为什么不能在 AppDelegate 中使用 if 语句？

Question

为什么我不能在 if 语句中传递常量？

 if UIDevice.current.orientation.isLandscape{
        let storyboard = UIStoryboard(name: "landscape", bundle: nil)
        let initialViewController = storyboard.instantiateViewController(withIdentifier: "landscape")
    }else{
        let storyboard = UIStoryboard(name: "Main", bundle: nil)
        let initialViewController = storyboard.instantiateViewController(withIdentifier: "ViewController")
    }
    

    self.window?.rootViewController = initialViewController

Answer 1

从作用域开始，您需要在 if else 块中插入错误行

if { 
  let initialViewController = //// 
  self.window?.rootViewController = initialViewController
}
else { 
  let initialViewController = //// 
  self.window?.rootViewController = initialViewController
}

或

var initialViewController:UIViewController!
if { 
  initialViewController = ////
}
else { 
  initialViewController = ////
}
// use it here

Answer 2

initialViewController仅在if else块的范围内可用，因此您需要在if else块开始之前声明initialViewController并设置initialViewController 属性在块中，稍后根据需要使用它。

let initialViewController: UIViewController
if UIDevice.current.orientation.isLandscape {
    let storyboard = UIStoryboard(name: "landscape", bundle: nil)
    initialViewController = storyboard.instantiateViewController(withIdentifier: "landscape")
} else {
    let storyboard = UIStoryboard(name: "Main", bundle: nil)
    initialViewController = storyboard.instantiateViewController(withIdentifier: "ViewController")
}
window?.rootViewController = initialViewController

注意：您可能应该对编程范围做更多研究。

Answer 3

仔细看Swift报错：在这部分代码中找不到名字为initialViewController的变量

变量仅在创建它们的块范围内可用。换句话说，如果要在 if 语句之外的其他地方使用它，则需要将 initialViewController 声明从 if 语句中移除。您可以通过将其改为 var 来做到这一点：

// Let Swift know that we’re going to make this a UIViewController
var initialViewController: UIViewController
if condition {
  // Now, initialViewController has been instantiated; it just needs a value
  initialViewController = storyboard.instantiateViewController(withIdentifier: "landscape")
} else {
  initialViewController = storyboard.instantiateViewController(withIdentifier: "portrait")
}

// We can safely use it outside of the if statement, but it may need unwrapping since it was declared as a var 
self.window?.rootViewController = initialViewController!

请注意，copy/pasting 由于我尚未定义 storyboard 或 condition 变量，因此此确切代码将不起作用：看看您是否可以更改当前代码以修复按照我在这里描述的方式发布！

为什么不能在 AppDelegate 中使用 if 语句？

Why cannot use if statement in AppDelegate?

xcode

swift

swift5

ios13