SwiftUI中@State配置SortDescriptor初始化报错的解决方法

问题说明 #

尝试通过按钮修改Core Data的SortDescriptor，将排序键路径sorting和排序顺序sortOrder改为@State变量时，触发错误：

Variable 'self.fetchRequest' used before being initialized

原代码如下：

@Environment(\.managedObjectContext) private var viewContext
    
@State var sorting: KeyPath = \Log.diveNr
@State var sortOrder: SortOrder = .forward
    
private var fetchRequest: FetchRequest<Log>
private var logs: FetchedResults<Log> {
    fetchRequest.wrappedValue
}
    
init() {
    fetchRequest = FetchRequest(sortDescriptors: [SortDescriptor(sorting, order: sortOrder)])
}

问题原因 #

@State变量的初始化时机晚于视图的init方法，在init中访问sorting和sortOrder时，这两个变量尚未完成初始化，导致无法正确创建FetchRequest。

解决方案 #

直接使用@FetchRequest属性包装器声明logs，无需手动初始化FetchRequest。后续可通过修改logs.sortDescriptors动态更新排序规则：

@FetchRequest(sortDescriptors: [SortDescriptor(\.diveNr, order: .forward)])
var logs: FetchedResults<Log>

var body: some View {
    NavigationStack {
        List{
            if logs.count == 0 {
                Text("未找到日志")
            }
            Button("切换排序顺序") {
                logs.sortDescriptors = [SortDescriptor(\.diveNr, order: .reverse)]
            }
            ForEach(logs){log in
                NavigationLink(destination: DiveLogDetailView(diveNo: 1, diveSite: "Vela Garska")) {
                    DiveLogListObject(log: log)
                }
            }
            .onDelete(perform: deleteLog)
        }
    }
}

内容的提问来源于stack exchange，提问作者luegm.dev