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feat: add OffscreenImages Audit (#1807)
* refactor: rename ContentWidth to ViewportDimensions
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147 changes: 147 additions & 0 deletions
147
lighthouse-core/audits/byte-efficiency/offscreen-images.js
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,147 @@ | ||
| /** | ||
| * @license | ||
| * Copyright 2017 Google Inc. All rights reserved. | ||
| * | ||
| * Licensed under the Apache License, Version 2.0 (the "License"); | ||
| * you may not use this file except in compliance with the License. | ||
| * You may obtain a copy of the License at | ||
| * | ||
| * http://www.apache.org/licenses/LICENSE-2.0 | ||
| * | ||
| * Unless required by applicable law or agreed to in writing, software | ||
| * distributed under the License is distributed on an "AS IS" BASIS, | ||
| * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
| * See the License for the specific language governing permissions and | ||
| * limitations under the License. | ||
| */ | ||
| /** | ||
| * @fileoverview Checks to see if images are displayed only outside of the viewport. | ||
| * Images requested after TTI are not flagged as violations. | ||
| */ | ||
| 'use strict'; | ||
|
|
||
| const Audit = require('./byte-efficiency-audit'); | ||
| const TTIAudit = require('../time-to-interactive'); | ||
| const URL = require('../../lib/url-shim'); | ||
|
|
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| const ALLOWABLE_OFFSCREEN_X = 100; | ||
| const ALLOWABLE_OFFSCREEN_Y = 200; | ||
|
|
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| const IGNORE_THRESHOLD_IN_BYTES = 2048; | ||
| const IGNORE_THRESHOLD_IN_PERCENT = 75; | ||
|
|
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| class OffscreenImages extends Audit { | ||
| /** | ||
| * @return {!AuditMeta} | ||
| */ | ||
| static get meta() { | ||
| return { | ||
| category: 'Images', | ||
| name: 'offscreen-images', | ||
| description: 'Offscreen images', | ||
| helpText: 'Images that are not above the fold should be lazily loaded after the page is ' + | ||
| 'interactive. Consider using the [IntersectionObserver](https://developers.google.com/web/updates/2016/04/intersectionobserver) API.', | ||
| requiredArtifacts: ['ImageUsage', 'ViewportDimensions', 'traces', 'networkRecords'] | ||
| }; | ||
| } | ||
|
|
||
| /** | ||
| * @param {!ClientRect} imageRect | ||
| * @param {{innerWidth: number, innerHeight: number}} viewportDimensions | ||
| * @return {number} | ||
| */ | ||
| static computeVisiblePixels(imageRect, viewportDimensions) { | ||
| const innerWidth = viewportDimensions.innerWidth; | ||
| const innerHeight = viewportDimensions.innerHeight; | ||
|
|
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| const top = Math.max(imageRect.top, -1 * ALLOWABLE_OFFSCREEN_Y); | ||
| const right = Math.min(imageRect.right, innerWidth + ALLOWABLE_OFFSCREEN_X); | ||
| const bottom = Math.min(imageRect.bottom, innerHeight + ALLOWABLE_OFFSCREEN_Y); | ||
| const left = Math.max(imageRect.left, -1 * ALLOWABLE_OFFSCREEN_X); | ||
|
|
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| return Math.max(right - left, 0) * Math.max(bottom - top, 0); | ||
| } | ||
|
|
||
| /** | ||
| * @param {!Object} image | ||
| * @param {{innerWidth: number, innerHeight: number}} viewportDimensions | ||
| * @return {?Object} | ||
| */ | ||
| static computeWaste(image, viewportDimensions) { | ||
| const url = URL.getDisplayName(image.src, {preserveQuery: true}); | ||
| const totalPixels = image.clientWidth * image.clientHeight; | ||
| const visiblePixels = this.computeVisiblePixels(image.clientRect, viewportDimensions); | ||
| const wastedRatio = 1 - visiblePixels / totalPixels; | ||
| const totalBytes = image.networkRecord.resourceSize; | ||
| const wastedBytes = Math.round(totalBytes * wastedRatio); | ||
|
|
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| if (!Number.isFinite(wastedRatio)) { | ||
| return new Error(`Invalid image sizing information ${url}`); | ||
| } | ||
|
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| return { | ||
| url, | ||
| preview: { | ||
| url: image.networkRecord.url, | ||
| mimeType: image.networkRecord.mimeType | ||
| }, | ||
| requestStartTime: image.networkRecord.startTime, | ||
| totalBytes, | ||
| wastedBytes, | ||
| wastedPercent: 100 * wastedRatio, | ||
| }; | ||
| } | ||
|
|
||
| /** | ||
| * @param {!Artifacts} artifacts | ||
| * @return {{results: !Array<Object>, tableHeadings: Object, | ||
| * passes: boolean=, debugString: string=}} | ||
| */ | ||
| static audit_(artifacts) { | ||
| const images = artifacts.ImageUsage; | ||
| const viewportDimensions = artifacts.ViewportDimensions; | ||
|
|
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| let debugString; | ||
| const resultsMap = images.reduce((results, image) => { | ||
| if (!image.networkRecord) { | ||
| return results; | ||
| } | ||
|
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| const processed = OffscreenImages.computeWaste(image, viewportDimensions); | ||
| if (processed instanceof Error) { | ||
| debugString = processed.message; | ||
| return results; | ||
| } | ||
|
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| // If an image was used more than once, warn only about its least wasteful usage | ||
| const existing = results.get(processed.preview.url); | ||
| if (!existing || existing.wastedBytes > processed.wastedBytes) { | ||
| results.set(processed.preview.url, processed); | ||
| } | ||
|
|
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| return results; | ||
| }, new Map()); | ||
|
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| return TTIAudit.audit(artifacts).then(ttiResult => { | ||
| const ttiTimestamp = ttiResult.extendedInfo.value.timestamps.timeToInteractive / 1000000; | ||
| const results = Array.from(resultsMap.values()).filter(item => { | ||
| const isWasteful = item.wastedBytes > IGNORE_THRESHOLD_IN_BYTES && | ||
| item.wastedPercent > IGNORE_THRESHOLD_IN_PERCENT; | ||
| const loadedEarly = item.requestStartTime < ttiTimestamp; | ||
| return isWasteful && loadedEarly; | ||
| }); | ||
| return { | ||
| debugString, | ||
| results, | ||
| tableHeadings: { | ||
| preview: '', | ||
| url: 'URL', | ||
| totalKb: 'Original', | ||
| potentialSavings: 'Potential Savings', | ||
| } | ||
| }; | ||
| }); | ||
| } | ||
| } | ||
|
|
||
| module.exports = OffscreenImages; |
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