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Add exception info for HTMLCanvasElement.transferControlToOffscreen() and OffscreenCanvas.transferToImageBitmap() #36631

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Nov 22, 2024
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Add exception info for HTMLCanvasElement.transferControlToOffscreen() and OffscreenCanvas.transferToImageBitmap() #36631

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Add exception info for `HTMLCanvasElement.transferControlToOffscreen(…
skyclouds2001 Nov 3, 2024
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Update files/en-us/web/api/htmlcanvaselement/getcontext/index.md
wbamberg Nov 16, 2024
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Update files/en-us/web/api/htmlcanvaselement/transfercontroltooffscre…
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2 changes: 1 addition & 1 deletion files/en-us/web/api/htmlcanvaselement/getcontext/index.md
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Expand Up @@ -117,7 +117,7 @@ If the context identifier is not supported, or the canvas has already been set t
### Exceptions

- `InvalidStateError` {{domxref("DOMException")}}
- : Throws if the canvas has transferred its control to offscreen via calling `HTMLCanvasElement.transferControlToOffscreen()`.
- : Throws if the canvas has transferred its control to offscreen via calling {{domxref("HTMLCanvasElement.transferControlToOffscreen()")}}.
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## Examples

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5 changes: 5 additions & 0 deletions files/en-us/web/api/htmlcanvaselement/transfercontroltooffscreen/index.md
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Expand Up @@ -24,6 +24,11 @@ None.

An {{domxref("OffscreenCanvas")}} object.

### Exceptions

- `InvalidStateError` {{domxref("DOMException")}}
- : Throws if the canvas has been set to a context mode via calling {{domxref("HTMLCanvasElement.getContext()")}} or has transferred its control to offscreen via calling {{domxref("HTMLCanvasElement.transferControlToOffscreen()")}}.
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I don't see in https://html.spec.whatwg.org/multipage/canvas.html#dom-canvas-transfercontroltooffscreen where we throw an exception if the canvas "has transferred its control to offscreen via calling {{domxref("HTMLCanvasElement.transferControlToOffscreen()")}}", can you point to it?

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The transferControlToOffscreen() method, when invoked, must run these steps:
If this canvas element's context mode is not set to none, throw an "InvalidStateError" DOMException.
Let offscreenCanvas be a new OffscreenCanvas object with its width and height equal to the values of the width and height content attributes of this canvas element.
Set the placeholder canvas element of offscreenCanvas to a weak reference to this canvas element.
Set this canvas element's context mode to placeholder.
Return offscreenCanvas.The transferControlToOffscreen() method, when invoked, must run these steps:
If this canvas element's context mode is not set to none, throw an "InvalidStateError" DOMException.
Let offscreenCanvas be a new OffscreenCanvas object with its width and height equal to the values of the width and height content attributes of this canvas element.
Set the placeholder canvas element of offscreenCanvas to a weak reference to this canvas element.
Set this canvas element's context mode to placeholder.
Return offscreenCanvas.

After calling transferControlToOffscreen(), the canvas's context mode will be set to "placeholder"; so if calling transferControlToOffscreen() again, it will fail at the first step.

Sample code is as follows:

image

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Thank you! (I didn't doubt you were right here, I just couldn't see it!)


## Examples

The following example shows how to transfer control to an {{domxref("OffscreenCanvas")}} object on the main thread.
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5 changes: 5 additions & 0 deletions files/en-us/web/api/offscreencanvas/transfertoimagebitmap/index.md
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Expand Up @@ -32,6 +32,11 @@ If your goal is to pass the `ImageBitmap` to other web APIs which do not consume

If you call `transferToImageBitmap()` and don't intend to pass it to {{domxref("ImageBitmapRenderingContext.transferFromImageBitmap()")}}, consider whether you need to call `transferToImageBitmap()` at all. Many web APIs which accept `ImageBitmap` also accept `OffscreenCanvas` as an argument.

### Exceptions

- `InvalidStateError` {{domxref("DOMException")}}
- : Throws if the canvas has transferred to another context scope, for example, to worker; or hasn't set to a context mode via calling {{domxref("OffscreenCanvas.getContext()")}}.
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## Examples

```js
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