ios Swift if 语句 - 多个条件以逗号分隔？

Swift if statement - multiple conditions separated by commas?

提问by Marcus Leon

Looking at a Swift example:

看一个 Swift示例：

if let sourceViewController = sender.sourceViewController as? MealViewController, meal = sourceViewController.meal {
    ...
}

The doc states:

该文档指出：

... the code assigns that view controller to the local constant sourceViewController, and checks to see if the meal property on sourceViewController is nil.

...代码将该视图控制器分配给本地常量 sourceViewController，并检查 sourceViewController 上的 meal 属性是否为零。

Question: Does Swift let you have multiple conditions in your if statement when separated by commas (as in this example with the comma after MealViewController)?

问题：当用逗号分隔时，Swift 是否允许您在 if 语句中有多个条件（如本示例中的 后有逗号MealViewController）？

Haven't seen this in the docs.

在文档中没有看到这个。

回答by Luca Angeletti

Yes when you write

是的，当你写

if let a = optA, let b = optB, let c = optC {

}

Swift does execute the body of the IFonly if all the assignments are properly completed.

IF只有当所有的分配都正确完成时，Swift 才会执行 的主体。

More

更多的

Another feature of this technique: the assignments are done in order.

这种技术的另一个特点是：分配是按顺序完成的。

So only if a value is properly assigned to a, Swift tries to assign a value to b. And so on.

因此，只有当一个值被正确分配给 时a，Swift 才会尝试为 分配一个值b。等等。

This allows you to use the previous defined variable/constant like this

这允许您像这样使用先前定义的变量/常量

if let a = optA, let b = a.optB {

}

In this case (in second assignment) we are safely using abecause we knowthat if that code is executed, then ahas been populated with a valid value.

在这种情况下（在第二次赋值中），我们可以安全地使用，a因为我们知道如果执行该代码，a则已填充了有效值。

回答by Joachim Isaksson

Yes. Swift: Documentation: Language Guide: The Basics: Optional Bindingsays:

是的。Swift：文档：语言指南：基础知识：可选绑定说：

You can include as many optional bindings and Boolean conditions in a single ifstatement as you need to, separated by commas. If any of the values in the optional bindings are nilor any Boolean condition evaluates to false, the whole ifstatement's condition is considered to be false. The following ifstatements are equivalent:

if let firstNumber = Int("4"), let secondNumber = Int("42"), firstNumber < secondNumber && secondNumber < 100 {
    print("\(firstNumber) < \(secondNumber) < 100")
}   
// Prints "4 < 42 < 100"

if let firstNumber = Int("4") {
    if let secondNumber = Int("42") {
        if firstNumber < secondNumber && secondNumber < 100 {
            print("\(firstNumber) < \(secondNumber) < 100")
        }   
    }   
}   
// Prints "4 < 42 < 100"

您可以根据需要在单个if语句中包含任意数量的可选绑定和布尔条件，并用逗号分隔。如果可选绑定中的nil任何值是或任何布尔条件的计算结果为false，则整个if语句的条件被视为false。以下if语句是等效的：

if let firstNumber = Int("4"), let secondNumber = Int("42"), firstNumber < secondNumber && secondNumber < 100 {
    print("\(firstNumber) < \(secondNumber) < 100")
}   
// Prints "4 < 42 < 100"

if let firstNumber = Int("4") {
    if let secondNumber = Int("42") {
        if firstNumber < secondNumber && secondNumber < 100 {
            print("\(firstNumber) < \(secondNumber) < 100")
        }   
    }   
}   
// Prints "4 < 42 < 100"