ios - defer 语句的不同行为

Question

我在注销按钮上有代码，我在其中使用 defer 语句。

我想知道何时更改操作方法范围内的 defer 语句代码的位置。

1. 我在方法末尾添加了 defer 语句，它向我显示警告。

'defer' statement before end of scope always executes immediately; replace with 'do' statement to silence this warning

代码:

override func sendButtonTapped(sender: Any) {

    self.deleteCoreData()
    self.clearUserDefaults()

    // Clear view context
    AppDelegate.shared.persistentContainer.viewContext.reset()

    ....
    ....

    // Call after all code execution completed in this block's Scope
    defer {
        // Set isUserLoggedIn and change root view controller.
        UserDefaults.Account.set(false, forKey: .isUserLoggedIn)
        AppDelegate.shared.setRootViewController()
    }
}

然后，我在方法开始处添加了 defer 语句，它什么也没显示。

代码:

override func sendButtonTapped(sender: Any) {

    // Call after all code execution completed in this block's Scope
    defer {
        // Set isUserLoggedIn and change root view controller.
        UserDefaults.Account.set(false, forKey: .isUserLoggedIn)
        AppDelegate.shared.setRootViewController()
    }

    self.deleteCoreData()
    self.clearUserDefaults()

    // Clear view context
    AppDelegate.shared.persistentContainer.viewContext.reset()

    ....
    ....
}

谁能解释一下 defer 语句到底发生了什么？

Answer 1

总而言之，defer 语句将在您所在的范围末尾执行。( .apple doc : https://docs.swift.org/swift-book/ReferenceManual/Statements.html#grammar_defer-statement )

来自苹果文档

func f() {
    defer { print("First defer") }
    defer { print("Second defer") }
    print("End of function")
}
f()
// Prints "End of function"
// Prints "Second defer"
// Prints "First defer"

defer 语句允许您定义一个操作，该操作将在您想要完成的其余操作之后执行，即在结束时执行>范围。

警告也非常明确，考虑到您将 defer 语句放在作用域的末尾，它没有任何作用:

func f() {
    print("TIC")
    defer { print("TAC") } // will be print at the end of the function
}
f()
// Prints "TIC"
// Prints "TAC""

这与:

完全相同

func f() {
    print("TIC")
    print("TAC") // no defer, same result
}
f()
// Prints "TIC"
// Prints "TAC""

---

走得更远

那么为什么警告会建议您使用 do block ？ 实际上，前面的两个示例并不是 100% 相同，当您使用 defer 语句时，它会创建自己的范围

func f() {
    // here you are in the scope of the `f()` function
    print("TIC")
    defer { 
        // here you are the scope of the `defer` statement
        print("First defer") 
    }
}

手动创建作用域的最接近方法是 do 语句

func f() {
    // here you are in the scope of the `f()` function
    print("TIC")
    do { 
        // here you are the scope of the `do` statement
        print("First defer") 
    }
}

来自苹果文档

The do statement is used to introduce a new scope and can optionally contain one or more catch clauses, which contain patterns that match against defined error conditions. Variables and constants declared in the scope of a do statement can be accessed only within that scope.

如果您想了解有关范围的更多信息，这里有一些讲座:https://andybargh.com/lifetime-scope-and-namespaces-in-swift/

Essentially, an objects scope defines the areas of our program from which the item can be accessed.