ios - "Attempt to present while already presenting"检查后还出现？

Question

 let appDelegate = UIKit.UIApplication.shared.delegate!

        if let tabBarController = appDelegate.window??.rootViewController as? UITabBarController {
            let storyboard = UIStoryboard.init(name: "Main", bundle: nil)
            let signInVC   = storyboard.instantiateViewController(withIdentifier: "SignInVC") as! SignInVC

            guard !signInVC.isBeingPresented else {
                log.warning("Attempt to present sign in sheet when it is already showing")
                return
            }

            signInVC.modalPresentationStyle = UIModalPresentationStyle.formSheet

            tabBarController.present(signInVC, animated: true, completion: nil)
        }

尽管出现了 signInVC，此代码仍可被多次调用。我已经添加了这张支票:

 guard !signInVC.isBeingPresented else {
     log.warning("Attempt to present sign in sheet when it is already showing")
     return
 }

但它似乎并不能防止这个错误:

Warning: Attempt to present <App.SignInVC: 0x101f2f280>  on <UITabBarController: 0x101e05880> which is already presenting <App.SignInVC: 0x101f4e4c0>

Answer 1

你的 guard 不是有效的支票。 isBeingPresented 在尚未呈现的全新 View Controller 实例上被调用。所以 isBeingPresented 将始终为 false。除此之外，该属性只能在 View Controller 的 view[Will|Did]Appear 方法中使用。

您要检查的是 tabBarController 是否已经呈现另一个 View Controller 。

最后，仅在应显示时创建和设置登录 View Controller 。

let appDelegate = UIKit.UIApplication.shared.delegate!

if let tabBarController = appDelegate.window?.rootViewController as? UITabBarController {
    if tabBarController.presentedViewController == nil {
        let storyboard = UIStoryboard.init(name: "Main", bundle: nil)
        let signInVC   = storyboard.instantiateViewController(withIdentifier: "SignInVC") as! SignInVC
        signInVC.modalPresentationStyle = UIModalPresentationStyle.formSheet

        tabBarController.present(signInVC, animated: true, completion: nil)
    }
}