Write document about canvas (contexts) #70
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sagudev
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I think it's enough baking for first round. |
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Reviewing now :) |
delan
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Jun 25, 2025
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Thanks for writing this new chapter, it’s very useful! For new content I think it makes sense to edit and polish before landing, so I’ve done some copyediting. Let me know if I’ve made any mistakes, since script and canvas are not my areas of expertise. |
Co-authored-by: shuppy <delan@azabani.com> Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>
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jdm
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Jun 25, 2025
Co-authored-by: Josh Matthews <josh@joshmatthews.net> Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>
Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>
Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>
sagudev
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Jun 25, 2025
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| As part of HTML event loop, script thread runs a task (parsing, script evaluating, callbacks, events, ...) and after that it [performs a microtask checkpoint](https://html.spec.whatwg.org/multipage/#perform-a-microtask-checkpoint) that drains microtasks queue. | ||
| In [window event loop](https://html.spec.whatwg.org/multipage/webappapis.html#event-loop-processing-model:window-event-loop-3) we queue a global task to [updates the rendering](https://html.spec.whatwg.org/multipage/webappapis.html#update-the-rendering) if there is an [rendering opportunity](https://html.spec.whatwg.org/multipage/webappapis.html#rendering-opportunity) (usually driven by compositor based on hardware refresh rate). | ||
| In servo we do not actually queue a task, but instead we [run updates the rendering on any IPC messages in the ScriptThread](https://github.com/servo/servo/blob/d970584332a3761009f672f975bfffa917513b85/components/script/script_thread.rs#L1418) and [then perform a microtask checkpoint to as event loop would done after a task is completed](https://github.com/servo/servo/blob/d970584332a3761009f672f975bfffa917513b85/components/script/script_thread.rs#L1371). | ||
| [Updates the rendering](https://github.com/servo/servo/blob/d970584332a3761009f672f975bfffa917513b85/components/script/script_thread.rs#L1201) does various resize, scroll and animations steps (which also includes performing a microtask checkpoint; to resolve promises) and then [run the animation frame callbacks](https://html.spec.whatwg.org/multipage/imagebitmap-and-animations.html#run-the-animation-frame-callbacks) (callbacks added with [`requestAnimationFrame`](https://developer.mozilla.org/en-US/docs/Web/API/Window/requestAnimationFrame)). | ||
| Here draw commands are issued to painters to create new frame of animation. | ||
| Then we update the rendering of those that do not require a reflow (animated images and WebGPU). | ||
| Finally we triggers reflow (layout), which takes the DOM and its styles, builds a `DisplayList`, and sends that to WebRender for rendering. | ||
| Dirty WebGL and 2D canvases are flushed as part of a reflow. |
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@gterzian could you check if my understanding is correct.
Also driving force of rendering opportunities is not just compositor (via any script msg), but also script itself, at least for now.
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