PDF Viewer Update: Clear thumbnails that are far out of view

Discard thumbnail image data for thumbnails that are far from view to
save memory.

The top margin of 500% was selected with the assumption that every
100% height of the thumbnail bar can fit ~4 thumbnails on an average
screen. Along with the 100% bottom margin and the visible area, around
30 thumbnails worth of image data would be kept in memory at any given
time. Metrics also show that ~90% of PDFs are less than 30 pages.

Bug: 652400
Change-Id: I92126374ad715b502fd6e052246e537d65c407e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2439645Reviewed-by: dpapad <dpapad@chromium.org>
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812752}

chrome/browser/resources/pdf/elements/viewer-thumbnail-bar.js

@@ -49,26 +49,29 @@ export class ViewerThumbnailBarElement extends PolymerElement {
     /** @private {!IntersectionObserver} */
     this.intersectionObserver_ = new IntersectionObserver(entries => {
       entries.forEach(entry => {
+        const thumbnail = /** @type {!ViewerThumbnailElement} */ (entry.target);
+
         if (!entry.isIntersecting) {
-          // TODO(crbug.com/652400): Unpaint thumbnails.
+          thumbnail.clearImage();
           return;
         }
 
-        const thumbnail = /** @type {!ViewerThumbnailElement} */ (entry.target);
-        if (thumbnail.isPending()) {
+        if (thumbnail.isPainted()) {
           return;
         }
 
-        thumbnail.setPending();
+        thumbnail.setPainted();
         this.dispatchEvent(new CustomEvent(
             'paint-thumbnail',
             {detail: thumbnail, bubbles: true, composed: true}));
       });
     }, {
       root: thumbnailsDiv,
-      // The vertical root margin is set to 100% to also track thumbnails that
-      // are one standard finger swipe away.
-      rootMargin: '100% 0%',
+      // The root margin is set to 100% on the bottom to prepare thumbnails that
+      // are one standard scroll finger swipe away.
+      // The root margin is set to 500% on the top to discard thumbnails that
+      // far from view, but to avoid regenerating thumbnails that are close.
+      rootMargin: '500% 0% 100%',
     });
 
     FocusOutlineManager.forDocument(document);

chrome/browser/resources/pdf/elements/viewer-thumbnail.js

@@ -16,6 +16,9 @@ const PORTRAIT_WIDTH = 108;
 /** @type {number} */
 const LANDSCAPE_WIDTH = 140;
 
+/** @type {string} */
+const PAINTED_ATTRIBUTE = 'painted';
+
 export class ViewerThumbnailElement extends PolymerElement {
   static get is() {
     return 'viewer-thumbnail';
@@ -66,7 +69,20 @@ export class ViewerThumbnailElement extends PolymerElement {
 
     const ctx = canvas.getContext('2d');
     ctx.putImageData(imageData, 0, 0);
-    this.removeAttribute('pending');
+  }
+
+  clearImage() {
+    if (!this.isPainted()) {
+      return;
+    }
+
+    const canvas = this.getCanvas_();
+    const ctx = canvas.getContext('2d');
+    ctx.clearRect(0, 0, canvas.width, canvas.height);
+    this.removeAttribute(PAINTED_ATTRIBUTE);
+
+    // For tests
+    this.dispatchEvent(new CustomEvent('clear-thumbnail-for-testing'));
   }
 
   /** @return {!HTMLElement} */
@@ -122,12 +138,12 @@ export class ViewerThumbnailElement extends PolymerElement {
   }
 
   /** @return {boolean} */
-  isPending() {
-    return this.hasAttribute('pending');
+  isPainted() {
+    return this.hasAttribute(PAINTED_ATTRIBUTE);
   }
 
-  setPending() {
-    this.toggleAttribute('pending', true);
+  setPainted() {
+    this.toggleAttribute(PAINTED_ATTRIBUTE, true);
   }
 
