使用Swift，我有一个在viewDidLoad覆盖函数中以编程方式创建的UILabel:varmiddleDate=UILabel(frame:CGRect(x:(screenWidth/2)-45,y:0,width:90,height:20))middleDate.text="3:13:46PM"middleDate.font=UIFont(name:"ArialMT",size:13)middleDate.textAlignment=.Center效果很好。然后我尝试更改覆盖函数viewDidAppear中的文本。我收到构建失败消息Useofunresolvedidentifi