如何将来自 QR 码扫描仪的字符串值分配给另一个 class 上的另一个变量

Question

我在将二维码扫描仪的字符串值分配给另一个 class 上的变量时遇到问题。

扫描二维码
二维码中的字符串需要赋给另一个class
对 class 的 viewController

所以我在另一个 class 上创建了一个初始化变量 id 并将其分配给 temp 正如你所看到的但是仍然没有将 temp 分配给 id 变量并且当我执行代码时变量 id 出现错误 nil...?

Class：QRScannerController

            if metadataObj.stringValue != nil {
            
            let destVC = MonitorimiViewController(temp: metadataObj.stringValue!)
            destVC.id = metadataObj.stringValue!
            
            do {
                
                guard let tabBarController = self.storyboard?.instantiateViewController(withIdentifier: "tabBarController") as? UITabBarController else {
                    return
                }
                
                (tabBarController.viewControllers![0] as? MonitorimiViewController)?.id = metadataObj.stringValue!
                tabBarController.modalTransitionStyle = .crossDissolve
                tabBarController.modalPresentationStyle = .custom
                self.present(tabBarController, animated: true, completion: nil)
            }
        }
        captureSession.stopRunning()
    }

Class: MonitorimiViewController

init(temp : String){
    id = temp
    super.init(nibName: nil, bundle: nil)
}

required init?(coder aDecoder: NSCoder) {
    super.init(coder: aDecoder)
}


var id:String?


    func fetchAndReloadData(){
    APICaller.shared.getVehicles(for: id!) {[weak self] (result) in     // error nil value at id! 
        guard let self = self else { return }
        
        switch result {
        case .success(let vehicle):
            self.listOfVechicle = vehicle
            DispatchQueue.main.async {
                self.monitorimiTableView.reloadData()
            }
        case .failure(let error):
            print(error)
        }
    }
}

如有任何疑问，请随时提出，因为我已经被这个问题困扰好几天了:(

Answer 1

如果自己不会debug（以后积累经验），千万不要写这种代码：

(tabBarController.viewControllers![0] as?MonitorimiViewController)?.id = metadataObj.stringValue!

所以，它说的是什么：

tabBarController.viewControllers!: 如果没有 viewControllers: crash.

(... as? SomeClass)：如果不是那个class（或者符合协议，或者子class等），转换就会失败，并且会nil.

所以在你的情况下，它是：

(someNilValue)?.id = someId

所以，什么也得不到 someId。

如何自己知道：

guard let viewcontrollers = tabBarController.viewControllers else {
    print("tabBarController.viewControllers is nil")
    return
}

guard let firstVC = viewControllers.first else {
    print("viewControllers is empty")
    return
}

guard let asMonitorimiVC = firstVC as? MonitorimiViewController else {
    print("firstVC is NOT a MonitorimiViewController instance: \(firstVC)") //by default, you should see its type in console, by you could also use type(of:)
    return
}

asMonitorimiVC.id = someId

你的应该会在 guard let asMonitorimiVC 失败，因为 firstVC 是 UINavigationController.

guard let asNavigationVC = firstVC as? UINavigationController else {
    print("firstVC is NOT a UINavigationController instance: \(firstVC)") //by default, you should see its type in console, by you could also use type(of:)
    return
}

guard let firtVCFromNavVC = asNavigationVC.viewcontrollers.first else {
    print("asNavigationVC.viewcontrollers is empty") //But that shouldn't be the case, I used viewcontrollers, but you could also use topViewController in this case
    return
}

guard let asMonitorimiVC = firtVCFromNavVC as? MonitorimiViewController else {
    print("firtVCFromNavVC is NOT a MonitorimiViewController instance: \(firstVC)") //by default, you should see its type in console, by you could also use type(of:)
    return
}

asMonitorimiVC.id = someId

当然，这是一个冗长的解决方案，你可能想削减它一点，但在调试时，不要犹豫，明确代码：

guard let navVC = tabBarController.viewControllers?.first as? UINavigationController else {
print("tabBarController.viewControllers is nil or empty, or the first of it is not a UINavigationController")
return 
}
guard let asMonitorimiVC = navVC.viewControllers.first as? MonitorimiViewController else {
    print("navVC.viewControllers is empty, or its first is not a is NOT a MonitorimiViewController instance: \(navVC.viewControllers.first)")
    return
}

旁注：您可能会注意到，如果您在上一个问题中回答正确 my first question in comment，我们会直接发现问题：

Is (tabBarController.viewControllers![0] as? MonitorimiViewController) nil?

因为 print("Is it nil: (tabBarController.viewControllers![0] as?MonitorimiViewController))` 应该显示它实际上是 nil，因为转换失败...

如何将来自 QR 码扫描仪的字符串值分配给另一个 class 上的另一个变量

How to assign the stringValue coming from QR code scanner to another variable on another class

xcode

ios

swift