SwiftUI-動態NSPredicateIf陳述句

如何創建謂詞，以便當用戶選擇“全身”時，它會回傳沒有謂詞的整個串列？現在，它正在回傳與我設定的肌肉群（Abs、Legs、Push、Pull）相對應的“部分”。我想在選擇“全身”時回傳所有選項。我如何撰寫一個 If 陳述句以便不使用謂詞？

import SwiftUI

var parts = ["Abs", "Legs", "Push", "Pull", "Full Body"]
struct ExerciseList: View {
    
    @State private var selectedPart = " "
    
    var body: some View {
        NavigationView {
            VStack (alignment: .leading) {
                
                NavigationLink(destination: AddExerciseView()){
                    Text("Add Exercise")
                        .fontWeight(.bold)
                }
                
                Picker("Body Part", selection: $selectedPart) {
                    ForEach(parts, id:\.self) { part in
                        Text(part)
                    }
                }.pickerStyle(.segmented)
                
                ListView(part:selectedPart)
            }    
        }
    }
}

import SwiftUI

struct ListView: View {
    
    var part: String
    
    @FetchRequest var exercises: FetchedResults<Exercise>
    
    init(part: String) {
        self.part = part
        self._exercises = FetchRequest(
            entity: Exercise.entity(),
            sortDescriptors: [],

            predicate: NSPredicate(format: "musclegroup == %@", part as any CVarArg)
        )
    }
    
    var body: some View {
        List(exercises) { e in
            Text(e.exercisename)
        }
    }
}

uj5u.com熱心網友回復：

你可以試試這個簡單的方法ListView：

init(part: String) {
    self.part = part
    self._exercises = FetchRequest(
        entity: Exercise.entity(),
        sortDescriptors: [],
        predicate: (part == "Full Body")
        ? nil
        : NSPredicate(format: "musclegroup == %@", part as any CVarArg)
    )
}

uj5u.com熱心網友回復：

在 View 結構中初始化物件不是一個好主意，因為堆分配會減慢速度。您可以預先創建所有謂詞，也可以在選擇器值更改時創建一個，例如：

// all the Picker samples in the docs tend to use enums.
enum Part: String, Identifiable, CaseIterable {
    case abs
    case legs
    case push
    case pull
    case fullBody
    
    var id: Self { self }

// Apple sometimes does it like this
//    var localizedName: LocalizedStringKey {
//        switch self {
//            case .abs: return "Abs"
//            case .legs: return "Legs"
//            case .push: return "Push"
//            case .pull: return "Pull"
//            case .fullBody: return "Full Body"
//        }
//    }
    
   
}

struct ExerciseListConfig {
    var selectedPart: Part = .fullBody {
        didSet {
            if selectedPart == .fullBody {
                predicate = nil
            }
            else {
                // note this will use the lower case string
                predicate = NSPredicate(format: "musclegroup == %@", selectedPart.rawValue)
            }
        }
    }
    var predicate: NSPredicate?
}

struct ExerciseList: View {
    
    @State private var config = ExerciseListConfig()
    
    var body: some View {
        NavigationView {
            VStack (alignment: .leading) {
                Picker("Body Part", selection: $config.selectedPart) {
                    //ForEach(Part.allCases) // Apple sometimes does this but means you can't easily change the display order.
                    Text("Abs").tag(Part.abs)
                    Text("Legs").tag(Part.legs)
                    Text("Push").tag(Part.push)
                    Text("Pull").tag(Part.pull)
                    Text("Full Body").tag(Part.fullBody)
                    
                }.pickerStyle(.segmented)
                
                ExerciseListView(predicate:config.predicate)
            }
        }
    }
}


struct ExerciseListView: View {
    
   // var part: String
    let predicate: NSPredicate?
  //  @FetchRequest var exercises: FetchedResults<Exercise>
    
    init(predicate: NSPredicate?) {
       self.predicate = predicate
//        self._exercises = FetchRequest(
//            entity: Exercise.entity(),
//            sortDescriptors: [],
//
//            predicate: NSPredicate(format: "musclegroup == %@", part as any CVarArg)
//        )
    }
    
    var body: some View {
        Text(predicate?.description ?? "")
//        List(exercises) { e in
//            Text(e.exercisename)
//        }
    }
}

由于您使用的是 Core Data，您可能希望在物體中使用 Int 列舉來減少存盤空間并加快查詢速度。

標籤：代码迅捷nspredicate谓词nsfetchrequest

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