Fetch more content as user is scrolling down - reactjs

Question

I have a page with a search input, once the user click on submit results come up.
There can be a lot of results and I don't want to load them all at once, how can I fetch more data into the page using Lodash throttle on mouse move?
This is my react component:
const getContacts = async (searchString) => {
const { data: contactsInfo} = await axios.get(`api/Contats/Search?contactNum=${searchString}`);
return contactsInfo;
};
export default class Home extends React.Component {
state = {
contactsInfo: [],
searchString: '',
};
handleSubmit = async () => {
const { searchString } = this.state;
const contactsInfo = await getContacts(searchString);
this.setState({ contactsInfo });
};
onInputChange = e => {
this.setState({
searchString: e.target.value,
});
};
onMouseMove = e => {
};
render() {
const { contactsInfo, searchString, } = this.state;
return (
<div css={bodyWrap} onMouseMove={e => this.onMouseMove(e)}>
<Header appName="VERIFY" user={user} />
{user.viewApp && (
<div css={innerWrap}>
<SearchInput
searchIcon
value={searchString || ''}
onChange={e => this.onInputChange(e)}
handleSubmit={this.handleSubmit}
/>
{contactsInfo.map(info => (
<SearchResultPanel
info={info}
isAdmin={user.isAdmin}
key={info.id}
/>
))}
</div>
)}
<Footer />
</div>
);
}
}

Answer 1

I supposed that, using getContacts() you retrieve ALL the contacts, and then you just want to show them at some rate, like showing the first 20, then when you reach the last one, another 20s appear.
Just asking because this is really different from "let's fetch the first 20 contacts, show them, and when the user reaches the last one, fetch another 20s".
So, if the first assumption I've made it's correct, I can raccomend you to use the Intersection Observer API https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API
This is really useful in case like yours (it's even written in the documentation "Lazy-loading of images or other content as a page is scrolled.").
The idea is that you should add this Intersection Observer, and start the observation on the last image: this observator will run a callback as soon as the last image appears on the screen (you can even decide the percentage of the image that must be on the screen).
For example, you can say that, as soon as 1px of the image appear on the screen, you add another 20s images!
Notice that, once another 20s images are shown, you must unobserve the currect observed image, and observe the new last image!
I can also suggest to not put the observer on the last image, but maybe on the third last.
EDIT: I'm not sure this answers your question. It does if I consider the title "Fetch more content as user is scrolling down", but it does not actually use mouseover (even though I think this implementation is the best one for your goal).
EDIT2: There it goes, I've added the fiddle, and here there is the codepen: https://codepen.io/Gesma94/pen/OqpOQb
Note that I've simulated the contacts with divs of different color. What is going on is that, when the third last contact (div) appear on the screen, new contacts are added in the state. Right now the contacts are just empty objects, but you can run a fetch or doing whatever you want inside fetchMoreContent(). Is this clear enough? :) I've commented the code too.
/* Just a function that create a random hex color. */
function randomColor() {
let randomColor = '#';
const letters = '0123456789ABCDEF';
for (let i = 0; i < 6; i++) {
randomColor += letters[Math.floor(Math.random() * 16)];
}
return randomColor;
}
class Home extends React.Component {
contactList = null; // Ref to the div containing the contacts.
contactObserved = null; // The contact which is observed.
intersectionObserver = null; // The intersectionObserver object.
constructor(props) {
super(props);
this.contactList = React.createRef();
this.state = {
loading: true,
contactsToShow: 0,
contacts: []
};
}
componentDidMount() {
/* Perform fetch here. I'm faking a fetch using setTimeout(). */
setTimeout(() => {
const contacts = [];
for (let i=0; i<100; i++) contacts.push({});
this.setState({loading: false, contacts, contactsToShow: 10})}, 1500);
}
componentDidUpdate() {
if (!this.state.loading) this.handleMoreContent();
}
render() {
if (this.state.loading) {
return <p>Loading..</p>
}
return (
<div ref={this.contactList}>
{this.state.contacts.map((contact, index) => {
if (index < this.state.contactsToShow) {
const color = contact.color || randomColor();
contact.color = color;
return (
<div
className="contact"
style={{background: color}}>
{color}
</div>
);
}
})}
</div>
);
}
handleMoreContent = () => {
/* The third last contact is retrieved. */
const contactsDOM = this.contactList.current.getElementsByClassName("contact");
const thirdLastContact = contactsDOM[contactsDOM.length - 3];
/* If the current third last contact is different from the current observed one,
* then the observation target must change. */
if (thirdLastContact !== this.contactObserved) {
/* In case there was a contact observed, we unobserve it and we disconnect the
* intersection observer. */
if (this.intersectionObserver && this.contactObserved) {
this.intersectionObserver.unobserve(this.contactObserved);
this.intersectionObserver.disconnect();
}
/* We create a new intersection observer and we start observating the new third
* last contact. */
this.intersectionObserver = new IntersectionObserver(this.loadMoreContent, {
root: null,
threshold: 0
});
this.intersectionObserver.observe(thirdLastContact);
this.contactObserved = thirdLastContact;
}
}
loadMoreContent = (entries) => {
entries.forEach(entry => {
if (entry.isIntersecting) {
let contactsCounter = this.state.contacts.length;
let contactsToShow = this.state.contactsToShow + 10;
if (contactsToShow > contactsToShow) contactsToShow = contactsToShow;
this.setState({contactsToShow});
}
})
}
}
ReactDOM.render(<Home />, document.getElementById('root'));
#import url(https://fonts.googleapis.com/css?family=Montserrat);
body {
font-family: 'Montserrat', sans-serif;
}
.contact {
width: 200px;
height: 100px;
border: 1px solid rgba(0,0,0,0.1);
}
.contact + .contact {
margin-top: 5px;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.4/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.4/umd/react-dom.production.min.js"></script>
<div id='root'></div>