VC专场生命周期
1、在显示新vc时, 旧vc的disappear必定调用
不论是push, pop, present modal,dimiss modal 全部适应这一个规则.
A-》B push / pop
A willDisappear -〉 B willAppear -》A DidDisappear -〉 B DidAppear
A-》B present modal / dimiss modal
A willDisappear -〉 B willAppear -》B DidAppear -〉A DidDisappear