Gamepad

2. Dependencies

This specification references interfaces from a number of other specifications:

• PerformanceTiming [NAVIGATION-TIMING].

3. Scope

Interfacing with external devices designed to control games has the potential to become large and intractable if approached in full generality. In this specification we explicitly choose to narrow scope to provide a useful subset of functionality that can be widely implemented and broadly useful.

Specifically, we choose to only support the functionality required to support gamepads. Support for gamepads requires two input types: buttons and axes. Both buttons and axes are reported as analog values, buttons ranging from [0..1], and axes ranging from [-1..1].

While the primary goal is support for gamepad devices, supporting these two types of analog inputs allows support for other similar devices common to current gaming systems including joysticks, driving wheels, pedals, and accelerometers. As such, the name "gamepad" is exemplary rather than trying to be a generic name for the entire set of devices addressed by this specification.

We specifically exclude support for more complex devices that may also be used in some gaming contexts, including those that that do motion sensing, depth sensing, video analysis, gesture recognition, and so on.

4. Model

A gamepad is a physical or virtual input device that provides button and/or axis inputs. The host is the operating system environment hosting the user agent. The host MUST provide an algorithm for enumerating connected gamepads. A gamepad is considered connected when the host is capable of reading new inputs. A gamepad that is not connected is disconnected.

The host MUST provide an algorithm for synchronously reading input data from the gamepad. If the host does not provide such an algorithm, the user agent MAY implement an algorithm that caches the most recent input data and synchronously returns the cached value.

When a gamepad transitions from disconnected to connected, it becomes connected. Likewise, when a gamepad transitions from connected to disconnected, it becomes disconnected. The host MUST provide algorithms to register listeners for these transitions. If these events are not available, the user agent MAY implement an algorithm that synthesizes the events by periodically enumerating connected gamepads.

The user agent MUST define a gamepad user gesture. A gamepad user gesture is a class of gamepad input state transitions that signify that the user is interacting with the gamepad. At a minimum, the gamepad user gesture MUST include state changes where any button transitions from unpressed to pressed. Other transitions MAY be included as user gestures. A transition MUST NOT be included if the transition could occur without user interaction.

The user agent MUST NOT expose information about connected gamepads until a gamepad user gesture has been received from any connected gamepad. The user gesture MUST be tracked independently for each gamepad context. A gamepad context MUST be created when a page first requests the current gamepad state or registers for gamepad connection events. The gamepad context MUST be destroyed when the tab is closed or navigates away from the current page. Reloading the current page MUST NOT destroy the gamepad context.

To register a user gesture, a gamepad context MUST be active. A gamepad context is active if the page associated with the context is visible and the page is capable of executing script. If the page is not visible or the page cannot execute script, the context is inactive. When a context is first created or when it transitions from inactive to active, it becomes active. Likewise, when it transitions from active to inactive, it becomes inactive.

5. Steps to execute when a gamepad context is created

The steps to register a new gamepad context.

1. Let context be a new gamepad context object.
2. Set context["seenGamepadGesture"] to false.
3. Run "Steps to execute when a gamepad context becomes active".

6. Steps to execute when a gamepad context becomes active

The steps to transition a gamepad context to an active state.

1. Let context be the gamepad context object.
2. Set context["active"] to true.
3. If context["seenGamepadGesture"] is false:
   1. Register a gamepad user gesture listener. When a gamepad user gesture is received, run "Steps to execute when a gamepad user gesture is received".

7. Steps to execute when a gamepad user gesture is received

1. Let context be the gamepad context object.
2. Set context["seenGamepadGesture"] to true.

8. Gamepad interface

This interface defines an individual gamepad device.

