Good question.

There's no built-in method for this, but you would need to do something along these lines:

var objs = canvas.getObjects().map(function(o) {

  return o.set('active', true);

});



var group = new fabric.Group(objs, {

  originX: 'center', 

  originY: 'center'

});



canvas._activeObject = null;



canvas.setActiveGroup(group.setCoords()).renderAll();

The code should be self-explanatory, and it's pretty much what's happening under the hood when you use mouse, shift+click, etc.