I want to use Combine in UIKit as well.

Introduction

Now that iOS14 has been released, Swft itself has been updated and Swift UI has been standardized, I think there are many people who can't cut iOS12 and can't use Combine in practice.

But I want to try new technology. SwiftUI + Combine is a bit high hurdle, but first of all, I would like to try Combine alone with the familiar UIKit! !!

So, this time I wrote an article with the following contents.

• About the basic concept of Combine
• Try using it with UIKit

I hope you find it useful. : bow:

• The top is good, so show me only the code! In that case, I would appreciate it if you could see the following. : bow: kawano108/CombineBookManagerApp

environment

This article has been confirmed to work in the following operating environment.

• Xcode12
• Swift5.3

What is Combine?

In a nutshell, it's Apple's genuine asynchronous framework`. I will quote the description of Apple Developer.

The Combine framework provides a declarative Swift API for processing values over time.

It means something like "The Combine framework provides a declarative API for processing values over time."

• Can be written declaratively
• Process values over time

This is the main function of Combine, and it seems to be the nature of the asynchronous framework.

Know the characters in Combine

Before trying to use Combine, I will introduce the overall flow and the characters that you need to know to use Combine.

Character

• Publisher: Publish the value.
• Operator: Convert the issued value.
• Subscriber: Receives a value and fires an event.

It looks like this in the figure.

In Rx and reactive programming, the above concept is often expressed by a marble diagram, Personally, it's easier to get the concept in the figure that shows the characters 1-1-1 as shown in the above figure.

Let's touch it for the time being

Now that you know the characters, let's touch them for the time being.

This is the login screen you often see. If you're designing with an MVVM, you need to bind the email address you receive from the TextField in your View to your ViewModel. Let's take the implementation at that time as an example to see the explanation and implementation example of Publisher, Operator, and Subscriber.

Publisher First is Publisher. https://developer.apple.com/documentation/combine/publisher

Publisher is a great guy who can publish values, so you can publish the email address you received in View.

First, let's create a Publisher that publishes a value when the text of the TextField changes.

import Combine

final class LoginViewController: UIViewController {

    @IBOutlet weak var mailAddressTextField: UITextField!

    private let viewModel = LoginViewModel()
    private var binding = Set<AnyCancellable>()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        //When textDidChangeNotification is notified`mailAddressTextField`Issue the object.
        NotificationCenter.default
            .publisher(for: UITextField.textDidChangeNotification, object: mailAddressTextField)

    }
}

Don't forget to import. Now when the textField changes, an object called mailAddressTextField will be issued.

Operator Next, let's go to Operator. The Operator is a great guy who can convert values, so you can also convert the UITextField object published by Publisher above. When binding to a ViewModel with a TextField value, just a String is enough to pass to the ViewModel. So let's use Operator to convert the UITextField to a String.

import Combine

final class LoginViewController: UIViewController {

    @IBOutlet weak var mailAddressTextField: UITextField!

    private let viewModel = LoginViewModel()
    private var binding = Set<AnyCancellable>()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        NotificationCenter.default
            .publisher(for: UITextField.textDidChangeNotification, object: mailAddressTextField)
            .compactMap { $0.object as? UITextField } //If you fail to cast UITextField and it becomes nil, play it
            .map { $0.text ?? "" } //Extract text from UITextField and convert to String
    }
}

• Exclude instances other than UITextField in .compactMap.
• Get text from UITextField with .map and convert it to String.

I am doing that.

In this way, when the value issued by Publisher and the value you want to receive are different, the Operator handles the conversion and handling of the value in the middle. See below for Operator types.

• [IOS] Combine framework summary

Subscriber The last is Sbscriber. Since Sbscriber is a great guy who can receive values and do various things, you can set the String of the email address sent from Publisher to the ViewModel.

import Combine

final class LoginViewController: UIViewController {

    @IBOutlet weak var mailAddressTextField: UITextField!
    
    private let viewModel = LoginViewModel()
    private var binding = Set<AnyCancellable>()

    override func viewDidLoad() {
        super.viewDidLoad()
        
        NotificationCenter.default
            .publisher(for: UITextField.textDidChangeNotification, object: mailAddressTextField)
            .compactMap { $0.object as? UITextField }
            .map { $0.text ?? "" }
            .removeDuplicates() //Eliminate duplicate values
            .eraseToAnyPublisher()
            .receive(on: RunLoop.main)
            .assign(to: \.mailAddress, on: viewModel) //viewModel\.Assign a value to mailAddress
            .store(in: &binding)
    }
}

final class LoginViewModel {
    var mailAddress: String = "" {
        didSet {
            print(mailAddress)
        }
    }
}

It looks like you're doing a lot, but there are two points.

• The String received by .assign (to: \ .mailAddress, on: viewModel) is set in ViewModel.
• The instance can be monitored and canceled with .store (in: & binding).

The assign method grows from Publisher and can generate a Subscriber while passing the received value to the object defined by keypath. However, if it is left as it is, the memory will not be released, so we are calling .store (in: & binding) to cancel the monitoring.

• I thought I couldn't explain this area with a vague understanding, so please refer to @ shiz's article for details. [IOS] Combine framework summary

Run

With the code so far, when you receive the value of TextField, it will be set in ViewModel!

Summary

Combine is Apple's genuine asynchronous framework and has the following features.

• Can be written declaratively
• Process values over time

Combine has the following major characters:

• Publisher: Publish the value.
• Operator: Convert the issued value.
• Subscriber: Receives a value and fires an event.

bonus

In order to get used to Combine, I am also making an application that manages books by hitting API with UIKit and Combine. I'm planning to include an API client that incorporates Combine and Publisher that has been made into an extension to make it easier to use, so please have a look if you like: bow:

kawano108/CombineBookManagerApp