React Virtual Rendering

Implementation of a list component using virtual rendering

by guillermo Quiros

HTML

<div id="app"></div>

CSS

.viewPort {
  position: relative;
  width: 510px;
  height: 300px;
  border: solid 1px;
  overflow-y: scroll;
}

.itemContainer {
  position: absolute;
  width: 500px;
  background-color: azure;

}

.item {
  position: absolute;
  background-color: beige;
  border: solid 1px;
  width: 500px;
  text-align:center;
}

.done {
  color: rgba(0, 0, 0, 0.3);
  text-decoration: line-through;
}

React

const data = []
function createData(){
	for (let i=0;i<1000000;i++){
  	data.push({name: `Row ${i}`});
  }
}
createData();

//Item class to render each of the list rows
class Item extends React.Component{
    constructor(props){
        super(props);
    }
    render(){
        return (
        <div className="item" style={{top:this.props.top,height:this.props.itemheight}}>
                {this.props.label}
        </div>)
          
    }
}


//The list component that contains the rows and manage the virtual rendering 
// using the vertical scroll position
class Vlist extends React.Component{
    constructor(props){
        super(props);
        this.numVisibleItems=Math.trunc(300 / this.props.itemheight);
        this.state={
            start:0,
            end:this.numVisibleItems         
        }
        this.containerStyle={height:data.length * this.props.itemheight}
        
        this.scollPos=this.scollPos.bind(this)
    }

    scollPos(){
        let currentIndx=Math.trunc(this.refs.viewPort.scrollTop/this.props.itemheight)
        currentIndx=currentIndx-this.numVisibleItems>=data.length?currentIndx-this.numVisibleItems:currentIndx;
        if (currentIndx!==this.state.start){
            console.log("Redraw");
            this.setState(
                this.state={
                    start:currentIndx,
                    end:currentIndx+this.numVisibleItems>=data.length ? data.length-1:currentIndx+this.numVisibleItems
                }
            )
        }
       
    }
    
    renderRows(){
        let result=[];
        for (let i=this.state.start;i<=this.state.end;i++){
            let item=data[i];
            result.push(<Item key={i} label={item.name} top={i*this.props.itemheight} itemheight={this.props.itemheight} />);
        }
        return result;
    }
    
    render(){
        return (
        <div ref="viewPort"  className="viewPort" onScroll={this.scollPos} >
            <div className="itemContainer"...