XAML in a Nutshell

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Chapter 5Resources

5

Resources

Every XAML element has a collection of resources. Resources provide a mecha-

nism for defining common styles or elements that can be reused throughout the

user interface. They also configure the actions that are carried out when a user

interacts with a display element.

The benefit of using resources to define reusable, common styles is that modifica-

tions can be applied to one element, but they will take effect throughout the entire

application. This reduces the chance of error and the possibility that an element

might be missed when changes are applied. For example, you may want to define

a specific

Point from which all geometric shapes will originate. By defining the

Point as a resource and referencing it as the appropriate attribute value of

geometric elements, the origination point can easily be changed in one place—the

resource declaration—without concern for mistakes made in multiple places

throughout the user interface.

Local resources are defined on the element, while global resources are defined on

the root element. Global resources can be used by all elements in the page while

local resources are reserved for use by the element in which they are declared.

Regardless of the type of resource (local or global), the syntax used to declare the

resources is the same.

Although every element has a collection of resources, they are usu-

ally declared only on the root element.

Using Resources

When adding resources, you must add the appropriate namespace to the root

element. You’ll also need to give it a name to differentiate it from the default

namespace. The default namespace, which references Avalon, contains the

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Chapter 5: Resources

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definitions of Avalon elements, such as Button, Page, and StackPanel. The

namespace that must be added to define resources is the XAML namespace and

describes the language itself.

Because resource definitions require the use of XAML-specific tags—which are

not described by the default namespace—you must declare a reference to the

XAML namespace and use it to prefix those attributes found only there, such as

Key.

The key is a fully qualified attribute comprising the namespace, a colon, and the

keyword

Key. Elements defined as a resource must have a declared “key name” to

be referenced by other elements. The value of the attribute is the name by which

the resource will be referenced by other elements.

Resources are added by explicitly declaring elements as children of the Resources

attribute of an element.

In Example 5-1, there are two instances of

SolidColorBrush defined as resources:

RedBrush and BlueBrush.

Resources must be declared in the file before they can be accessed. This is because

the runtime engine interprets XAML as a stream of binary input and doesn’t

understand that a resource might be defined later in the stream. It can’t render an

element if a resource is required but hasn’t yet been declared. In Example 5-2, the

resource

RedBrush is referenced before it is declared.

Example 5-1. Using resources to define global styles

<Page

xmlns="http://schemas.microsoft.com/winfx/avalon/2005"

xmlns:x="http://schemas.microsoft.com/winfx/xaml/2005"

<Page.Resources>

<SolidColorBrush

x:Key="RedBrush"

Color="red"/>

<SolidColorBrush

x:Key="BlueBrush"

Color="blue"/>

</Page.Resources>

<StackPanel>

<Button

Background="{StaticResource RedBrush}" />

<Ellipse

Fill="{StaticResource BlueBrush}"

Margin="40"

Width="15"

Height="25"/>

</StackPanel>

</Page>

Example 5-2. Illegal use of a local resource

<Button

Content="Click Me"

Background="{StaticResource RedBrush}" >