如何在不中断删除的情况下在 SwiftUI 中实现 TextField 列表

Posted 2023-02-24

【中文标题】如何在不中断删除的情况下在 SwiftUI 中实现 TextField 列表

【英文标题】：How to implement a List of TextFields in SwiftUI without breaking deletion

【发布时间】：2019-12-10 15:24:26

【问题描述】：

当我

在 XCode 中创建主从应用程序 使用核心数据 向事件模型添加一个新字段（例如 title 为 String） 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)
            
        
    

，即用适当的绑定到event 对象的title 属性将Text 替换为TextField，XCode 可以愉快地编译，只要我不这样做，应用程序就会以预期的行为运行t 尝试从列表中删除。

当我尝试删除 Event 时，它会因堆栈跟踪而崩溃

* 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 holding reference to deleted core data object causing crash 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？

【参考方案1】：

代替

TextField("标题", text: Binding(ObservedObject(wrappedValue: 事件).projectedValue.title)!)

使用

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

经过测试并适用于 Xcode 11.2、ios 13.2

【讨论】：

这只是简单的透明包装器，带有明确控制的可选

【参考方案2】：

对此的一些想法：

选项 1 （内联绑定）

（我猜可能会有更简洁的实现，但这段代码运行良好）

// Somewhere in MasterView...
// A function that returns a binding for the title of an event
func titleBindingFor(_ event: Event) -> Binding<String> 
    Binding<String>(get:  () -> String in
        event.title ?? ""
    )  (title) in
        event.title = title
    

然后

// Bind it to the textfield
TextField("Title", text: self.titleBindingFor(event))

---

选项 2 （单独视图）

extension Optional where Wrapped == String 
    var safe: String 
        get  self ?? "" 
        set  self = newValue 
    


struct TitleEditor: View 
    @ObservedObject var event: Event

    var body: some View 
        TextField("Title", text: $event.title.safe)
    

然后

TitleEditor(event: event)

注意：我还必须在 DetailView 中处理可选的时间戳强制解包（真的是 Apple 吗？！），因为我在删除时遇到崩溃：

struct DetailView: View 
    @ObservedObject var event: Event

    var body: some View 
        let timestamp = event.timestamp ?? Date()
        return Text("\(timestamp, formatter: dateFormatter)")
            .navigationBarTitle(Text("Detail"))
    

请注意，如果标题为 nil，我将返回一个空字符串，它由文本字段优雅地处理（它在新创建的项目上显示占位符）