Swift 2之guard语法

[v5tech]

语法介绍

Guard Statement

guard condition else {
    statements
    return
}

示例

func printArea(x: Int, y: Int) {
    // Validate that "x" argument is valid.
    guard x >= 1 else {
        print("X invalid")
        return
    }
    // Validate "y" argument.
    guard y >= 1 else {
        print("Y invalid")
        return
    }
    // Print area.
    let area = x * y
    print(area)
}

// Call printArea.
printArea(5, y: 10)
// Use invalid "X" then invalid "Y" arguments.
printArea(0, y: 1)
printArea(2, y: 0)

Output

50
X invalid
Y invalid

错误用法

• else语句块缺失return, break,continue, throw,fatalError()等语句

func test(size: Int) {
    guard size >= 10 else {
        print(1)
    }
    print(2)
}

Output

'guard' body may not fall through, consider using 'return' or 'break' to exit the scope

• 缺失else关键字

func test(size: Int) {
    guard size >= 10 {
        print(1)
        return
    }
    print(2)
}

Output

Expected 'else' after 'guard' condition

Guard Optional Binding Statement

guard let constantName = someOptional else {
    statements
    return
}

示例

func greet(person: [String: String]) {
    guard let name = person["name"] else {
        return
    }
    print("Hello \(name)!")
    guard let location = person["location"] else {
        print("I hope the weather is nice near you.")
        return
    }
    print("I hope the weather is nice in \(location).")
}

greet(["name": "John"])
// prints "Hello John!"
// prints "I hope the weather is nice near you."

greet(["name": "Jane", "location": "Cupertino"])
// prints "Hello Jane!"
// prints "I hope the weather is nice in Cupertino."

注意

guard语句结构中的else不可缺失，在else子句中可以使用return, break,continue, throw,fatalError()等。当guard语句条件表达式不为true时执行else语句块中的代码。

参考

http://nshipster.cn/guard-and-defer/
http://radex.io/swift/guard/
http://stackoverflow.com/questions/30791488/swift-2-guard-keyword
https://www.hackingwithswift.com/new-syntax-swift-2-guard
http://ericcerney.com/swift-guard-statement/
http://www.codingexplorer.com/the-guard-statement-in-swift-2/
http://stackoverflow.com/questions/32256834/swift-2-0-guard-vs-if-let
https://www.natashatherobot.com/swift-guard-better-than-if/
https://www.codebeaulieu.com/40/How-to-use-Guard-in-Swift-2-to-parse-JSON
http://www.appcoda.com/swift2/
http://www.dotnetperls.com/guard-swift
http://www.dotnetperls.com/keyword-swift