// 实现正方形的拖拽
// 记录鼠标位置，赋予正方形position 属性 的 top 和 left
// 记录事件 mousedown mousemove mouseup

window.onload = function() {
  const DragDrop = (function() {
    let dragging = null // 记录拖拽的对象
    var diffX = 0 // 记录鼠标与正方形边缘的距离
    var diffY = 0 // 同上

    function dragEvent(event) {
      var that = event.target

      // 判断事件类型
      switch (event.type) {
        // 记录鼠标点击处的位置，与目标的位置做差值
        case 'mousedown':
          dragging = that
          diffX = event.clientX - that.offsetLeft
          diffY = event.clientY - that.offsetTop
          break

        // 实时修改正方形的position中的 top 和 left值
        case 'mousemove':
          if (dragging !== null && event.clientX >= diffX && event.clientY >= diffY) {
            dragging.style.left = event.clientX - diffX + 'px'
            dragging.style.top = event.clientY - diffY + 'px'
          }
          break

        // 将拖拽对象置为 null
        case 'mouseup':
          dragging = null
          break
      }
      // ... do something
    }

    // 返回对象
    return {
      // 触发，监听以上三个事件，监听对象document
      fire: function() {
        document.addEventListener('mousedown', dragEvent, false)
        document.addEventListener('mousemove', dragEvent, false)
        document.addEventListener('mouseup', dragEvent, false)
      },
    }
  })()

  DragDrop.fire()
}