Scenic Input System

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

Zircon - gives us inputs as HID reports.

RootPresenter - routes input from Zircon to Scenic

Scenic - routes input from RootPresenter to UI Clients (touch and mouse inputs), and from RootPresenter to TextService (text inputs only).

TextService - routes text input from Scenic to IME

IME - routes and transforms text input from TextService to UI Clients

UI Client - consumes inputs from Scenic and IMEs to drive UI

Zircon Sends Raw Inputs

Zircon provides access to input peripherals through the file system under /dev/class/input. These are presented as HID devices, with associated controls to retrieve the description report. Simple reads from these HID devices will return HID reports, which generally are expected to follow the HID Usage Tables.

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 HID reports from Zircon, 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.

The general transformation for an input event through RootPresenter is from HID report, to InputReport, to InputEvent. The InputEvent is sent to Scenic.


The InputReader library is the code responsible for actually monitoring /dev/class/input 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 here.

For each new peripheral (an input device), InputReader assigns a new InputInterpreter object that reads the HID descriptor report for a single input device, and performs bookkeeping by pushing a DeviceDescriptor and its designated event forwarding channel, an InputDevice, to the InputDeviceRegistry FIDL protocol. (The InputDeviceRegistry protocol also enables programmatic input injection from outside RootPresenter.) The InputDeviceRegistry protocol is vended by RootPresenter, and in addition to bookkeeping (details below), informs each Presentation about the new peripheral.

For each new event, InputInterpreter reads and decodes a HID report, transforms it into an InputReport, and forwards it on InputDevice::DispatchReport.

The implementation of DispatchReport forwards the InputReport to the registered InputDeviceImpl::Listener, typically the RootPresenter itself. In turn, the InputReport is forwarded to the active Presentation.

For internal bookkeeping, each Presentation keeps a mapping of InputDevice ID to an associated DeviceState. The 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 Presentation, the InputReport is routed to its relevant 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).

The 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 Inputs

Sensor HID reports are handled in an analogous fashion. Some differences are:

  • Sensors typically don't have state to manage, so they have no DeviceState.
  • The InputReport is typically enough for plumbing out to clients.
  • Interfaces for sensor data is vended by RootPresenter itself; this may change in the future.

Scenic Routes 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), as well as handling input dispatch to each UI client.

Scenic accepts commands from a client over its session. RootPresenter is a privileged client that may submit input commands, each of which encapsulates an InputEvent. The Scenic-side implementation of session logic has an InputCommandDispatcher that farms out different types of events to appropriate dispatch logic.

We outline some representative event flows below.

Pointer Event Handling

Pointer events, such as touch, typically follow an ADD → DOWN → MOVE* → UP → REMOVE state sequence, encoded as PointerEventPhase.

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 send a FocusEvent with 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.

Keyboard Event Handling

Keyboard events are a little more involved, due to the need for mediation by an IME (“soft keyboard”). We distinguish hard key events, generated by a physical keyboard, from soft key events, generated by an IME.

Scenic deals exclusively with hard key events, but must typically not forward them directly to clients. Instead, Scenic sends all hard key events to the TextService, which vends IMEs to UI clients. The TextService routes hard key events to an IME associated with a particular UI client that has received the FocusEvent.

Some clients have a real need for hard key events (e.g., games and software platforms). These clients may use the SetHardKeyboardDeliveryCmd to trigger direct dispatch from Scenic.