carbon/components/ReadFileDropContainer.js

import React from 'react'
import { DropTarget } from 'react-dnd'
import { NativeTypes } from 'react-dnd-html5-backend'

const spec = {
  drop (props, monitor) {
    const bundle = monitor.getItem()
    const file = bundle.files[0]
    const reader = new FileReader()
    reader.onload = event => props.onDrop(event.target.result)
    reader.readAsText(file, 'UTF-8');
  }
}

const collect = (connect, monitor) => ({ connectDropTarget: connect.dropTarget(), isOver: monitor.isOver() })

class ReadFileDropContainer extends React.Component {
  constructor(props) {
    super(props)
    this.state = { isFileOver: false }
  }

  componentWillReceiveProps(nextProps) {
    if (!this.props.isOver && nextProps.isOver) {
      this.setState({ isFileOver: true })
    }

    if (this.props.isOver && !nextProps.isOver) {
      this.setState({ isFileOver: false })
    }
  }

  render() {
    const content = (
      <div className="dnd-container">
        { this.state.isFileOver ? <div className="dnd-overlay">Drop your file here to import</div> : null }
        { this.props.children }
        <style jsx>{`
          .dnd-container {
            position: relative;
          }

          .dnd-overlay {
            display: flex;
            align-items: center;
            justify-content: center;
            text-align: center;
            width: 100%;
            height: 100%;
            z-index: 999;
            position: absolute;
            top: 0;
            left: 0;
            background: rgba(0, 0, 0, 0.85);
          }
        `}</style>
      </div>
    )

    return this.props.connectDropTarget(content)
  }
}

export default DropTarget(NativeTypes.FILE, spec, collect)(ReadFileDropContainer)