Why does a custom view in a SwiftUI List trigger eager cell rendering and how can I fix it?

The post content has been automatically edited by the Moderator Agent for consistency and clarity.

I'm experimenting with SwiftUI and noticed unexpected behavior when using a custom view inside a List. In the following simplified code, using a direct Text view renders only about 18 cells on initial load due to SwiftUI's lazy loading, while switching to a custom Cell view results in all 1000 items being created immediately, which affects performance.

SWIFT
import SwiftUI

struct ContentView: View {
    var body: some View {
        NavigationView {
            List(0..<1000, id: \.self) { index in
                let _ = print(index)
                Text("\(index)")        // On initial load ~18 cells are created
//                Cell(index: index)    // On initial load 1000 cells are created
            }
        }
    }
}

struct Cell: View {
    let index: Int
    var body: some View {
        Text("\(index)")
    }
}

#Preview {
    ContentView()
}

Observed Behavior:

• Using Text("\(index)") directly leads to lazy rendering of around 18 cells on initial load.
• Using Cell(index: index) eagerly creates all 1000 cells immediately.

Question:

Why does embedding a custom view cause this eager rendering, and how can I adjust my code to maintain lazy loading while still using a custom view?

Comments 0

•

Answers 2

•

2 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

When you add a custom view in the List’s closure, the extra code (even something as simple as a print statement) or the structure of the view builder can break the lazy behavior. In your example the line

let _ = print(index)

forces the closure to execute completely for each element when the List is being built. This in turn causes SwiftUI to construct every Cell immediately rather than lazily when they’re about to appear on screen.

To fix this while still using your custom view you have a couple of options:

• Remove the debug side effect (or move it inside the custom view’s onAppear or initializer so that it doesn’t interfere with the closure’s laziness).

For example, you can change your List to simply:

List(0..<1000, id: .self) { index in
Cell(index: index)
}

and if you need to log when a cell becomes visible, add an .onAppear modifier inside Cell:

struct Cell: View {
let index: Int
var body: some View {
Text("(index)")
.onAppear {
print("Appeared: (index)")
}
}
}

• Alternatively, wrap the cell’s contents inside a Group so that the closure returns a single view without the extra binding, preserving the lazy loading.

In short, extra statements or bindings in the List’s view builder closure may force SwiftUI to compute all cells immediately. Moving side effects into the view (or removing them) restores the laziness and greatly improves performance.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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Answer by StarlitRanger318 • 1 month ago

Note, NavigationView is deprecated use NavigationStack instead.

List are lazy by default, but your side effect
let _ = print(item) is responsible for showing all the index, it is eagerly evaluated.
Remove it to ...fix this while still using the custom view?

To see this in action, remove let _ = print(item) from the loop, and
instead add

SWIFT
.onAppear { print(index) }

to your Cell, such as

SWIFT
struct Cell: View {
    let index: Int
    var body: some View {
        Text("\(index)")
            .onAppear {
                print(index)  // <-- here
            }
    }
}

The eager evaluation (of print) does not happens when Text is used, because
Text is a built-in view in SwiftUI that has knowledge and optimizations for handling primitive views. The eager evaluation is not done because it is not a custom view.

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Why does a custom view in a SwiftUI List trigger eager cell rendering and how can I fix it?

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