runtime error when try to read fetchrequest<> - swift

Question

I want to get data form coredata ,and show it in Charts,
there is a runtime error at the code in init func when app is loaded:
let x=self.marks.wrappedValue.count
the error message is :Thread 1: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)
the whole code is following:
import SwiftUI
import Charts
import CoreData
struct CareDetails: View {
#Environment(\.managedObjectContext) var context
let timeSpan=["day","week","month","year"]
let care:Care
var marks:FetchRequest<Mark>
#State private var timeSpanIndex = 0
#State private var showSheet = false
private var sessions:[Int]=[]
private var accuracy:[Double]=[]
init(care:Care){
self.care=care
self.marks = FetchRequest(entity:Mark.entity(),
sortDescriptors: [],
predicate: NSPredicate(format:"care_id == %#",care.id!.uuidString))
let x=self.marks.wrappedValue.count
}
var body: some View {
VStack{
Picker(selection: $timeSpanIndex, label: Text("timeSpan")) {
ForEach(0..<timeSpan.count,id:\.self){ idx in
Text(self.timeSpan[idx])
}
}
.pickerStyle(SegmentedPickerStyle())
.padding()
LineChartViewSwiftUI(dataLabel: self.care.unit!,sessions: self.sessions,accuracy: self.accuracy)
Form{
Section{
List{
NavigationLink(destination:
CareDetailsList(marks: self.marks)
.environment(\.managedObjectContext, self.context)
){
Text("显示所有数据")
}
}
}
.multilineTextAlignment(.leading)
}
.padding()
}
}
}

Answer 1

It hasn't been evaluated yet. It resolves when update() gets called by your view's body function. Try structuring it slightly differently and using it directly in body.
struct CareView: View {
let care: Care
#FetchRequest var marks: FetchedResult<Mark>
#State private var timeSpanIndex = 0
#State private var showSheet = false
init(care: Care) {
self.care = care
self._marks = FetchRequest(...) // set up as before now. note the `_` though!
}
var body: some View {
// make sure you can ID each `Mark` by something
ForEach(marks, id: \.id) { mark in
// create your `MarkView` here
}
}
}