|RFC-0147: View System|
Describes the windowing system for the Fuchsia platform and ratifies its role in the platform.
|Date submitted (year-month-day)||2021-11-03|
|Date reviewed (year-month-day)||2022-01-05|
This RFC provides an overview of the Fuchsia View System: the set of APIs to reason about and interact with visual regions ("Views") and their lifecycle. This set of functionality is also commonly referred to on other platforms as a windowing system. The scope of this RFC is limited to products with a single display. Changes to accommodate headless devices and/or multiple displays will be covered in future RFCs.
A View is a region of graphical content, and is the basic unit of graphics and user interaction on Fuchsia. Views connect to form a View tree hierarchy, with a distinguished root View. Fuchsia's graphics compositor, Scenic, renders graphical contents from each View in the View tree to produce output for the display. Scenic and the input pipeline are responsible for routing UI-targeted user input such as keyboard, mouse and touch to the correct View.
The View System is a critical part of the Fuchsia platform's support for "bring your own runtime" and the mechanism through which product developers use graphical content from multiple runtimes to build secure visual user experiences on Fuchsia. Fuchsia's composition APIs (Flatland and GFX) are built on top of the View System.
The goal of this RFC is to document and ratify the "state of the world" with respect to the View System. Specifically, we'd like to ratify the following decisions.
- Components that wish to display graphics on Fuchsia must create a View using the View System. The only exception to this is single-window utility UIs such as Virtcon and the Recovery UI, which do not use windowing features and are designed run in resource-limited situations when Scenic is not running. These UIs talk directly to the display controller.
- The View System is the only way for components to integrate with the Fuchsia platform's support for user touch, mouse, and keyboard and accessibility services.
- The View System is agnostic about the composition strategy being used (GFX vs. Flatland), and provides a shared foundation for all compositor implementations on Fuchsia.
Modifying these decisions requires an additional RFC and/or an update to this RFC.
This section expected to be updated during the review
- Graphics: email@example.com, firstname.lastname@example.org, email@example.com
- Input: firstname.lastname@example.org
- Components: email@example.com
- Security: firstname.lastname@example.org
- General: email@example.com
firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
Draft doc was send to the Scenic and Input teams for discussion.
- The graphical compositing component in the Fuchsia platform
- The sole implementer of the View System APIs, including its tie-ins to the graphical composition APIs and HCI APIs.
- A visual region of graphical content.
- It has a coordinate system, a bounding box, and a defined spatial relationship to its ancestors, via the View Tree.
- View Tree
- The structure of Views on the system, connected by parent-child relationship. The root View of the View Tree is typically attached to the display.
- A pixel-based output device; also called a screen. The View System currently supports one screen attached to the system at a time.
- The screen is managed through a display controller.
- Display controller
- Driver that manages screens attached to the computer.
- Only talks to Scenic.
- Is not available to out of tree clients.
- Scene Manager
- A platform component that attaches the root View to the screen.
- Accessibility Manager
- A platform component that implements accessibility APIs.
- It has privileged access to the View System.
- System shell ("Sys UI", "System UI")
- A component responsible for a product's user experience, for example Ermine. This component is typically specific to a particular product.
- A system shell is responsible for window management, including focus handling and management of top-level Views, while the View system handles the windowing system.
- Developer shell ("tiles", "present_view")
- A system shell used for testing. Typically this allows a developer to launch a Fuchsia component in isolation.
- UI client
- This is a generic way of referring to the owner of a View, used throughout this RFC.
- Examples: A flutter app, chromium with multiple top-level Views.
Definition of a View
A Fuchsia View is the basic unit of graphics and interaction on Fuchsia. A View defines a visual region for displaying graphical content to the user. Not all Views are visible at a given time. The graphical contents for each View are supplied by a Fuchsia component. Platform support for managing Views and compositing their contents onto the screen is implemented by Scenic.
- Can incorporate the contents of another View (its child View), forming a View tree.
- Defines a coordinate system for placing child content.
- Has a bounding box that defines the visible (and optionally interactive) part of that View.
- May be connected and disconnected from the Scenic View Tree. (Unconnected Views may exist, but do not have their content rendered to the screen, nor do they receive input.)
Any graphical composition API supported by Scenic (such as fuchsia.ui.composition and fuchsia.ui.scenic) must provide methods to create a View and manage its lifecycle. Any future composition APIs must also operate on top of the View System. Fuchsia platform HCI APIs (such as pointer and keyboard) use Views to route input. (See Input below for details.)
There is one global View Tree, which is owned and implemented by Scenic. The View Tree has a distinguished root View that connects to the screen. Scenic is the only Fuchsia component that directly manipulates all Views and their positions.
