Handle detached arrays in XRView

[Manishearth]

There are a bunch of places in the WebXR spec where arrays are returned. If you call .transfer() on these, their buffers are "detached", and need to be recomputed. You can check if a buffer is detached by calling is_null() on it.

We handle most of these cases, except for XRView.projectionMatrix

We need to store the projection matrix on the XRView and use a null check in ProjectionMatrix to recompute it if necessary:

servo/components/script/dom/xrview.rs

Line 83 in 5e91e48

NonNull::new(self.proj.get()).unwrap()

[highfive]

Hi! If you have any questions regarding this issue, feel free to make a comment here, or ask it in the #servo channel in IRC.

If you intend to work on this issue, then add @highfive: assign me to your comment, and I'll assign this to you. 😄

[daemon1024]

Hi! I wanted to get started with contributing to servo and this issue was listed on Servo Starters. I am familiar with rust and I want to work on this issue.

We handle most of these cases, except for XRView.projectionMatrix

Can you link me with a example where this case is already being handled, it will help me figure out what exactly I need to implement here!

[Manishearth]

@daemon1024 in all the other cases the array is lazy initialized so fetching it when it's null after detachment just reinitializes it

servo/components/script/dom/xrrigidtransform.rs

Line 118 in 5e91e48

fn Matrix(&self, _cx: JSContext) -> NonNull<JSObject> {

[daemon1024]

@Manishearth Thanks for reference. Got it now, will get started to work on it.

[daemon1024]

@highfive: assign me

[highfive]

Hey @daemon1024! Thanks for your interest in working on this issue. It's now assigned to you!

[daemon1024]

From what I understood I need to initialise self.proj using create_typed_array with the underlying view's projection matrix as source.

Underlying view can be referenced as self.view but no field of proj/projectionmatrix/projections exists in self.view.

Also view field in the struct XRView is Heap here, I think it should be of the type webxr_api::View for me to be able to reference the property.

@Manishearth ,can you assist me with where am I going wrong?

[Manishearth]

Yes, you need to copy the projection matrix into the XRView as a new field during construction.

You still need the Heap to be able to make the array.

[daemon1024]

Logically it should be something like this i guess

if self.proj.get().is_null() {
let cx = self.global().get_cx();
let projsrc = self.view.projection;
create_typed_array(cx, &projsrc, &self.proj);
}

But I can't seem to figure how access the projection matrix source ( projsrc here ) that i need to copy.

[Manishearth]

You have view in new() during construction, you should copy out view.projection and store it on a field.

[daemon1024]

I added a new field as u suggested

pub struct XRView {
....
viewsrc: &'static [f32],
}

I tried to store it out from new() using

let projm = view.projection.to_row_major_array();
let ret = reflect_dom_object(
Box::new(XRView::new_inherited(session, &transform, eye, &projm )), global, );

But I am getting an error

borrowed value does not live long enough
| cast requires that projm is borrowed for 'static

I am not sure how else do I copy it out.

[Manishearth]

It needs to be a Vec<f32>, you can't hold on to borrowed slices there

[daemon1024]

@Manishearth Thank You so much for the help, I have made the required changes and made a PR. Can you please review it?