arrays - 如何使用 Swift 递增地轮换数组？

Question

刚刚学习 swift 并且想像这样旋转一系列颜色:

class ColorSwitcher
{
    let colors:String[] = ["red", "blue", "green"]
    var currIndex:Int?

    var selectedColor:String{
        return self.colors[currIndex!]
    }

    init(){
        currIndex = 0
    }

    func changeColor()
    {
        currIndex++ //this doesn't work
    }
}

当我尝试像这样调用函数时:

var switcher:ColorSwitcher = ColorSwitcher()
switcher.selectedColor // returns red

switcher.changeColor()

switcher.selectedColor // still returns red

问题出在 changeColor 函数上。我得到的错误是:

Could not find an overload for '++' that accepts the supplied arguments

我做错了什么？

Answer 1

问题是 currIndex 是可选的。我建议像这样重构:

class ColorSwitcher {
    let colors:String[] = ["red", "blue", "green"]
    var currIndex:Int = 0

    var selectedColor:String {
        return self.colors[currIndex]
    }

    func changeColor() {
        currIndex++
    }
}

如果你想让它成为可选的，你需要这样做:

currIndex = currIndex! + 1

但这当然不安全，所以您可能应该这样做:

if let i = currIndex {
    currIndex = i + 1
}
else {
    currIndex = 1
}

此外，请记住，如果您要在 init() 中设置值，则不需要使用可选项。以下是好的:

class ColorSwitcher {
    let colors:String[] = ["red", "blue", "green"]
    var currIndex:Int

    init(startIndex: Int) {
        currIndex = startIndex
    }

    var selectedColor:String {
        return self.colors[currIndex]
    }

    func changeColor() {
        currIndex++
    }
}