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SOLVED: Preview Crashing When Trying To Iterate Relationship

Forums > SwiftUI

I am currently teaching myself Swift and Swiftui, to that end I am creating an app that is semi useful to me. I have created a peice of functionality that works within the simulator as well as on a physical device but seems to cause a crash in the preview.

I have created the following model:

final class Hosts{
    var ip: String?

    var api_url: URL

    var connected_too: Hosts? = nil

    @Relationship(inverse: \Hosts.connected_too)
    var connected_from: [Hosts] = []

    var host_description: String?
    var hostname: String

    init(api_url: URL, hostname: String, ip: String?, host_description: String?){
        self.api_url = api_url
        self.host_description = host_description
        self.hostname = hostname.lowercased()
        self.ip = ip

I have a view that lists each instance of an object of this model type and I pass in the instance to a further view so that I can see the full details.

so the detail view is is like:

struct HostDetailView: View {
    public var host: Hosts

During processing of this view a method is called:

func get_connected_too(host: Hosts) -> [Hosts]{
    var current_host: Hosts = host
    var connected_hosts: [Hosts] = []
    while (current_host.connected_too != nil) {
        current_host = current_host.connected_too!
    return connected_hosts

this produces a list of connected too hosts or an empty list (so think device a is connected to switch b which is connected to router c). As I say this works perfectly on a defvice or on the simulator.

The issue is coming in when I try to create a preview. I have created the following:

struct HostDetailView_Previews: PreviewProvider {
    static var previews: some View {
        Group {
            HostDetailView(host: test_data())
                .previewDisplayName("Default mode")

            HostDetailView(host: test_data())
                .previewDisplayName("Dark mode")

func test_data() -> Hosts{
    let router: Hosts = Hosts(api_url: URL(string: "")!, hostname: "router", ip: "", host_description: "Main router")
    let host: Hosts = Hosts(api_url: URL(string: "")!, hostname: "router", ip: "", host_description: "Main router")
    host.connected_too = router
    return host

Unfortunately this causes the preview to crash, the pertinent part of the crash report is:

5   HostDetailView.1.preview-thunk.dylib           0x14edb51b4 __preview__get_connected_too(host:) + 140 (HostDetailView.swift:73)
6   HostDetailView.1.preview-thunk.dylib           0x14edb57cc closure #1 in HostLinksStruct.__preview__body.getter + 372 (HostDetailView.swift:44)
7   HostDetailView.1.preview-thunk.dylib           0x14edb5968 partial apply for closure #1 in HostLinksStruct.__preview__body.getter + 16

If I remove the while clause in the get_connected_too function the preview will work, but if I either comment out the content of the while or only the while start and end block the preview wil crash.

I suspect the issue might be with how I am constructing or providing the test data. Can anyone identfy what I might be doing wrong.


Turns out I have actually just managed to fix it myself. by creating a preview in this way instead:

#Preview {
    let config_mem = ModelConfiguration(isStoredInMemoryOnly: true)
    let container = try! ModelContainer(for: Hosts.self, configurations: config_mem)
    let router: Hosts = Hosts(api_url: URL(string: "")!, hostname: "router", ip: "", host_description: "Main router")
    let host: Hosts = Hosts(api_url: URL(string: "")!, hostname: "router", ip: "", host_description: "Main router")
    return HostDetailView(host: host)


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