can add drag limit?

[fashman]

is there anyway to limit drag at the same parent?
ps

tree = {
  module: 'tree',
  children: [
    { 
      module: 'parentA',
      children: [
        { module: 'a1'},
        { module: 'a2'}
      ]
    },
    { 
      module: 'parentB',
      children: [
        { module: 'b1'},
        { module: 'b2'}
      ]
    }
  ]
}

parentA's children can only drag in parentA;
can add drag limit ? such as beforeDrag callback to stop dragging.