Why Can't I Append to a State Array Within View in SwiftUI?

Question

I'm not sure why this code doesn't work. It builds and runs fine, but nothing is displayed in the view. While debugging, I can see that the append() calls don't actually add the items to the array, but there's no error:

import SwiftUI

struct Test: View {
    @State private var array: [String] = []

    var body: some View {
        self.array.append("A")
        self.array.append("B")
        self.array.append("C")

        return VStack {
            ForEach(self.array, id: \.self) {string in
                Text(string)
            }
        }
    }
}

struct Test_Previews: PreviewProvider {
    static var previews: some View {
        Test()
    }
}

I've also tried doing the appends in an init(_:) with the same result. It works if I initialize the array with the values like @State private var array: [String] = ["A", "B", "C"] so it seems like there's some immutability of state variables nuance that I'm missing. I'm pretty new to Swift. Can someone please explain what I'm doing wrong here?

Answer 1

I tried to run your code on Xcode 11.4.1, I got a warning saying;
Modifying state during view update, this will cause undefined behavior

This error occurs because you’re trying to modify the state of a SwiftUI view while it is actually being rendered.

So as an alternative you can try appending items in onAppear block

struct Test: View {

    State private var array: [String] = []

    var body: some View {
        VStack {
            ForEach(self.array, id: \.self) {string in
                Text(string)
            }
        }.onAppear { // Prefer, Life cycle method
            self.array.append("A")
            self.array.append("B")
            self.array.append("C")
        }
    }
}

Then you'll be able to see your items in the screen.

Also here is a bit detailed explanation