Swift的guard关键字

Swift 2引入了guard关键字，它可以用来确保各种数据配置就绪。我在这个网站上看到的一个例子演示了一个submitTapped函数:

func submitTapped() {
    guard username.text.characters.count > 0 else {
        return
    }

    print("All good")
}

我想知道使用guard与使用if条件的老式方法是否有任何不同。它是否提供了简单支票无法获得的好处?

当前回答

来自苹果文档:

保安声明

守卫语句用于在一个或多个条件不满足时将程序控制转移到作用域之外。

Synatx：

guard condition else {
    statements
}

优势:

1. 通过使用guard语句，我们可以摆脱深度嵌套的条件，这些条件的唯一目的是验证一组需求。

2. 它是专门为提前退出方法或函数而设计的。

如果你使用If，下面是它看起来的代码。

  let task = URLSession.shared.dataTask(with: request) { (data, response, error) in

        if error == nil {
            if let  statusCode = (response as? HTTPURLResponse)?.statusCode, statusCode >= 200 && statusCode <= 299 {
                if let data = data {

                    //Process Data Here.
                    print("Data: \(data)")

                } else {
                    print("No data was returned by the request!")
                }
            } else {
                print("Your request returned a status code other than 2XX!")
            }
        } else {
            print("Error Info: \(error.debugDescription)")
        }
    }
    task.resume()

如果一个或多个条件不满足，可以使用守卫将控制转移到作用域之外。

let task = URLSession.shared.dataTask(with: request) { (data, response, error) in

            /* GUARD: was there an error? */
            guard (error == nil) else {
                print("There was an error with your request: \(error)")
                return
            }

            /* GUARD: Did we get a successful 2XX response? */
            guard let statusCode = (response as? HTTPURLResponse)?.statusCode, statusCode >= 200 && statusCode <= 299 else {
                print("Your request returned a status code other than 2XX!")
                return
            }

            /* GUARD: was there any data returned? */
            guard let data = data else {
                print("No data was returned by the request!")
                return
            }

            //Process Data Here.
            print("Data: \(data)")
}
task.resume()

参考:

1. Swift 2:带着警卫早点离开 2. Udacity 3.保安声明

2016-11-17 07:14:24

其他回答

一个好处是消除了大量嵌套的if let语句。请在15:30左右观看WWDC“Swift新功能”视频，其中一节名为“末日金字塔”。

2015-06-11 21:12:06

当使用守护满足条件时，它将在守护块中声明的变量暴露给代码块的其余部分，将它们带入其作用域。如前所述，这将在嵌套的if let语句中派上用场。

注意，guard在其else语句中要求return或throw。

使用Guard解析JSON

下面是一个如何使用guard而不是if-let解析JSON对象的示例。这是一篇博客文章的节选，其中包括一个playground文件，你可以在这里找到:

如何使用守卫在Swift 2解析JSON

func parseJSONWithGuard(data : [String : AnyObject]) throws -> Developer {

    guard let firstname = data["First"] as? String  else {
        return Developer() // we could return a nil Developer()
    }

    guard let lastname = data["Last"] as? String else {
        throw ParseError.BadName // or we could throw a custom exception and handle the error
    }

    guard let website = data["WebSite"] as? String else {
        throw ParseError.BadName
    }

    guard let iosDev = data["iosDeveloper"] as? Bool else {
        throw ParseError.BadName
    }



    return Developer(first: firstname, last: lastname, site: website, ios: iosDev)

}

下载操场:守卫操场

更多信息:

以下是Swift编程语言指南的节选:

If the guard statement’s condition is met, code execution continues after the guard statement’s closing brace. Any variables or constants that were assigned values using an optional binding as part of the condition are available for the rest of the code block that the guard statement appears in. If that condition is not met, the code inside the else branch is executed. That branch must transfer control to exit the code block that that guard statement appears in. It can do this with a control transfer statement such as return, break, or continue, or it can call a function or method that doesn’t return, such as fatalError().

2015-06-11 21:55:48

这确实有两大好处需要保护。一个是避免厄运金字塔，就像其他人提到的那样——许多恼人的if语句嵌套在彼此之间，越来越向右移动。

另一个好处是你想要实现的逻辑通常是“if not let”而不是“if let {} else”。

这里有一个例子:假设你想要实现accumulate -一个map和reduce之间的交叉，它会返回一个运行的reduce数组。下面是guard:

extension Sliceable where SubSlice.Generator.Element == Generator.Element {

    func accumulate(combine: (Generator.Element,Generator.Element)->Generator.Element) -> [Generator.Element] {
        // if there are no elements, I just want to bail out and
        // return an empty array
        guard var running = self.first else { return [] }

        // running will now be an unwrapped non-optional
        var result = [running]

        // dropFirst is safe because the collection
        // must have at least one element at this point
        for x in dropFirst(self) {
            running = combine(running, x)
            result.append(running)
        }
        return result
    }

}


let a = [1,2,3].accumulate(+)  // [1,3,6]
let b = [Int]().accumulate(+)  // []

你怎么写它没有守卫，但仍然使用第一，返回一个可选?就像这样:

extension Sliceable where SubSlice.Generator.Element == Generator.Element {

    func accumulate(combine: (Generator.Element,Generator.Element)->Generator.Element) -> [Generator.Element] {

        if var running = self.first  {
            var result = [running]

            for x in dropFirst(self) {
                running = combine(running, x)
                result.append(running)
            }
            return result
        }
        else {
            return []
        }
    }

}

额外的嵌套是恼人的，而且，if和else相隔太远也不符合逻辑。让空case提前退出，然后继续函数的其余部分，就好像这是不可能的一样，可读性要高得多。

2015-06-11 22:01:59