SwiftUI 试图从 FetchedResults 获取对象但没有找到方法
Posted
技术标签:
【中文标题】SwiftUI 试图从 FetchedResults 获取对象但没有找到方法【英文标题】:SwiftUI Trying to get object from FetchedResults but not finding a way 【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】:
【参考方案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】:
以上是关于SwiftUI 试图从 FetchedResults 获取对象但没有找到方法的主要内容,如果未能解决你的问题,请参考以下文章
SwiftUI 试图从列表中删除对象但无法在不可变值上使用变异成员:是一个“让”常量