tl;dr: Elements are components added to a session as a result of user interaction, similar to “apps” on other platforms.
This document provides a brief conceptual overview of elements, element annotations, and their relationship to components.
Elements are components with key-value annotations. All elements are implemented as components. While any component could hypothetically be instantiated as an element, elements contribute more directly to the user experience of a product, much like an “app” would on other platforms.
Since elements are components, they inherit all component properties, such as:
Elements are typically instantiated in component collections within the session component.
Elements contribute to the user experience of a product by providing a visual, audible, or other discernible interaction with the user. The specific modes of interaction available to, and expected from, an element, are defined by the product and its configuration. The most common mode of output is graphical.
Every element has an associated list of mutable key-value annotations, set first when the element instance is added to a product experience at runtime. Element annotations are used by the product‘s “user experience presenter” to control details of the element’s presentation. For example, for graphical elements annotations may specify if presentation should hide system chrome, allow for certain types of gestures, and so on.
Elements are an abstraction intended to make it easier to interactively run components on many different Fuchsia products and varying product configurations. An element's primary interaction surface with a product are defined by:
Although none of these mechanisms are strictly defined by the Fuchsia platform, some are standardized and intended to be re-used across all products built with Fuchsia.
For example, the specific meanings of different annotation keys and their respective values are not prescribed by the Fuchsia platform. Some keys have recommended meanings, with recommended standard schemas.
