Pageable.swift

import Foundation

protocol Pageable {
    typealias PagedContentType
    
    var pageLimit: Int { get set }
    var pageOffset: Int { get set }
    
    func getPage(limit limit: Int, offset: Int, completion: (result: [PagedContentType]?, error: NSError?) -> Void)
    mutating func nextPage(completion: (result: [PagedContentType]?, error: NSError?) -> Void)
    mutating func resetPagingCounter()
}

extension Pageable {
    mutating func nextPage(completion: (result: [PagedContentType]?, error: NSError?) -> Void) {
        getPage(limit: pageLimit, offset: pageOffset) {
            (result: [PagedContentType]?, error: NSError?) in
            self.pageOffset += self.pageLimit
            completion(result: result, error: error)
        }
    }
    
    mutating func resetPagingCounter() {
        self.pageOffset = 0
    }
}

PageableTest.swift

import XCTest

class PageableTestObject: NSObject, Pageable {
    typealias PagedContentType = String
    var pageLimit = 2
    var pageOffset = 0
    
    var dataSource = [
        "first",
        "second",
        "third",
        "fourth",
        "fifth",
        "sixth"
    ]
    
    func getPage(limit limit: Int, offset: Int, completion: (result: [String]?, error: NSError?) -> Void) {
        let results = Array(dataSource[offset..<(offset + limit)])
        completion(result: results, error: nil)
    }
}

class PageableTest: CRTestCase {
    var testObject = PageableTestObject()
    
    func testNextPage() {
        XCTAssertEqual(2, testObject.pageLimit)
        XCTAssertEqual(0, testObject.pageOffset)
        
        testObject.nextPage {
            (result, error) in
            XCTAssertEqual(2, result!.count)
            XCTAssertEqual("first", result![0])
            XCTAssertEqual("second", result![1])
        }
        
        XCTAssertEqual(2, testObject.pageLimit)
        XCTAssertEqual(2, testObject.pageOffset)
        
        testObject.nextPage {
            (result, error) in
            XCTAssertEqual(2, result!.count)
            XCTAssertEqual("third", result![0])
            XCTAssertEqual("fourth", result![1])
        }
        
        XCTAssertEqual(2, testObject.pageLimit)
        XCTAssertEqual(4, testObject.pageOffset)
        
        testObject.nextPage {
            (result, error) in
            XCTAssertEqual(2, result!.count)
            XCTAssertEqual("fifth", result![0])
            XCTAssertEqual("sixth", result![1])
        }
    }
    
    func testResetPagingCounter() {
        XCTAssertEqual(2, testObject.pageLimit)
        XCTAssertEqual(0, testObject.pageOffset)
        
        testObject.nextPage {
            (result, error) in
            // do nothing
        }
        
        XCTAssertEqual(2, testObject.pageLimit)
        XCTAssertEqual(2, testObject.pageOffset)
        
        testObject.resetPagingCounter()
        
        XCTAssertEqual(2, testObject.pageLimit)
        XCTAssertEqual(0, testObject.pageOffset)
    }
}