拖動 SVG 元素

使用滑鼠拖動 SVG 元素（或元素組）可以通過以下方式完成：

1. 新增 mousedown 處理程式以啟動拖動：在要拖動時使用的元素上新增轉換（如果需要），跟蹤 mousemove 事件，並新增 mouseup 處理程式以結束拖動。
2. 在 mousemove 期間，將滑鼠的位置從螢幕座標轉換為你要拖動的物件的本地座標，並相應地更新轉換。
3. 在 mouseup 期間，刪除了 mousemove 和 mouseup 處理程式。

// Makes an element in an SVG document draggable.
// Fires custom `dragstart`, `drag`, and `dragend` events on the
// element with the `detail` property of the event carrying XY
// coordinates for the location of the element.
function makeDraggable(el){
  if (!el) return console.error('makeDraggable() needs an element');
  var svg = el;
  while (svg && svg.tagName!='svg') svg=svg.parentNode;
  if (!svg) return console.error(el,'must be inside an SVG wrapper');
  var pt=svg.createSVGPoint(), doc=svg.ownerDocument;

  var root = doc.rootElement || doc.body || svg;
  var xlate, txStartX, txStartY, mouseStart;
  var xforms = el.transform.baseVal;

  el.addEventListener('mousedown',startMove,false);

  function startMove(evt){
    // We listen for mousemove/up on the root-most
    // element in case the mouse is not over el.
    root.addEventListener('mousemove',handleMove,false);
    root.addEventListener('mouseup',  finishMove,false);

    // Ensure that the first transform is a translate()
    xlate = xforms.numberOfItems>0 && xforms.getItem(0);
    if (!xlate || xlate.type != SVGTransform.SVG_TRANSFORM_TRANSLATE){
      xlate = xforms.createSVGTransformFromMatrix( svg.createSVGMatrix() );
      xforms.insertItemBefore( xlate, 0 );
    }
    txStartX=xlate.matrix.e;
    txStartY=xlate.matrix.f;
    mouseStart = inElementSpace(evt);
    fireEvent('dragstart');
  }

  function handleMove(evt){
    var point = inElementSpace(evt);
    xlate.setTranslate(
      txStartX + point.x - mouseStart.x,
      txStartY + point.y - mouseStart.y
    );
    fireEvent('drag');
  }

  function finishMove(evt){
    root.removeEventListener('mousemove',handleMove,false);
    root.removeEventListener('mouseup',  finishMove,false);
    fireEvent('dragend');
  }

  function fireEvent(eventName){
    var event = new Event(eventName);
    event.detail = { x:xlate.matrix.e, y:xlate.matrix.f };
    return el.dispatchEvent(event);
  }

  // Convert mouse position from screen space to coordinates of el
  function inElementSpace(evt){
    pt.x=evt.clientX; pt.y=evt.clientY;
    return pt.matrixTransform(el.parentNode.getScreenCTM().inverse());
  }
}