在 Swift 中为 Collection 下标中的 Index 创建变异集

Question

我正在尝试修改此扩展以获得可变性：

extension Collection where Element: Equatable {
    
    /// Returns the element at the specified index iff it is within count, otherwise nil.
    subscript (safe index: Index) -> Element? {
            indices.contains(index) ? self[index] : nil
    }
}

我想实现这个工作：

var array = ["Hello", "World"]
array[safe: 5] = "Other word" // This will not be setted because index is out of bounds, but error won't be throwed.

当我尝试修改下标扩展时...

extension Collection where Element: Equatable {
    
    /// Returns the element at the specified index iff it is within count, otherwise nil.
    subscript (safe index: Index) -> Element? {
        get {
            indices.contains(index) ? self[index] : nil
        }
        mutating set {
            if indices.contains(index) {
                self[index] = newValue        <<<-- ERROR
            }
        }
    }
}

...我收到此错误：Missing argument label 'safe:' in subscript

Answer 1

问题是 Collection 没有可变下标 setter。为此，您需要改为扩展 MutableCollection。

extension MutableCollection where Element: Equatable {
    /// Returns the element at the specified index iff it is within count, otherwise nil.
    subscript(safe index: Index) -> Element? {
        get {
            indices.contains(index) ? self[index] : nil
        }
        mutating set {
            if indices.contains(index), let value = newValue {
                self[index] = value
            }
        }
    }
}

Answer 2

Collection 的 subscript 是只获取的。

MutableCollection is the protocol that Array conforms to, that declares the settable subscript 我们都知道并喜爱。毕竟，您的扩展程序要求集合是可变的，并且并非所有 Collection 都是可变的，只有 MutableCollection 是可变的。

extension MutableCollection {
    
    /// Returns the element at the specified index iff it is within count, otherwise nil.
    subscript (safe index: Index) -> Element? {
        get {
            indices.contains(index) ? self[index] : nil
        }
        mutating set {
            if indices.contains(index), let value = newValue {
                self[index] = value
            }
        }
    }
}

请注意，我还在 setter 中为 newValue == nil 的情况添加了检查（因为下标的类型为 Element?，您可以将 nil 分配给它！）。在那种情况下，下标不会做任何事情。也不需要 Element : Equatable 约束。

从技术上讲，这也可以在 RangeReplaceableCollections 上完成，这是 Array 遵守的另一种协议。但是下标应该是O(1)时间，而replaceSubrange不一定如此。

extension RangeReplaceableCollection {
    
    /// Returns the element at the specified index iff it is within count, otherwise nil.
    subscript (safe index: Index) -> Element? {
        get {
            indices.contains(index) ? self[index] : nil
        }
        mutating set {
            if indices.contains(index), let value = newValue {
                self.replaceSubrange(index...index), with: CollectionOfOne(value))
            }
        }
    }
}

在 Swift 中为 Collection 下标中的 Index 创建变异集

Create mutating set for Index in Collection subscript in Swift

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