运行

github地址
https://github.com/STRML/react-grid-layout

下载完代码之后，
npm install
npm run dev
首先让代码运行起来

lib/components/WidthProvider.jsx 解读

// @flow
import React from "react";
import PropTypes from "prop-types";
import ReactDOM from "react-dom";
import type { ComponentType as ReactComponentType } from "react";

type WPProps = {
  className?: string,
  measureBeforeMount: boolean,
  style?: Object
};

type WPState = {
  width: number
};

/*
 * A simple HOC that provides facility for listening to container resizes.
 */
// typescript  类型检查
export default function WidthProvider<
  Props,
  ComposedProps: { ...Props, ...WPProps }
>(
  ComposedComponent: ReactComponentType<Props>
): ReactComponentType<ComposedProps> {
  return class WidthProvider extends React.Component<ComposedProps, WPState> {
    static defaultProps = {
      measureBeforeMount: false
    };

    static propTypes = {
      // If true, will not render children until mounted. Useful for getting the exact width before
      // rendering, to prevent any unsightly resizing.
      measureBeforeMount: PropTypes.bool
    };

    state = {
      width: 1280
    };

    mounted: boolean = false;

    componentDidMount() {
      this.mounted = true;

      window.addEventListener("resize", this.onWindowResize);
      // Call to properly set the breakpoint and resize the elements.
      // Note that if you're doing a full-width element, this can get a little wonky if a scrollbar
      // appears because of the grid. In that case, fire your own resize event, or set `overflow: scroll` on your body.
      this.onWindowResize();
    }

    componentWillUnmount() {
      this.mounted = false;
      window.removeEventListener("resize", this.onWindowResize);
    }

    onWindowResize = () => {
      if (!this.mounted) return;
      // eslint-disable-next-line
      const node = ReactDOM.findDOMNode(this); // Flow casts this to Text | Element
      if (node instanceof HTMLElement)
        this.setState({ width: node.offsetWidth });
    };

    render() {
      const { measureBeforeMount, ...rest } = this.props;
      if (measureBeforeMount && !this.mounted) {
        return (
          <div className={this.props.className} style={this.props.style} />
        );
      }

      return <ComposedComponent {...rest} {...this.state} />;
    }
  };
}

一个特别简单的高级函数，主要作用是绑定onresize事件，当组件改变的时候，重新计算元素的宽度
最主要的是这里

onWindowResize = () => {
      if (!this.mounted) return;
      // eslint-disable-next-line
      const node = ReactDOM.findDOMNode(this); // Flow casts this to Text | Element
      if (node instanceof HTMLElement)
        this.setState({ width: node.offsetWidth });
    };

lib/GridItem.jsx 解读

/**
   * Return position on the page given an x, y, w, h.
   * left, top, width, height are all in pixels.
   * @param  {Number}  x             X coordinate in grid units.
   * @param  {Number}  y             Y coordinate in grid units.
   * @param  {Number}  w             W coordinate in grid units.
   * @param  {Number}  h             H coordinate in grid units.
   * @return {Object}                Object containing coords.
   */
  calcPosition(
    x: number,
    y: number,
    w: number,
    h: number,
    state: ?Object
  ): Position {
    const { margin, containerPadding, rowHeight } = this.props;
    const colWidth = this.calcColWidth();

    const out = {
      left: Math.round((colWidth + margin[0]) * x + containerPadding[0]),
      top: Math.round((rowHeight + margin[1]) * y + containerPadding[1]),
      // 0 * Infinity === NaN, which causes problems with resize constraints;
      // Fix this if it occurs.
      // Note we do it here rather than later because Math.round(Infinity) causes deopt
      width:
        w === Infinity
          ? w
          : Math.round(colWidth * w + Math.max(0, w - 1) * margin[0]),
      height:
        h === Infinity
          ? h
          : Math.round(rowHeight * h + Math.max(0, h - 1) * margin[1])
    };

    if (state && state.resizing) {
      out.width = Math.round(state.resizing.width);
      out.height = Math.round(state.resizing.height);
    }

    if (state && state.dragging) {
      out.top = Math.round(state.dragging.top);
      out.left = Math.round(state.dragging.left);
    }

    return out;
  }

根据x，y计算出位置