1 files changed, 130 insertions, 0 deletions
diff --git a/app/javascript/mastodon/components/intersection_observer_article.jsx b/app/javascript/mastodon/components/intersection_observer_article.jsx
new file mode 100644
index 000000000..77957a21d
--- /dev/null
+++ b/app/javascript/mastodon/components/intersection_observer_article.jsx
@@ -0,0 +1,130 @@
+import React from 'react';
+import PropTypes from 'prop-types';
+import scheduleIdleTask from '../features/ui/util/schedule_idle_task';
+import getRectFromEntry from '../features/ui/util/get_rect_from_entry';
+
+// Diff these props in the "unrendered" state
+const updateOnPropsForUnrendered = ['id', 'index', 'listLength', 'cachedHeight'];
+
+export default class IntersectionObserverArticle extends React.Component {
+
+  static propTypes = {
+    intersectionObserverWrapper: PropTypes.object.isRequired,
+    id: PropTypes.oneOfType([PropTypes.string, PropTypes.number]),
+    index: PropTypes.oneOfType([PropTypes.string, PropTypes.number]),
+    listLength: PropTypes.oneOfType([PropTypes.string, PropTypes.number]),
+    saveHeightKey: PropTypes.string,
+    cachedHeight: PropTypes.number,
+    onHeightChange: PropTypes.func,
+    children: PropTypes.node,
+  };
+
+  state = {
+    isHidden: false, // set to true in requestIdleCallback to trigger un-render
+  };
+
+  shouldComponentUpdate (nextProps, nextState) {
+    const isUnrendered = !this.state.isIntersecting && (this.state.isHidden || this.props.cachedHeight);
+    const willBeUnrendered = !nextState.isIntersecting && (nextState.isHidden || nextProps.cachedHeight);
+    if (!!isUnrendered !== !!willBeUnrendered) {
+      // If we're going from rendered to unrendered (or vice versa) then update
+      return true;
+    }
+    // If we are and remain hidden, diff based on props
+    if (isUnrendered) {
+      return !updateOnPropsForUnrendered.every(prop => nextProps[prop] === this.props[prop]);
+    }
+    // Else, assume the children have changed
+    return true;
+  }
+
+  componentDidMount () {
+    const { intersectionObserverWrapper, id } = this.props;
+
+    intersectionObserverWrapper.observe(
+      id,
+      this.node,
+      this.handleIntersection,
+    );
+
+    this.componentMounted = true;
+  }
+
+  componentWillUnmount () {
+    const { intersectionObserverWrapper, id } = this.props;
+    intersectionObserverWrapper.unobserve(id, this.node);
+
+    this.componentMounted = false;
+  }
+
+  handleIntersection = (entry) => {
+    this.entry = entry;
+
+    scheduleIdleTask(this.calculateHeight);
+    this.setState(this.updateStateAfterIntersection);
+  };
+
+  updateStateAfterIntersection = (prevState) => {
+    if (prevState.isIntersecting !== false && !this.entry.isIntersecting) {
+      scheduleIdleTask(this.hideIfNotIntersecting);
+    }
+    return {
+      isIntersecting: this.entry.isIntersecting,
+      isHidden: false,
+    };
+  };
+
+  calculateHeight = () => {
+    const { onHeightChange, saveHeightKey, id } = this.props;
+    // save the height of the fully-rendered element (this is expensive
+    // on Chrome, where we need to fall back to getBoundingClientRect)
+    this.height = getRectFromEntry(this.entry).height;
+
+    if (onHeightChange && saveHeightKey) {
+      onHeightChange(saveHeightKey, id, this.height);
+    }
+  };
+
+  hideIfNotIntersecting = () => {
+    if (!this.componentMounted) {
+      return;
+    }
+
+    // When the browser gets a chance, test if we're still not intersecting,
+    // and if so, set our isHidden to true to trigger an unrender. The point of
+    // this is to save DOM nodes and avoid using up too much memory.
+    // See: https://github.com/mastodon/mastodon/issues/2900
+    this.setState((prevState) => ({ isHidden: !prevState.isIntersecting }));
+  };
+
+  handleRef = (node) => {
+    this.node = node;
+  };
+
+  render () {
+    const { children, id, index, listLength, cachedHeight } = this.props;
+    const { isIntersecting, isHidden } = this.state;
+
+    if (!isIntersecting && (isHidden || cachedHeight)) {
+      return (
+        <article
+          ref={this.handleRef}
+          aria-posinset={index + 1}
+          aria-setsize={listLength}
+          style={{ height: `${this.height || cachedHeight}px`, opacity: 0, overflow: 'hidden' }}
+          data-id={id}
+          tabIndex={0}
+        >
+          {children && React.cloneElement(children, { hidden: true })}
+        </article>
+      );
+    }
+
+    return (
+      <article ref={this.handleRef} aria-posinset={index + 1} aria-setsize={listLength} data-id={id} tabIndex={0}>
+        {children && React.cloneElement(children, { hidden: false })}
+      </article>
+    );
+  }
+
+}