I made a small project that freezes after the following steps:
Run the app
Turn on VoiceOver
Tap on "Header" once so it is read out loud
Turn off VoiceOver
Scroll up and down really quickly
Using the Time Profiler, the items in the Main Thread followed by significant drops are:
19.11 s 85.1% 0 s closure #2 in closure #1 in ViewRendererHost.render(interval:updateDisplayList:targetTimestamp:)
13.05 s 58.1% 4.00 ms ViewGraph.updateOutputs(async:)
7.97 s 35.5% 83.00 ms AG::Graph::UpdateStack::update()
5.76 s 25.6% 19.00 ms LayoutScrollableTransform.updateValue()
1.73 s 7.7% 2.00 ms specialized NativeDictionary.setValue(:forKey:isUnique:)
579.00 ms 2.6% 58.00 ms 0x100a8c908
311.00 ms 1.4% 165.00 ms __thread_stack_pcs
I see the memory usage increase by about 0.6 MB per second while frozen.
And there are a few things that prevent the freeze:
Changing LazyVStack for VStack
Removing the VStack from the Section
Removing the header: parameter from the Section
Reducing the range of the ForEach
Move the Text(verbatim: "Text at same level as ForEach containing Section") outside of the LazyVStack
However, these are all things I need to keep for my actual app. So what is causing this hang? Am I using SwiftUI in any way it's not intended to be used?
import SwiftUI
struct ContentView: View {
var body: some View {
VStack {
VStack(alignment: .leading) {
Image(systemName: "pencil.circle.fill")
Text("Title")
Text("Label")
}
ScrollView {
LazyVStack {
Section {
VStack {
ForEach(0..<70) { _ in
Text(verbatim: "A")
}
}
} header: {
Text(verbatim: "Header")
}
Text(verbatim: "Text at same level as ForEach containing Section")
}
}
}
}
}
AppDelegate:
@main
class AppDelegate: UIResponder, UIApplicationDelegate {
var window: UIWindow?
func application(
_ application: UIApplication,
didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
) -> Bool {
let window = UIWindow(frame: UIScreen.main.bounds)
window.rootViewController = UINavigationController(rootViewController: MainViewController())
window.makeKeyAndVisible()
self.window = window
return true
}
}
MainViewController:
import UIKit
import SwiftUI
final class MainViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
let swiftUIView = ContentView()
let hostingController = UIHostingController(rootView: swiftUIView)
addChild(hostingController)
hostingController.view.translatesAutoresizingMaskIntoConstraints = false
view.addSubview(hostingController.view)
NSLayoutConstraint.activate([
hostingController.view.topAnchor.constraint(equalTo: view.topAnchor),
hostingController.view.bottomAnchor.constraint(equalTo: view.bottomAnchor),
hostingController.view.leadingAnchor.constraint(equalTo: view.leadingAnchor),
hostingController.view.trailingAnchor.constraint(equalTo: view.trailingAnchor)
])
hostingController.didMove(toParent: self)
view.backgroundColor = .white
}
}
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Initially, I had a property with the @State wrapper to define whether or not a DisclosureGroup is expanded. When expanded, the numbers have a nice animation where they appear one by one (this does not show on the Canvas, it has to be run on a sim or device).
struct ExampleView: View {
@State var isExpanded = false
var body: some View {
DisclosureGroup(isExpanded: $isExpanded) {
ForEach(1..<10) { num in
Text("\(num)")
}
} label: {
Text("Example")
.foregroundColor(.black)
}
}
}
When I replaced @State for @AppStorage("isExpanded") I expected the behavior to be the same, but when expanding I got shown all numbers at once, while the title of the DisclosureGroup was still moving up.
Is this expected? Should I file a bug report?