swift change theme using delegate and protocol on many view controller

Question

I'm trying to use delegate and protocol first time.

I want to change the theme across many view controller.

Then on any controller which has protocol to change theme

When I go to this controller now I expect theme to be new but is old.

I do not go from theme controller to where them has changed

My code

protocol ThemeDelegate: class {
    func changeTheme(theme: UIColor)
}


class FirstController: UICollectionViewController, UICollectionViewDelegateFlowLayout, ThemeDelegate {

    var newTheme: UIColor  = .red

    func changeTheme(theme: UIColor) {
        newTheme = theme
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = newTheme
    }
}

ThemeController {

    weak var themeDelegate: ThemeDelegate?

    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {        
        let theme = .blue        
        themeDelegate?.changeTheme(theme: theme)        
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {

        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: themeCellId, for: indexPath) as! ThemeCell
        cell.themeImageView.image = UIImage(named: "theme cell image")
        return cell
    }
}

this is order

image2

Did you created this protocol in your CollectionViewCell class? — Wings, Jul 17 '19 at 05:38
And where you created this `weak var themeDelegate: ThemeDelegate?`. ?? — Wings, Jul 17 '19 at 05:40
The in `cellForItem` method call `cell.themeDelegate = self` — Wings, Jul 17 '19 at 05:42
Its because you didn't called themeDelegate where you calling this delegate function just call delegate. check this answer https://stackoverflow.com/questions/52327377/delegate-seems-to-not-be-working-according-to-the-console/52327562#52327562 — Wings, Jul 17 '19 at 05:48
"you need to pass that delegate from one view controller to another view controller" i am not using storyboard. — boobmaster, Jul 17 '19 at 06:03
Just to answer your question, and why it doesn't work, I've posted an answer. But, all of these seem like bits and pieces since you have not made your objective clear. Please post more about the two controllers, how you handle their instances and such so that we can probably be of more help — Lokesh SN, Jul 17 '19 at 06:43
thank you for the answer. i understand some more. i will work now to add more information to the question. thanks — boobmaster, Jul 17 '19 at 06:52

score 1 · Answer 1 · answered Jul 17 '19 at 06:33

I'll try to make it simple:-

1) First, declare a protocol:-

protocol ThemeDelegate{
    func changeTheme(theme: String, fontColor: UIColor, alpha: CGFloat)
}

2) Have a variable of that protocol in your ThemeController:

open var themeDelegate: ThemeDelegate? = nil

3) Call the delegate functions through themeDelegate:- (you've done this right till this step)

func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
    themeDelegate?.changeTheme(theme: theme, fontColor: fontColor, alpha: alpha)
}

4) You need to conform your AnyController as the delegate, like yourThemeControllerInstance.delegate = self and you've done that as well.

It doesn't work because you've declared a new instance of ThemeController and have conformed AnyController to be the delegate of that new instance which supposedly has the same old theme:

override func viewDidLoad() {
    let themeController = ThemeController() // This is a new instance which you have created, so gets initialised with old theme. You have made your class to be the delegate of this instance
    themeController.themeDelegate = self
}

In order for your delegate to work as expected, you need the same instance of ThemeController where you change your theme

score 0 · Answer 2 · answered Jul 17 '19 at 06:45

You can set delegate but careful view needs a be load.

You do this also with notifications and send all viewControllers with conforms Theme

1 - Your Step

import UIKit

protocol ThemeDelegate: class {
    func changeTheme(theme: String, fontColor: UIColor, alpha: CGFloat)
}

class ThemeController: UIViewController {

    weak var themeDelegate: ThemeDelegate?

    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        themeDelegate?.changeTheme(theme: theme, fontColor: fontColor, alpha: alpha)
    }

}

class XViewController: UIViewController {


    // Create an instance
    lazy var themeController = ThemeController()


    override func viewDidLoad() {
        super.viewDidLoad()

        // We need call loadView becase collectionView needs to be show
        themeController.loadView()
        themeController.themeDelegate = self

    }

}
extension XViewController: ThemeDelegate {
    // And we conform ThemeDelegate
    func changeTheme(theme: String, fontColor: UIColor, alpha: CGFloat) {
        // TODO UI
    }
}

2 - Observer Step

// Sometimes we need update multiple viewControllers and we should use notification and observers


// We create a model
struct Theme {
    let theme: String
    let fontColor: UIColor
    let alpha: CGFloat
}

// We need a protocol for we don't want all view controller listen theme
protocol Themeable: class {
    func listenTheme()
    func didThemeChange(theme: Theme)
}


// Global notification name
let themeableNotificationName = Notification.Name(rawValue: "ThemeableNotification")

// Our protocol extension and observer notification
extension Themeable where Self: UIViewController {
    func listenTheme() {
        NotificationCenter.default.addObserver(forName: themeableNotificationName, object: nil, queue: nil) { [weak self] notification in
            guard let theme = notification.object as? Theme else { return }
            self?.didThemeChange(theme: theme)
        }
    }

}


// Notification sender themeController
class NotifyThemeController: UIViewController {


    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        // Create a model and post
        NotificationCenter.default.post(name: themeableNotificationName, object: Theme(theme: "Lorem", fontColor: .red, alpha: 1.0), userInfo: nil)
    }

}

// YViewController
class YViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        // We need call this method for observer
        listenTheme()
    }


}
// YViewController conforms Themeable
extension YViewController: Themeable {
    func didThemeChange(theme: Theme) {
        // TODO UI
    }

}
// ZViewController
class ZViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // We need call this method for observer
        listenTheme()
    }


}
// ZViewController conforms Themeable
extension ZViewController: Themeable {
    func didThemeChange(theme: Theme) {
        // TODO UI
    }

}

Have Fun!

really thank you for this answer. i am understanding a lot more. very helpful to know 2 ways. — boobmaster, Jul 17 '19 at 07:11

swift change theme using delegate and protocol on many view controller

2 Answers2

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