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Saturday, April 22, 2017

Value Converter in Xamarin.Forms

Introduction:
Databind two control properties or view model property that have incompatible types / Validation, you need a piece of code in between, that converts the value from source to target type and back. This piece of code is called ValueConverter. A value converter is a class, that implements the interface IValueConverter with the two methods object Convert(object value) and object ConvertBack(object value).
In this article, I will share about IValueConverter in xamarin.forms application
Create New Xamarin Forms Application:
Let's start with creating a new Xamarin Forms Project in Visual Studio.

Open Run - Type Devenev.Exe and enter - New Project (Ctrl+Shift+N) - select Blank Xaml App (Xamarin.Forms Portable) template.
You can refer my previous article for create new xamarin forms application from here - http://www.c-sharpcorner.com/article/how-to-create-first-xamarin-form-application/
Create New Value Convert Class:
You can create new Value Converter class now, Right Click Project > Add New Item > Add Class File, Name as “ MaxLengthConverter” .
In the following sample, I am getting value from Editor control and checking text length ,if text length less than 100 words  means label color changed green or Red color .
Inherit IvalueConverter interface and add xamarin.Forms name space .
  1. using System;
  2. using System.Globalization;
  3. using Xamarin.Forms;
  4. namespace MVVMXamarin
  5. {
  6.    class MaxLengthColorConverter : IValueConverter
  7.    {
  8.        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
  9.        {
  10.            if (value != null)
  11.            {
  12.                int count = value.ToString().Length;
  13.                if (count > 100)
  14.                {
  15.                    return Color.Red;
  16.                }
  17.            }
  18.            return Color.Green;
  19.        }
  20.        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
  21.        {
  22.            throw new NotImplementedException();
  23.        }
  24.    }
  25. }


Implement Value Convert in Xaml:
In the following xaml page, added converter class within Resource Dictionary and added convert class key from control
  1. <?xml version="1.0" encoding="utf-8" ?>
  2. <ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
      1. xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
      2. x:Class="MVVMXamarin.ConvertDemo"
  3. xmlns:local="clr-namespace:MVVMXamarin;assembly=MVVMXamarin">
  4. <ContentPage.Resources>
  5. <ResourceDictionary>
  6. <local:MaxLengthColorConverter x:Key="cnmaxlength"></local:MaxLengthColorConverter>
  7. </ResourceDictionary>
  8. </ContentPage.Resources>
  9. <StackLayout>
  10. <Label Text="About You"></Label>
  11. <Editor  x:Name ="txtname"></Editor>
  12. <Label BindingContext="{x:Reference Name=txtname}" Text="Please Say about you within 100 words" BackgroundColor="{Binding Path=Text,Converter={StaticResource cnmaxlength}}"
      1. VerticalOptions="Center"
      2. HorizontalOptions="Center" />
  13. </StackLayout>
  14. </ContentPage>
Implement value Converter in C#:
In the following sample code shown, how add converter class into c# binding control .You can add your converter class into Setbinding method.
  1. StackLayout layout = new StackLayout();
  2. Label lbltitle = new Label();
  3. lbltitle.Text = "About Me";
  4. layout.Children.Add(lbltitle);
  5. var editor = new Editor();
  6. layout.Children.Add(editor);
  7. var label = new Label();
  8. label.Text = "Please Say about you within 100 words";
  9. label.SetBinding(Label.BackgroundColorProperty, new Binding("BackgroundColor",mode:BindingMode.Default, source: editor,converter: new MaxLengthColorConverter(),stringFormat:null));
  10. layout.Children.Add(label);
  11. this.Content = layout;

I believe this article will help you for how to use IValueConverter  

Download Source code :