext:MyConverter>
<DataTemplate x:Key="dt">
<local:View1></local:View1>
</DataTemplate>
</ResourceDictionary>
</Application.Resources>
</Application>
Page1.xaml
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="App1.Page1"
xmlns:local="clr-namespace:App1;assembly=App1">
<StackLayout>
<Label Text="Page1" VerticalOptions="Center" HorizontalOptions="Center" TextColor="White" />
<ListView ItemTemplate="{StaticResource dt}" ItemsSource="List">
</ListView>
</StackLayout>
</ContentPage>
View1.xaml
<?xml version="1.0" encoding="utf-8" ?>
<ContentView xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="App1.View1"
xmlns:ext="App1.Extensions">
<Label Text="{Binding MainText, Converter={StaticResource conv}" VerticalOptions="Center" HorizontalOptions="Center" TextColor="White"/>
</ContentView>
App1.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Xamarin.Forms;
namespace App1
{
public partial class App : Application
{
public App()
{
InitializeComponent();
// The root page of your application
Page1 p = new Page1();
p.BindingContext = new {
MainText = "test",
List = new List<string>() { "test"}
};
var navContainer = new NavigationPage(p);
navContainer.BarBackgroundColor = Color.Red;
navContainer.BarTextColor = Color.White;
MainPage = navContainer;
}
}
}
The error is thrown at runtime using the VS emulator

Answer 1

From what you mention you want to be able to define a IValueConverter the once, and use it from any Xamarin.Forms ContentPage, without the need to keep specifying the converter in the local XAML page.
This can be achieved by doing the following:-
In your PCL you normally have App.cs.
You will need to delete this, and add a new Forms Xaml Page called App.cs.
This will generate both the App.xaml and related App.cs files.
In this question, (How can I databind an image?), there is a converter called MyByteToImageSourceConverter.
I will illustrate using this:-
App.xaml
<?xml version="1.0" encoding="utf-8" ?>
<Application xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="SampleApp.App"
xmlns:local="clr-namespace:{namespace reference goes here to the converter}"
>
<Application.Resources>
<ResourceDictionary>
<local:MyByteToImageSourceConverter x:Key="kyByteToImageSourceConverter"/>
</ResourceDictionary>
</Application.Resources>
</Application>
So in the above we have defined our converter with a key, that we will then be able to reference from all other Xamarin.Forms ContentPage's.
App.cs
namespace SampleApp
{
public partial class App
: Xamarin.Forms.Application
{
public App()
{
InitializeComponent();
//
this.MainPage = new ByteToImageExample2();
}
}
}
In the code-behind we need to change the default inheritance from ContentPage and specify Xamarin.Forms.Application.
We also specify our launch page, via the this.MainPage = ...
ByteToImageExample2.xaml:-
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="SampleApp.ByteToImageExample2"
>
<StackLayout>
<Image Source="{Binding MyImageAsBytes, Converter={StaticResource kyByteToImageSourceConverter}}"/>
</StackLayout>
</ContentPage>
In our ContentPage we can see above we are referencing the converter that we specified in App.xaml via the StaticResource kyByteToImageSourceConverter.
As that was definied in App.xaml, we can re-use this in all our pages without the need to specify the location of the converter locally.
For completeness the code-behind is:-
ByteToImageExample2.cs:-
public partial class ByteToImageExample2 : ContentPage
{
public ByteToImageExample2()
{
InitializeComponent();
//
byte[] bytImage = { your image as a byte collection }
//
this.BindingContext = new MyImageViewModel()
{
MyImageAsBytes = bytImage
};
}
}
Update 1:-
You can have the following in your App.xaml:-
<DataTemplate x:Key="kyByteToImage3ExampleDataTemplate2">
<ViewCell>
<local2:MyCustomView1/>
</ViewCell>
</DataTemplate>
with local2:MyCustomView1 referencing your custom view, which for this example is defined as:-
<ContentView xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="SampleApp.MyCustomView1">
<Image Source="{Binding MyImage, Converter={StaticResource kyByteToImageSourceConverter}}" Aspect="AspectFit" />
</ContentView>
This custom view still uses a Converter as specified from the App.xaml and does still render, without the need to specify the Converter locally within the ContentView class.