   /** @private */

chrome/test/data/pdf/viewer_thumbnail_bar_test.js

@@ -18,6 +18,18 @@ function createThumbnailBar() {
   return thumbnailBar;
 }
 
+/** @return {number} */
+function getTestThumbnailBarHeight() {
+  // Create a viewer-thumbnail element to get the standard height.
+  document.body.innerHTML = '';
+  const sizerThumbnail = document.createElement('viewer-thumbnail');
+  sizerThumbnail.pageNumber = 1;
+  document.body.appendChild(sizerThumbnail);
+  // Add 24 to cover padding between thumbnails.
+  const thumbnailBarHeight = sizerThumbnail.offsetHeight + 24;
+  return thumbnailBarHeight;
+}
+
 // Unit tests for the viewer-thumbnail-bar element.
 const tests = [
   // Test that the thumbnail bar has the correct number of thumbnails and
@@ -60,15 +72,10 @@ const tests = [
     });
   },
   function testTriggerPaint() {
-    // Create a viewer-thumbnail element to get the standard height.
-    document.body.innerHTML = '';
-    const sizerThumbnail = document.createElement('viewer-thumbnail');
-    document.body.appendChild(sizerThumbnail);
-    // Add 24 to cover padding between thumbnails.
-    const thumbnailBarHeight = sizerThumbnail.offsetHeight + 24;
+    const thumbnailBarHeight = getTestThumbnailBarHeight();
 
     // Clear HTML for just the thumbnail bar.
-    const testDocLength = 4;
+    const testDocLength = 8;
     const thumbnailBar = createThumbnailBar();
     thumbnailBar.docLength = testDocLength;
 
@@ -77,6 +84,10 @@ const tests = [
     thumbnailBar.style.height = `${thumbnailBarHeight}px`;
     thumbnailBar.style.display = 'block';
 
+    // Remove any padding from the scroller.
+    const scroller = thumbnailBar.shadowRoot.querySelector('#thumbnails');
+    scroller.style.padding = '';
+
     flush();
 
     const thumbnails =
@@ -99,35 +110,52 @@ const tests = [
       });
     }
 
-    const whenRequestedPaintingFirst = [
-      paintThumbnailToPromise(thumbnails[0]),
-      paintThumbnailToPromise(thumbnails[1]),
-    ];
-
     testAsync(async () => {
+      // Only two thumbnails should be "painted" upon load.
+      const whenRequestedPaintingFirst = [
+        paintThumbnailToPromise(thumbnails[0]),
+        paintThumbnailToPromise(thumbnails[1]),
+      ];
       await Promise.all(whenRequestedPaintingFirst);
 
-      // Only two thumbnails should be pending.
+      chrome.test.assertEq(testDocLength, thumbnails.length);
+      for (let i = 0; i < thumbnails.length; i++) {
+        chrome.test.assertEq(i < 2, thumbnails[i].isPainted());
+      }
+
+      // Test that scrolling to the sixth thumbnail triggers 'paint-thumbnail'
+      // for thumbnails 3 through 7. When on the sixth thumbnail, five
+      // thumbnails above and one thumbnail below should also be painted because
+      // of the 500% top and 100% bottom root margins.
+      const whenRequestedPaintingNext = [];
+      for (let i = 2; i < 7; i++) {
+        whenRequestedPaintingNext.push(paintThumbnailToPromise(thumbnails[i]));
+      }
+      scroller.scrollTop = 5 * thumbnailBarHeight;
+      await Promise.all(whenRequestedPaintingNext);
+
+      // First seven thumbnails should be painted.
       for (let i = 0; i < thumbnails.length; i++) {
-        chrome.test.assertEq(i < 2, thumbnails[i].hasAttribute('pending'));
+        chrome.test.assertEq(i < 7, thumbnails[i].isPainted());
       }
 
-      // Test that scrolling to the bottom triggers 'paint-thumbnail' events for
-      // the remaining thumbnails.
+      // Test that scrolling down to the eighth thumbnail will clear the
+      // thumbnails outside the root margin, namely the first two. A paint
+      // should also be triggered for the eighth thumbnail.
       const whenRequestedPaintingLast = [
-        paintThumbnailToPromise(thumbnails[2]),
-        paintThumbnailToPromise(thumbnails[3]),
+        paintThumbnailToPromise(thumbnails[7]),
+        eventToPromise('clear-thumbnail-for-testing', thumbnails[0]),
+        eventToPromise('clear-thumbnail-for-testing', thumbnails[1]),
       ];
-
-      thumbnailBar.shadowRoot.querySelector('#thumbnails').scrollTop =
-          2 * thumbnailBarHeight;
+      scroller.scrollTop = 7 * thumbnailBarHeight;
       await Promise.all(whenRequestedPaintingLast);
 
-      for (const thumbnail of thumbnails) {
-        chrome.test.assertTrue(thumbnail.hasAttribute('pending'));
+      // Only first two thumbnails should not be painted.
+      for (let i = 0; i < thumbnails.length; i++) {
+        chrome.test.assertEq(i > 1, thumbnails[i].isPainted());
       }
     });
-  }
+  },
 ];
 
 chrome.test.runTests(tests);