Swift constrained Protocol for a subclass and superclass - swift

Question

I wanted to implement my own HUD for a UIViewCntroller and a UIView, so I did this:
protocol ViewHudProtocol {
func showLoadingView()
func hideLoadingView()
}
extension ViewHudProtocol where Self: UIView {
func showLoadingView() { //Show HUD by adding a custom UIView to self.}
}
func hideLoadingView() {
}
}
Now I can easily adopt ViewHudProtocol on any UIView to call showLoadingView and hideLoadingView. The problem is I want to use the same protocol for UIViewController, so I did this:
extension ViewHudProtocol where Self: UIViewController {
func showLoadingView() {
self.view.showLoadingView() //Error: UIView has no member showLoadingView
}
func hideLoadingView() {
self.view.hideLoadingView() //Error: UIView has no member hideLoadingView
}
}
I agree to the error that UIView has not adopted the protocol yet. So I did this:
extension UIView: ViewHudProtocol {}
And it works. Is there a better way to do this? I mean it feels wrong to extend every view with ViewHudProtocol, where not all of them will use it. If I could do something like, "only adopt ViewHudProtocol implicitly for a UIView, if its UIViewController demands for it. Else you could adopt ViewHUDProtocol manually on any UIView when required."

Answer 1

I would solve this with the following approach, using associatedtype, defined only for needed views and/or controllers (tested in Xcode 11.2 / swift 5.1):
protocol ViewHudProtocol {
associatedtype Content : ViewHudProtocol
var content: Self.Content { get }
func showLoadingView()
func hideLoadingView()
}
extension ViewHudProtocol where Self: UIView {
var content: some ViewHudProtocol {
return self
}
func showLoadingView() { //Show HUD by adding a custom UIView to self.}
}
func hideLoadingView() {
}
}
extension ViewHudProtocol where Self: UIViewController {
func showLoadingView() {
self.content.showLoadingView() //NO Error
}
func hideLoadingView() {
self.content.hideLoadingView() //NO Error
}
}
//Usage
extension UITableView: ViewHudProtocol { // only for specific view
}
extension UITableViewController: ViewHudProtocol { // only for specific controller
var content: some ViewHudProtocol {
return self.tableView
}
}

Answer 2

The problem
So you want to constraint the conformance of a UIViewController to the protocol ViewHudProtocol only when the UIViewController.view property conforms to ViewHudProtocol.
I am afraid this is not possible.
Understanding the problem
Let's have a better look at your problem
You have 2 types (UIView and UIViewController) and you want to add to both the same functionalities
func showLoadingView()
func hideLoadingView()
What Mick West teaches us
This kind of scenario is somehow similar to what Mick West faced during the development of the Tony Hawks series Mick West and an elegant solution is described in its article Evolve your hierarchy.
Solution
We can apply that approach to your problem and here's the solution
struct HudViewComponent {
let view: UIView
private let hud: UIView
init(view: UIView) {
self.view = view
self.hud = UIView(frame: view.frame)
self.hud.isHidden = true
self.view.addSubview(hud)
}
func showLoadingView() {
self.hud.isHidden = false
}
func hideLoadingView() {
self.hud.isHidden = true
}
}
protocol HasHudViewComponent {
var hidViewComponent: HudViewComponent { get }
}
extension HasHudViewComponent {
func showLoadingView() {
hidViewComponent.showLoadingView()
}
func hideLoadingView() {
hidViewComponent.hideLoadingView()
}
}
That's it, now you can add the hud functionalities to any Type conforming to HasHudViewComponent.
class SomeView: UIView, HasHudViewComponent {
lazy var hidViewComponent: HudViewComponent = { return HudViewComponent(view: self) }()
}
or
class MyViewController: UIViewController, HasHudViewComponent {
lazy var hidViewComponent: HudViewComponent = { return HudViewComponent(view: self.view) }()
}
Considerations
As you can see the idea is to thinking in terms of components.
You build a component (HudViewComponent) with your hud functionalities. The component only asks for the minimum requirements: it needs a UIView.
Next you define the HasHudViewComponent which states that the current type has a HudViewComponent property.
Finally you can add your hud functionalities to any Type which has a view (UIView, UIViewController, ...) simply conforming your type to HasHudViewComponent.
Notes
You asked an interesting question and I know this does not answers 100% what you were looking for, but by a practical point of view it should provides you with a tool to achieve what you need.

Answer 3

I would have taken this approach:
Create a UIView Class,
setup the view
Declare a shared object.
A function to show the view
A function to remove the view. and then call it in view controllers as IndicatorView.shared.show() , IndicatorView.shared.hide()
import Foundation
import UIKit
import Lottie
class IndicatorView : UIView {
static let shared = IndicatorView()
var loadingAnimation : AnimationView = {
let lottieView = AnimationView()
lottieView.translatesAutoresizingMaskIntoConstraints = false
lottieView.layer.masksToBounds = true
return lottieView
}()
var loadingLabel : UILabel = {
let label = UILabel()
label.textColor = .white
label.translatesAutoresizingMaskIntoConstraints = false
label.font = UIFont(name: "SegoeUI", size: 12)
return label
}()
override init(frame: CGRect) {
super.init(frame: frame)
translatesAutoresizingMaskIntoConstraints = false
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
public func show() {
setupLoadingView()
self.alpha = 0
UIView.animate(withDuration: 0.5, animations: {
self.isHidden = false
self.alpha = 1
}, completion: nil)
applyLottieAnimation()
}
public func hide() {
self.alpha = 1
UIView.animate(withDuration: 0.5, animations: {
self.alpha = 0
}, completion: { _ in
self.isHidden = true
self.removeFromSuperview()
}) }
private func setupLoadingView() {
let controller = UIApplication.shared.keyWindow!.rootViewController!
controller.view.addSubview(self)
//setup your views here
self.setNeedsLayout()
self.reloadInputViews()
}
}

Answer 4

For this particular scenario, a Decorator would work better, and result in a better design:
final class HUDDecorator {
private let view: UIView
init(_ view: UIView) {
self.view = view
}
func showLoadingView() {
// add the spinner
}
func hideLoadingView() {
// remove the spinner
}
}
Using the Decorator would then be as easy as declaring a property for it:
class MyViewController: UIViewController {
lazy var hudDecorator = HUDDecorator(view)
}
This will allow any controller to decide if it wants support for showing/hiding a loading view by simply exposing this property.
Protocols are too invasive for simple tasks like enhancing the looks on a UI component, and they have the disadvantage of forcing all views of a certain class to expose the protocol functionality, while the Decorator approach allows you to decide which view instances to receive the functionality.