So here is an app that only enables the submit button once both terms and conditions switches have been accepted, and the user enters a valid username in the text field.
A good way to start building this would be to define some outlets from our Storyboard.
@IBOutlet var termsSwitch: UIStackView!
@IBOutlet var privacySwitch: UISwitch!
@IBOutlet var nameField: UITextField!
@IBOutlet var submitButton: UIButton!Define some properties to hold state.
private var acceptedTerms: Bool = false
private var acceptedPrivacy: Bool = false
private var name: String = ""And then the target-action methods for the two switches and the text field for whenever something changes.
@IBAction private func acceptTerms(_ sender: UISwitch) {
acceptedTerms = sender.isOn
}
@IBAction private func acceptPrivacy(_ sender: UISwitch) {
acceptedPrivacy = sender.isOn
}
@IBAction private func nameChanged(_ sender: UITextField) {
name = sender.text ?? ""
}Now if we weren’t using Combine, this is where we could create some method name configureView, and call it from the didSet observer on each property.
private var acceptedTerms: Bool = false {
didSet { configureView() }
}And then add the logic there to check and see if all the conditions have been met. But we are here to learn Combine. So let’s see how using Combine would work.
import CombineThe way we turn properties into publishers is by adding the @Published keyword to each property.
@Published private var acceptedTerms: Bool = false
@Published private var acceptedPrivacy: Bool = false
@Published private var name: String = ""The @Published keyword is a property wrapper, which creates a publisher for any property of a class.
You can’t use it with value types like
struct. It can only be used onclass.
What’s great about @Published is you can still access variables just as if it were a regular property.
@Published var password: String = ""
self.password = "123"
let newPassword = self.password
let printerSubscription = $password.sink {
print("The published value is \($0)")
}But when you use the $ prefix, you get access to the published wrapper, which lets you sign yourself as a subscriber, and be notified when any values change like this.
// Keep a reference to the subscription so it's
// only cancelled when we are deallocated.
private var termsStream: AnyCancellable?
// Simple sink subscription to print value
termsStream = $acceptedTerms.sink { value in
print("Received \(value)")
}In order to enable the submit button we need to ‘combine’ the results of three publishers:
acceptedTerms acceptedPrivacy, and name
The we way do that in Combine is to use CombineLatest. CombineLatest is a publisher that takes the results from multiple publishers, and combines them and their latest values into one.
Publishers.CombineLatest3($acceptedTerms, $acceptedPrivacy, $name)Note how we are using the binding $ here. They gives us a tuple of type (Bool, Bool, String). A subscribe to this publisher will then receive these values and is able to use them for processing.
Next we can use the map operation to convert our tuple into a single Bool indicating whether we can enable the submit button.
.map { terms, privacy, name in
terms && privacy && !name.isEmpty
}Publishers can have really complicated signatures. And if we have to expose those signatures to the outside world, we would end up with some really messy and wacky APIs. So to keep our APIs simple and neat, Apple create the eraseToAnyPublisher() method expose only the generic type AnyPublisher<Bool, Never> to the outside world, and hide the internals.
Last step. Here we create the subscription to receive from our publisher and assign the output to the state of the submit button.
Because we are working with classes, don’t want the stream to be cancelled until the view control goes away we store a reference to our subscription as a type AnyCancellable.
private var stream: AnyCancellable?This also prevents memory leaks from occurring, and ensures the subscription gets nicely cleaned up later on deinit().
Then we create the subscription to our publisher in viewDidLoad.
stream = validToSubmit
.receive(on: RunLoop.main)
.assign(to: \.isEnabled, on: submitButton)Since we’re interacting with UIKit, we use receive(on:) to make sure we receive from our publisher on the main run loop. The assign(to:) operator assigns the received value to a keypath of an object. We use it to assign the received Bool to the isEnabled property of the submit button.
And that’s basically it! When both switches are on, and there is text in the text field, the submit button will be enabled and the text will be blue.
I am not really sure yet best how to debug Combine. Until then I simply rely on good on print statements.
.map { terms, privacy, name in
print("foo - terms: \(terms) privacy: \(privacy) name: \(name)")
return terms && privacy && !name.isEmpty
}.eraseToAnyPublisher()import UIKit
import Combine
class ViewController: UIViewController {
@IBOutlet var termsSwitch: UIStackView!
@IBOutlet var privacySwitch: UISwitch!
@IBOutlet var nameField: UITextField!
@IBOutlet var submitButton: UIButton!
// Define publishers
@Published private var acceptedTerms = false
@Published private var acceptedPrivacy = false
@Published private var name = ""
private var stream: AnyCancellable?
override func viewDidLoad() {
super.viewDidLoad()
nameField.delegate = self
stream = validToSubmit
.receive(on: RunLoop.main)
.assign(to: \.isEnabled, on: submitButton)
}
private var validToSubmit: AnyPublisher<Bool, Never> {
return Publishers.CombineLatest3($acceptedTerms, $acceptedPrivacy, $name)
.map { terms, privacy, name in
print("foo - terms: \(terms) privacy: \(privacy) name: \(name)")
return terms && privacy && !name.isEmpty
}.eraseToAnyPublisher()
}
@IBAction func acceptTerms(_ sender: UISwitch) {
acceptedTerms = sender.isOn
}
@IBAction func acceptPrivacy(_ sender: UISwitch) {
acceptedPrivacy = sender.isOn
}
@IBAction func nameChanged(_ sender: UITextField) {
name = sender.text ?? ""
}
@IBAction func submitAction(_ sender: UIButton) {
print("foo - Submit: \(name)")
}
}
extension ViewController: UITextFieldDelegate {
func textFieldShouldReturn(_ textField: UITextField) -> Bool {
textField.resignFirstResponder()
return true;
}
}