Allow XRDevice to override viewSpaces

src/api/XRSession.js

@@ -20,7 +20,6 @@ import XRReferenceSpace, {
   XRReferenceSpaceTypes
 } from './XRReferenceSpace';
 import XRRenderState from './XRRenderState';
-import XRWebGLLayer from './XRWebGLLayer';
 import XRInputSourceEvent from './XRInputSourceEvent';
 import XRSessionEvent from './XRSessionEvent';
 import XRSpace from './XRSpace';
@@ -63,6 +62,11 @@ export default class XRSession extends EventTarget {
       inlineVerticalFieldOfView: immersive ? null : Math.PI * 0.5
     });
 
+    const defaultViewSpaces = immersive ?
+      [new XRViewSpace('left'), new XRViewSpace('right')] :
+      [new XRViewSpace('none')];
+    Object.freeze(defaultViewSpaces);
+
     this[PRIVATE] = {
       device,
       mode,
@@ -77,17 +81,10 @@ export default class XRSession extends EventTarget {
       activeRenderState: initialRenderState,
       pendingRenderState: null,
       viewerSpace: new XRReferenceSpace("viewer"),
-      viewSpaces: [],
+      get viewSpaces() { return device.getViewSpaces(mode) || defaultViewSpaces; },
       currentInputSources: []
     };
 
-    if (immersive) {
-      this[PRIVATE].viewSpaces.push(new XRViewSpace('left'),
-                                    new XRViewSpace('right'));
-    } else {
-      this[PRIVATE].viewSpaces.push(new XRViewSpace('none'));
-    }
-
     // Single handler for animation frames from the device. The spec says this must
     // run on every candidate frame even if there are no callbacks queued up.
     this[PRIVATE].onDeviceFrame = () => {

src/devices/XRDevice.js

@@ -118,6 +118,13 @@ export default class XRDevice extends EventTarget {
    */
   endSession(sessionId) { throw new Error('Not implemented'); }
 
+  /**
+   * Allows the XRDevice to override the XRSession's view spaces.
+   *
+   * @param {XRSessionMode} mode
+   */
+  getViewSpaces(mode) { return undefined; }
+
   /**
    * Takes a XREye and a target to apply properties of
    * `x`, `y`, `width` and `height` on. Returns a boolean