WebIDL[Exposed=Window]
interface Gamepad {
  readonly attribute DOMString id;
  readonly attribute long index;
  readonly attribute boolean connected;
  readonly attribute DOMHighResTimeStamp timestamp;
  readonly attribute GamepadMappingType mapping;
  readonly attribute FrozenArray<double> axes;
  readonly attribute FrozenArray<GamepadButton> buttons;
};

id attribute

An identification string for the gamepad. This string identifies the brand or style of connected gamepad device. Typically, this will include the USB vendor and a product ID.

index attribute

The index of the gamepad in the Navigator. When multiple gamepads are connected to a user agent, indices MUST be assigned on a first-come, first-serve basis, starting at zero. If a gamepad is disconnected, previously assigned indices MUST NOT be reassigned to gamepads that continue to be connected. However, if a gamepad is disconnected, and subsequently the same or a different gamepad is then connected, the lowest previously used index MUST be reused.

connected attribute

Indicates whether the physical device represented by this object is still connected to the system. When a gamepad becomes unavailable, whether by being physically disconnected, powered off or otherwise unusable, the connected attribute MUST be set to false.

timestamp attribute

Last time the data for this gamepad was updated. Timestamp is a monotonically increasing value that allows the author to determine if the axes and button data have been updated from the hardware. The value must be relative to the navigationStart attribute of the PerformanceTiming interface. Since values are monotonically increasing they can be compared to determine the ordering of updates, as newer values will always be greater than or equal to older values. If no data has been received from the hardware, the value of the timestamp attribute should be the time relative to navigationStart when the Gamepad object was first made available to script.

mapping attribute

The mapping in use for this device. If the user agent has knowledge of the layout of the device, then it SHOULD indicate that a mapping is in use by setting this property to a known mapping name. Currently the only known mapping is "standard", which corresponds to the Standard Gamepad layout. If the user agent does not have knowledge of the device layout and is simply providing the controls as represented by the driver in use, then it MUST set the mapping property to the empty string.

axes attribute

Array of values for all axes of the gamepad. All axis values MUST be linearly normalized to the range [-1.0 .. 1.0]. If the controller is perpendicular to the ground with the directional stick pointing up, -1.0 SHOULD correspond to "forward" or "left", and 1.0 SHOULD correspond to "backward" or "right". Axes that are drawn from a 2D input device SHOULD appear next to each other in the axes array, X then Y. It is RECOMMENDED that axes appear in decreasing order of importance, such that element 0 and 1 typically represent the X and Y axis of a directional stick. The same object MUST be returned until the user agent needs to return different values (or values in a different order).

buttons attribute

Array of button states for all buttons of the gamepad. It is RECOMMENDED that buttons appear in decreasing importance such that the primary button, secondary button, tertiary button, and so on appear as elements 0, 1, 2, ... in the buttons array. The same object MUST be returned until the user agent needs to return different values (or values in a different order).

9. GamepadButton Interface

This interface defines the state of an individual button on a gamepad device.

WebIDL[Exposed=Window]
interface GamepadButton {
  readonly attribute boolean pressed;
  readonly attribute boolean touched;
  readonly attribute double value;
};

pressed attribute

The pressed state of the button. This property MUST be true if the button is currently pressed, and false if it is not pressed. For buttons which do not have a digital switch to indicate a pure pressed or released state, the user agent MUST choose a threshold value to indicate the button as pressed when its value is above a certain amount. If the platform API gives a recommended value, the user agent SHOULD use that. In other cases, the user agent SHOULD choose some other reasonable value.

touched attribute

The touched state of the button. If the button is capable of detecting touch, this property MUST be true if the button is currently being touched, and false otherwise. If the button is not capable of detecting touch and is capable of reporting an analog value, this property MUST be true if the value property is greater than 0, and false if the value is 0. If the button is not capable of detecting touch and can only report a digital value, this property MUST mirror the pressed attribute.

value attribute

For buttons that have an analog sensor, this property MUST represent the amount which the button has been pressed. All button values MUST be linearly normalized to the range [0.0 .. 1.0]. 0.0 MUST mean fully unpressed, and 1.0 MUST mean fully pressed. For buttons without an analog sensor, only the values 0.0 and 1.0 for fully unpressed and fully pressed respectively, MUST be provided.

10. GamepadMappingType enum

This enum defines the set of known mappings for a Gamepad.

WebIDLenum GamepadMappingType {
  "",
  "standard",
};

""

The empty string indicates that no mapping is in use for this gamepad.

standard

The Gamepad's controls have been mapped to the Standard Gamepad layout.

