ampproject · mszylkowski · Jul 12, 2021 · Jun 11, 2021 · Jun 11, 2021 · Jun 15, 2021
diff --git a/examples/amp-story/performance/lcp-compressed.html b/examples/amp-story/performance/lcp-compressed.html
@@ -0,0 +1,105 @@
+<!-- Use #load=first to enable the experiment. -->
+<!doctype html>
+<html ⚡ lang="en">
+<head>
+  <meta charset="utf-8">
+  <link rel="canonical" href="lcp-compressed.html">
+  <meta name="viewport" content="width=device-width,minimum-scale=1,initial-scale=1">
+  <script async src="https://cdn.ampproject.org/v0.js"></script>
+  <title>LCP test</title>
+
+  <script async custom-element="amp-story" src="https://cdn.ampproject.org/v0/amp-story-1.0.js"></script>
+  <!-- Fonts -->
+  <link rel="preconnect" href="https://fonts.googleapis.com">
+  <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
+  <link href="https://fonts.googleapis.com/css2?family=Source+Code+Pro&family=Source+Sans+Pro:wght@400;600&display=swap" rel="stylesheet">
+
+  <style amp-boilerplate>body{-webkit-animation:-amp-start 8s steps(1,end) 0s 1 normal both;-moz-animation:-amp-start 8s steps(1,end) 0s 1 normal both;-ms-animation:-amp-start 8s steps(1,end) 0s 1 normal both;animation:-amp-start 8s steps(1,end) 0s 1 normal both}@-webkit-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-moz-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-ms-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-o-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}</style><noscript><style amp-boilerplate>body{-webkit-animation:none;-moz-animation:none;-ms-animation:none;animation:none}</style></noscript>
+  <style amp-custom>
+    * {
+      box-sizing: border-box;
+    }
+    amp-story-page {
+      font-family: 'Source Sans Pro', sans-serif;
+      color: white;
+      background: black;
+    }
+    code {
+      font-family: 'Source Code Pro', monospace;
+      background: #fff2;
+    }
+    amp-story p {
+      line-height: 1.5;
+    }
+    .bold {
+      font-weight: bold;
+    }
+    .bottom {
+      align-content: end;
+    }
+    .last {
+      padding-bottom: 8.83vh;
+    }
+    .byline {
+      letter-spacing: 1.28px;
+    }
+    amp-img {
+      margin: 1em;
+      margin-bottom: 5em;
+    }
+  </style>
+</head>
+  <body>
+    <script>
+      const loadFirst = location.hash.includes('load=first');
+      (self.AMP = self.AMP || []).push(function(AMP) {
+        AMP.toggleExperiment('amp-story-load-first-page-only', loadFirst);
+      });
+    </script>
+    <amp-story standalone id="cats"
+        title="LCP test" publisher="The AMP team"
+        publisher-logo-src="./img/amplogo.png"
+        poster-portrait-src="./img/overview.jpg">
+
+      <amp-story-page id="page-1" title="Lorem ipsum dolor sit amet">
+        <amp-story-grid-layer template="fill">
+          <amp-img
+            id="img1"
+            width="400"
+            height="750"
+            src="https://images.unsplash.com/photo-1515705576963-95cad62945b6?w=1750&q=30&fm=webp"
+            layout="fill">
+          </amp-img>
+        </amp-story-grid-layer>
+        <amp-story-grid-layer template="vertical" class="bottom">
+          <p class="bold">LCP for images (compressed)</p>
+          <p class="byline">Add <code>#load=first</code> to activate experiment</p>
+        </amp-story-grid-layer>
+      </amp-story-page>
+
+      <amp-story-page id="page-2">
+        <amp-story-grid-layer template="fill">
+          <amp-img
+            id="img2"
+            width="400"
+            height="750"
+            src="https://images.unsplash.com/photo-1502318217862-aa4e294ba657?w=1080&q=30&fm=webp"
+            layout="fill">
+          </amp-img>
+        </amp-story-grid-layer>
+      </amp-story-page>
+
+      <amp-story-page id="page-3">
+        <amp-story-grid-layer template="fill">
+          <amp-img
+            id="img3"
+            width="400"
+            height="750"
+            src="https://images.unsplash.com/photo-1513809491260-0e192158ae44?w=1080&q=30&fm=webp"
+            layout="fill">
+          </amp-img>
+        </amp-story-grid-layer>
+      </amp-story-page>
+    </amp-story>
+  </body>
+</html>
diff --git a/examples/amp-story/performance/lcp.html b/examples/amp-story/performance/lcp.html
