Swift - func viewWillAppear

Question

I have a standard SingleViewApplication project.

ViewController.swift

import UIKit
class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        println("viewDidLoad");
    }
}

When I start the application, viewDidLoad is called.

My scenario:
- press Home button (applicationDidEnterBackground)
- recall application (applicationWillEnterForeground)
and viewDidLoad is not called.

Is there another func to override?

Spicy

Answer 1

If you want to call viewWillAppear in swift use this.

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated) // No need for semicolon
}

Strongarm

I am curious: can you explain why we need to pass false? – Minny

Don't pass false. Pass on the value of "animated" to the super class. – Neogothic

old habits die hard (';') :) – Bookplate

Answer 2

The best practice is to register for UIApplicationWillEnterForegroundNotification and UIApplicationWillEnterBackgroundNotification in your ViewController

public override func viewDidLoad()
{
    super.viewDidLoad()

    NSNotificationCenter.defaultCenter().addObserver(self, selector: "applicationWillEnterForeground:", name: UIApplicationWillEnterForegroundNotification, object: nil)
    NSNotificationCenter.defaultCenter().addObserver(self, selector: "applicationWillEnterBackground:", name: UIApplicationDidEnterBackgroundNotification, object: nil)
}

deinit {
    NSNotificationCenter.defaultCenter().removeObserver(self)
}

func applicationWillEnterForeground(notification: NSNotification) {
    println("did enter foreground")
}

func applicationWillEnterBackground(notification: NSNotification) {
    println("did enter background")
}

Redness

Answer 3

Based on Noah response:
on ViewController.swift add refresh function and call it from AppDelegate.swift > applicationWillEnterForeground

ViewController.swift  

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        println("viewDidLoad");
        refresh();
    }

    func refresh(){
        println("refresh");
    }
}

.

AppDelegate.swift
func applicationWillEnterForeground(application: UIApplication!) {
    // Called as part of the transition from the background to the active state; here you can undo many of the changes made on entering the background.
    ViewController().refresh();
}

Output:

viewDidLoad  
refresh  
refresh  
refresh  
refresh  

Spicy

Note that ViewController().refresh() creates a new instance of Viewcontroller. Also, since this is not stored anywhere ARC will deallocate it quite immediately. – Redness

Answer 4

viewDidLoad is only called when your view controller’s view is first loaded—it remains in memory after that, so in general it will never be called again until you create another instance of the view controller. If you need to refresh content in your view controller when the application enters the foreground, you should create a method to do that and call it from applicationWillEnterForeground.

Saponify

Answer 5

Same as @Sunkas but in Swift 4:

open override func viewDidLoad()
{
    super.viewDidLoad()
    NotificationCenter.default.addObserver(self, selector: #selector(applicationWillEnterForeground(notification:)), name: .UIApplicationWillEnterForeground, object: nil)
    NotificationCenter.default.addObserver(self, selector: #selector(applicationDidEnterBackground(notification:)), name: .UIApplicationDidEnterBackground, object: nil)
}

deinit {
    NotificationCenter.default.removeObserver(self)
}

@objc private func applicationWillEnterForeground(notification: NSNotification) {
    print("will enter foreground")
}

@objc private func applicationDidEnterBackground(notification: NSNotification) {
    print("did enter background")
}

Chilli

Not sure in which version it got changed but I had to change name: .UIApplicationWillEnterForeground to name: UIApplication.willEnterForegroundNotification – Hawkie

Answer 6

If you are using page view controllers and swiping between the controllers or if you are coming back from background, your viewDidLoad does not get called which means your viewDidLoad will get called only once. You need to set the data setup in viewWillAppear() since it gets called every time the view appears.

Purchase

viewWillAppear does not get called for me on iOS 15 simulator when an app gets in background, then back into foreground – Hawkie