PaginationNetworkLogic.swift

//
//  PaginationNetworkLogic.swift
//
//  Created by Daniel Tartaglia on 4/9/17.
//  Copyright © 2019 Daniel Tartaglia. MIT License
//

import RxSwift

struct PaginationUISource {
    /// reloads first page and dumps all other cached pages.
    let refresh: Observable<Void>
    /// loads next page
    let loadNextPage: Observable<Void>
}

struct PaginationSink<T> {
    /// true if network loading is in progress.
    let isLoading: Observable<Bool>
    /// elements from all loaded pages
    let elements: Observable<[T]>
    /// fires once for each error
    let error: Observable<Error>
}

extension PaginationSink {
    
    init(ui: PaginationUISource, loadData: @escaping (Int) -> Observable<[T]>)
    {
        let loadResults = BehaviorSubject<[Int: [T]]>(value: [:])
        
        let maxPage = loadResults
            .map { $0.keys }
            .map { $0.max() ?? 1 }
        
        let reload = ui.refresh
            .map { -1 }
        
        let loadNext = ui.loadNextPage
            .withLatestFrom(maxPage)
            .map { $0 + 1 }
        
        let start = Observable.merge(reload, loadNext, Observable.just(1))
        
        let page = start
            .flatMap { page in
                Observable.combineLatest(Observable.just(page), loadData(page == -1 ? 1 : page)) { (pageNumber: $0, items: $1) }
                    .materialize()
                    .filter { $0.isCompleted == false }
            }
            .share()
        
        _ = page
            .compactMap { $0.element }
            .withLatestFrom(loadResults) { (pages: $1, newPage: $0) }
            .filter { $0.newPage.pageNumber == -1 || !$0.newPage.items.isEmpty }
            .map { $0.newPage.pageNumber == -1 ? [1: $0.newPage.items] : $0.pages.merging([$0.newPage], uniquingKeysWith: { $1 }) }
            .subscribe(loadResults)
        
        let _isLoading = Observable.merge(start.map { _ in 1 }, page.map { _ in -1 })
            .scan(0, accumulator: +)
            .map { $0 > 0 }
            .distinctUntilChanged()
        
        let _elements = loadResults
            .map { $0.sorted(by: { $0.key < $1.key }).flatMap { $0.value } }
        
        let _error = page
            .map { $0.error }
            .filter { $0 != nil }
            .map { $0! }
        
        isLoading = _isLoading
        elements = _elements
        error = _error
    }
}

PaginationNetworkLogicTests.swift

class PaginationTests: XCTestCase {
    
    var testScheduler: TestScheduler!
    var bag: DisposeBag!
    var isLoading: TestableObserver<Bool>!
    var elements: TestableObserver<[Int]>!
    var error: TestableObserver<Error>!
    var dataLoader: DataLoader!
    
    override func setUp() {
        super.setUp()
        testScheduler = TestScheduler(initialClock: 0)
        bag = DisposeBag()
        isLoading = testScheduler.createObserver(Bool.self)
        elements = testScheduler.createObserver([Int].self)
        error = testScheduler.createObserver(Error.self)
        dataLoader = DataLoader(testScheduler: testScheduler)
    }
    
    func testDefault() {
        let refreshTrigger = testScheduler.createColdObservable([Recorded<Event<Void>>.completed(30)])
        let loadNextPageTrigger = testScheduler.createColdObservable([Recorded<Event<Void>>.completed(30)])
        let source = PaginationUISource(refresh: refreshTrigger.asObservable(), loadNextPage: loadNextPageTrigger.asObservable())
        let sink = PaginationSink(ui: source, loadData: dataLoader.loadData(page:))
        
        let foundErrors = testScheduler.createObserver(Bool.self)
        bag.insert(
            sink.isLoading.subscribe(isLoading),
            sink.elements.subscribe(elements),
            sink.error.map { _ in true }.subscribe(foundErrors)
        )
        testScheduler.start()
        
        XCTAssertEqual(isLoading.events, [.next(0, true), .next(10, false), .completed(30)])
        XCTAssertEqual(elements.events, [.next(0, [Int]()), .next(10, [1, 2, 3]), .completed(30)])
        XCTAssertEqual(foundErrors.events, [.completed(30)])
        XCTAssertEqual(dataLoader.pages, [1])
    }
    
    func testRefresh() {
        let refreshTrigger = testScheduler.createColdObservable([.next(20, ())])
        let loadNextPageTrigger = testScheduler.createColdObservable([Recorded<Event<Void>>]())
        let source = PaginationUISource(refresh: refreshTrigger.asObservable(), loadNextPage: loadNextPageTrigger.asObservable())
        let sink = PaginationSink(ui: source, loadData: dataLoader.loadData(page:))
        
        bag.insert(
            sink.isLoading.subscribe(isLoading),
            sink.elements.subscribe(elements),
            sink.error.subscribe(error)
        )
        testScheduler.start()
        
        XCTAssertEqual(isLoading.events, [.next(0, true), .next(10, false), .next(20, true), .next(30, false)])
        XCTAssertEqual(elements.events, [.next(0, [Int]()), .next(10, [1, 2, 3]), .next(30, [1, 2, 3])])
        XCTAssertTrue(error.events.isEmpty)
        XCTAssertEqual(dataLoader.pages, [1, 1])
    }
    
    func testRefreshNewLoadEmpty() {
        let refreshTrigger = testScheduler.createColdObservable([.next(20, ())])
        let loadNextPageTrigger = testScheduler.createColdObservable([Recorded<Event<Void>>]())
        let source = PaginationUISource(refresh: refreshTrigger.asObservable(), loadNextPage: loadNextPageTrigger.asObservable())
        let sink = PaginationSink(ui: source, loadData: dataLoader.loadPageThenEmpty(page:))
        
        bag.insert(
            sink.isLoading.subscribe(isLoading),
            sink.elements.subscribe(elements),
            sink.error.subscribe(error)
        )
        testScheduler.start()
        
        XCTAssertEqual(isLoading.events, [.next(0, true), .next(10, false), .next(20, true), .next(30, false)])
        XCTAssertEqual(elements.events, [.next(0, [Int]()), .next(10, [1, 2, 3]), .next(30, [])])
        XCTAssertTrue(error.events.isEmpty)
        XCTAssertEqual(dataLoader.pages, [1, 1])
    }
    
    func testNextPage() {
        let refreshTrigger = testScheduler.createColdObservable([Recorded<Event<Void>>]())
        let loadNextPageTrigger = testScheduler.createColdObservable([.next(20, ())])
        let source = PaginationUISource(refresh: refreshTrigger.asObservable(), loadNextPage: loadNextPageTrigger.asObservable())
        let sink = PaginationSink(ui: source, loadData: dataLoader.loadData(page:))
        
        bag.insert(
            sink.isLoading.subscribe(isLoading),
            sink.elements.subscribe(elements),
            sink.error.subscribe(error)
        )
        testScheduler.start()
        
        XCTAssertEqual(isLoading.events, [.next(0, true), .next(10, false), .next(20, true), .next(30, false)])
        XCTAssertEqual(elements.events, [.next(0, [Int]()), .next(10, [1, 2, 3]), .next(30, [1, 2, 3, 4, 5, 6])])
        XCTAssertTrue(error.events.isEmpty)
        XCTAssertEqual(dataLoader.pages, [1, 2])
    }
    
    func testNextPageThenRefresh() {
        let refreshTrigger = testScheduler.createColdObservable([.next(40, ())])
        let loadNextPageTrigger = testScheduler.createColdObservable([.next(20, ())])
        let source = PaginationUISource(refresh: refreshTrigger.asObservable(), loadNextPage: loadNextPageTrigger.asObservable())
        let sink = PaginationSink(ui: source, loadData: dataLoader.loadData(page:))
        
        bag.insert(
            sink.isLoading.subscribe(isLoading),
            sink.elements.subscribe(elements),
            sink.error.subscribe(error)
        )
        testScheduler.start()
        
        XCTAssertEqual(isLoading.events, [.next(0, true), .next(10, false), .next(20, true), .next(30, false), .next(40, true), .next(50, false)])
        XCTAssertEqual(elements.events, [.next(0, [Int]()), .next(10, [1, 2, 3]), .next(30, [1, 2, 3, 4, 5, 6]), .next(50, [1, 2, 3])])
        XCTAssertTrue(error.events.isEmpty)
        XCTAssertEqual(dataLoader.pages, [1, 2, 1])
    }
    
    func testNextPageBeforeInitialPage() {
        let refreshTrigger = testScheduler.createColdObservable([Recorded<Event<Void>>]())
        let loadNextPageTrigger = testScheduler.createColdObservable([.next(5, ())])
        let source = PaginationUISource(refresh: refreshTrigger.asObservable(), loadNextPage: loadNextPageTrigger.asObservable())
        let sink = PaginationSink(ui: source, loadData: dataLoader.loadData(page:))
        
        bag.insert(
            sink.isLoading.subscribe(isLoading),
            sink.elements.subscribe(elements),
            sink.error.subscribe(error)
        )
        testScheduler.start()
        
