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Windows Phone 8 : XAML Overview - What Is XAML?

12/21/2012 6:31:04 PM
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What is eXtensible Application Markup Language (or XAML)? For Windows Phone, XAML is used to design the user interfaces (both the look and feel of applications), but it does quite a lot more. The main concept to learn here is that XAML can be thought of as a serialization format that works well with tools. It allows us to declare the structure of a user interface. Declaring the interface in this way makes it easy for tools to create the user interfaces and have applications consume the files at runtime.

What do I mean by a serialization format? XAML is quite simple; let’s take a very basic piece of XAML:

<UserControl x:Class="WinningTheLottery.Sample"
    <TextBlock Text="Hello" />
    <Rectangle Width="100"
               Fill="Blue" />

XAML is an XML file that obeys basic XML rules (e.g., single top-level container, case sensitivity). In this file we are declaring a UserControl root that contains a Grid element that contains two elements (a TextBlock and a Rectangle). This is the basic hierarchy of this simple user interface. When parsed, this XAML document is used to create that same hierarchy in memory. Literally, the name of the element ties itself to the name of a class. So when the XAML is parsed, the UserControl element informs the system to create a new UserControl instance. To be used here all the classes must allow for empty constructors (in the .NET sense) so that the UserControl class can be created. After it creates the UserControl itself, it looks at its subelements (the Grid) and creates that element as a child inside the UserControl. Finally, it creates the TextBlock and Rectangle and places them as children inside the Grid. When the TextBlock is created, it sees the attribute (Text) and calls the property setter of the new TextBlock with the contents of the attribute. It does this with the multiple attributes of the Rectangle as well. In this way it uses the XAML to build an in-memory object graph that follows the same structure as the XAML. Understanding that the XAML you are using is the basis for your runtime design is very important in understanding how XAML works.

XAML Object Properties

Most objects’ properties you will set in XAML are simple and string-based:

<Rectangle Fill="Blue" />

Not all properties can be set using the simple, string-based syntax, however. Under the covers, many properties (during XAML parsing) are attempting to convert a string attribute to a property value (using a special type of class called a TypeConverter). For example, Fill is a property that accepts a Brush value, not a color. When Fill="Blue" is parsed as XAML, a conversion is done between the string (i.e., “Blue”) and a brush called a SolidColorBrush. For a more complex value type (like a Brush), there is a verbose syntax for setting property values:

    <SolidColorBrush Color="Blue" />

This verbose syntax is identical at runtime to the earlier example. By adding an element inside the Rectangle whose name is the name of the object, a dot, and the name of the property (e.g., Rectangle.Fill), we can define the value for the property using XAML instead of being stuck using just strings. Since not all complex property values can be defined in such a way that a conversion can be made, this syntax allows for property values to be set when the value is a complex type that would be difficult or impossible to describe in a single string. For example, let’s replace the SolidColorBrush with a LinearGradientBrush:

      <GradientStop Color="Blue" Offset="0" />
      <GradientStop Color="White" Offset="0.5" />
      <GradientStop Color="Blue" Offset="1" />

In this example, we can see that defining a fill by specifying the colors and offsets not only would be difficult in a simple string, but would make the XAML even harder to read. In this way, XAML allows you to set very complex properties without having to invent conversions. You will see how this is used in many different places in XAML as we continue.

Understanding XAML Namespaces

Inside the root element are two namespaces. Namespaces in XAML are XML namespaces. The default namespace (xmlns) declares that this is a XAML document. The second namespace (xmlns:x) brings in several elements and attribute types that are all prefixed with the x alias. So when you see x:Class, that is a convention that is defined in the second namespace.

You can think of the namespace aliasing as similar to namespaces in .NET. When you add a namespace, it brings in those new types of things that can be described in XAML. Unlike .NET, though, you have to use an alias (since all other namespaces are not the “default” namespace) and then use that alias everywhere you want to refer to information from that namespace. In the x alias’s case, the alias here as “x” is just a convention that XAML tends to use. In the XML namespace sense, you can change the alias to whatever you want, but you would have to change it everywhere it’s referenced as well. For example:

<UserControl foo:Class="WinningTheLottery.Sample"

The alias is just that: an alias so that the parser can determine which of the namespaces your element or attribute is from. When we changed the name of the alias, it is what you would use to alias that namespace in the rest of the document.

While these namespaces represent the basic XAML namespaces, you can extend the XAML by using namespaces to bring in arbitrary .NET types as well. If you define a namespace that points at a .NET namespace and assembly, those types will also be available in the XAML:

<UserControl x:Class="WinningTheLottery.Sample"

In this example, the XAML “imports” the System namespace that exists inside the mscorlib.dll assembly. Once that .NET namespace is imported, all the types in that namespace are creatable in the XAML. Any type that is created in XAML must conform to the following rules:

• Has an empty, public constructor

• Has public properties

If any .NET objects follow these rules, they are creatable in XAML (and therefore can be part of your initial object graph).

Naming in XAML

Unlike other platforms, XAML does not require that every object in the XAML be specifically named. In fact, it is probably a bad idea to name every object in the XAML. Naming objects in the XAML becomes important once you need to refer to an object by name (e.g., from code or via data binding). Naming objects in XAML takes the form of an attribute that can be applied to most XAML elements: x:Name. For example:

<UserControl x:Class="WinningTheLottery.Sample"
  <Grid x:Name="LayoutRoot">
    <TextBlock Text="Hello"
               x:Name="theTextBlock" />
    <Rectangle Width="100"
               x:Name="theRectangle" />

You will notice that the naming attribute starts with the x: prefix (or alias). As was explained in the namespaces discussion, this means that attribute is available through the x namespace that is included on the top of every XAML document (by default). Once these objects are named, they will be available to other XAML elements by name or via code . The names used here must be unique. Each name can occur only once within a single XAML document. This simplifies the naming strategy but also means there is no sense of naming scope (like HTML has).

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