swift - How can I use Combine to track UITextField changes in a UIViewRepresentable class?

Question

I have created a custom text field and I'd like to take advantage of Combine. In order to be notified whenever text changes in my text field, I currently use a custom modifier. It works well, but I want this code could inside my CustomTextField struct.

My CustomTextField struct conforms to UIViewRepresentable. Inside this struct, there is a NSObject class called Coordinator and it conforms to UITextFieldDelegate.

I'm already using other UITextField delegate methods, but couldn't find one that does exactly what I already do with my custom modifier. Some methods are close, but don't quite behave the way I want them to. Anyway, I feel it would be best to put this new custom textFieldDidChange method in the Coordinator class.

Here is my custom modifier

private let textFieldDidChange = NotificationCenter.default
    .publisher(for: UITextField.textDidChangeNotification)
    .map { $0.object as! UITextField}


struct CustomModifer: ViewModifier {

     func body(content: Content) -> some View {
         content
             .tag(1)
             .onReceive(textFieldDidChange) { data in

                //do something

             }
    }
}

My CustomTextField is used in a SwiftUI view, with my custom modifier attached to it. I’m able to do things when ever there are changes to the text field. The modifier is also using Combine. It works great, but I don't want this functionality to be in the form of a modifier. I want to use it in my Coordinator class, along with my UITextFieldDelegate methods.

This is my CustomTextField

struct CustomTextField: UIViewRepresentable {

    var isFirstResponder: Bool = false
    @EnvironmentObject var authenticationViewModel: AuthenticationViewModel

    func makeCoordinator() -> Coordinator {
        return Coordinator(authenticationViewModel: self._authenticationViewModel)
    }

    class Coordinator: NSObject, UITextFieldDelegate {

        var didBecomeFirstResponder = false
        @EnvironmentObject var authenticationViewModel: AuthenticationViewModel

        init(authenticationViewModel: EnvironmentObject<AuthenticationViewModel>)
        {
            self._authenticationViewModel = authenticationViewModel
        }

        // Limit the amount of characters that can be typed in the field
        func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {

            let currentText = textField.text ?? ""
            guard let stringRange = Range(range, in: currentText) else { return false }
            let updatedText = currentText.replacingCharacters(in: stringRange, with: string)
            return updatedText.count <= 14
        }

        /* I want to put my textFieldDidChange method right here */

        /* * * * * * * * * * * * * * * * * * * * * * * * * * * * */


        func textFieldDidEndEditing(_ textField: UITextField) {

            textField.resignFirstResponder()
            textField.endEditing(true)
        }

    }

    func makeUIView(context: Context) -> UITextField {

        let textField = UITextField()
        textField.delegate = context.coordinator
        textField.placeholder = context.coordinator.authenticationViewModel.placeholder
        textField.font = .systemFont(ofSize: 33, weight: .bold)
        textField.keyboardType = .numberPad

        return textField
    }

    func updateUIView(_ uiView: UITextField, context: Context) {

        let textField = uiView
        textField.text = self.authenticationViewModel.text
    }
}

struct CustomTextField_Previews: PreviewProvider {

    static var previews: some View {
        CustomTextField()
            .previewLayout(.fixed(width: 270, height: 55))
            .previewDisplayName("Custom Textfield")
            .previewDevice(.none)
    }
}

I've been watching videos about Combine and I'd like to start utilising it in a new app I'm building. I really think it's the right thing to use in this situation, but still not quite sure how to pull this off. I'd really appreciate an example.

To summarise:

I want to add a function called textFieldDidChange to my Coordinator class, and it should be triggered every time there is a change to my text field. It must utilise Combine.

Thanks in advance

Answer 1

Updated Answer

After looking at your updated question, I realized my original answer could use some cleaning up. I had collapsed the model and coordinator into one class, which, while it worked for my example, is not always feasible or desirable. If the model and coordinator cannot be the same, then you can't rely on the model property's didSet method to update the textField. So instead, I'm making use of the Combine publisher we get for free using a @Published variable inside our model.

The key things we need to do are to:

1. Make a single source of truth by keeping model.text and textField.text in sync

   1. Use the publisher provided by the @Published property wrapper to update textField.text when model.text changes

   2. Use the .addTarget(:action:for) method on textField to update model.text when textfield.text changes

2. Execute a closure called textDidChange when our model changes.

(I prefer using .addTarget for #1.2 rather than going through NotificationCenter, as it's less code, worked immediately, and it is well known to users of UIKit).

Here is an updated example that shows this working:

Demo

import SwiftUI
import Combine

// Example view showing that `model.text` and `textField.text`
//     stay in sync with one another
struct CustomTextFieldDemo: View {
    @ObservedObject var model = Model()

    var body: some View {
        VStack {
            // The model's text can be used as a property
            Text("The text is "(model.text)"")
            // or as a binding,
            TextField(model.placeholder, text: $model.text)
                .disableAutocorrection(true)
                .padding()
                .border(Color.black)
            // or the model itself can be passed to a CustomTextField
            CustomTextField().environmentObject(model)
                .padding()
                .border(Color.black)
        }
        .frame(height: 100)
        .padding()
    }
}

Model

class Model: ObservableObject {
    @Published var text = ""
    var placeholder = "Placeholder"
}

