Thanks! Waiting for the new version. > 在 2017年2月20日，上午7:42，Xiaodi Wu <xiaodi...@gmail.com> 写道： > > That is a new feature in Swift 3.1. > > > On Sun, Feb 19, 2017 at 5:32 PM, 曹剑楠 <frog...@163.com > <mailto:frog...@163.com>> wrote: > Sorry about my typo: > > public struct Album { > public let sectionedMediaItemInfos: [Sectioned<MediaItemInfo>]?} > > > public struct Sectioned<Item> : SectionProtocol { > public let title: String? > public let items: [Item] > > public init() { > items = [] > title = nil > } > } > > public extension Array where Element == Sectioned { > > var itemsCount: Int { > return reduce(0) { (result, section) in result + section.items.count } > } > > var items: [Element.Item] { > return self.flatMap { $0.items } > } > } > > Allow extension Array with strict its Element with a struct type (may be > generic) is highly wanted feature. > >> 在 2017年2月20日，上午7:28，曹剑楠 <frog...@163.com <mailto:frog...@163.com>> 写道： >> >> >> OK, my fault. That solved the problem. >> >>> var items: [Element.SectionItemType] >> >> I should use this. >> >> How about to allow using generic type Section directly instead of declare a >> protocol >> >> ```Swift >> public extension Array where Element == Section { >> >> var itemsCount: Int { >> return reduce(0) { (result, section) in result + section.items.count >> } >> } >> >> var items: [Element.SectionItemType] { >> return self.flatMap { $0.items } >> } >> } >> ``` >> >>> 在 2017年2月20日，上午7:14，Xiaodi Wu <xiaodi...@gmail.com >>> <mailto:xiaodi...@gmail.com>> 写道： >>> >>> On Sun, Feb 19, 2017 at 5:04 PM, 曹剑楠 via swift-evolution >>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: >>> Hi All, >>> >>> I’m coding for section like data structure. >>> For example: >>> >>> I have an Album struct, which has many MediaItems. >>> It has a sectioned property called "sectionedMediaItemInfos", which is an >>> array of sections. >>> Each section represents for a disc, and has an "items" array contains all >>> MedaItems in that disc. >>> >>> The define code is like: >>> >>> ```Swift >>> public struct Album { >>> public let sectionedMediaItemInfos: [Sectioned<MediaItemInfo>]? >>> } >>> >>> public struct Sectioned<Item> : SectionProtocol { >>> public let title: String? >>> public let items: [Item] >>> >>> public init() { >>> items = [] >>> title = nil >>> } >>> } >>> >>> public protocol SectionProtocol { >>> associatedtype SectionItemType >>> var items: [SectionItemType] { get } >>> } >>> ``` >>> >>> Now I want to define some extra properties for sections array, like >>> "sectionMediaItemInfos"."itemsCount" that count all items in each sections. >>> So I can write that extension: >>> >>> ```Swift >>> public extension Array where Element : SectionProtocol { >>> >>> var itemsCount: Int { >>> return reduce(0) { (result, section) in result + >>> section.items.count } >>> } >>> >>> } >>> ``` >>> >>> So I can get my itemsCount with code like: >>> >>> ```Swift >>> album.sectionedMediaItemInfos.itemsCount >>> ``` >>> >>> That looks good. >>> >>> Then I want to define code to return all items in this sectioned property. >>> >>> >>> ```Swift >>> public extension Array where Element : SectionProtocol { >>> var items: [SectionProtocol.SectionItemType] { >>> return .flatMap { $0.items } >>> } >>> } >>> ``` >>> >>> Sorry, I'm reading this quickly, but I'm confused as to why you're not >>> writing `var items: [Element.SectionItemType]`. That seems to be what you >>> want, no? `SectionProtocol.SectionItemType` has no constraints and, even if >>> the grammar allowed you to write it, would have to be equivalent to `Any`. >>> >>> This doesn’t work. It reported as "Cannot use associated type >>> 'SectionItemType' outside of its protocol" >>> >>> The only way to achieve my goals is to untyped the extended "items" >>> property: >>> >>> ```Swift >>> public extension Array where Element : SectionProtocol { >>> var items: [Any] { >>> return self.flatMap { $0.items } >>> } >>> } >>> ``` >>> >>> Which is not perfect for this case. >>> >>> >>> So in this special case, I think allow use associated type outside of its >>> protocol is necessary. >>> >>> And we may allow define protocol with generic type. That would be more >>> convenient. >>> >>> >>> ```Swift >>> public struct Album { >>> public let sectionedMediaItemInfos: [Sectioned<MediaItemInfo>]? >>> } >>> >>> >>> public struct Sectioned<Item> : SectionProtocol { >>> public let title: String? >>> public let items: [Item] >>> >>> public init() { >>> items = [] >>> title = nil >>> } >>> } >>> >>> public protocol SectionProtocol<SectionItemType> { >>> var items: [SectionItemType] { get } >>> } >>> >>> >>> public extension Array where Element : SectionProtocol { >>> var items: [Element.SectionItemType] { >>> return self.flatMap { $0.items } >>> } >>> } >>> >>> ``` >>> >>> Thank all! >>> >>> Jiannan >>> >>> >>> >>> >>> _______________________________________________ >>> swift-evolution mailing list >>> swift-evolution@swift.org <mailto:swift-evolution@swift.org> >>> https://lists.swift.org/mailman/listinfo/swift-evolution >>> <https://lists.swift.org/mailman/listinfo/swift-evolution> >>> >>> >> > >
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