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我正在使用 SwiftUI 为基于 iOS 的旧项目编写 macOS 目标。是一个核心数据驱动的应用程序，对于 macOS 目标，我已经使用动态 @FetchRequest 成功实现了通用列表，主要如 Paul Hudson 在他的博客中所描述的那样。

我主要通过遵循 Apple 的SwiftUI 教程并复制提供的示例代码来构建目标。

之前使用的条件 if 语句List根据 3 个 UI 控件主动过滤每个 SwiftUI。

三个 UI 控件的图像

// PART 1
    if (!self.appData.showFavouritesOnly
        || fetchedEvent.isFavourite)
// PART 2
    && (self.searchText.count == 0
        || (fetchedEvent.eventName?.contains(self.searchText) == true))
// PART 3
    && (self.filter == .all
        || self.filter.name == fetchedEvent.eventCategory
        || (self.filter.category == .featured && fetchedEvent.isFeatured)) {

现在我有一个使用谓词的通用 @FetchRequest，我想将此条件 if 语句转换为NSCompoundPredicate.

我将包含整个初始化程序，以便您可以看到动态@FetchRequest 是如何构建的，但它是我需要帮助的谓词...

init(sortDescriptors: [NSSortDescriptor],
     searchKey: String,
     searchValue: String?,
     showFavourites: Bool,
     filterKey: String,
     filter: FilterType,
     @ViewBuilder content: @escaping (T) -> Content) {

    let entity = T.entity
    let predicateTrue = NSPredicate(value: true)
// PART 1
    let predicateFavourite = showFavourites == false ? predicateTrue : NSPredicate(format: "isFavourite == TRUE")
// PART 2
    let predicateSearch = searchValue?.count == 0 ? predicateTrue : NSPredicate(format: "%K CONTAINS[cd] %@", searchKey, searchValue!)

    // The initialiser works perfectly down to this point...then...
// PART 3
    let predicateFilterName = filter == .all ? predicateTrue : NSPredicate(format: "%K == %@", filterKey, filter.name as CVarArg)
    let predicateFilterFeature = filter.category == .featured ? NSPredicate(format: "isFeatured == TRUE") : predicateTrue

    let predicateOr = NSCompoundPredicate(orPredicateWithSubpredicates: [predicateFilterName, predicateFilterFeature])
    let predicate = NSCompoundPredicate(andPredicateWithSubpredicates: [predicateFavourite, predicateSearch, predicateOr])

    fetchRequest =
        FetchRequest<T>(entity: entity,
                        sortDescriptors: sortDescriptors,
                        predicate: predicate)

    self.content = content
}

第 3 部分中包含的代码部分工作。在之间切换FilterType.all并FilterType.featured进行预期的更改，但是我正在努力为选择另一个类别的“其他”情况编写谓词 - 即 - NOT Featured，但是.lakesEITHER.rivers或.mountains.

为了完整起见，我还包括了 enumCategory和 struct FilterType...

enum Category: String, CaseIterable, Codable, Hashable {

    case featured = "Featured"
    case lakes = "Lakes"
    case rivers = "Rivers"
    case mountains = "Mountains"
}

struct FilterType: CaseIterable, Hashable, Identifiable {

    var name: String
    var category: Category?

    init(_ category: Category) {
        self.name = category.rawValue
        self.category = category
    }

    init(name: String) {
        self.name = name
        self.category = nil
    }

    static var all = FilterType(name: "All")

    static var allCases: [FilterType] {
        return [.all] + Category.allCases.map(FilterType.init)
    }

    var id: FilterType {
        return self
    }

}
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2 回答 2

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我认为问题出在这里：

 let predicateFilterFeature = filter.category == .featured ? NSPredicate(format: "isFeatured == TRUE") : predicateTrue

如果过滤器是.lakesetc，那么这个子谓词将为 TRUE，当 ORed 时predicateFilterName会覆盖它。尝试返回 FALSE：

 let predicateFilterFeature = filter.category == .featured ? NSPredicate(format: "isFeatured == TRUE") : predicateFalse
于 2020-06-12T08:09:45.173 回答
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我的答案是欺诈性的，因为这是我在尝试以不同方式构建谓词后偶然发现的黑客解决方案。（它没有表现出对理解我写的谓词语法的实际问题的任何仔细思考！）

所以这个回复没有回答我原来的问题——@pbasdf 已经回答了——尽管这个回复达到了相同的结果。

所以也许是另一种解决方案？！？

let predicateTrue = NSPredicate(value: true)

let predicateFavourite = showFavourites == false ? predicateTrue : NSPredicate(format: "isFavourite == TRUE")

let predicateSearch = searchValue?.count == 0 ? predicateTrue : NSPredicate(format: "%K CONTAINS[cd] %@", searchKey, searchValue!)

let predicateFilterFeatured = NSPredicate(format: "isFeatured == TRUE")
let predicateFilterName = NSPredicate(format: "%K == %@", filterKey, filter.name as CVarArg)
let predicateFilterCategory = filter.category == .featured ? predicateFilterFeatured : predicateFilterName

let predicateFilter = filter == .all ? predicateTrue : predicateFilterCategory

let predicate = NSCompoundPredicate(andPredicateWithSubpredicates: [predicateFavourite, predicateSearch, predicateFilter])
于 2020-06-12T11:59:33.540 回答