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Table of Contents


Introduction

A SceneGraph application consists of one or more custom SceneGraph components defined in XML files. These component XML files define the appearance and behavior of the component as needed for the application design. Defining these custom components consists of selecting node classes and previously-defined custom components that have the appearance and behavior of the application design sub-elements that will make up the component, and then defining custom attributes and scripting for the component.

You can select the node classes for your component from the built-in node classes (as described in SceneGraph API Reference), and also from custom components that you have created yourself by defining the custom component in an XML file. In both cases, you must identify the node class or component to be defined for your component, either in BrightScript using the createObject() or createChild() or similar functions, or as child nodes of the <children> element in the XML file. In both cases, you begin the new XML file definition of the component by identifying either the basic abstract node class the component will be extended from, either a Scene node class or the Group node class, or a built-in node class, or another custom component defined in an XML file.

For example, you may want to use the built-in node classes Rectangle and Label for a simple custom component that displays some text in an on-screen box. You can define the box and the text as follows:

Defining Custom Components in XML Markup

To use this custom component directly in a Scene node, add this XML markup to an XML component file extended from one of the built-in abstract Scene node classes. To use it as a custom component in as many different custom components in your application where it would be useful, add the XML markup to an XML component file extended from the built-in abstract Group node class, with a descriptive name such as textbox. After that, you can use the component in any other XML component file by creating it using that name. For example, to use it by declaring it in the XML markup in the <children> element of an XML component file:

<children>

...

<textbox />

...

</children>

Focus Handling in Custom Components

There has been some confusion about how to set focus to a specific RSG element in custom components / views.

Best Practices:

Best practice is setting focus to view and allowing view to handle the proper child focus.
Focus should be handled by observing focusedchild and then checking if the required node does not have focus.

MainScene.brs file

What to Avoid:

A common mistake is setting focus to an element in the custom component's init() function.

A Node should never set focus to any of it’s children if not asked. Creating additional objects of your node will create problems.

Extending Custom Components

After you have created a custom component that contains one or more of the built-in node classes, you can also extend that component into one or more new custom components. There are some special considerations concerning custom components extended from other custom components.

Custom Component Initialization Order

The process by which custom components are initialized is similar to how C++ object constructors work. First, the component being extended is completely constructed, then the component or components extended from it are constructed.

As each component being extended is constructed, the following sequence of events occurs:

  1. The <children> nodes defined in the component being extended are created, and their fields are set to their initial values, either to a default value, or to the value specified in the XML markup.
  2. The <interface> fields in the component being extended are created, and their initial values are set, either to a default value, or to the value specified by the value attribute.
  3. The <script> element init() function of the component being extended is called, and all initializations contained in the function are performed.

At this point, the component being extended is completely initialized, and the component or components to be extended from it are constructed, following the same three steps as above. Note these are the same steps described in Component Initialization Order, and note the implications of the initialization order described there.

Characteristics of Components Extended from Custom Components

The following describes the characteristics of components extended from custom components:

  • Calling a function in an extended component with the same name as in the custom component from which it is extended will call the function in the extended component. This allows you to override (or overload) a function name to perform differently depending on the component from which it is called.
  • <interface> fields accumulate: the extended component includes all of the interface fields of the component from which it is extended in addition to its own. In the case where an extended component field name is the same as the component from which it is extended, the definition of the extended component field is used, similar to functions.
  • Any component object declared with the m object reference (see SceneGraph Data Scoping) can be accessed in either the component that is extended or any components extended from it.
  • All functions defined in a component that is extended can be called directly from any components extended from it.

Example

For an example of extending custom components, download the sample channel ExtendingCustomComponents.zip.  This sample defines a custom component called CenteredRectangle. This component creates a rectangle centered on the screen.  It also defines a subroutine called SetColor, which sets the color property of the rectangle.  The sample also defines a component that extends CenteredRectangle, called RedCenteredRectangle.  This new component overrides the SetColor subroutine to make the rectangle red.

 

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