React countup animation starts immediately after the page loading , should start when scrolled to the component (without jquery)

Question

I have a react single page app, with multiple components. For the 5th component(visible only when scrolled down) I have a counter . Now I am using react-countup library to achieve the counter function. However , the counter starts soon as the page is loaded . Is it possible for countup to begin once we scroll down to the component. Animation happens only once(which is good)after the page is loaded, but I would like the counter not to begin soon after the page is loaded, but when user scrolls down to the component the first time. My code looks like this:

    render() {
         return (
         <div className={style.componentName}>
         <h2>Heading</h2>
         <div className={style.col}>
         <div>My counter</div>
         <CountUp className={style.countup} decimals={1} start={0} end={25} suffix=" %" duration={3} />
        </div>
        </div>)}

Updated code:

    import CountUp, { startAnimation } from 'react-countup';
    import VisibilitySensor from 'react-visibility-sensor';

    class className extends Component {

        state = {
            scrollStatus: true
        };


        onVisibilityChange = isVisible => {
            if (isVisible) {
                if (this.state.scrollStatus) {
                    startAnimation(this.myCountUp);
                    this.setState({ scrollStatus: false });
                }
            }
        }
    render() {
             return (
            <div className={style.componentName}>
             <h2>Heading</h2>
             <VisibilitySensor onChange={this.onVisibilityChange} offset = {{ top: 
              10}} delayedCall>
             <CountUp className={style.countup} decimals={1} start={0} end={25} 
             suffix=" %" duration={3} ref={countUp => { this.myCountUp= countUp;}}/>
             </VisibilitySensor>
            </div>)}
}

Answer 1

The API may have changed since last year. I manage to make this work with this code now :

import React from "react";
import CountUp from "react-countup";
import VisibilitySensor from 'react-visibility-sensor';

const MyComponent = () => (
  <>
    <CountUp end={100} redraw={true}>
        {({ countUpRef, start }) => (
            <VisibilitySensor onChange={start} delayedCall>
                <span ref={countUpRef} />
            </VisibilitySensor>
        )}
    </CountUp>
  </>
);

export default App;

I use this component inside a tab, so the redraw={true} prop is only here to redraw the animation on tabChange.

Answer 2

Per React CountUp's README, you can use the startAnimation hook to manually kick off the animation. Combine this with something like react-visibility-sensor, and you can wait to kick off the animation until it is visible in the user's browser.

import React, {Component} from 'react';
import CountUp, {startAnimation} from 'react-countup';
import './App.css';
import VisibilitySensor from 'react-visibility-sensor';

const style = {
  componentName: {},
  col: {},
  countup: {},
};

class App extends Component {
  constructor(props) {
    super(props);
    this.onVisibilityChange = this.onVisibilityChange.bind(this); // Bind for appropriate 'this' context
  }

  onVisibilityChange(isVisible) {
    if (isVisible) {
      startAnimation(this.myCountUp);
    }
  }

  render() {
    return (
      <div className={style.componentName}>
        <h2>Heading</h2>
        <div className={style.col}>
          <div>My counter</div>
          <VisibilitySensor
            onChange={this.onVisibilityChange}
            delayedCall // Prevents react apps triggering elements as visible before styles are loaded
          >
            <CountUp className={style.countup} decimals={1} start={0} end={25} suffix=" %" duration={3}
                     ref={countUp => { this.myCountUp = countUp; }} // From react-countup README 
            />
          </VisibilitySensor>
        </div>
      </div>
    );
  }
}

export default App;

As is, it will startAnimation every time you scroll to the countup. If you want to only do that once, just add a piece of state that gets set after the first render (and then prevent it from doing startAnimation again based on that altered state).

Less elegant (not recommended) ways to accomplish the same effect might include:

• Use the built-in animation triggers (i.e. changing the props duration, end, start) by setting them equal to some state that changes when the user scrolls down
• Leveraging the onStart prop, called before the animation starts, to delay starting the animation until the user scrolls down

EDIT: Update to address your second question

Unfortunately, it looks like the react-countup library doesn't expose a way to prevent startAnimation on startup.

But we can hack together a fairly elegant fix by manipulating the end prop using state instead:

import React, {Component} from 'react';
import CountUp, {startAnimation} from 'react-countup';
import './App.css';
import VisibilitySensor from 'react-visibility-sensor';

const style = {
  componentName: {},
  col: {},
  countup: {},
};

class App extends Component {
  state = {
    didViewCountUp: false
  };


  onVisibilityChange = isVisible => {
    if (isVisible) {
      this.setState({didViewCountUp: true});
    }
  }

  render() {
    return (
      <div className={style.componentName}>
        <h2 style={{fontSize: '40em'}}>Heading</h2>
        <VisibilitySensor onChange={this.onVisibilityChange} offset={{
          top:
            10
        }} delayedCall>
          <CountUp className={style.countup} decimals={1} start={0} end={this.state.didViewCountUp ? 25 : 0}
                   suffix=" %" duration={3} />
        </VisibilitySensor>
      </div>)
  }
}

export default App;