ios - Why viewWillAppear is not call while present the other ViewController as a FormSheet -

i have 2 vc, , b, when present b formsheet type, method viewwilldisappear in not called. when in b, , press a, viewwillappear not called. how make viewwillappear in called? i've seen somewhere, din't find answer.

in b set:

protocol callbackviewwillappeardelegate: class {     func callbackviewwillappear(controller: userrightroletableviewcontroller) } 

and delegate variable:

weak var callbackviewwillappeardelegate: callbackviewwillappeardelegate? 

and in button:

@ibaction func backtouserrightbuttonaction(sender: anyobject) {         callbackviewwillappeardelegate?.callbackviewwillappear(self)         dismissviewcontrolleranimated(true, completion: nil)              } 

in import protocol , set protocol function:

func callbackviewwillappear(controller: userrightroletableviewcontroller) {         viewwillappear(true)     } 

the method viewwilldisappear not called controller because stays on view hierarchy.