ios - How to dynamically create sections in a SwiftUI List/ForEach and avoid "Unable to infer complex closure return type"

Question

I'm trying to recreate a view that matches the event list in the stock iOS Calendar app. I have the following code which generates a list of events with each event separated into its own section by Date:

var body: some View {
    NavigationView {
        List {
            ForEach(userData.occurrences) { occurrence in
                Section(header: Text("(occurrence.start, formatter: Self.dateFormatter)")) {
                    NavigationLink(
                        destination: OccurrenceDetail(occurrence: occurrence)
                            .environmentObject(self.userData)
                    ) {
                        OccurrenceRow(occurrence: occurrence)
                    }
                }
            }
        }
        .navigationBarTitle(Text("Events"))
    }.onAppear(perform: populate)
}

The problem with this code is that if there are two events on the same date, they are separated into different sections with the same title instead of being grouped together into the same section.

As a Swift novice, my instinct is to do something like this:

ForEach(userData.occurrences) { occurrence in
                if occurrence.start != self.date {
                    Section(header: Text("(occurrence.start, formatter: Self.dateFormatter)")) {
                        NavigationLink(
                            destination: OccurrenceDetail(occurrence: occurrence)
                                .environmentObject(self.userData)
                        ) {
                            OccurrenceRow(occurrence: occurrence)
                        }
                    }
                } else {
                    NavigationLink(
                        destination: OccurrenceDetail(occurrence: occurrence)
                            .environmentObject(self.userData)
                    ) {
                        OccurrenceRow(occurrence: occurrence)
                    }
                }
                self.date = occurrence.start

But in Swift, this gives me the error "Unable to infer complex closure return type; add explicit type to disambiguate" because I'm calling arbitrary code (self.date = occurrence.start) inside ForEach{}, which isn't allowed.

What's the correct way to implement this? Is there a more dynamic way to execute this, or do I need to abstract the code outside of ForEach{} somehow?

Edit: The Occurrence object looks like this:

struct Occurrence: Hashable, Codable, Identifiable {
    var id: Int
    var title: String
    var description: String
    var location: String
    var start: Date
    var end: String
    var cancelled: Bool
    var public_occurrence: Bool
    var created: String
    var last_updated: String

    private enum CodingKeys : String, CodingKey {
        case id, title, description, location, start, end, cancelled, public_occurrence = "public", created, last_updated
    }
}

Update: The following code got me a dictionary which contains arrays of occurrences keyed by the same date:

let myDict = Dictionary( grouping: value ?? [], by: { occurrence -> String in
                            let dateFormatter = DateFormatter()
                            dateFormatter.dateStyle = .medium
                            dateFormatter.timeStyle = .none
                            return dateFormatter.string(from: occurrence.start)
                        })
            self.userData.latestOccurrences = myDict

However, if I try and use this in my View as follows:

ForEach(self.occurrencesByDate) { occurrenceSameDate in
//                    Section(header: Text("(occurrenceSameDate[0].start, formatter: Self.dateFormatter)")) {
                    ForEach(occurrenceSameDate, id: occurrenceSameDate.id){ occurrence in
                        NavigationLink(
                            destination: OccurrenceDetail(occurrence: occurrence)
                                .environmentObject(self.userData)
                        ) {
                            OccurrenceRow(occurrence: occurrence)
                            }
                        }
//                    }
                }

(Section stuff commented out while I get the main bit working)

I get this error: Cannot convert value of type '_.Element' to expected argument type 'Occurrence'

Answer 1

In reference to my comment on your question, the data should be put into sections before being displayed.

The idea would be to have an array of objects, where each object contains an array of occurrences. So we simplify your occurrence object (for this example) and create the following:

struct Occurrence: Identifiable {
    let id = UUID()
    let start: Date
    let title: String
}

Next we need an object to represent all the occurrences that occur on a given day. We'll call it a Day object, however the name is not too important for this example.

struct Day: Identifiable {
    let id = UUID()
    let title: String
    let occurrences: [Occurrence]
    let date: Date
}

So what we have to do is take an array of Occurrence objects and convert them into an array of Day objects.

I have created a simple struct that performs all the tasks that are needed to make this happen. Obviously you would want to modify this so that it matches the data that you have, but the crux of it is that you will have an array of Day objects that you can then easily display. I have added comments through the code so that you can clearly see what each thing is doing.

struct EventData {
    let sections: [Day]

    init() {
        // create some events
        let first = Occurrence(start: EventData.constructDate(day: 5, month: 5, year: 2019), title: "First Event")
        let second = Occurrence(start: EventData.constructDate(day: 5, month: 5, year: 2019, hour: 10), title: "Second Event")
        let third = Occurrence(start: EventData.constructDate(day: 5, month: 6, year: 2019), title: "Third Event")

        // Create an array of the occurrence objects and then sort them
        // this makes sure that they are in ascending date order
        let events = [third, first, second].sorted { $0.start < $1.start }

        // create a DateFormatter 
        let dateFormatter = DateFormatter()
        dateFormatter.dateStyle = .medium
        dateFormatter.timeStyle = .none

        // We use the Dictionary(grouping:) function so that all the events are 
        // group together, one downside of this is that the Dictionary keys may 
        // not be in order that we require, but we can fix that
        let grouped = Dictionary(grouping: events) { (occurrence: Occurrence) -> String in
            dateFormatter.string(from: occurrence.start)
        }

        // We now map over the dictionary and create our Day objects
        // making sure to sort them on the date of the first object in the occurrences array
        // You may want a protection for the date value but it would be 
        // unlikely that the occurrences array would be empty (but you never know)
        // Then we want to sort them so that they are in the correct order
        self.sections = grouped.map { day -> Day in
            Day(title: day.key, occurrences: day.value, date: day.value[0].start)
        }.sorted { $0.date < $1.date }
    }

    /// This is a helper function to quickly create dates so that this code will work. You probably don't need this in your code.
    static func constructDate(day: Int, month: Int, year: Int, hour: Int = 0, minute: Int = 0) -> Date {
        var dateComponents = DateComponents()
        dateComponents.year = year
        dateComponents.month = month
        dateComponents.day = day
        dateComponents.timeZone = TimeZone(abbreviation: "GMT")
        dateComponents.hour = hour
        dateComponents.minute = minute

        // Create date from components
        let userCalendar = Calendar.current // user calendar
        let someDateTime = userCalendar.date(from: dateComponents)
        return someDateTime!
    }

}

This then allows the ContentView to simply be two nested ForEach.

struct ContentView: View {

    // this mocks your data
    let events = EventData()

    var body: some View {
        NavigationView {
            List {
                ForEach(events.sections) { section in
                    Section(header: Text(section.title)) {
                        ForEach(section.occurrences) { occurrence in
                            NavigationLink(destination: OccurrenceDetail(occurrence: occurrence)) {
                                OccurrenceRow(occurrence: occurrence)
                            }
                        }
                    }
                }
            }.navigationBarTitle(Text("Events"))
        }
    }
}

// These are sample views so that the code will work
struct OccurrenceDetail: View {
    let occurrence: Occurrence

    var body: some View {
        Text(occurrence.title)
    }
}

struct OccurrenceRow: View {
    let occurrence: Occurrence

    var body: some View {
        Text(occurrence.title)
    }
}

This is the end result.