A source code to demo view did load behavior on SwiftUI view

CustomViewDidLoad.swift

import SwiftUI

struct CustomViewDidLoad: View {

    var body: some View {
        VStack {
            Text("Hello")
        }
    }
}

struct CustomViewDidLoadModifier: ViewModifier {
    @State private var didViewLoad = false

    private let action: (() -> Void)?

    init(perform action: (() -> Void)? = nil) {
        self.action = action
    }

    func body(content: Content) -> some View {
        content.onAppear {
            if !didViewLoad {
                didViewLoad = true
                action?()
            }
        }
    }
}

extension View {
    func onLoad(perform action: (() -> Void)? = nil) -> some View {
        modifier(CustomViewDidLoadModifier(perform: action))
    }
}

struct CustomViewDidLoad_Previews: PreviewProvider {
    static var previews: some View {
        CustomViewDidLoad().onLoad {
            print("YES")
        }
    }
}