【发布时间】：2020-07-21 10:36:04
【问题描述】：
我确定这很愚蠢，但是当另一个状态发生变化时，应该如何重置子视图的状态值？
例如下面的代码显示了2个文件夹，分别有2个和3个项目，可以编辑。
如果您选择第二个文件夹 (Work) 及其第三个项目 (Peter)，然后选择第一个文件夹 (Home)，则应用程序会崩溃，因为 selectedItemIndex 超出范围。
我尝试在视图初始化时“重置”状态值，但似乎像这样更改状态会触发“运行时：SwiftUI：在视图更新期间修改状态，这将导致未定义的行为。”警告。
init(items: Binding<[Item]>) {
self._items = items
self._selectedItemIndex = State(wrappedValue: 0)
}
这样做的正确方法是什么？谢谢！
代码如下：
AppDelegate.swift
import Cocoa
import SwiftUI
@NSApplicationMain
class AppDelegate: NSObject, NSApplicationDelegate {
var window: NSWindow!
func applicationDidFinishLaunching(_ aNotification: Notification) {
// Create the SwiftUI view that provides the window contents.
let store = ItemStore()
let contentView = ContentView(store: store)
// Create the window and set the content view.
window = NSWindow(
contentRect: NSRect(x: 0, y: 0, width: 480, height: 300),
styleMask: [.titled, .closable, .miniaturizable, .resizable, .fullSizeContentView],
backing: .buffered, defer: false)
window.center()
window.setFrameAutosaveName("Main Window")
window.contentView = NSHostingView(rootView: contentView)
window.makeKeyAndOrderFront(nil)
}
func applicationWillTerminate(_ aNotification: Notification) {
// Insert code here to tear down your application
}
}
ContentView.swift
import SwiftUI
final class ItemStore: ObservableObject {
@Published var data: [Folder] = [Folder(name: "Home",
items: [Item(name: "Mark"), Item(name: "Vincent")]),
Folder(name: "Work",
items:[Item(name: "Joseph"), Item(name: "Phil"), Item(name: "Peter")])]
}
struct Folder: Identifiable {
var id = UUID()
var name: String
var items: [Item]
}
struct Item: Identifiable {
static func == (lhs: Item, rhs: Item) -> Bool {
return true
}
var id = UUID()
var name: String
var content = Date().description
init(name: String) {
self.name = name
}
}
struct ContentView: View {
@ObservedObject var store: ItemStore
@State var selectedFolderIndex: Int?
var body: some View {
HSplitView {
// FOLDERS
List(selection: $selectedFolderIndex) {
Section(header: Text("Groups")) {
ForEach(store.data.indexed(), id: \.1.id) { index, folder in
Text(folder.name).tag(index)
}
}.collapsible(false)
}
.listStyle(SidebarListStyle())
// ITEMS
if selectedFolderIndex != nil {
ItemsView(items: $store.data[selectedFolderIndex!].items)
}
}
.frame(minWidth: 800, maxWidth: .infinity, maxHeight: .infinity)
}
}
struct ItemsView: View {
@Binding var items: [Item]
@State var selectedItemIndex: Int?
var body: some View {
HSplitView {
List(selection: $selectedItemIndex) {
ForEach(items.indexed(), id: \.1.id) { index, item in
Text(item.name).tag(index)
}
}
.frame(width: 300)
if selectedItemIndex != nil {
DetailView(item: $items[selectedItemIndex!])
.padding()
.frame(minWidth: 200, maxHeight: .infinity)
}
}
}
init(items: Binding<[Item]>) {
self._items = items
self._selectedItemIndex = State(wrappedValue: 0)
}
}
struct DetailView: View {
@Binding var item: Item
var body: some View {
VStack {
TextField("", text: $item.name)
}
}
}
// Credit: https://swiftwithmajid.com/2019/07/03/managing-data-flow-in-swiftui/
struct IndexedCollection<Base: RandomAccessCollection>: RandomAccessCollection {
typealias Index = Base.Index
typealias Element = (index: Index, element: Base.Element)
let base: Base
var startIndex: Index { base.startIndex }
var endIndex: Index { base.endIndex }
func index(after i: Index) -> Index {
base.index(after: i)
}
func index(before i: Index) -> Index {
base.index(before: i)
}
func index(_ i: Index, offsetBy distance: Int) -> Index {
base.index(i, offsetBy: distance)
}
subscript(position: Index) -> Element {
(index: position, element: base[position])
}
}
extension RandomAccessCollection {
func indexed() -> IndexedCollection<Self> {
IndexedCollection(base: self)
}
}
【问题讨论】：
-
这能回答你的问题吗？ Handling derived state in SwiftUI
-
我尝试将您的代码粘贴到 Xcode 11.4 中，但出现了很多编译器错误，因此很难进行试验。 IndexedCollection 上应该有类型参数吗？有些东西可能没有成为您粘贴到问题中的内容。例如，我看到
Binding<[Item]>变成了Binding。