Don't require action within a Promise reaction microtask. #570
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jyasskin
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May 8, 2025
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Thanks @jan-ivar. I've left a bunch of suggestions for @martinthomson to review, and you're welcome to apply any that you agree with or make other changes in response, but we can also take it from here.
Co-authored-by: Jeffrey Yasskin <[email protected]>
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| <p>If an algorithm requires that callback functions respond | ||
| synchronously (e.g. to take or veto some action), then that is a | ||
| synchronous API, and should not be modelled with a promise. Instead, |
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Suggested change
| synchronous API, and should not be modelled with a promise. Instead, | |
| synchronous API, and should not be modeled with a Promise. Instead, |
| you may not require action within | ||
| a fixed number of microtasks.</p> | ||
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| <p>This protects promise composition: patterns like <code>await</code>, |
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| <p>This protects promise composition: patterns like <code>await</code>, | |
| <p>This protects Promise composition: patterns like <code>await</code>, |
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| If developer-supplied code needs to be able to call <code>event.{{Event/preventDefault()}}</code> | ||
| or choose a response object with | ||
| <code>fetchEvent.{{FetchEvent/respondWith()}}</code>, | ||
| it needs to be a callback rather than a reaction passed to <code>Promise.{{Promise/then()}}</code>. |
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Is this a good example or a bad one?
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| A non-event example is the <code>captureController.{{CaptureController/setFocusBehavior()}}</code> | ||
| method to push a window the user has chosen to screen-capture, to | ||
| the front. Applications can decide this based on the window chosen. |
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Suggested change
| A non-event example is the <code>captureController.{{CaptureController/setFocusBehavior()}}</code> | |
| method to push a window the user has chosen to screen-capture, to | |
| the front. Applications can decide this based on the window chosen. | |
| A non-event example is the <code>captureController.{{CaptureController/setFocusBehavior()}}</code> | |
| method, which pushes a window the user has chosen to screen-capture to | |
| the front. Applications can decide this based on the window chosen. |
| A non-event example is the <code>captureController.{{CaptureController/setFocusBehavior()}}</code> | ||
| method to push a window the user has chosen to screen-capture, to | ||
| the front. Applications can decide this based on the window chosen. | ||
| But any delay risks click-jacking. |
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Suggested change
| But any delay risks click-jacking. | |
| The challenge is that allowing delay creates an opportunity for click-jacking attacks from sites. |
A link defining click-jacking would be helpful.
| But any delay risks click-jacking. | ||
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| Instead of requiring the method be called synchronously on the | ||
| reaction microtask of the promise from `getDisplayMedia()`, the |
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Suggested change
| reaction microtask of the promise from `getDisplayMedia()`, the | |
| reaction microtask of the promise from `getDisplayMedia()`, the |
A link to getDisplayMedia() would also be helpful.
| <h4 id="sync-callbacks">When to use callback functions instead</h4> | ||
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| <p>If an algorithm requires that callback functions respond | ||
| synchronously (e.g. to take or veto some action), then that is a |
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Suggested change
| synchronously (e.g. to take or veto some action), then that is a | |
| synchronously (e.g., to take or veto some action), then that is a |
I'm not sure about our conventions here; this is the one that I've been taught to use.
... checks ...
damn. Well.
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Fixes #342.
Preview (#sync-callbacks) (#promises-and-reacti…) | Diff