@@ -0,0 +1,105 @@
+<!-- Use #load=first to enable the experiment. -->
+<!doctype html>
+<html ⚡ lang="en">
+<head>
+  <meta charset="utf-8">
+  <link rel="canonical" href="lcp.html">
+  <meta name="viewport" content="width=device-width,minimum-scale=1,initial-scale=1">
+  <script async src="https://cdn.ampproject.org/v0.js"></script>
+  <title>LCP test</title>
+
+  <script async custom-element="amp-story" src="https://cdn.ampproject.org/v0/amp-story-1.0.js"></script>
+  <!-- Fonts -->
+  <link rel="preconnect" href="https://fonts.googleapis.com">
+  <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
+  <link href="https://fonts.googleapis.com/css2?family=Source+Code+Pro&family=Source+Sans+Pro:wght@400;600&display=swap" rel="stylesheet">
+
+  <style amp-boilerplate>body{-webkit-animation:-amp-start 8s steps(1,end) 0s 1 normal both;-moz-animation:-amp-start 8s steps(1,end) 0s 1 normal both;-ms-animation:-amp-start 8s steps(1,end) 0s 1 normal both;animation:-amp-start 8s steps(1,end) 0s 1 normal both}@-webkit-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-moz-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-ms-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-o-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}</style><noscript><style amp-boilerplate>body{-webkit-animation:none;-moz-animation:none;-ms-animation:none;animation:none}</style></noscript>
+  <style amp-custom>
+    * {
+      box-sizing: border-box;
+    }
+    amp-story-page {
+      font-family: 'Source Sans Pro', sans-serif;
+      color: white;
+      background: black;
+    }
+    code {
+      font-family: 'Source Code Pro', monospace;
+      background: #fff2;
+    }
+    amp-story p {
+      line-height: 1.5;
+    }
+    .bold {
+      font-weight: bold;
+    }
+    .bottom {
+      align-content: end;
+    }
+    .last {
+      padding-bottom: 8.83vh;
+    }
+    .byline {
+      letter-spacing: 1.28px;
+    }
+    amp-img {
+      margin: 1em;
+      margin-bottom: 5em;
+    }
+  </style>
+</head>
+  <body>
+    <script>
+      const loadFirst = location.hash.includes('load=first');
+      (self.AMP = self.AMP || []).push(function(AMP) {
+        AMP.toggleExperiment('amp-story-load-first-page-only', loadFirst);
+      });
+    </script>
+    <amp-story standalone id="cats"
+        title="LCP test" publisher="The AMP team"
+        publisher-logo-src="./img/amplogo.png"
+        poster-portrait-src="./img/overview.jpg">
+
+      <amp-story-page id="page-1" title="Lorem ipsum dolor sit amet">
+        <amp-story-grid-layer template="fill">
+          <amp-img
+            id="img1"
+            width="400"
+            height="750"
+            src="https://images.unsplash.com/photo-1515705576963-95cad62945b6?w=1750&q=100&fm=webp"
+            layout="fill">
+          </amp-img>
+        </amp-story-grid-layer>
+        <amp-story-grid-layer template="vertical" class="bottom">
+          <p class="bold">LCP for images (uncompressed)</p>
+          <p class="byline">Add <code>#load=first</code> to activate experiment</p>
+        </amp-story-grid-layer>
+      </amp-story-page>
+
+      <amp-story-page id="page-2">
+        <amp-story-grid-layer template="fill">
+          <amp-img
+            id="img2"
+            width="400"
+            height="750"
+            src="https://images.unsplash.com/photo-1502318217862-aa4e294ba657?w=1080&q=100&fm=webp"
+            layout="fill">
+          </amp-img>
+        </amp-story-grid-layer>
+      </amp-story-page>
+
+      <amp-story-page id="page-3">
+        <amp-story-grid-layer template="fill">
+          <amp-img
+            id="img3"
+            width="400"
+            height="750"
+            src="https://images.unsplash.com/photo-1513809491260-0e192158ae44?w=1080&q=100&fm=webp"
+            layout="fill">
+          </amp-img>
+        </amp-story-grid-layer>
+      </amp-story-page>
+    </amp-story>
+  </body>
+</html>
diff --git a/extensions/amp-story/1.0/amp-story-page.js b/extensions/amp-story/1.0/amp-story-page.js
@@ -1299,6 +1299,9 @@ export class AmpStoryPage extends AMP.BaseElement {
     if (this.isAd()) {
       distance = Math.min(distance, 2);
     }
+    if (distance == this.getDistance()) {
+      return;
+    }
 
