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;
5. namespace MVVMXamarin
6. {
7.    class MaxLengthColorConverter : IValueConverter
8.    {
9.        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
10.        {
11.            if (value != null)
12.            {
13.                int count = value.ToString().Length;
14.                if (count > 100)
15.                {
16.                    return Color.Red;
18.                }
19.            }
20.            return Color.Green;
21.        }
23.        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
24.        {
25.            throw new NotImplementedException();
26.        }
27.    }
28. }

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 :

Binding Control to Control in Xamarin Forms

Introduction:

Databinding is a mechanism in Xamarin Forms Application.  Binding is common context, is the process of mapping a property on a Page, to a property in a view or ViewModel. In this Article I will show how bind Control property to Control property  binding in xamarin Forms.

 

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/

Binding Control to Control using xaml :

Data bindings can be defined to link properties of two Controls on the same page. In this case, you set the BindingContext of the target object using the x: Reference markup extension.

In the following sample added one Entry control and Label control for print text

1. <StackLayout>
3.  <Entry x:Name ="txtname"></Entry>
4.  <Label  Text="Welcome Mr.."
5.           VerticalOptions="Center"
6.           HorizontalOptions="Center" />
7.  <Label BindingContext="{x:Reference Name=txtname}" Text="{Binding Path=Text}"
8.           VerticalOptions="Center"
9.           HorizontalOptions="Center" />
10.  </StackLayout>

Entry Control Name : txtname

Label control adding Binding Context and Binding the value.

While assign binding Context, we can use following two way

BindingContext="{x:Reference Name=txtname} OR BindingContext="{x:Reference txtname} // Name attribute is not required

Same like while assign value to property, we can use following two way

Text="{Binding Path=Text}" OR Text="{Binding Text}" // Path attribute is not required

Binding Control with String Builder:

All the database data or other srorage data is not formatted data so you Can bind Property with string or any type like below  

<Label BindingContext="{x:Reference Name=txtname}" Text="{Binding Path=Text, StringFormat='Welcome Mr {0}'}"/>

1. <StackLayout>
2. <Entry x:Name ="txtname"></Entry>
3. <Label BindingContext="{x:Reference Name=txtname}" Text="{Binding Path=Text, StringFormat='Welcome Mr {0}'}"
1. VerticalOptions="Center"
2. HorizontalOptions="Center" />
4. </StackLayout>

Binding Control in C#:

You can do it effectively the same way as the XAML example simply by setting your view's BindingContext or Source to the other view.

label.SetBinding(Label.TextProperty, new Binding("<Property Name>", stringFormat: "Welcome Mr {0}", source: <BindingContext Control  name>));

or

var label = new Label{ BindingContext = <Control Name> };

label.SetBinding(Label.TextProperty, new Binding("<Property Name>", stringFormat: "Welcome Mr {0}"));

1. StackLayout layout = new StackLayout();
2. var editor = new Editor();
3. layout.Children.Add(editor);
4. var label = new Label();
5. label.SetBinding(Label.TextProperty, new Binding("Text", stringFormat: "Welcome Mr {0}", source: editor));
6. layout.Children.Add(label);
7. this.Content = layout;

Binding User Control:

Xamarin Forms doesn’t have a control called a User Control. However, we can make any VisualElement, or combination of Visual Element’s into a reusable control, to use on different pages, just like a User Control.

Create User Control:

Right Click PCL project > Select” Forms XAML View”  and click on Add

In xaml and C# file, change content View to any layout control like Grid, Stack layout

In Xaml Page:

8. <?xml version="1.0" encoding="UTF-8"?>
9. <Grid xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="MVVMXamarin.MyControl">
11.    <Label Text="{Binding Text, Source={x:Reference this}}" />
12.  
13. </Grid>

In Code behind file, you might want to make your control, have a bindable property to pass data through to it, on each page it is used. First, create a Bindable Property in your UserControl. It doesn’t have to be bindable if you just want to pass a static value through, however if you want to bind to it, you must make it bindable.

14. using System;
15. using System.Collections.Generic;
16. using System.Linq;
17. using System.Text;
18. using System.Threading.Tasks;
20. using Xamarin.Forms;
22. namespace MVVMXamarin
23. {
24.    public partial class MyControl : Grid
25.    {
26.        public MyControl()
27.        {
28.            InitializeComponent();
29.        }
30.        public static readonly BindableProperty TextProperty = BindableProperty.Create(nameof(Text), typeof(string), typeof(MyControl));
32.        public string Text
33.        {
34.            get
35.            {
36.                return (string)GetValue(TextProperty);
37.            }
38.            set
39.            {
40.                SetValue(TextProperty, value);
41.            }
42.        }
43.    }
44. }

Going to your MainPage, you can now assign a value to the Text property. You can also use {Binding PropertyName} if you want, as this is a bindable property.

45. <StackLayout>
46. <Entry x:Name ="txtname"></Entry>
47. <local:MyControl BindingContext="{x:Reference Name=txtname}" Text="{Binding Path=Text, StringFormat='Welcome Mr {0}'}">
48. </local:MyControl>
49.  </StackLayout>