Default Implementation

Introduction

The Default Implementation property can be used to indicate which component is the default implementation for a behavior. This property is not used by the Gum tool, but instead exists for runtime implementations (such as FlatRedBall) to decide which type of component to create when an instance of a behavior is requested.

ListBoxItemBehavior with a Default Implementation set to Controls/ListBoxItem

As of March 2025 this property is used in the following runtimes:

• FlatRedBall

• MonoGame/Kni/FNA

Additional runtimes may add support for this property in the future. If you need it for your project please make a request on GitHub or Discord.

Common Usage

Behaviors are often used to help with the creation of components which need to have a certain set of states and instances. For example, the Button type in Gum Forms is associated with the ButtonBehavior.

At runtime, a game may need to create an instance of the Button type without specifying a component. For example, the following code can be used to create a button:

var button = new Button();
button.Text = "Hello";
StackLayoutInstance.AddChild(button);

The Default Implementation property can help runtime libraries determine which component to create for the Button's visual.

The Button type is a good example of why this property might be needed because the default Forms components include multiple components which use ButtonBehavior.

Multiple button components

To resolve this ambiguity, the ButtonBehavior's Default Implementation is automatically set to Controls/ButtonStandard.

ButtonBehavior using ButtonStandard as the Default Implementation

For more information about whether you should set the Default Implementation, refer to the documentation for your particular runtime.

If you have created a custom runtime, such as a new ListBoxItem, you may need to change the Default Implementation for the ListBoxItem behavior.