Tuesday, 16 July 2013

WPF Introduction to XAML



XAML stands for Extensible Application Markup Language. It’s a simple language based on XML to create and initialize .NET objects with hierarchical relations. Although it was originally invented for WPF it can be used to create any kind of object trees.
Today XAML is used to create user interfaces in WPF, Silverlight, declare workflows in WF and for electronic paper in the XPS standard.
All classes in WPF have parameterless constructors and make excessive usage of properties. That is done to make it perfectly fit for XML languages like XAML.

Advantages of XAML
All you can do in XAML can also be done in code. XAML is just another way to create and initialize objects. You can use WPF without using XAML. It's up to you if you want to declare it in XAML or write it in code. Declare your UI in XAML has some advantages:
1)    XAML code is short and clear to read.
2)    Separation of designer code and logic.
3)    Graphical design tools like Expression Blend require XAML as source.
4)    The separation of XAML and UI logic allows it to clearly separate the roles of designer and developer.

XAML vs. Code
As an example we build a simple StackPanel with a textblock and a button in XAML and compare it to the same code in C#.
<StackPanel>
    <TextBlock Margin="20">Welcome to the World of XAML</TextBlock>
    <Button Margin="10" HorizontalAlignment="Right">OK</Button>
</StackPanel>

The same expressed in C# will look like this:
// Create the StackPanel
StackPanel stackPanel = new StackPanel();
this.Content = stackPanel;

// Create the TextBlock
TextBlock textBlock = new TextBlock();
textBlock.Margin = new Thickness(10);
textBlock.Text = "Welcome to the World of XAML";
stackPanel.Children.Add(textBlock);

// Create the Button
Button button = new Button();
button.Margin= new Thickness(20);
button.Content = "OK";
stackPanel.Children.Add(button);

As you can see is the XAML version much shorter and clearer to read. And that's the power of XAMLs expressiveness.

Properties as Elements
Properties are normally written inline as known from XML <Button Content="OK" />. But what if we want to put a more complex object as content like an image that has properties itself or maybe a whole grid panel? To do that we can use the property element syntax. This allows us to extract the property as an own child element.
<Button>
  <Button.Content>
     <Image Source="Images/OK.png" Width="50" Height="50" />
  </Button.Content>
</Button>

Implicit Type conversion
A very powerful construct of WPF are implicit type converters. They do their work silently in the background. When you declare a BorderBrush, the word "Blue" is only a string. The implicit BrushConverter makes a System.Windows.Media.Brushes.Blue out of it. The same regards to the border thickness that is being converted implicit into a Thickness object. WPF includes a lot of type converters for built-in classes, but you can also write type converters for your own classes.
<Border BorderBrush="Blue" BorderThickness="0,10">
</Border>



Markup Extensions
Markup extensions are dynamic placeholders for attribute values in XAML. They resolve the value of a property at runtime. Markup extensions are surrounded by curly braces (Example: Background="{StaticResource NormalBackgroundBrush}"). WPF has some built-in markup extensions, but you can write your own, by deriving from MarkupExtension. These are the built-in markup extensions:
1)    Binding: To bind the values of two properties together.
2)    StaticResource: One time lookup of a resource entry.
3)    DynamicResource: Auto updating lookup of a resource entry.
4)    TemplateBinding: To bind a property of a control template to a dependency property of the control.
5)    x:Static: Resolve the value of a static property.
6)    x:Null: Return null.
The first identifier within a pair of curly braces is the name of the extension. All preceding identifiers are named parameters in the form of Property=Value. The following example shows a label whose Content is bound to the Text of the textbox. When you type a text into the text box, the text property changes and the binding markup extension automatically updates the content of the label.
<TextBox x:Name="textBox"/>
<Label Content="{Binding Text, ElementName=textBox}"/>



Namespaces
At the beginning of every XAML file you need to include two namespaces.
The first is:
It is mapped to all WPF controls in System.Windows.Controls.
The second is:
It is mapped to System.Windows.Markup that defines the XAML keywords.
The mapping between an XML namespace and a CLR namespace is done by the XmlnsDefinition attribute at assembly level. You can also directly include a CLR namespace in XAML by using the clr-namespace: prefix.
 
<Window xmlns=”http://schemas.microsoft.com/winfx/2006/xaml/presentation”
        xmlns:x=”http://schemas.microsoft.com/winfx/2006/xaml”>
</Window>




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