调整光标位置的方法
let userPaddingView =UIView(frame:CGRectMake(0,0,44,self.userNameTextField.frame.height))
self.userNameTextField.leftView= userPaddingView
self.userNameTextField.leftViewMode= .Always
在左侧添加图标的方法
let userImageView =UIImageView(image:UIImage(named:"User"))
//设置图标的位置
userImageView.frame.origin=CGPointMake(10,9)
self.userNameTextField.addSubview(userImageView)