问题:

当尝试通过例如跨越 String.CharacterView.Index 索引时2 的一大步

extension String.CharacterView.Index : Strideable { }

let str = "01234"
for _ in str.startIndex.stride(to: str.endIndex, by: 2) { } // fatal error

fatal error: cannot increment endIndex

let ret = current
current += stride  // <-- here
return ret

(0..<4).startIndex.advancedBy(4) // OK, -> 4
"foo".startIndex.advancedBy(4)   // fatal error: cannot increment endIndex

...

func stride(to end: Self, by stride: Self.Stride) -> StrideTo<Self>

Returns the sequence of values (self, self + stride, self + stride + stride, ... last) where last is the last value in the progression that is less than end.

extension String.CharacterView.Index : Strideable { }

var str = "0123"

// stride by 1: all good
str.startIndex.stride(to: str.endIndex, by: 1).forEach {
    print($0,str.characters[$0])
} /* 0 0 
     1 1 
     2 2
     3 3 */

// stride by 2: OK for even number of characters in str.
str.startIndex.stride(to: str.endIndex, by: 2).forEach {
    print($0,str.characters[$0])
} /* 0 0
     2 2 */

// stride by 2: fatal error for odd number of characters in str.
str = "01234"
str.startIndex.stride(to: str.endIndex, by: 2).forEach {
    print($0,str.characters[$0])
} /* 0 0
     2 2
     fatal error: cannot increment endIndex */

public func += <T : Strideable>(inout lhs: T, rhs: T.Stride) {
  lhs = lhs.advancedBy(rhs)
}

// ... in StrideToGenerator
public mutating func next() -> Element? {
    if stride > 0 ? current >= end : current <= end {
        return nil
    }
    let ret = current
    current += stride /* <-- will increase current to larger or equal to end 
                               if stride is large enough (even if this last current
                               will never be returned in next call to next()) */
    return ret
}

(0..<4).startIndex.advancedBy(4) // OK, -> 4
"foo".startIndex.advancedBy(4)   // fatal error: cannot increment endIndex

最佳答案

简单地声明协议(protocol)一致性

extension String.CharacterView.Index : Strideable { }

extension Strideable {
    // ...
    public func stride(to end: Self, by stride: Self.Stride) -> StrideTo<Self>
}

extension String.CharacterView.Index {
    typealias Index = String.CharacterView.Index

    func stride(to end: Index, by stride: Int) -> AnySequence<Index> {

        precondition(stride != 0, "stride size must not be zero")

        return AnySequence { () -> AnyGenerator<Index> in
            var current = self
            return AnyGenerator {
                if stride > 0 ? current >= end : current <= end {
                    return nil
                }
                defer {
                    current = current.advancedBy(stride, limit: end)
                }
                return current
            }
        }
    }
}

let str = "01234"
str.startIndex.stride(to: str.endIndex, by: 2).forEach {
    print($0,str.characters[$0])
} 

0 0
2 2
4 4

关于swift - 使 String.CharacterView.Index 符合 Strideable : fatal error when using stride(to:by:): "cannot increment endIndex "，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/36205309/