Question

我在Swift 2中使用此代码。

extension PHFetchResult: Sequence {
     public func makeIterator() -> NSFastEnumerationIterator {
         return NSFastEnumerationIterator(self)
     }
}

自从我升级到Swift 3

通用Objective-C类的扩展无法在运行时访问类的通用参数

我不知道如何解决这个问题。非常感谢任何帮助！

Answer 1

此处报告了问题：https://bugs.swift.org/browse/SR-1576

但最终你不能在Swift 3.0中使用for in和PHFetchResult。让我们看一些例子：

let collections = PHAssetCollection.fetchAssetCollections(with: .album, subtype: .any, options: nil)
let collectionLists = PHCollectionList.fetchCollectionLists(with: .momentList, subtype: .momentListYear, options: nil)
let assets = PHAsset.fetchAssets(with: .image, options: nil)

您可以使用PHFetchResult的内置枚举（我推荐的解决方案）：

collections.enumerateObjects(_:) { (collection, count, stop) in
    //...
}
collectionLists.enumerateObjects(_:) { (collectionList, count, stop) in
    //...
}
assets.enumerateObjects(_:) { (asset, count, stop) in
    //...
}

或者通过索引访问每个对象：

for idx in 0 ..< collections.count {
    let collection = collections[idx]
    // ...
}
for idx in 0 ..< collectionLists.count {
    let collectionList = collectionLists[idx]
    // ...
}
for idx in 0 ..< assets.count {
    let asset = assets[idx]
    // ...
}

Answer 2

从Realm中拉出来，你可以通过扩展你想要符合Sequence的子类并将makeIerator函数放在那里来解决这个问题。

// Sequence conformance for ClassA is provided by ProtocolX's `makeIterator()` implementation.
extension ClassA: Sequence {}

extension ProtocolX {
    // Support Sequence-style enumeration
    public func makeIterator() -> RLMIterator {
        return RLMIterator(collection: self)
    }
}

您可以在https://github.com/realm/realm-cocoa/blob/master/Realm/Swift/RLMSupport.swift

查看完整代码

Answer 3

您可以在错误报告中使用包装类型suggested by Swift engineer Jordan Rose：

import Photos

struct ResultSequence<Element: AnyObject>: Sequence {
  var result: PHFetchResult<Element>
  init(_ result: PHFetchResult<Element>) {
    self.result = result
  }
  func makeIterator() -> NSFastEnumerationIterator {
    return NSFastEnumerationIterator(self.result)
  }
}


func test(_ request: PHFetchResult<PHCollection>) {
  for elem in ResultSequence(request) {
    print(elem)
  }
}

Answer 4

没有办法解决这个问题，您可能需要重构代码或使用其他技术。

你可以参考：

How to use a swift class with a generic type in objective c

文档：

https://developer.apple.com/library/content/documentation/Swift/Conceptual/BuildingCocoaApps/MixandMatch.html#//apple_ref/doc/uid/TP40014216-CH10-XID_78

SWIFT 3.0迁移错误 - 泛型Obj-C类的扩展无法访问该类＆＃39;运行时的泛型参数

4 个答案: