Introduction to Combine | Supporting Combine in a custom class

Combine is a framework to implement asynchronous operations. It is created by Apple, it is also used in Swift Data Binding. It is an important framework.

If you new to Combine, see next article.

現役のプログラマーが書くプログラミング情報

Combine入門 | CombineでNotificationを受け取る方法

Combineはアップル純正の非同期処理を実装するためのフレームワークです。SwiftUIのバインディングなどでも使われ…

If you want other samples, see following articles.

現役のプログラマーが書くプログラミング情報

Combine入門 | CombineでTimer処理を行う方法

TimerもCombineに対応しています。Timerで定期的に実行される処理をCombineを使って設定出来るようにな…

This article explains how to support Combine in a custom class.

Adopt to ObservableObject protocol.

To make a class to support Combine, adopt to ObservableObject protocol.

To notify a changes of properties with Combine, mark the property with @Published attribute.

class PlayerStatus : ObservableObject {
    @Published var name: String = ""
    @Published var hitPoint: Int = 100
}

@Published

The property which is marked with @Published attribute, can make the Publisher.

Use $ operator to make the Publisher.

let status = PlayerStatus()
let hitPointPublisher = status.$hitPoint

Receive changes of properties of a custom class

Use sink() to receive changes.

let cancellable = status.$hitPoint.sink { (value) in 
    // Block when a property is changed.
}

The value of the property is not changed at the block is executing, so the value of the property is an old value yet. You should use a block argument “value”.

Sample Application

I made a sample application. “Hit” button decrement HP and “Heal” button recover it. I think this code will make you understand an effect of the ObservableObject.

import UIKit
import Combine

class PlayerStatus: ObservableObject {
    @Published var name:String = ""
    @Published var hitPoint: Int = 100
}

class ViewController: UIViewController {
    @IBOutlet var hitPointLabel: UILabel!
    
    var playerStatus: PlayerStatus = PlayerStatus()
    var hitPointCancellable: AnyCancellable? = nil

    private func refresh(hitPoint: Int) {
        self.hitPointLabel.text = "(hitPoint)"
    }
    
    @IBAction func hit(_ sender: Any?) {
        if self.playerStatus.hitPoint > 0 {
            self.playerStatus.hitPoint -= 10
        }
    }
    
    @IBAction func heal(_ sender: Any?) {
        self.playerStatus.hitPoint = 100
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        refresh(hitPoint: self.playerStatus.hitPoint)
        
        hitPointCancellable = self.playerStatus.$hitPoint.sink { (value) in
            self.refresh(hitPoint: value)
        }
    }
}

Author Profile

Akira HayashiA professional developer specializing in macOS Apps, iOS Apps, SDKs and middleware development.

Representative of RK Kaihatsu. I am a professional developer specializing in macOS Apps, iOS Apps, SDKs and middleware development. I often use ObjC, Swift and C++. Based on development experience, I provide e-learning contents, technical books and technical guidance. I am also a technical seminar instructor, in-house training instructor, and administrative / local goverment staff training instructor.

Latest entries

• App Developer2021.01.092 types of Life cycle of SwiftUI
• Developer Tools2021.01.03Setup Homebrew on Windows
• Developer Tools2021.01.03Docker: Check the Privacy Configuration if you use Docker on macOS Catalina 10.15
• App Developer2021.01.03Python: Output the log with logging module