     this.element.setAttribute('distance', distance);
     this.element.setAttribute('aria-hidden', distance != 0);

diff --git a/extensions/amp-story/1.0/amp-story.js b/extensions/amp-story/1.0/amp-story.js
@@ -457,6 +457,11 @@ export class AmpStory extends AMP.BaseElement {
         );
       });
     }
+    if (isExperimentOn(this.win, 'amp-story-load-first-page-only')) {
+      Services.performanceFor(this.win).addEnabledExperiment(
+        'amp-story-load-first-page-only'
+      );
+    }
 
     if (this.maybeLoadStoryDevTools_()) {
       return;
@@ -1532,7 +1537,7 @@ export class AmpStory extends AMP.BaseElement {
       // the navigation happened, like preloading the following pages, or
       // sending analytics events.
       () => {
-        this.preloadPagesByDistance_();
+        this.preloadPagesByDistance_(/* prioritizeActivePage */ !oldPage);
         this.triggerActiveEventForPage_();
 
         this.systemLayer_.resetDeveloperLogs();
@@ -2225,8 +2230,11 @@ export class AmpStory extends AMP.BaseElement {
     return map;
   }
 
-  /** @private */
-  preloadPagesByDistance_() {
+  /**
+   * @param {boolean=} prioritizeActivePage
+   * @private
+   */
+  preloadPagesByDistance_(prioritizeActivePage = false) {
     if (this.platform_.isBot()) {
       this.pages_.forEach((page) => {
         page.setDistance(0);
@@ -2236,13 +2244,33 @@ export class AmpStory extends AMP.BaseElement {
 
     const pagesByDistance = this.getPagesByDistance_();
 
-    this.mutateElement(() => {
+    const preloadAllPages = () => {
       pagesByDistance.forEach((pageIds, distance) => {
         pageIds.forEach((pageId) => {
           const page = this.getPageById(pageId);
           page.setDistance(distance);
         });
       });
+    };
+
+    this.mutateElement(() => {
+      if (
+        !isExperimentOn(this.win, 'amp-story-load-first-page-only') ||
+        !prioritizeActivePage
+      ) {
+        return preloadAllPages();
+      }
+
+      const activePageId = devAssert(pagesByDistance[0][0]);
+      new Promise((res, rej) => {
+        const page = this.getPageById(activePageId);
+        page.setDistance(0);
+        page.signals().whenSignal(CommonSignals.LOAD_END).then(res);
+        // Don't call preload if user navigates before page loads, since the navigation will call preload properly.
+        this.storeService_.subscribe(StateProperty.CURRENT_PAGE_ID, rej);
+      })
+        .then(() => preloadAllPages())
+        .catch();
     });
   }
 

diff --git a/extensions/amp-story/1.0/test/test-amp-story.js b/extensions/amp-story/1.0/test/test-amp-story.js
@@ -31,7 +31,9 @@ import {Keys} from '#core/constants/key-codes';
 import {LocalizationService} from '#service/localization';
 import {MediaType} from '../media-pool';
 import {PageState} from '../amp-story-page';
+import {Performance} from '#service/performance-impl';
 import {Services} from '#service';
+import {Signals} from '#core/data-structures/signals';
 import {VisibilityState} from '#core/constants/visibility-state';
 import {createElementWithAttributes} from '#core/dom';
 import {registerServiceBuilder} from '../../../../src/service-helpers';
@@ -58,6 +60,8 @@ describes.realWin(
     let replaceStateStub;
     let win;
 
+    const nextTick = () => new Promise((resolve) => win.setTimeout(resolve, 0));
+
     /**
      * @param {number} count
      * @param {Array<string>=} ids
@@ -1837,5 +1841,62 @@ describes.realWin(
         ).to.be.false;
       });
     });
+
+    describe('experiment for amp-story-load-first-page-only', () => {
+      let pages;
+      let performanceImpl;
+      beforeEach(async () => {
+        toggleExperiment(win, 'amp-story-load-first-page-only', true);
+        pages = await createStoryWithPages(2, ['page-1', 'page-2'], false);
+        performanceImpl = new Performance(env.win);
+        env.sandbox.stub(story, 'mutateElement').callsFake((fn) => fn());
+        env.sandbox.stub(Services, 'performanceFor').returns(performanceImpl);
+      });
+
+      it('should position the active page so it preloads', async () => {
+        story.buildCallback();
+        await story.layoutCallback();
+
+        // Check page 0 is loaded with distance 0.
+        expect(pages[0].getAttribute('distance')).to.be.equal('0');
+      });
+
+      it('should not position the inactive page so it preloads before the active page is loaded', async () => {
+        const signals = new Signals();
+        pages[0].signals = () => signals;
+        story.buildCallback();
+        await story.layoutCallback();
+
+        // Check page 1 is not loaded.
+        expect(pages[1].hasAttribute('distance')).to.be.false;
+      });
+
+      it('should position the inactive page so it preloads after the active page is loaded', async () => {
+        const signals = new Signals();
+        pages[0].signals = () => signals;
+        story.buildCallback();
+        await story.layoutCallback();
+
+        // Check page 1 is not loaded.
+        expect(pages[1].hasAttribute('distance')).to.be.false;
+
+        signals.signal(CommonSignals.LOAD_END);
+        await nextTick();
+
+        // Check page 1 is loaded with distance 1.
+        expect(pages[1].getAttribute('distance')).to.be.equal('1');
+      });
+
+      it('should enable the CSI experiment', async () => {
+        const enableSpy = env.sandbox.spy(
+          performanceImpl,
+          'addEnabledExperiment'
+        );
+        story.buildCallback();
+        await story.layoutCallback();
+
+        expect(enableSpy).to.be.calledWith('amp-story-load-first-page-only');
+      });
+    });
   }
 );