View

struct CustomTextField: UIViewRepresentable {
    @EnvironmentObject var model: Model

    func makeCoordinator() -> CustomTextField.Coordinator {
        Coordinator(model: model)
    }

    func makeUIView(context: UIViewRepresentableContext<CustomTextField>) -> UITextField {
        let textField = UITextField()

        // Set the coordinator as the textField's delegate
        textField.delegate = context.coordinator

        // Set up textField's properties
        textField.text = context.coordinator.model.text
        textField.placeholder = context.coordinator.model.placeholder
        textField.autocorrectionType = .no

        // Update model.text when textField.text is changed
        textField.addTarget(context.coordinator,
                            action: #selector(context.coordinator.textFieldDidChange),
                            for: .editingChanged)

        // Update textField.text when model.text is changed
        // The map step is there because .assign(to:on:) complains
        //     if you try to assign a String to textField.text, which is a String?
        // Note that assigning textField.text with .assign(to:on:)
        //     does NOT trigger a UITextField.Event.editingChanged
        let sub = context.coordinator.model.$text.receive(on: RunLoop.main)
                         .map { Optional($0) }
                         .assign(to: UITextField.text, on: textField)
        context.coordinator.subscribers.append(sub)

        // Become first responder
        textField.becomeFirstResponder()

        return textField
    }

    func updateUIView(_ textField: UITextField, context: UIViewRepresentableContext<CustomTextField>) {
        // If something needs to happen when the view updates
    }
}

View.Coordinator

extension CustomTextField {
    class Coordinator: NSObject, UITextFieldDelegate, ObservableObject {
        @ObservedObject var model: Model
        var subscribers: [AnyCancellable] = []

        // Make subscriber which runs textDidChange closure whenever model.text changes
        init(model: Model) {
            self.model = model
            let sub = model.$text.receive(on: RunLoop.main).sink(receiveValue: textDidChange)
            subscribers.append(sub)
        }

        // Cancel subscribers when Coordinator is deinitialized
        deinit {
            for sub in subscribers {
                sub.cancel()
            }
        }

        // Any code that needs to be run when model.text changes
        var textDidChange: (String) -> Void = { text in
            print("Text changed to "(text)"")
            // * * * * * * * * * * //
            // Put your code here  //
            // * * * * * * * * * * //
        }

        // Update model.text when textField.text is changed
        @objc func textFieldDidChange(_ textField: UITextField) {
            model.text = textField.text ?? ""
        }

        // Example UITextFieldDelegate method
        func textFieldShouldReturn(_ textField: UITextField) -> Bool {
            textField.resignFirstResponder()
            return true
        }
    }
}

Original Answer

It sounds like you have a few goals:

1. Use a UITextField so you can use functionality like .becomeFirstResponder()
2. Perform an action when the text changes
3. Notify other SwiftUI views that the text has changed

I think you can satisfy all these using a single model class, and the UIViewRepresentable struct. The reason I structured the code this way is so that you have a single source of truth (model.text), which can be used interchangeably with other SwiftUI views that take a String or Binding<String>.

Model

class MyTextFieldModel: NSObject, UITextFieldDelegate, ObservableObject {
    // Must be weak, so that we don't have a strong reference cycle
    weak var textField: UITextField?

    // The @Published property wrapper just makes a Combine Publisher for the text
    @Published var text: String = "" {
        // If the model's text property changes, update the UITextField
        didSet {
            textField?.text = text
        }
    }

    // If the UITextField's text property changes, update the model
    @objc func textFieldDidChange() {
        text = textField?.text ?? ""

        // Put your code that needs to run on text change here
        print("Text changed to "(text)"")
    }

    // Example UITextFieldDelegate method
    func textFieldShouldReturn(_ textField: UITextField) -> Bool {
        textField.resignFirstResponder()
        return true
    }
}

View

struct MyTextField: UIViewRepresentable {
    @ObservedObject var model: MyTextFieldModel

    func makeUIView(context: UIViewRepresentableContext<MyTextField>) -> UITextField {
        let textField = UITextField()

        // Give the model a reference to textField
        model.textField = textField

        // Set the model as the textField's delegate
        textField.delegate = model

        // TextField setup
        textField.text = model.text
        textField.placeholder = "Type in this UITextField"

        // Call the model's textFieldDidChange() method on change
        textField.addTarget(model, action: #selector(model.textFieldDidChange), for: .editingChanged)

        // Become first responder
        textField.becomeFirstResponder()

        return textField
    }

    func updateUIView(_ textField: UITextField, context: UIViewRepresentableContext<MyTextField>) {
        // If something needs to happen when the view updates
    }
}

If you don't need #3 above, you could replace

@ObservedObject var model: MyTextFieldModel

with

@ObservedObject private var model = MyTextFieldModel()

Demo

Here's a demo view showing all this working

struct MyTextFieldDemo: View {
    @ObservedObject var model = MyTextFieldModel()

    var body: some View {
        VStack {
            // The model's text can be used as a property
            Text("The text is "(model.text)"")
            // or as a binding,
            TextField("Type in this TextField", text: $model.text)
                .padding()
                .border(Color.black)
            // but the model itself should only be used for one wrapped UITextField
            MyTextField(model: model)
                .padding()
                .border(Color.black)
        }
        .frame(height: 100)
        // Any view can subscribe to the model's text publisher
        .onReceive(model.$text) { text in
                print("I received the text "(text)"")
        }

    }
}