11. Navigator Interface extension

This partial interface defines an extension to the Navigator interface.

WebIDL[Exposed=Window]
partial interface Navigator {
  sequence<Gamepad?> getGamepads();
};

getGamepads

Retrieve a snapshot of the data for the the currently connected and interacted-with gamepads. Gamepads MUST only appear in the list if they are currently connected to the user agent, and at least one device has been interacted with by the user. If no devices have been interacted with, devices MUST NOT appear in the list to avoid a malicious page from fingerprinting the user. The length of the array returned MUST be one more than the maximum index value of the Gamepad objects returned in the array. The entries in the array MUST be the set of Gamepad objects that are visible to the current page, with each Gamepad present at the index in the array specified by its index attribute. Array indices for which there is no connected Gamepad with the corresponding index should return null.

The gamepad state returned from getGamepads() does not reflect disconnection or connection until after the gamepaddisconnected or gamepadconnected events have fired.

As an example, if there is one connected gamepad with an index of 1, then the following code snippet describes the expected behavior:

Example 1

// gamepads should look like [null, [object Gamepad]]
var gamepads = navigator.getGamepads();
// The following statements should all evaluate to true.
gamepads[0] == null;
gamepads[1].index == 1;
gamepads.length == 2;

12. GamepadEvent Interface

WebIDL[Exposed=Window]

interface GamepadEvent: Event {
  constructor(DOMString type, GamepadEventInit eventInitDict);
  [SameObject] readonly attribute Gamepad gamepad;
};

gamepad

The single gamepad attribute provides access to the associated gamepad data for this event.

WebIDLdictionary GamepadEventInit : EventInit {
  required Gamepad gamepad;
};

13. Remapping

Each device manufacturer creates many different products and each has unique styles and layouts of buttons and axes. It is intended that the user agent support as many of these as possible.

Additionally there are de facto standard layouts that have been made popular by game consoles. When the user agent recognizes the attached device, it is RECOMMENDED that it be remapped to a canonical ordering when possible. Devices that are not recognized should still be exposed in their raw form.

There is currently one canonical device, the "Standard Gamepad". The standard gamepad has 4 axes, and up to 17 buttons. When remapping, the indices in axes[] and buttons[] should correspond as closely as possible to the physical locations in the diagram below. Additionally, the mapping property of the Gamepad SHOULD be set to the string "standard".

The "Standard Gamepad" physical button locations are layed out in a left cluster of four buttons, a right cluster of four buttons, a center cluster of three buttons, and a pair of front facing buttons on the left and right side of the gamepad. The four axes of the "Standard Gamepad" are associated with a pair of analog sticks, one on the left and one on the right. The following table describes the buttons/axes and their physical locations.

15. The gamepadconnected event

User agents implementing this specification must provide a new DOM event, named gamepadconnected. The corresponding event MUST be of type GamepadEvent and MUST fire on the window object. Registration for and firing of the gamepadconnected event MUST follow the usual behavior of DOM Events. [DOM]

A user agent MUST dispatch this event type to indicate the user has connected a gamepad. If a gamepad was already connected when the page was loaded, the gamepadconnected event SHOULD be dispatched when the user presses a button or moves an axis.

16. The gamepaddisconnected event

User agents implementing this specification must provide a new DOM event, named gamepaddisconnected. The corresponding event MUST be of type GamepadEvent and MUST fire on the window object. Registration for and firing of the gamepaddisconnected event MUST follow the usual behavior of DOM Events. [DOM]

When a gamepad is disconnected from the user agent, if the user agent has previously dispatched a gamepadconnected event for that gamepad to a window, a gamepaddisconnected event MUST be dispatched to that same window.

17. Other events

More discussion needed, on whether to include or exclude axis and button changed events, and whether to roll them more together (gamepadchanged?), separate somewhat (gamepadaxischanged?), or separate by individual axis and button.

18. Conformance

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words MAY, MUST, MUST NOT, RECOMMENDED, and SHOULD in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

This specification defines conformance criteria that apply to a single product: the user agent that implements the interfaces that it contains.