ios - Swift - 重点从何而来？

Question

我很难理解下面的简单函数。神秘的“点”从何而来？

typealias Position = CGPoint
typealias Distance = CGFloat
typealias Region = Position -> Bool


func circle(radius: Distance) -> Region {
    return { point in
        sqrt(point.x * point.x + point.y * point.y) <= radius
    }
}

Answer 1

circle 函数返回一个包含以下声明的闭包:

(Position) -> Bool

point 是传递给闭包的参数名称，Position 类型 - 也许重写函数如下更清楚:

func circle(radius: Distance) -> Region {
    let closure: Region = { (point: Position) -> Bool in
        return sqrt(point.x * point.x + point.y * point.y) <= radius
    }

    return closure
}

您可以将 circle 函数的返回值赋给一个变量:

let closure = circle(1.0)

然后调用它:

let p = CGPoint(x: 1.0, y: 1.0)
closure(p)

p 参数在闭包声明中被称为(并被引用为)point