fuchsia.ui.views

PROTOCOLS

View

A View is an interface that a component implements to offer Scenic view(s) to its clients.

A Scenic view contains a tree of Scenic graph nodes which is used to render a graphical user interface, such as a module, shell, or on-screen keyboard. Other Scenic views can be embedded within this tree of nodes. This creates a scene graph describing the UI of the entire system, rooted at the top-most view. Different processes can contribute branches to the scene graph, and their content will be rendered together in a shared space.

# Offering a View

A component that wishes to offer a View can do so in one of two ways:

Expose a View interface as a service. This is usually done by components that provide a single view, or have a clearly defined "main" view. In this case, the component may obtain relevant properties for configuring the view using services that may be available in its namespace, such as:
- fuchsia.intl.PropertyProvider
- fuchsia.accessibility.PropertyProvider
Offer a domain-specific interface that provides Views using a request<View> parameter. In this case, the component may obtain relevant properties for configuring the view using services that may be provided by its client as part of the request.

For example, an encyclopedia component might offer a method to expose article views:

GetArticleView(string article_id, fuchsia.intl.PropertyProvider intl_properties, fuchsia.accessibility.PropertyProvider accessibility_properties, request view_request);

This latter case is probably less common, as controlling domain-specific views tends to be the job of the component that creates them.

Presenting a View

A client of the View interface will:
Launch (or bind to) the component that provides the interface.
Connect to the component's View interface.
Call Present() to give the View an attachment point into the scene graph via the view_token. Subsequent calls to Present() will generate an error and cause the connection to be closed.

When the client no longer needs the View, it should disconnect from the interface.

NOTE: The client owns the View instance and must retain it for the lifetime of the UI that it displays. If the View instance is destroyed, the connection will be dropped and all UI content will be destroyed.

Implementing a View

On the implementation side, a component that exposes the View interface has the following responsibilities:

When Present() is called, create a root for the View's content in the Scenic scene graph by passing the provided view_token.
Provide graphical content for the view and attach it to the root.
Adjust the appearance and/or contents of the view's content based on relevant internationalization and/or accessibility properties as described above.
Handle user interface events such as touches, key presses, and fuchsia.ui.gfx.ViewProperty changes using other Scenic interfaces such as fuchsia.ui.Scenic and fuchsia.ui.SessionListener.

See also: fuchsia.intl.PropertyProvider, fuchsia.accessibility.PropertyProvider.

TODO(SCN-1198): Migrate all implementations of ViewProvider to use View.

Present

Provides the View with an attachment point to Scenic's scene graph.

When Present() is called the View's implementation should create a View resource within Scenic by providing it with the view_token (using a fuchsia.ui.gfx.CreateResourceCmd and fuchsia.ui.gfx.ViewArgs).

Then the implementation should attach its graphical content to the newly-created View resource using a fuchsia.ui.gfx.AddChildCmd.

If the implementation already owns a View resource (because Present() had already been called before), then it should terminate the connection with an error.

TODO(SCN-1271): Allow re-parenting Views with a new Present() call.

Request

Name	Type
`view_token`	`ViewToken`

STRUCTS

ViewRef

Defined in fuchsia.ui.views/view.fidl

A ViewRef is a handle to a kernel object which identifies a unique View across the system. Two ViewRefs to the same View have the same KOID.

Clients use a ViewRef to identify a View, to validate a View, and to receive a View invalidation signal.

As part of View creation, the client creates a linked ViewRef/ViewRefControl pair and hands the pair to Scenic (ViewRefControl is described below). The client must remove the ViewRef's signal capabilities; otherwise the View is not created.

The client may freely clone its ViewRef and share it, even before sending it to Scenic.

Example 1. Accessibility accepts a ViewRef from a client to group the semantic nodes, and semantic operations, associated with a client's View. It must validate a client's ViewRef with Scenic.

Example 2. We use ViewRefs to create a FocusChain, which identifies Views considered as "in-focus" down the View hierarchy. When a View is destroyed, Scenic signals to all FocusChain holders that the ViewRef is now invalid.

Name	Type	Description	Default
`reference`	`handle<eventpair>`		No default

ViewRefControl

Defined in fuchsia.ui.views/view.fidl

A ViewRefControl is the peer to a ViewRef. Their references are linked.

Like ViewRef, a ViewRefControl is a typed handle to an eventpair. Unlike ViewRef, a ViewRefControl's handle is unique. Scenic uses this property when it ties a ViewRefControl to a View, arranged to share fate. When a View is destroyed, the associated destruction of its ViewRefControl triggers an automatic ZX_ERR_PEER_CLOSED signal sent to all ViewRef holders; hence ViewRef holders may track View lifetime.

As part of View creation, the client creates a linked ViewRef/ViewRefControl pair and hands the pair to Scenic (ViewRef is described above). The client must not clone the ViewRefControl. It must not remove or modify the ViewRefControl's capabilities; otherwise the View is not created.

Name	Type	Description	Default
`reference`	`handle<eventpair>`		No default

ViewHolderToken

Defined in fuchsia.ui.views/view_token.fidl

Token that uniquely identifies an attachment point for a View in the global scene graph. Each ViewHolderToken has exactly one corresponding ViewToken.

A Scenic client can reference contents from another client by creating a ViewHolder resource using this token. The other client must also create a View resource using the corresponding ViewToken.

Name	Type	Description	Default
`value`	`handle<eventpair>`		No default

ViewToken

Defined in fuchsia.ui.views/view_token.fidl

Token that uniquely identifies a View, which is the root point for a subgraph in the global scene graph. Each ViewToken has exactly one corresponding ViewHolderToken.

A Scenic client can have its contents referenced from another client by creating a View resource using this token. The other client must also create a ViewHolder resource using the corresponding ViewHolderToken.

Name	Type	Description	Default
`value`	`handle<eventpair>`		No default

UNIONS

Command

Defined in fuchsia.ui.views/commands.fidl

DO NOT USE - Retained for ABI stability in fuchsia.ui.scenic.Command

DO NOT USE

Name	Type	Description
`empty`	`int32`