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
其他回答
与if不同，guard创建的变量可以从其块外访问。打开大量的可选项是有用的。
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
和if语句一样，guard也根据表达式的布尔值执行语句。与if语句不同，guard语句只在条件不满足时运行。您可以将守卫更像Assert，但是可以优雅地退出，而不是崩溃。
请参考: http://ericcerney.com/swift-guard-statement/
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().
用保护我们的意图是明确的。如果这个特定条件不满足，我们就不想执行其余的代码。 在这里我们也可以扩展链，请看看下面的代码:
guard let value1 = number1, let value2 = number2 else { return }
// do stuff here
