Implemented support for creating a render surface from a canvas or offscreencanvas

[anlumo]

Objective

• bevy uses raw-window-handle for creating a rendering context. That crate does not really work for the web target (issue).

Solution

• Builds on PR Enable creating "virtual" windows without corresponding OS window #6256.
• This PR bypasses raw-window-handle to allow applications to specify the canvas directly.

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Changelog

• Added two new variants to the enum called bevy_window::window::AbstractWindowHandle that can take either an HtmlCanvasElement or OffscreenCanvas.
• Added three new functions to bevy_window::window::Window:
  • new_canvas for creating a Window based on a canvas element directly.
  • new_canvas_selector for creating a Window based on a CSS-style selector.
  • new_offscreen_canvas for creating a Window with an offscreen canvas.
• Added WindowDescriptor::set_canvas_from_selector and WindowDescriptor::set_canvas to allow winit to use a canvas element directly instead of creating its own (which is still supported).
• All of this only applies to the wasm target! winit only supports a regular HtmlCanvasElement, but not an OffscreenCanvas. This is a limitation of winit.

Migration Guide

• The canvas field in Window does no longer exists for non-wasm targets.
• If you want to specify a canvas for winit, you now have to use WindowDescriptor::set_canvas_from_selector or WindowDescriptor::set_canvas instead.

[mockersf]

How does this relate to the existing canvas field?

It's using with_canvas on the window builder from winit to use that canvas

[anlumo]

How does this relate to the existing canvas field?

The existing canvas field is a String, which is not helpful for referencing a JsValue. I thought about just replacing that field, but that would potentially break existing code.

It's using with_canvas on the window builder from winit to use that canvas

The goal of this PR is to allow OffscreenCanvas usage in bevy, mostly for moving bevy into a Web Worker and thus avoiding blocking the UI thread with rendering operations. This is not something winit supports (rust-windowing/winit#1518).

btw, I just confirmed that this is working with bevy running in a web worker in a test application using this PR.

[anlumo]

Here is a minimal example for using bevy in a web worker using this PR: https://github.com/anlumo/bevy-webworker-test

[alice-i-cecile]

I thought about just replacing that field, but that would potentially break existing code.

This PR is out of my area of expertise, but if you think this is a better solution, just replace that field. We're deliberately open to breaking changes to improve the API at this stage.

[mockersf]

I agree with Alice, breaking is not an issue, but having the api offers two ways around the same thing isn't nice.

This api seems a little harder to use for the simple case of just passing the canvas id though.

Here is a minimal example for using bevy in a web worker using this PR: https://github.com/anlumo/bevy-webworker-test

Thanks ! That's quite interesting

[anlumo]

This api seems a little harder to use for the simple case of just passing the canvas id though.

True, I've added a WindowDescriptor::set_canvas_from_selector function to simplify the transition. It's simply a wrapper around web_sys::Window::query_selector that sets the same member of the struct.

I've also removed the old selector-based entry!

[rparrett]

The decision to include canvas and fit_canvas_to_parent in non-web targets was intentional.

See #4726 and #4683

[anlumo]

The decision to include canvas and fit_canvas_to_parent in non-web targets was intentional.

That's unfortunately no longer possible once it's using a web-sys type for canvas, and since it's already a special case there's no point to include the other flag on other platforms.

An alternative would be to have a fake flag there for non-wasm32, like a PhantomData<()>, an Option<()> or bool. Would that be an acceptable solution?

[rparrett]

I doubt it's a big deal if it's unavoidable, just wanted to surface that history.

[anlumo]

Needs some testing from my side before I can remove the draft flag.

[anlumo]

Rebased to new version of #6256 and also added winit support (without OffscreenCanvas, because winit doesn't support that at all right now).

[anlumo]

Holding off on this until #6256 is merged, since it changes around all the time at the moment, causing merge conflicts.