en/builder/src/zrender/mixin/Draggable.js

// TODO Draggable for group
// FIXME Draggable on element which has parent rotation or scale
function Draggable() {
  this.on('mousedown', this._dragStart, this);
  this.on('mousemove', this._drag, this);
  this.on('mouseup', this._dragEnd, this); // `mosuemove` and `mouseup` can be continue to fire when dragging.
  // See [Drag outside] in `Handler.js`. So we do not need to trigger
  // `_dragEnd` when globalout. That would brings better user experience.
  // this.on('globalout', this._dragEnd, this);
  // this._dropTarget = null;
  // this._draggingTarget = null;
  // this._x = 0;
  // this._y = 0;
}

Draggable.prototype = {
  constructor: Draggable,
  _dragStart: function (e) {
    var draggingTarget = e.target; // Find if there is draggable in the ancestor

    while (draggingTarget && !draggingTarget.draggable) {
      draggingTarget = draggingTarget.parent;
    }

    if (draggingTarget) {
      this._draggingTarget = draggingTarget;
      draggingTarget.dragging = true;
      this._x = e.offsetX;
      this._y = e.offsetY;
      this.dispatchToElement(param(draggingTarget, e), 'dragstart', e.event);
    }
  },
  _drag: function (e) {
    var draggingTarget = this._draggingTarget;

    if (draggingTarget) {
      var x = e.offsetX;
      var y = e.offsetY;
      var dx = x - this._x;
      var dy = y - this._y;
      this._x = x;
      this._y = y;
      draggingTarget.drift(dx, dy, e);
      this.dispatchToElement(param(draggingTarget, e), 'drag', e.event);
      var dropTarget = this.findHover(x, y, draggingTarget).target;
      var lastDropTarget = this._dropTarget;
      this._dropTarget = dropTarget;

      if (draggingTarget !== dropTarget) {
        if (lastDropTarget && dropTarget !== lastDropTarget) {
          this.dispatchToElement(param(lastDropTarget, e), 'dragleave', e.event);
        }

        if (dropTarget && dropTarget !== lastDropTarget) {
          this.dispatchToElement(param(dropTarget, e), 'dragenter', e.event);
        }
      }
    }
  },
  _dragEnd: function (e) {
    var draggingTarget = this._draggingTarget;

    if (draggingTarget) {
      draggingTarget.dragging = false;
    }

    this.dispatchToElement(param(draggingTarget, e), 'dragend', e.event);

    if (this._dropTarget) {
      this.dispatchToElement(param(this._dropTarget, e), 'drop', e.event);
    }

    this._draggingTarget = null;
    this._dropTarget = null;
  }
};

function param(target, e) {
  return {
    target: target,
    topTarget: e && e.topTarget
  };
}

export default Draggable;