Each View in a View Tree has bounds defined in its own coordinate system, and a position and orientation expressed in the parent View's coordinate system. Together, these determine the regions where graphical content is ultimately visible on-screen, and regions which are responsive to user input.
A View can embed additional graphical content from another View by creating an empty placeholder for it in its coordinate system; the placeholder is called a viewport. The two Views form a parent-child relationship in the View Tree, where the parent View's Viewport embeds a child View's graphical content. To establish the relationship, the parent and child must provide matching tokens when creating their View and Viewport. These tokens are implemented as kernel objects, and are not clonable. The parent and child may obtain these matching tokens in a variety of ways, which are external to the View System.
Attaching or detaching a parent View from the View Tree also attaches/detaches its child View(s). Connections between Views within a subtree remain even when a subtree is detached from the global View Tree.
While a View can embed another View, the View System does not give a View access to the graphical content of any other View. This isolation guarantee forms one of the bases of View security.
Scenic maps Views to the screen
Scenic is the sole component that talks to the display controller. It takes the graphical content of the entire View Tree, along with the relationships encoded in the tree, creates a single image (possibly with multiple layers). It then programs the hardware to display the final on-screen image.
The contents in a View are supplied by a single FIDL channel to a graphical composition API, which is implemented by Scenic. Each client endpoint of a FIDL channel to Scenic is termed a "UI client", and a UI client can create at most one View.
Views may come from a variety of components including user-facing components (e.g. a browser), the system UI, as well as components that are part of the Fuchsia platform, such as the Accessibility Manager. A component may create multiple channels and thus multiple Views. Consequently, a single component may vend both a parent View and that View's child View in some cases.
In order to combine graphics from multiple components, developers should create a View or Views in each component and use child View and the View Tree to combine them. It is important to note that the View Tree hierarchy does not need to match the component instance hierarchy. In many cases, these structures intentionally look quite different.
Window Management vs. Window System vs. Composition
Fuchsia separates three functions that are sometimes combined on other platforms.
- Window management incorporates product-specific policy and specific choices about how windows behave. On Fuchsia, this is the responsibility of the system UI and lives entirely outside of the platform.
- The windowing system refers to low-level window management. On Fuchsia this is handled by the View System.
- Composition refers to combining images from multiple sources to produce images for display. This is currently also a responsibility of the Scenic component, although this could change in the future.
The choice to keep the windowing system and composition separate provides a fast-path for graphical presentation and allows for an efficient implementation. While both the View System and the compositor(s) are currently implemented primarily within Scenic, they are separate both in terms of APIs and code. This separation makes it possible for the View System to support multiple composition strategies (Flatland and GFX).
By making window management a product-level concern we keep policy logic outside of the platform, ensuring a clean separation between platform and product.
The View System is the primary mechanism through which the Fuchsia platform determines how to route user input, such as pointer events or keyboard events. Fuchsia's user input APIs are designed such that each channel is scoped to a particular View. Input will be routed to a View based on either the current View focus (in the case of keyboard events) and/or to the View or Views corresponding to the location of the input event (as in the case of touch or mouse events). A View may only receive user input when conntected to the View Tree.
Multiple Views may participate in input processing for the same event. The window manager for a particular product may configure this routing based on product policy, for example by granting global access to mouse events to the system UI. For additional details see User Input Architecture. This system allows seamless user experiences across Views from multiple runtimes, including the ability to disambiguate touch gestures.
One important implication of this is that components outside of the Fuchsia platform do not get direct access to input events from the driver. They must receive this information mediated by the View System.
At any given time the View Tree has one distinguished View, called the focused View. The focused View is typically the View that the user expects will receive user input. Fuchsia's input subsystems rely on View focus as a means of determining where to route input. For additional information see the Focus Chain documentation. A parent View owner may control View focus within its subtree.
The platform uses tokens called ViewRefs to identify and communicate about Views. A ViewRef is a unique reference to a particular View that remains unique until system reboot. It is implemented as a kernel object, the handle of which can be freely duplicated and sent to other components over arbitrary protocols.
The View System APIs make heavy use of ViewRefs to provides a stable cross-component reference for each View. ViewRefs are also used to signal lifecycle events about the associated View. This allows other components inside and outside of the platform to communicate about Views, their lifecycles, and to route user input and accessibility protocols to Views.
Scenic is the source of authority for the View System because the View Tree and core View management APIs are implemented in Scenic. Each client uses its ViewRef to register to receive user input and accessibility events for the associated View. Scenic currently supports two graphical composition APIs: fuchsia.ui.scenic (legacy) and fuchsia.ui.composition (in development). The View System is independent of, but works in close conjunction, with each.
API implementation details are out of scope for this RFC but the relevant APIs can be found below. It's worth noting that these APIs evolved incrementally over multiple years. Some aspects may be simplified in the future.
