swift - 'Fatal error: index out of range' when deleting bound object in view

Question

I am having some trouble avoiding index out of range errors when modifying an array that a child view depends on a bound object from.

I have a parent view called WorkoutList. WorkoutList has an EnvironmentObject of ActiveWorkoutStore. ActiveWorkoutStore is an ObservableObject that has an array of Workout objects. I have a list of active workouts being retrieved from ActiveWorkoutStore. I'm using a ForEach loop to work with the indices of these active workouts and pass an object binding to a child view called EditWorkout as a destination for a NavigationLink. EditWorkout has a button to finish a workout, which removes it from ActiveWorkoutStore's array of workouts and adds it to WorkoutHistoryStore. I'm running into trouble when I remove this object from ActiveWorkoutStore's activeWorkouts array, immediately causing an index out of range error. I'm suspecting this is because the active view relies on a bound object that I've just deleted. I've tried a couple permutations of this, including passing a workout to EditWorkout, then using its id to reference a workout in ActiveWorkoutStore to perform my operations, but run into similar troubles. I've seen a lot of examples online that follow this pattern of leveraging ForEach to iterate over indices and I've mirrored it as best I can tell, but I suspect I may be missing a nuance to the approach.

I've attached code samples below. Let me know if you have any questions or if there's anything else I should include! Thanks in advance for your help!

WorkoutList (Parent View)

import SwiftUI

struct WorkoutList: View {
    @EnvironmentObject var activeWorkoutsStore: ActiveWorkoutStore
    @State private var addExercise = false
    @State private var workoutInProgress = false

    var newWorkoutButton: some View {
        Button(action: {
            self.activeWorkoutsStore.newActiveWorkout()
        }) {
            Text("New Workout")
            Image(systemName: "plus.circle")
        }
    }

    var body: some View {
        NavigationView {
            Group {
                if activeWorkoutsStore.activeWorkouts.isEmpty {
                    Text("No active workouts")
                } else {
                    List {
                        ForEach(activeWorkoutsStore.activeWorkouts.indices.reversed(), id: .self) { activeWorkoutIndex in
                            NavigationLink(destination: EditWorkout(activeWorkout: self.$activeWorkoutsStore.activeWorkouts[activeWorkoutIndex])) {
                                Text(self.activeWorkoutsStore.activeWorkouts[activeWorkoutIndex].id.uuidString)
                            }
                        }
                    }
                }
            }
            .navigationBarTitle(Text("Active Workouts"))
            .navigationBarItems(trailing: newWorkoutButton)
        }
    }
}

EditWorkout (Child View)

//
//  EditWorkout.swift
//  workout-planner
//
//  Created by Dominic Minischetti III on 11/2/19.
//  Copyright ? 2019 Dominic Minischetti. All rights reserved.
//

import SwiftUI

struct EditWorkout: View {
    @EnvironmentObject var workoutHistoryStore: WorkoutHistoryStore
    @EnvironmentObject var activeWorkoutStore: ActiveWorkoutStore
    @EnvironmentObject var exerciseStore: ExerciseStore
    @Environment(.presentationMode) var presentationMode
    @State private var addExercise = false
    @Binding var activeWorkout: Workout
    
    var currentDayOfWeek: String {
        let weekdayIndex = Calendar.current.component(.weekday, from: Date())
        return Calendar.current.weekdaySymbols[weekdayIndex - 1]
    }

    var chooseExercisesButton: some View {
        Button (action: {
            self.addExercise = true
        }) {
            HStack {
                Image(systemName: "plus.square")
                Text("Choose Exercises")
            }
        }
        .sheet(isPresented: self.$addExercise) {
            AddWorkoutExercise(exercises: self.$activeWorkout.exercises)
                .environmentObject(self.exerciseStore)

        }
    }
    
    var saveButton: some View {
        Button(action: {
            self.workoutHistoryStore.addWorkout(workout: self.$activeWorkout.wrappedValue)
            self.activeWorkoutStore.removeActiveWorkout(workout: self.$activeWorkout.wrappedValue)
            self.presentationMode.wrappedValue.dismiss()
        }) {
            Text("Finish Workout")
        }
        .disabled(self.$activeWorkout.wrappedValue.exercises.isEmpty)
    }

    var body: some View {
        Form {
            Section(footer: Text("Choose which exercises are part of this workout")) {
                chooseExercisesButton
            }
            Section(header: Text("Exercises")) {
                if $activeWorkout.wrappedValue.exercises.isEmpty {
                    Text("No exercises")
                } else {
                    ForEach(activeWorkout.exercises.indices, id: .self) { exerciseIndex in
                        NavigationLink(destination: EditWorkoutExercise(exercise: self.$activeWorkout.exercises[exerciseIndex])) {
                            VStack(alignment: .leading) {
                                Text(self.activeWorkout.exercises[exerciseIndex].name)
                                Text("(self.activeWorkout.exercises[exerciseIndex].sets.count) Set(self.activeWorkout.exercises[exerciseIndex].sets.count == 1 ? "" : "s")")
                                    .font(.footnote)
                                    .opacity(0.5)
                            }
                        }
                    }
                    saveButton
                }
            }
        }
        .navigationBarTitle(Text("Edit Workout"), displayMode: .inline )
    }
}

ActiveWorkoutStore

import Foundation
import Combine

class ActiveWorkoutStore: ObservableObject {
    @Published var activeWorkouts: [Workout] = []
    
    func newActiveWorkout() {
        activeWorkouts.append(Workout())
    }
    
    func saveActiveWorkout(workout: Workout) {
        let workoutIndex = activeWorkouts.firstIndex(where: { $0.id == workout.id })!
        
        activeWorkouts[workoutIndex] = workout
    }
    
    func removeActiveWorkout(workout: Workout) {
        if let workoutIndex = activeWorkouts.firstIndex(where: { $0.id == workout.id }) {
            activeWorkouts.remove(at: workoutIndex)
        }
    }
}

Workout

import SwiftUI

struct Workout: Hashable, Codable, Identifiable {
    var id = UUID()
    var date = Date()
    var exercises: [WorkoutExercise] = []
}

Answer 1

ForEach<Range> is constant range container (pay attention on below description of constructor), it is not allowed to modify it after construction.

extension ForEach where Data == Range<Int>, ID == Int, Content : View {

    /// Creates an instance that computes views on demand over a *constant*
    /// range.
    ///
    /// This instance only reads the initial value of `data` and so it does not
    /// need to identify views across updates.
    ///
    /// To compute views on demand over a dynamic range use
    /// `ForEach(_:id:content:)`.
    public init(_ data: Range<Int>, @ViewBuilder content: @escaping (Int) -> Content)
}

If you want to modify container, you have to use ForEach(activeWorkout.exercises)