Monitoring An Element As It Enters Or Leaves the Viewport Efficiently in React

import { useInView } from "react-cool-inview";

const App = () => {
  const { observe, unobserve, inView, scrollDirection, entry } = useInView({
    threshold: 0.25, // Default is 0
    onChange: ({ inView, scrollDirection, entry, observe, unobserve }) => {
      // Triggered whenever the target meets a threshold, e.g. [0.25, 0.5, ...]

      unobserve(); // To stop observing the current target element
      observe(); // To re-start observing the current target element
    },
    onEnter: ({ scrollDirection, entry, observe, unobserve }) => {
      // Triggered when the target enters the viewport
    },
    onLeave: ({ scrollDirection, entry, observe, unobserve }) => {
      // Triggered when the target leaves the viewport
    },
    // More useful options...
  });

  return <div ref={observe}>{inView ? "Hello, I am 🤗" : "Bye, I am 😴"}</div>;
};

import { InView } from "react-cool-inview";

const HelloText = ({ inView, observe }) => (
  <div ref={observe}>{inView ? "Hello, I am 🤗" : "Bye, I am 😴"}</div>
);

const App = () => (
  <InView unobserveOnEnter>
    <HelloText />
  </InView>
);

import { useInView } from "react-cool-inview";

const LazyImage = ({ width, height, ...rest }) => {
  const { observe, inView } = useInView({
    // Stop observe when the target enters the viewport, so the "inView" only triggered once
    unobserveOnEnter: true,
    // For better UX, we can grow the root margin so the image will be loaded before it comes to the viewport
    rootMargin: "50px",
  });

  return (
    <div className="placeholder" style={{ width, height }} ref={observe}>
      {inView && <img {...rest} />}
    </div>
  );
};

import { useState } from "react";
import { useInView } from "react-cool-inview";
import axios from "axios";

const App = () => {
  const [todos, setTodos] = useState(["todo-1", "todo-2", "..."]);
  const { observe } = useInView({
    // For better UX, we can grow the root margin so the data will be loaded earlier
    rootMargin: "50px 0px",
    // When the last item comes to the viewport
    onEnter: ({ unobserve }) => {
      // Pause observe when loading data
      unobserve();
      // Load more data
      axios.get("/todos").then((res) => {
        setTodos([...todos, ...res.todos]);
      });
    },
  });

  return (
    <div>
      {todos.map((todo, idx) => (
        <div ref={idx === todos.length - 1 ? observe : null}>{todo}</div>
      ))}
    </div>
  );
};

import { useInView } from "react-cool-inview";

const App = () => {
  const { observe, inView } = useInView({
    // Stop observe when the target enters the viewport, so the "inView" only triggered once
    unobserveOnEnter: true,
    // Shrink the root margin, so the animation will be triggered once the target reach a fixed amount of visible
    rootMargin: "-100px 0px",
  });

  return (
    <div className="container" ref={observe}>
      <div className={inView ? "fade-in" : ""}>I'm a 🍟</div>
    </div>
  );
};

import { useInView } from "react-cool-inview";

const App = () => {
  const { observe } = useInView({
    // For an element to be considered "seen", we'll say it must be 100% in the viewport
    threshold: 1,
    onEnter: ({ unobserve }) => {
      // Stop observe when the target enters the viewport, so the callback only triggered once
      unobserve();
      // Fire an analytic event to your tracking service
      someTrackingService.send("🍋 is seen");
    },
  });

  return <div ref={observe}>I'm a 🍋</div>;
};

import useInView from "react-cool-inview";

const App = () => {
  const {
    observe,
    inView,
    // vertical will be "up" or "down", horizontal will be "left" or "right"
    scrollDirection: { vertical, horizontal },
  } = useInView({
    // Scroll direction is calculated whenever the target meets a threshold
    // more trigger points the calculation will be more instant and accurate
    threshold: [0.2, 0.4, 0.6, 0.8, 1],
    onChange: ({ scrollDirection }) => {
      // We can also access the scroll direction from the event object
      console.log("Scroll direction: ", scrollDirection.vertical);
    },
  });

  return (
    <div ref={observe}>
      <div>{inView ? "Hello, I am 🤗" : "Bye, I am 😴"}</div>
      <div>{`You're scrolling ${vertical === "up" ? "⬆️" : "⬇️"}`}</div>
    </div>
  );
};

import { useInView } from "react-cool-inview";

const App = () => {
  // With Intersection Observer v2, the "inView" not only tells you the target
  // is intersecting with the root, but also guarantees it's visible on the page
  const { observe, inView } = useInView({
    // Track the actual visibility of the target
    trackVisibility: true,
    // Set a minimum delay between notifications, it must be set to 100 (ms) or greater
    // For performance perspective, use the largest tolerable value as much as possible
    delay: 100,
    onEnter: () => {
      // Triggered when the target is visible and enters the viewport
    },
    onLeave: () => {
      // Triggered when the target is visible and leaves the viewport
    },
  });

  return <div ref={observe}>{inView ? "Hello, I am 🤗" : "Bye, I am 😴"}</div>;
};

$ yarn add react-cool-inview
# or
$ npm install --save react-cool-inview