Custom NSURLCache for web services which don't provide expiration info.

CustomURLCache.swift

import Foundation

class CustomURLCache: NSURLCache {
    
    let createdAtKey = "CreatedAt"
    
    override func cachedResponseForRequest(request:NSURLRequest) -> NSCachedURLResponse? {        
        let cachedResponse = expiredCachedResponseForRequest(request)

        if cachedResponse == nil {
            removeCachedResponseForRequest(request)
        }
        
        return cachedResponse
    }
    
    override func storeCachedResponse(cachedResponse: NSCachedURLResponse, forRequest request: NSURLRequest) {
        
        var newUserInfo = cachedResponse.userInfo ?? [NSObject : AnyObject]()
        
        if let expiredCachedResponse = expiredCachedResponseForRequest(request),
            userInfo = expiredCachedResponse.userInfo,
            createdAt = userInfo[createdAtKey] as? NSDate {
            newUserInfo[createdAtKey] = createdAt
        } else {
            newUserInfo[createdAtKey] = NSDate()
        }
        
        let newCachedResponse = NSCachedURLResponse(response: cachedResponse.response, data: cachedResponse.data, userInfo: newUserInfo, storagePolicy: cachedResponse.storagePolicy)
        
        super.storeCachedResponse(newCachedResponse, forRequest: request)

    }
    
    func expiredCachedResponseForRequest(request:NSURLRequest) -> NSCachedURLResponse? {
        
        var cachedResponse: NSCachedURLResponse? = nil
        
        if let actualCachedResponse = super.cachedResponseForRequest(request),
            userInfo = actualCachedResponse.userInfo,
            createdAt = userInfo[createdAtKey] as? NSDate {
            
            if createdAt.timeIntervalSinceNow > -3600 {
                cachedResponse = actualCachedResponse
            }
        }
        
        return cachedResponse
    }
        
}