当父节点选中，所有子节点也都将被选中；当先选中子节点，父节点也自动被选中；当所有子节点都取消选中后，父节点也也自动取消选中-When the parent node is selected, all child nodes also will be selected　the head select child nodes, parent nodes are automatically selected　When all child nodes are unchecked, the parent node is also automatically uncheck