Better define gamepad placeholder buttons and axes (#661)

toji · web-flow · commit 93516d13ce9b · 2019-05-21T14:08:22.000-07:00
Define what values are expected for placeholder elements and when it is appropriate to omit them.
diff --git a/index.bs b/index.bs
@@ -77,6 +77,8 @@ spec: Gamepad; urlPrefix: https://w3c.github.io/gamepad/#
     type: attribute; text: buttons; for: Gamepad; url: dom-gamepad-buttons
     type: attribute; text: axes; for: Gamepad; url: dom-gamepad-axes
     type: attribute; text: touched; for: GamepadButton; url: dom-gamepadbutton-touched
+    type: attribute; text: pressed; for: GamepadButton; url: dom-gamepadbutton-pressed
+    type: attribute; text: value; for: GamepadButton; url: dom-gamepadbutton-value
 spec:html; type:method; for:HTMLCanvasElement; text:getContext(contextId); url: https://html.spec.whatwg.org/multipage/canvas.html#dom-canvas-getcontext
 </pre>
 
@@ -1476,7 +1478,9 @@ In order to report a {{Gamepad/mapping}} of {{GamepadMappingType/xr-standard}} t
   </tbody>
 </table>
 
-Additional buttons or axes may be exposed after these reserved indices, and SHOULD appear in order of decreasing importance. Related axes (such both axes of a joystick) SHOULD be grouped and, if applicable, SHOULD appear in X, Y, Z order. Devices that lack one of the inputs listed in the tables above MUST still preserve their place in the {{Gamepad/buttons}} or {{Gamepad/axes}} array. If a device has both a touchpad and a joystick the UA MAY designate whichever it chooses to be the primary axis-based input. Buttons reserved by the UA or platform MUST NOT be exposed.
+Devices that lack one of the optional inputs listed in the tables above MUST preserve their place in the {{Gamepad/buttons}} or {{Gamepad/axes}} array, reporting a <dfn>placeholder button</dfn> or <dfn>placeholder axis</dfn>, respectively. A [=placeholder button=] MUST report <code>0</code> for {{GamepadButton/value}}, <code>false</code> for {{GamepadButton/pressed}}, and <code>false</code> for {{GamepadButton/touched}}. A [=placeholder axis=] MUST report <code>0</code>. [=Placeholder buttons=] and [=placeholder axis|axes=] MUST be omitted if they are the last element in the array or all following elements are also [=placeholder buttons=] or [=placeholder axis|axes=].
+
+Additional buttons or axes may be exposed after these reserved indices, and SHOULD appear in order of decreasing importance. Related axes (such both axes of a joystick) SHOULD be grouped and, if applicable, SHOULD appear in X, Y, Z order. If a device has both a touchpad and a joystick the UA MAY designate whichever it chooses to be the primary axis-based input. Buttons reserved by the UA or platform MUST NOT be exposed.
 
 NOTE: This diagram demonstrates how two potential controllers would be exposed with the `xr-standard` mapping. <img src="images/xr-standard-mapping.svg" alt="Simple 'xr-standard' controller and Advanced 'xr-standard' controller">
 </section>
