SwiftUI

Question

所以我是 SwiftUI 的新手。通常我会在 viewdidload 方法中收到获取请求，但现在不确定如何处理它，因为该方法似乎没有等效方法。

所以我有一个视图，我正在将一个变量传递给另一个视图，就像这样

NavigationLink(destination: ItemAddView(series: series)) {
    Label("Add Item", systemImage: "plus")
}

我现在想在获取请求的谓词中使用这个变量。获取请求设置如下

struct ItemAddView: View {
@Environment(\.managedObjectContext) private var viewContext

@FetchRequest(
    sortDescriptors: [NSSortDescriptor(keyPath: \Item.name, ascending: true)],
    animation: .default)
private var items: FetchedResults<Item>

var series: Series

@State private var searchText = ""

var body: some View {
    List {
        ForEach(items) { item in
            NavigationLink(destination: ItemDetailView(item: item)) {
                Image(item.mainImage ?? "")
                    .renderingMode(.original)
                    .resizable()
                    .scaledToFit()
                    .frame(width: 100.0)
                Text("\(item.name ?? "Error")")
            }
        }
    }
    .searchable(text: $searchText)
    .onSubmit(of: .search) {
        items.nsPredicate = NSPredicate(format: "series = %@", series)
        if(!searchText.isEmpty) {
            items.nsPredicate = NSPredicate(format: "series = %@ AND amount = 0 AND name CONTAINS[cd] %@", series, searchText)
        }
    }
    .onChange(of: searchText) { _ in
        items.nsPredicate = NSPredicate(format: "series = %@", series)
        if(!searchText.isEmpty) {
            items.nsPredicate = NSPredicate(format: "series = %@ AND amount = 0 AND name CONTAINS[cd] %@", series, searchText)
        }
    }
    .navigationBarTitle("\(series.name ?? "Error") Add Items", displayMode: .inline)
    .onAppear() {
        items.nsPredicate = NSPredicate(format: "series = %@ AND amount = 0", series)
    }
}
}

如果我在这里使用谓词，我会得到一个错误

Cannot use instance member 'series' within property initializer; property initializers run before 'self' is available

我已经习惯于在 onAppear 方法中使用谓词

.onAppear() {
    items.nsPredicate = NSPredicate(format: "series = %@ AND amount > 0", series)
}

但问题是我看到了完整列表，然后才转到过滤后的请求。这样做的正确方法是什么？

Answer 1

您只需在header中声明变量即可。然后您可以在 init() 中进行初始化并使用传入的变量。如果您随后不需要 series，则不需要在视图中有任何变量来分配给它。此外，由于您没有 post 完整视图，我不得不猜测 series.

的类型

struct ItemAddView: View {
    @Environment(\.managedObjectContext) private var viewContext

    @FetchRequest private var items: FetchedResults<Item>


    init(series: Series) {
        let request = Item.fetchRequest(sortDescriptors: [NSSortDescriptor(keyPath: \Item.name, ascending: true)], predicate: NSPredicate(format: "series = %@ AND amount > 0", series), animation: .default)
        _items = FetchRequest(fetchRequest: request)

        // Initialize anything else that is necessary
    }

    ...
}

SwiftUI - 在谓词中使用从先前视图传递的值

SwiftUI - Use value passed from previous view in predicate

nspredicate