        XCTAssertEqual(isLoading.events, [.next(0, true), .next(15, false)])
        XCTAssertEqual(elements.events, [.next(0, [Int]()), .next(10, [1, 2, 3]), .next(15, [1, 2, 3, 4, 5, 6])])
        XCTAssertTrue(error.events.isEmpty)
        XCTAssertEqual(dataLoader.pages, [1, 2])
    }
    
    func testNextPageComesBeforeInitialPage() {
        let refreshTrigger = testScheduler.createColdObservable([Recorded<Event<Void>>]())
        let loadNextPageTrigger = testScheduler.createColdObservable([.next(3, ())])
        let source = PaginationUISource(refresh: refreshTrigger.asObservable(), loadNextPage: loadNextPageTrigger.asObservable())
        let sink = PaginationSink(ui: source, loadData: dataLoader.reverseLoadData(page:))
        
        bag.insert(
            sink.isLoading.subscribe(isLoading),
            sink.elements.subscribe(elements),
            sink.error.subscribe(error)
        )
        testScheduler.start()
        
        XCTAssertEqual(isLoading.events, [.next(0, true), .next(10, false)])
        XCTAssertEqual(elements.events, [.next(0, [Int]()), .next(8, [4, 5, 6]), .next(10, [1, 2, 3, 4, 5, 6])])
        XCTAssertTrue(error.events.isEmpty)
        XCTAssertEqual(dataLoader.pages, [1, 2])
    }
    
    func testLoadError() {
        let refreshTrigger = testScheduler.createColdObservable([Recorded<Event<Void>>]())
        let loadNextPageTrigger = testScheduler.createColdObservable([Recorded<Event<Void>>]())
        let source = PaginationUISource(refresh: refreshTrigger.asObservable(), loadNextPage: loadNextPageTrigger.asObservable())
        let sink = PaginationSink(ui: source, loadData: dataLoader.errorLoadData(page:))
        
        bag.insert(
            sink.isLoading.subscribe(isLoading),
            sink.elements.subscribe(elements),
            sink.error.subscribe(error)
        )
        testScheduler.start()
        
        XCTAssertEqual(isLoading.events, [.next(0, true), .next(10, false)])
        XCTAssertEqual(elements.events, [.next(0, [Int]())])
        XCTAssertEqual(error.events.count, 1)
        XCTAssertEqual(dataLoader.pages, [1])
    }
    
    func testErrorThenRetry() {
        let refreshTrigger = testScheduler.createColdObservable([.next(40, ())])
        let loadNextPageTrigger = testScheduler.createColdObservable([.next(20, ())])
        let source = PaginationUISource(refresh: refreshTrigger.asObservable(), loadNextPage: loadNextPageTrigger.asObservable())
        let sink = PaginationSink(ui: source, loadData: dataLoader.errorThenSuccessLoadData(page:))
        
        bag.insert(
            sink.isLoading.subscribe(isLoading),
            sink.elements.subscribe(elements),
            sink.error.subscribe(error)
        )
        testScheduler.start()
        
        XCTAssertEqual(isLoading.events, [.next(0, true), .next(10, false), .next(20, true), .next(30, false), .next(40, true), .next(50, false)])
        XCTAssertEqual(elements.events, [.next(0, [Int]()), .next(10, [1, 2, 3]), .next(50, [1, 2, 3])])
        XCTAssertEqual(error.events.map { $0.time }, [30])
        XCTAssertEqual(dataLoader.pages, [1, 2, 1])
    }
}

class DataLoader {
    private (set) var pages: [Int] = []
    private let testScheduler: TestScheduler
    
    init(testScheduler: TestScheduler) {
        self.testScheduler = testScheduler
    }
    
    func loadEmpty(page: Int) -> Observable<[Int]> {
        pages.append(page)
        return self.testScheduler.createColdObservable([.next(10, []), .completed(10)])
            .asObservable()
    }
    
    func loadData(page: Int) -> Observable<[Int]> {
        pages.append(page)
        let result = Array((0..<3).map { page * 3 - (2 - $0) })
        return self.testScheduler.createColdObservable([.next(10, result), .completed(10)])
            .asObservable()
    }
    
