Understanding .onAppear and .onDisappear in SwiftUI

SwiftUI simplifies the way developers think about the lifecycle of views with its declarative syntax. Unlike UIKit, which requires you to manage the lifecycle through methods like viewDidLoad, viewWillAppear, and viewWillDisappear, SwiftUI provides .onAppear and .onDisappear modifiers for executing code when a view appears or disappears. This tutorial demonstrates how to use these modifiers in a real case scenario.

The Basics

.onAppear: This modifier allows you to perform actions right after the view has been added to the view hierarchy.
.onDisappear: Conversely, this modifier lets you execute code just as the view is about to be removed from the view hierarchy.

Example Scenarios

Let us take an example of an app that is used for Splitting expenses between friends. The main screen will be showing list of shared expenses and this can be fetched from the API or database using the .onAppear modifier. Similarly when the user creates a new expense and leaves the screen the .onDisAppear modifier can be used to create a new expense and clean up the data.

import SwiftUI

struct ExpensesListView: View {
    @State private var expenses: [Expense] = []

    var body: some View {
        NavigationView {
            List(expenses) { expense in
                ExpenseRow(expense: expense)
            }
            .navigationBarTitle("Shared Expenses")
            .onAppear {
                loadExpenses()
            }
            .onDisappear {
                saveExpenses()
            }
        }
    }
    
    func loadExpenses() {
        // Add Logic to load expenses from your data source
        print("Expenses loaded.")
    }
    
    func saveExpenses() {
        // Add Logic to save any changes to the expenses
        print("Expenses saved.")
    }
}

Other real world examples use cases could be

Analytics – Track screen views and user interactions by triggering analytics events in .onAppear and .onDisappear.
Releasing Resources: Use .onDisappear to cancel network requests, timers, or free up any resources that are no longer needed.