CoalesceGroup.swift

//
//  CoalesceOperation.swift
//  RequestCoalesce
//
//  Created by Nicholas Cross on 4/12/2016.
//  Copyright © 2016 Nicholas Cross. All rights reserved.
//

import Foundation

public class CoalesceGroup {
    private var coalesceGroup: DispatchGroup
    private var coalesceQueue: DispatchQueue
    
    init() {
        self.coalesceGroup = DispatchGroup()
        self.coalesceQueue = DispatchQueue(label: "OperationQueue")
    }

    public func performAsync(coalesce coalesceable: Coalesceable, withForceRefresh forceRefresh: Bool = false, completed: @escaping ()->()) {
        DispatchQueue.global(qos: .background).async {
            self.perform(coalesce: coalesceable, withForceRefresh: forceRefresh, completed: completed)
        }
    }
    
    public func perform(coalesce coalesceable: Coalesceable, withForceRefresh forceRefresh: Bool = false, completed: @escaping ()->()) {
        guard !forceRefresh else {
            self._perform(coalesce: coalesceable, withForceRefresh: true, completed: completed)
            return
        }
        
        self.coalesceQueue.async {
            self._perform(coalesce: coalesceable, completed: completed)
        }
    }
    
    private func _perform(coalesce coalesceable: Coalesceable, withForceRefresh forceRefresh: Bool = false, completed: @escaping ()->()) {
        if (!forceRefresh) {
            self.coalesceGroup.wait()
        }
        
        self.coalesceGroup.enter()
        
        self.coalesceGroup.notify(queue: DispatchQueue.global(qos: .background), execute: completed)
        
        if !coalesceable.isCached || forceRefresh {
            coalesceable.performOperation() {
                DispatchQueue.global(qos: .background).async {
                    self.coalesceGroup.leave()
                }
            }
        }
        else {
            self.coalesceGroup.leave()
        }
    }

}

public protocol Coalesceable {
    var isCached: Bool {get}
    func performOperation(complete: @escaping ()->())
}

RequestCoalesce.md

Request coalescing

Queue up async requests for the same resource and retrieving first from a remote source and any queued requests from a cache

Load from cache if possible

• Dispatch async background queue
  • Dispatch group wait
    • Dipsatch group enter
    • Fetch from cache
    • Else load from url
    • Dispatch group leave

Force reload

• Dipsatch group enter
• Load from url
• Dispatch group leave