    func reverseLoadData(page: Int) -> Observable<[Int]> {
        pages.append(page)
        let result = Array((0..<3).map { page * 3 - (2 - $0) })
        let time = 15 - page * 5
        return self.testScheduler.createColdObservable([.next(time, result), .completed(time)])
            .asObservable()
    }
    
    func errorLoadData(page: Int) -> Observable<[Int]> {
        pages.append(page)
        let error = NSError(domain: "testing", code: -1, userInfo: nil)
        return self.testScheduler.createColdObservable([Recorded<Event<[Int]>>.error(10, error)])
            .asObservable()
    }
    
    func loadPageThenEmpty(page: Int) -> Observable<[Int]> {
        if pages.count == 0 {
            return loadData(page: page)
        }
        else {
            return loadEmpty(page: page)
        }
    }
    
    func errorThenSuccessLoadData(page: Int) -> Observable<[Int]> {
        if pages.count == 1 {
            return errorLoadData(page: page)
        }
        else {
            return loadData(page: page)
        }
    }
}

It worked!
Thank you so much Daniel for your quick response and support :)

Would this work for a pagination with cursors?

@danielt1263 thanks again for a great sink! any chance of this being made into a pod?

I'm having trouble where by the pagination.isLoading stops emitting events.
I've simplified the reproducible state code as example:

let pagination = PaginationSink<Show>(ui: source, loadData: { [unowned showsFilter] page in
  if let filter = showsFilter.value, filter.isEmpty == false {
    return Single<[Show]>
      .create(subscribe: { (single) -> Disposable in
        DispatchQueue.global(qos: .userInitiated).async {
          single(.success([.init(title: "\(page) - \(showsFilter.value ?? "no filter")")]))
        }
        return Disposables.create()
      })
      .asObservable()
  } else {
    return Single<[Show]>
      .create(subscribe: { (single) -> Disposable in
        single(.success([.init(title: "\(page) - \(showsFilter.value ?? "no filter")")]))
        return Disposables.create()
      })
      .asObservable()
  }
})

After returning filter Single, no 'loading' events are ever emitted again regardless of filter value.
What resolves the above chunk of code is starting with empty array .asObservable().startWith([])

---

I've tried to create a test to reflect my issue, but not so familiar with RxTest framework to produce appropriate events.
Best I can do for now:

func testLoadingEventsError() {
  
  let showsFilter = BehaviorRelay<String?>(value: nil)
  let refreshRelay = PublishRelay<Void>.init()

  let refreshTrigger = Observable.merge(showsFilter.skip(1).map({ _ in }), refreshRelay.asObservable())
  let loadNextPageTrigger = Observable<Void>.empty()
  let source = PaginationUISource(refresh: refreshTrigger.asObservable(), loadNextPage: loadNextPageTrigger.asObservable())

  let expectationOne = XCTestExpectation(description: "Fetch filter")
  let expectationTwo = XCTestExpectation(description: "Fetch reload")
  
  let sink = PaginationSink(ui: source) { [unowned showsFilter] (page) -> Observable<[Int]> in
    if let filter = showsFilter.value, filter.isEmpty == false {
      return Single<[Int]>
        .create(subscribe: { (single) -> Disposable in
          DispatchQueue.global(qos: .userInitiated)
            .asyncAfter(deadline: .now() + 5, execute: {
              expectationOne.fulfill()
              single(.success(filter.map({ Int(String($0))! }) ))
            })
          return Disposables.create()
        })
        .asObservable()
    } else {
      return Single<[Int]>
        .create(subscribe: { (single) -> Disposable in
          DispatchQueue.global(qos: .userInitiated)
            .asyncAfter(deadline: .now() + 5, execute: {
              expectationTwo.fulfill()
              single(.success([]))
            })
          return Disposables.create()
        })
        .asObservable()
    }
  }
  
  bag.insert(
    sink.isLoading.subscribe(isLoading),
    sink.elements.subscribe(elements),
    sink.error.subscribe(error)
  )
  
  showsFilter.accept("123")
  refreshRelay.accept(())

  wait(for: [expectationOne, expectationTwo], timeout: 15.0)
  XCTAssertEqual(isLoading.events, [.next(0, true), .next(0, false), .next(0, true), .next(0, false)])
  XCTAssertEqual(elements.events, [.next(0, [Int]()), .next(0, [1, 2, 3]), .next(0, [1, 2, 3])])
}