Platform components involved in the View System
- Input Pipeline
- Scene Manager (or Root Presenter on legacy systems)
- Accessibility Manager
- Text Manager
APIs for View management
- View refs, View tokens definitions
- View focus management
- Create a View, move a View, resize a View, connect/disconnect a View
- Connect System UI View to root View
- Surface View creation from a component
- Create web client Views
APIs for user input
- Pointer events
- Keyboard input
- Keyboard shortcuts
APIs for accessibility
- Allows a View to send semantic information for accessibility
- Allows accessibility to highlight the focused UI component
Many aspects of the View System influence graphics and input performance. We describe some important aspects below.
- All runtimes are equal: Fuchsia's principle of bring your own runtime means that all runtimes on Fuchsia use the same event routing, and no runtime gets preferential treatment.
- Render once: Scenic makes it possible to combine graphical content for different Views without necessarily re-rendering, because it knows view positions and can correctly forward graphical content to the display driver.
- Efficient dispatch: Scenic exposes focus information, allowing non-graphical user input (e.g. keyboard events) to be dispatched directly from the component(s) involved in processing it.
Security and Privacy considerations
The View System makes several guarantees for securely composing graphical content on Fuchsia, and these guarantees form the foundation for a secure UX. However, the View System alone cannot guarantee that every UX is secure - only that the View System respects its own security guarantees.
The View System attempts to enable secure UX by:
- Delegating sole responsibility for owning and manipulating the View Tree to Scenic.
- Delegating sole responsibility for displaying graphical content on the screen to Scenic.
- Disallowing UI clients from manipulating another View without a matching Viewport or ViewRef.
- Disallowing UI clients from using the View System to inspect the contents of any other UI clients' Views.
- Only allowing a UI client to inject input into its View subtree.
- Only allowing a View to gain focus and receive input when connected to the view tree.
UI security relies on the guarantees provided by Component Framework with respect to capability routing. The details of View security will be addressed in future RFCs.
The View System is implemented at multiple layers of abstraction, so it is tested using a layered approach.
- Unit tests of the View Tree and View lifecycle, in the Scenic component's codebase.
- UI Integration tests, in /src/ui/tests, that exercise the
contracts around View System APIs.
- These APIs are vended and used by platform components like Scenic, Scene Manager, Input Pipeline, Accessibility Manager, and Text Manager.
- The fuchsia.ui.observation.test APIs are used within in-tree integration tests to inspect View System behavior.
Out of tree:
- Runtimes such as Chromium and Flutter write integration tests to exercise View System APIs
- Products write end-to-end tests that indirectly exercise the View System.
This RFC provides a high-level overview of the View System's role and responsibilities. Docs and possibly additional RFCs will be written to address details.
Some of the Scenic documentation will also need to be updated in light of this RFC and upcoming changes for Flatland.
Drawbacks, alternatives, and unknowns
The View System currently lacks a mechanism for synchronizing updates between multiple Views.
The View System is partially responsible for routing capabilities to runtimes that expose Views (using ViewRefs). This has some conceptual overlap with capability routing via Fuchsia's Component Framework, but is currently entirely separate. In the future, it may be advantageous to support View capabilities in the Component Framework.
The current APIs only allow View owners to create child Views. This isn't ideal for something like an application that wishes to create multiple top-level windows.
Because the View System APIs evolved organically over multiple years with many different contributors, the cognitive burden on runtime developers is currently quite high. This should be mitigated in the future by better documentation and examples, and possible API simplification.
The View System architecture makes a number of architectural choices.
- The View System is required for any product with graphics.
- Alternatively, Scenic could be optional and products could include their own compositor. This would require exposing many low-level APIs out of tree.
- Advantages of current approach:
- Homogeneous behavior (including accessibility and input) across runtimes
- Consistent security guarantees
- Display API can evolve without breaking clients
- Disadvantages of current approach:
- Less customization for products
- Complexity for runtime integrations
- This could change in the future if needed by a product building on Fuchsia.
- The View System is distributed across several components including Scenic and
the Input Pipeline.
- Alternatively, we could have built the entire View System into Scenic.
- Advantages of current approach:
- Separation of concerns: graphical composition, window management, product policy are all separate.
- Allows high-performance graphics path via delegated composition
- UI-related components can evolve independently
- Allows graphics to keep working even if a sub-system (e.g. text input) crashes
- Cons of current approach:
- Multiple processes can add latency with IPC
- Added coordination/synchronization complexity between components
Prior art and references
- The View System and Scenic share some architectural similarities with the X Windows System.
- Other aspects have more in common with the Wayland protocol.
- The input architecture RFC describes details of how the View System interacts with Fuchsia's user-input subsystems.