扩展Swift数组以按类型过滤元素 [英] Extend Swift Array to Filter Elements by Type

问题描述

如何将swift数组扩展为访问特定类型的成员？

How can a swift array be extended to access members of a particular type?

如果数组包含从同一个超类继承的多个类的实例，则此方法相关。理想情况下，它将适当地执行类型检查。

This is relevant if an array contains instances of multiple classes which inherit from the same superclass. Ideally it would enforce type checking appropriately.

使用 filter（_：）方法可以正常工作，但确实可以保证类型安全。例如：

Using the filter(_:) method works fine, but does enforce type safety. For example:

protocol MyProtocol { }
struct TypeA: MyProtocol { }
struct TypeB: MyProtocol { }

let myStructs:[MyProtocol] = [ TypeA(), TypeA(), TypeB() ]

let filteredArray = myStructs.filter({ $0 is TypeA })

filteredArray 包含正确的值，但是类型保持为 [MyProtocol] 而不是 [TypeA] 。我希望将最后一个替换为 letfilteredArray = myStructs.filter（{$ 0是TypeA}）！ [TypeA] 可以解决该问题，但是项目失败，并显示 EXEC_BAD_INSTRUCTION ，我不明白。也许类型转换数组是不可能的？

the filteredArray contains the correct values, but the type remains [MyProtocol] not [TypeA]. I would expect replacing the last with let filteredArray = myStructs.filter({ $0 is TypeA }) as! [TypeA] would resolve that, but the project fails with EXEC_BAD_INSTRUCTION which I do not understand. Perhaps type casting arrays is not possible?

理想情况下，此行为可以包装在数组扩展中。以下内容无法编译：

Ideally this behavior could be wrapped up in an array extension. The following doesn't compile:

extension Array {
    func objectsOfType<T:Element>(type:T.Type) -> [T] {
        return filter { $0 is T } as! [T]
    }
}

这里似乎至少有两个问题：类型约束 T：Element 似乎不起作用。我不确定基于泛型类型添加约束的正确方法是什么。我的目的是说 T 是 Element 的子类型。另外，在第3行上存在编译时错误，但这可能与传播错误相同。

Here there seem to be at least two problems: the type constraint T:Element doesn't seem to work. I'm not sure what the correct way to add a constraint based on a generic type. My intention here is to say T is a subtype of Element. Additionally there are compile time errors on line 3, but this could just be the same error propagating.

推荐答案

SequenceType 具有 flatMap（）方法，该方法用作可选过滤器：

SequenceType has a flatMap() method which acts as an "optional filter":

extension SequenceType {
    /// Return an `Array` containing the non-nil results of mapping
    /// `transform` over `self`.
    ///
    /// - Complexity: O(*M* + *N*), where *M* is the length of `self`
    ///   and *N* is the length of the result.
    @warn_unused_result
    @rethrows public func flatMap<T>(@noescape transform: (Self.Generator.Element) throws -> T?) rethrows -> [T]
}

结合了Matt的建议使用作为？代替是，您
可以将其用作

Combined with matt's suggestion to use as? instead of is you can use it as

let myStructs:[MyProtocol] = [ TypeA(), TypeA(), TypeB() ]
let filteredArray = myStructs.flatMap { $0 as? TypeA }

现在 filteredArray 的类型为推断为 [TypeA] 。

作为扩展方法，应为

extension Array {
    func objectsOfType<T>(type:T.Type) -> [T] {
        return flatMap { $0 as? T }
    }
}

let filteredArray = myStructs.objectsOfType(TypeA.self)

注意：对于 Swift> = 4.1，用 compactMap替换 flatMap code>。

Note: For Swift >= 4.1, replace flatMap by compactMap.

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