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We also recommend that a 185 | file or class name and description of purpose be included on the 186 | same "printed page" as the copyright notice for easier 187 | identification within third-party archives. 188 | 189 | Copyright [yyyy] [name of copyright owner] 190 | 191 | Licensed under the Apache License, Version 2.0 (the "License"); 192 | you may not use this file except in compliance with the License. 193 | You may obtain a copy of the License at 194 | 195 | http://www.apache.org/licenses/LICENSE-2.0 196 | 197 | Unless required by applicable law or agreed to in writing, software 198 | distributed under the License is distributed on an "AS IS" BASIS, 199 | WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 200 | See the License for the specific language governing permissions and 201 | limitations under the License. 202 | -------------------------------------------------------------------------------- /README.md: -------------------------------------------------------------------------------- 1 | # isFramePending 2 | spec proposal for new web API: boolean navigator.scheduling.isFramePending() 3 | 4 | [Explainer](./explainer.md) 5 | -------------------------------------------------------------------------------- /explainer.md: -------------------------------------------------------------------------------- 1 | # navigator.scheduling.isFramePending 2 | 3 | ## Author 4 | szager@chromium.org 5 | 6 | ## Introduction 7 | The purpose of this API is to allow developers to make informed decisions about when to yield from javascript code, in order to give the rendering pipeline an opportunity to run (and thereby avoid visual jank). It provides insight into whether the browser needs to refresh the visual display by running its rendering steps. 8 | 9 | ## Problem Statement 10 | Browsers cannot update the rendering of a page while javascript is running; those tasks must run serially (as described in the [HTML processing model](https://html.spec.whatwg.org/multipage/webappapis.html#event-loop-processing-model)). If an uninterruptible javascript task runs too long, it may cause the browser to fall behind in updating the rendering, resulting in dropped or missed animation frames ("jank"). 11 | 12 | To ensure smooth animations and visual updates, web developers must schedule their application logic to run without delaying necessary rendering updates ([more info](https://developers.google.com/web/fundamentals/performance/rendering/optimize-javascript-execution)). Presently, the only real strategy for doing this is to break up application logic into small chunks and yield control to the browser frequently. This can be quite limiting for developers that want to write scheduling logic into their javascript code and yield to the browser less frequently: doing so increases the likelihood that the browser will fall behind in updating the rendering. The web platform doesn't provide any signal that rendering is falling behind, or that the browser needs to update the rendering. 13 | 14 | ## Proposed Solution 15 | isFramePending returns a boolean indicating whether any [browsing contexts](https://html.spec.whatwg.org/multipage/browsers.html#concept-document-bc) currently have a [rendering opportunity](https://html.spec.whatwg.org/multipage/webappapis.html#rendering-opportunity). If the return value is `true`, the browser will run the "Update the Rendering" steps on the next iteration of the event loop, as specified in step 11 of the HTML [processing model](https://html.spec.whatwg.org/multipage/webappapis.html#event-loop-processing-model). 16 | 17 | Here's how the API might be used by an application that has a main loop in javascript: 18 | 19 | ```javascript 20 | function mainLoop() { 21 | // As long as there is work to do, don't yield back to the browser until it 22 | // needs to update the rendering. 23 | while (thereIsWorkToDo() && !navigator.scheduling.isFramePending()) { 24 | getNextWorkItem().run(); 25 | } 26 | 27 | // If there is more work to do, but you want to give the browser a chance to 28 | // update the rendering, re-schedule the main loop to run again later. 29 | if (thereIsWorkToDo()) { 30 | setTimeout(mainLoop, 0); 31 | } 32 | }; 33 | 34 | button.addEventListener('click', e => { 35 | let workItem = createWorkItemToHandleEvent(e); 36 | if (isUrgent(e)) { 37 | // Don't yield to the browser, even at risk of delaying rendering. 38 | workItem.run(); 39 | } else { 40 | // Let the mainLoop logic decide whether to run the event handler code 41 | // immediately or postpone it until later. 42 | enqueueWorkItem(workItem); 43 | mainLoop(); 44 | } 45 | }); 46 | ``` 47 | 48 | One limitation of the above code is that event listeners might be delayed while `mainLoop` runs through all pending work. Another proposed API, [navigator.scheduling.isInputPending](https://github.com/WICG/is-input-pending), can be included to make the app more responsive: 49 | 50 | ```javascript 51 | function mainLoop() { 52 | while (thereIsWorkToDo() && !navigator.scheduling.isFramePending() && 53 | !navigator.scheduling.isInputPending()) { 54 | getNextWorkItem().run(); 55 | } 56 | 57 | ``` 58 | 59 | ## Security Considerations 60 | 61 | isFramePending reveals the fact that the browser has scheduled an animation frame, indicating that the page contents are dirty. More investigation is required to determine whether this information is potentially exploitable. 62 | 63 | ## Design Decisions and Non-Goals 64 | 65 | This API is not intended to be predictive in any way -- it won't tell you if a rendering opportunity will happen "soon", or provide any budget or estimated time until the next rendering opportunity. It only indicates whether the browser has *already* identified a rendering opportunity that it is waiting to service. 66 | 67 | Originally, a `scheduling.timeUntilNextFrame()` API was considered, but it was concluded that this information was impossible to determine accurately in some important scenarios. 68 | -------------------------------------------------------------------------------- /security_and_privacy_questionnaire.md: -------------------------------------------------------------------------------- 1 | Answers to [Self-Review Questionnaire: Security and Privacy](https://www.w3.org/TR/security-privacy-questionnaire/) 2 | 3 | ### What information might this feature expose to Web sites or other parties, and for what purposes is that exposure necessary? 4 | 5 | This API reveals the fact that the browser has scheduled an animation frame, indicating that the page contents are dirty. This information is inseparable from the API's primary purpose. 6 | 7 | ### Is this specification exposing the minimum amount of information necessary to power the feature? 8 | 9 | Yes 10 | 11 | ### How does this specification deal with personal information or personally-identifiable information or information derived thereof? 12 | 13 | N/A 14 | 15 | ### How does this specification deal with sensitive information? 16 | 17 | N/A 18 | 19 | ### Does this specification introduce new state for an origin that persists across browsing sessions? 20 | 21 | No 22 | 23 | ### What information from the underlying platform, e.g. configuration data, is exposed by this specification to an origin? 24 | 25 | None 26 | 27 | ### Does this specification allow an origin access to sensors on a user’s device 28 | 29 | No 30 | 31 | ### What data does this specification expose to an origin? Please also document what data is identical to data exposed by other features, in the same or different contexts. 32 | 33 | N/A 34 | 35 | ### Does this specification enable new script execution/loading mechanisms? 36 | 37 | No 38 | 39 | ### Does this specification allow an origin to access other devices? 40 | 41 | No 42 | 43 | ### Does this specification allow an origin some measure of control over a user agent’s native UI? 44 | 45 | No 46 | 47 | ### What temporary identifiers might this this specification create or expose to the web? 48 | 49 | None 50 | 51 | ### How does this specification distinguish between behavior in first-party and third-party contexts? 52 | 53 | N/A 54 | 55 | ### How does this specification work in the context of a user agent’s Private \ Browsing or "incognito" mode? 56 | 57 | No difference 58 | 59 | ### Does this specification have a "Security Considerations" and "Privacy Considerations" section? 60 | 61 | The [explainer](./explainer.md#security-considerations) has such a section. 62 | 63 | ### Does this specification allow downgrading default security characteristics? 64 | 65 | No 66 | --------------------------------------------------------------------------------