ios - 如何符合 MutableCollection？

Question

我找不到有关如何符合 MutableCollection 的文档. Google 在该主题上完全空白。

示例，我想为 GMSPath 添加一致性/GMSMutablePath :

import CoreLocation
import GoogleMaps

extension GMSPath: RandomAccessCollection {

    public var startIndex: Int {
        return 0
    }

    public var endIndex: Int {
        return count
    }

    public func index(before i: Int) -> Int {
        return i-1
    }

    public func index(after i: Int) -> Int {
        return i+1
    }

    public subscript(position: Int) -> CLLocationCoordinate2D {
        return coordinate(at: UInt(position))
    }
}

extension GMSMutablePath: MutableCollection {  // Error!

    public override subscript(position: Int) -> CLLocationCoordinate2D {
        get {
            return coordinate(at: UInt(position))
        }
        set {
            replaceCoordinate(at: UInt(position), with: newValue)
        }
    }
}

Error: Type 'GMSMutablePath' does not conform to a) 'MutableIndexable', b) 'MutableCollection'.

MutableCollection 的文档状态:

To add conformance to the MutableCollection protocol to your own custom collection, upgrade your type’s subscript to support both read and write access.

我做到了。

MutableCollection 继承自 MutableIndexable，文档说明:

In most cases, it’s best to ignore this protocol and use the MutableCollection protocol instead, because it has a more complete interface.

嗯？

Answer 1

这里的问题是我还遵循了 RandomAccessCollection 而不仅仅是 Collection/MutableCollection。在那种情况下，与文档所 promise 的相反，我们要做的不仅仅是提供一个下标 setter 。具体来说，切片下标需要实现。

我最终得到了以下结果。典型化是必要的，因为编译器似乎并不总能推断出它们。

 extension GMSPath: RandomAccessCollection {

    public typealias Index = Int
    public typealias Indices = DefaultRandomAccessIndices<GMSPath>

    public var startIndex: Index {
        return 0
    }

    public var endIndex: Index {
        return count
    }

    public func index(before i: Index) -> Index {
        return i-1
    }

    public func index(after i: Index) -> Index {
        return i+1
    }

    public subscript(position: Index) -> CLLocationCoordinate2D {
        return coordinate(at: UInt(position))
    }
}

extension GMSMutablePath: MutableCollection {

    public subscript(bounds: Range<Index>) -> RandomAccessSlice<GMSPath> {
        get { return .init(base: self, bounds: bounds) }
        set {
            assert(newValue.count == bounds.count)
            newValue.enumerated().forEach { self[$0] = $1 }
        }
    }

    public override subscript(position: Index) -> CLLocationCoordinate2D {
        get { return coordinate(at: UInt(position)) }
        set { replaceCoordinate(at: UInt(position), with: newValue) }
    }
}

使用 Swift 的条件默认协议(protocol)实现，我发现很难找出究竟需要实现什么。