README.md

Lesson 3.8: side-effects: doOn

Problem statement

Modiy the solution from Lesson 3.6 to produce a side-effect for each "hello" which is generated, using func doOn.

Problem project

You can use the project in the problem folder of this repo as a starting point.

Note: I have omitted the Carthage folder from the problem project, because it includes large binary files. In order to use the this project, you will need to run carthage update --platform iOS.

Solution

ViewController.swift:

import UIKit
import RxSwift
import RxCocoa

class TickHelloGenerator
{
    class func generate() -> Observable<String>
    {
        let tickerObservable = interval(1, MainScheduler.sharedInstance)
        
        let sideEffectingObservable = tickerObservable.doOn { (_) -> Void in
            debugPrint("side-effect")
        }
        
        let helloObservable = sideEffectingObservable.map({ (_) -> String in
            return "hello"
        })
        
        return helloObservable
    }
}

class ViewController: UIViewController {
    
    let disposeBag = DisposeBag()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        TickHelloGenerator.generate().subscribeNext { (s) -> Void in
            debugPrint(s)
        }.addDisposableTo(disposeBag)
    }
}

Discussion:

Here, we use func doOn to produce a side-effect for each Event which propogates through the Observable chain.

Start up the app and verify that you see a side-effect message for each "hello" message:

"side-effect"
"hello"
"side-effect"
"hello"
"side-effect"
"hello"

We can write a wrapper function which effectively renames func doOn as func sideEffect to make this more explicit. Here's an example of doing so, along with some more terse Observable chaining syntax:

extension ObservableType
{
    public func sideEffect(eventHandler: (RxSwift.Event<Self.E>) throws -> Void) -> RxSwift.Observable<Self.E>
    {
        return doOn(eventHandler)
    }
}

class TickHelloGenerator2
{
    class func generate() -> Observable<String>
    {
        return interval(1, MainScheduler.sharedInstance)
            .sideEffect({ (_) -> Void in
                debugPrint("side-effect")
            })
            .map({ (_) -> String in
                return "hello"
            })
    }
}

New concepts to explore

• Open up RxExample.xcodeproj.
  • Take a look at func doOn in Observable+Single.swift
  • Take a look at class Do in Do.swift

Solution project

My solution is included in the solution folder of this repo.

Note: I have omitted the Carthage folder from the solution project, because it includes large binary files. In order to run the this project, you will need to run carthage update --platform iOS.