This document describes how RootPresenter and Scenic process visually-related input, such as touch, mouse, and keyboard. We'll work roughly bottom up through the layers of abstraction, from device to gesture.
Other inputs, such as buttons, audio, and video, are out of scope for this document.
Major Entities and Their High-Level Role
- gives us inputs as InputReport FIDL tables
- routes touch and mouse input from Drivers to Scenic
- routes touch and mouse input from RootPresenter to UI Clients
- Keyboard Subsystem
- routes and transforms text input to UI Clients
- UI Client
- consumes inputs from Scenic and Keyboard Subsystem to drive UI
Drivers Send Structured Input
Input Drivers provide access to input peripherals through the file system under
/dev/class/input-report. These are presented as structured FIDL tables that
describe the device and the reports that will be sent.
RootPresenter Transforms and Routes Inputs
Generally, the RootPresenter is the singleton process that has detailed and specific knowledge about the entire device, such as details about the display, peripherals, sensors, etc. It takes care of device management details, such as reading out InputReports reports from Drivers, and packages them into FIDL structs for consumption by Scenic or other entities.
It also instructs Scenic to create the top-level (or "root") elements of the scene graph, and vends the Presenter API that UI clients use to attach their visual content to the scene graph.
is the code responsible for actually monitoring
/dev/class/input-report for new
peripherals, and reacting to new reports from existing peripherals. It forwards
new events for processing to other parts of RootPresenter.
More information on
InputReader can be found
For each new peripheral (an input device),
InputReader assigns a new
InputInterpreter object that reads the InputReport descriptor report for a single
input device, and performs bookkeeping by pushing a
and its designated event forwarding channel, an
FIDL protocol. (The
InputDeviceRegistry protocol also enables programmatic
input injection from outside RootPresenter.) The
is vended by RootPresenter, and in addition to bookkeeping (details below),
Presentation about the new peripheral.
For each new event,
InputInterpreter reads a
transforms it into a
fuchsia.ui.input:InputReport, and forwards it on
InputReport to the registered
typically the RootPresenter itself. In turn, the
InputReport is forwarded to
For internal bookkeeping, each
Presentation keeps a mapping of
ID to an associated
DeviceState is used to create a little
persistent state for each peripheral, e.g., keeping track of a mouse device's
DOWN/MOVE/UP state. In
InputReport is routed to its
DeviceState, where it is transformed into an appropriate
InputEvent, and is sent to the
OnEventCallback that was registered at the
DeviceState's constructor (when the peripheral was first added).
InputEvent is now handled by RootPresenter's
OnEvent callback. It looks
for global hooks, displays a mouse cursor, adjusts for predetermined screen
rotation, and finally enqueues the
InputEvent as an
InputCmd to Scenic.
Sensor HID reports are handled in an analogous fashion. Some differences are:
- Sensors typically don't have state to manage, so they have no
InputReportis typically enough for plumbing out to clients.
- Interfaces for sensor data is vended by RootPresenter itself; this may change in the future.
Scenic Routes Pointer Inputs to UI Clients
In contrast to RootPresenter, Scenic has less knowledge about the device.
Instead of knowing about peripherals, it receives
InputEvent FIDL structs from
RootPresenter. Generally, it owns and manages the large-scale visual elements
that each UI client creates (the scene graph), and handles mouse and touch input
(hereafter: "pointer input") dispatch to each UI client.
Root Presenter performs pointer event injections into Scenic over the
fuchsia.ui.pointerinjector] protocol, which is a privileged capability.
Pointer events, such as touch, typically follow an ADD → DOWN → MOVE*
→ UP → REMOVE state sequence, encoded as
On ADD, we identify the set of potential clients by performing a hit test, and forward this event to these clients. To associate future touch events by the same finger to the same clients, we track the set of clients for that particular finger. Parallel dispatch is used to enable gesture disambiguation (TBD), where the touch events should eventually be owned by a single client.
On DOWN, we send a
FocusEvent to the single client that is "on top". We also
focused=false to the previously focused client.
On MOVE and UP, we merely forward them to existing clients.
On REMOVE, we forward it to existing clients, and then remove the tracking association.
For an overview of pointer coordinate mapping, see Ray Casting and Hit Testing.
The Keyboard Subsystem routes key events based on which client(s) have requested such events, and whether or not those clients have received focus.