SwiftUI 试图从 FetchedResults 获取对象但没有找到方法答案

【问题标题】：SwiftUI Trying to get object from FetchedResults but not finding a waySwiftUI 试图从 FetchedResults 获取对象但没有找到方法
【发布时间】：2020-05-07 15:17:11
【问题描述】：

我有一个 FetchedResults 列表，我正在通过 Picker 从该 FetchedResults 中挑选一个对象，但不知道如何存储该对象。我正在通过变量 listOfPolymers 中的 FetchedRequest 从 CoreData 实体中获取对象列表。现在我想拾取一个对象作为父家庭，但我无法找到一种方法来从 Picker 中拾取所选对象。如果我在 Picker 中使用对象类型，则会出现错误，因为绑定对象需要从我要从该视图传递到该视图的视图传递，但我没有提供从该视图传递该对象的规定。这是我的代码：

struct Polymer: View {
@Environment(\.managedObjectContext) var mocPolymer
@FetchRequest(entity: AECPolymer.entity(), sortDescriptors: [NSSortDescriptor(keyPath: \AECPolymer.name, ascending: true)]) var listOfPolymers: FetchedResults<AECPolymer>
@State var toAddPolymerName: String = ""
@State var toAddPolymerShortName: String = ""
@State var addPolymer: Bool = false
@State var parentPolymerName: String = "Select"
@State var hasParent: Bool = false

var body: some View {
    List {
        if addPolymer {
            Section(header: Text("Add Polymer")) {
                TextField("Name", text: $toAddPolymerName)
                TextField("Short Name", text: $toAddPolymerShortName)
                Toggle(isOn: $hasParent) {
                    Text("Belongs to a Parent Polymer Family?")
                }
                VStack {
                    if hasParent {
                        Picker(selection: $parentPolymerName, label: EmptyView()) {
                        ForEach(listOfPolymers, id: \.self) {(parent: AECPolymer) in
                            Text(parent.name ?? "Unknown")
                        }
                        }
                    } else {
                        EmptyView()
                    }
                }
                if self.toAddPolymerName.isEmpty || self.toAddPolymerShortName.isEmpty {
                    EmptyView()
                } else {
                    Button(action: {
                        let toAddPolymer = AECPolymer(context: self.mocPolymer)
                        toAddPolymer.name = self.toAddPolymerName
                        toAddPolymer.shortName = self.toAddPolymerShortName
                        let selectedParentPolymerName = self.parentPolymerName
                        if let parentPolymer = self.listOfPolymers.first(where: { "name == @%", selectedParentPolymerName }) {
                            toAddPolymer.polymerFamily = parentPolymer
                        }

                        do {
                            try self.mocPolymer.save()
                        } catch {
                            let nserror = error as NSError
                            fatalError("Unresolved error \(nserror), \(nserror.userInfo)")
                        }
                        self.toAddPolymerName = ""
                        self.toAddPolymerShortName = ""
                        self.addPolymer.toggle()
                    }, label: {
                        Text("Save")
                    }).buttonStyle(BorderlessButtonStyle())
                }
            }
        } else {
            EmptyView()
        }
        Section(header: Text("Polymers")) {
            if self.listOfPolymers.count != 0 {
                ForEach(listOfPolymers, id: \.self) {(polymer: AECPolymer) in
                    VStack(alignment: .leading) {
                        Text(polymer.name ?? "Unknown")
                        Text("(\(polymer.shortName ?? "Unknown"))").font(.footnote)
                    }
                }
            } else {
                Text("List is empty...")
            }
        }
    }.navigationBarTitle("List")
        .navigationBarItems(trailing: Button(action: {
            self.addPolymer.toggle()
        }, label: {
            if self.addPolymer {
                Text("Cancel")
            } else {
                Image(systemName: "plus.circle.fill")
                    .imageScale(.large)
            }
        }))
}

}

有人可以帮助我如何获取用户从选择器中选择的对象...

【问题讨论】：

标签： core-data swiftui

【解决方案1】：

CoreData 问题有点困难，因为很难让它们在本地编译。不过，试试这个：（不编译，只是给你一个方向！）

struct PickerView: View {
  @State private var selection: Int? = nil     // Int? state 
  var body: some View {
    Picker(selection: $selection, label: EmptyView()) {
      ForEach(YourModel.indices) { index in    // Loop over your model's Int index  
        Text("\(YourModel.name)").tag(index).  // Tag the text with the index
      } 
    } 
  }
}

用户做出选择后，您应该通过索引找到您的对象：YourModel[$selection!]。当然，请确保提防nil 或提供默认值。

【讨论】：