如何在不中断删除的情况下在 SwiftUI 中实现文本字段列表

Question

当我

1. 在 XCode 中创建主从应用程序
2. 使用核心数据
3. 一个新字段添加到事件模型（说title的String）
4. 将MasterView实现更改为此

struct MasterView: View {
    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \Event.timestamp, ascending: true)], 
        animation: .default)
    var events: FetchedResults<Event>

    @Environment(\.managedObjectContext)
    var viewContext

    var body: some View {
        List {
            ForEach(events, id: \.self) { event in
                NavigationLink(
                    destination: DetailView(event: event)
                ) {
                    TextField("Title", text: Binding(ObservedObject<Event>(wrappedValue: event).projectedValue.title)!)
                }
            }.onDelete { indices in
                self.events.delete(at: indices, from: self.viewContext)
            }
        }
    }
}

，即，使用与对象属性的适当绑定换出Textfor TextField，只要我不尝试从列表中删除，XCode 就会愉快地编译并且应用程序以预期的行为运行。titleevent

当我尝试删除 an 时Event，它会因此堆栈跟踪而崩溃

* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)
    #0: [33m`SwiftUI.BindingOperations.ForceUnwrapping.get(base: Swift.Optional<A>) -> A[0m
    #1: [33m`protocol witness for SwiftUI.Projection.get(base: A.Base) -> A.Projected in conformance SwiftUI.BindingOperations.ForceUnwrapping<A> : SwiftUI.Projection in SwiftUI[0m
    #2: [33m`SwiftUI.(ProjectedLocation in _5A9440699EF65619D724050F1A6941EE).update(context: AttributeGraph.AttributeContext<SwiftUI.VoidAttribute>, state: inout Swift.Optional<Any>) -> (B.Projected, Swift.Bool)[0m
    #3: [33m`SwiftUI.LocationBox.update(context: AttributeGraph.AttributeContext<SwiftUI.VoidAttribute>, state: inout Swift.Optional<Any>) -> (A.Value, Swift.Bool)[0m
    #4: [33m`SwiftUI.Binding.(ScopedLocation in _5436F2B399369BE3B016147A5F8FE9F2).update(context: AttributeGraph.AttributeContext<SwiftUI.VoidAttribute>, state: inout Swift.Optional<Any>) -> (A, Swift.Bool)[0m
    #5: [33m`protocol witness for SwiftUI.Location.update(context: AttributeGraph.AttributeContext<SwiftUI.VoidAttribute>, state: inout Swift.Optional<Any>) -> (A.Value, Swift.Bool) in conformance SwiftUI.Binding<A>.(ScopedLocation in _5436F2B399369BE3B016147A5F8FE9F2) : SwiftUI.Location in SwiftUI[0m
    #6: [33m`SwiftUI.LocationBox.update(context: AttributeGraph.AttributeContext<SwiftUI.VoidAttribute>, state: inout Swift.Optional<Any>) -> (A.Value, Swift.Bool)[0m
    #7: [33m`SwiftUI.Binding.(Box in _5436F2B399369BE3B016147A5F8FE9F2).update(property: inout SwiftUI.Binding<A>, context: AttributeGraph.AttributeContext<SwiftUI.VoidAttribute>) -> Swift.Bool[0m
    #8: [33m`static SwiftUI.(BoxVTable in _68550FF604D39F05971FE35A26EE75B0).update(ptr: Swift.UnsafeMutableRawPointer, property: Swift.UnsafeMutableRawPointer, context: AttributeGraph.AttributeContext<SwiftUI.VoidAttribute>) -> Swift.Bool[0m
    #9: [33m`SwiftUI._DynamicPropertyBuffer.update(container: Swift.UnsafeMutableRawPointer, context: AttributeGraph.AttributeContext<SwiftUI.VoidAttribute>) -> Swift.Bool[0m
    #10: [33m`SwiftUI.(DynamicPropertyBody in _9F92ACD17B554E8AB7D29ABB1E796415).update(context: inout AttributeGraph.AttributeContext<SwiftUI.(DynamicPropertyBody in _9F92ACD17B554E8AB7D29ABB1E796415)<A>>) -> ()[0m
    #11: [33m`protocol witness for static AttributeGraph.UntypedAttribute._update(_: Swift.UnsafeMutableRawPointer, graph: __C.AGGraphRef, attribute: __C.AGAttribute) -> () in conformance SwiftUI.(DynamicPropertyBody in _9F92ACD17B554E8AB7D29ABB1E796415)<A> : AttributeGraph.UntypedAttribute in SwiftUI[0m
    #12: [33m`partial apply forwarder[0m
    #13: [33m`AG::Graph::UpdateStack::update[0m
    #14: [33m`AG::Graph::update_attribute[0m
    #15: [33m`AG::Subgraph::update[0m
    #16: [33m`SwiftUI.ViewGraph.(runTransaction in _D63C4EB7F2B205694B6515509E76E98B)(in: __C.AGGraphRef) -> ()[0m
    #17: [33m`closure #1 (__C.AGGraphRef) -> (prefs: Swift.Bool, idealSize: Swift.Bool, outputs: SwiftUI.ViewGraph.Outputs) in SwiftUI.ViewGraph.updateOutputs(at: SwiftUI.Time) -> ()[0m
    #18: [33m`SwiftUI.ViewGraph.updateOutputs(at: SwiftUI.Time) -> ()[0m
    #19: [33m`closure #1 () -> () in closure #1 () -> () in (extension in SwiftUI):SwiftUI.ViewRendererHost.render(interval: Swift.Double, updateDisplayList: Swift.Bool) -> ()[0m
    #20: [33m`closure #1 () -> () in (extension in SwiftUI):SwiftUI.ViewRendererHost.render(interval: Swift.Double, updateDisplayList: Swift.Bool) -> ()[0m
    #21: [33m`(extension in SwiftUI):SwiftUI.ViewRendererHost.render(interval: Swift.Double, updateDisplayList: Swift.Bool) -> ()[0m
    #22: [33m`closure #1 () -> () in SwiftUI._UIHostingView.requestImmediateUpdate() -> ()[0m
    #23: [33m`reabstraction thunk helper from @escaping @callee_guaranteed () -> () to @escaping @callee_unowned @convention(block) () -> ()[0m
    #24: [33m`_dispatch_call_block_and_release[0m
    #25: [33m`_dispatch_client_callout[0m
    #26: [33m`_dispatch_main_queue_callback_4CF[0m
    #27: [33m`__CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__[0m
    #28: [33m`__CFRunLoopRun[0m
    #29: [33m`CFRunLoopRunSpecific[0m
    #30: [33m`GSEventRunModal[0m
    #31: [33m`UIApplicationMain[0m
  * #32: [33m`main[0m at AppDelegate.swift:13
    #33: [33m`start[0m

我试过这个：SwiftUI 持有对已删除核心数据对象的引用，导致崩溃但无济于事。我还尝试将文本字段提取到单独的视图类中并使用适当的属性包装器。

我想我以某种方式滥用了基础设施（绑定、可观察对象等）。我应该如何使用这些来实现列表中的文本字段？

Answer 1

代替

TextField("Title", text: Binding(ObservedObject(wrappedValue: event).projectedValue.title)!)

用

TextField("Title", text: Binding<String>(
                   get: {event.title ?? "<none>"}, set: {event.title = $0}))

经测试适用于 Xcode 11.2、iOS 13.2