SVG Scripting Dragging SVG Elements

Example

Using the mouse to drag an SVG element (or group of elements) can be accomplished by:

1. Adding mousedown handler to starts the drag: adding a translation on the element to use during dragging (if needed), tracking mousemove events, and adding a mouseup handler to end the drag.
2. During mousemove, transforming the position of the mouse from screen coordinates into the local coordinates for the object you are dragging, and update the translation accordingly.
3. During mouseup, removing the mousemove and mouseup handlers.

// 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());
  }
}