Latest 0.2.2
License MIT
Platforms ios 9.0, requires ARC
Dependencies RxSwift

Build status
Platform iOSSwift 4 compatible
Carthage compatible
CocoaPods compatible
License: MIT


Droste is a lightweight composable caching library which leverages RxSwift’s Observable for its API.

Example usage

Using out of the box caches

import Droste

let aDisposeBag = DisposeBag()
let url = URL(string: "")!

    .subscribe(onNext: { jsonObject in
        //use jsonObject
    }).disposed(by: aDisposeBag)

Or composing your own cache

import Droste

let aDisposeBag = DisposeBag()
let url = URL(string: "")!

let networkFetcher = NetworkFetcher()
    .mapKeys { (url: URL) -> URLRequest in //map url to urlrequest
        return URLRequest(url: url)

let diskCache = DiskCache<URL, NSData>()
let ramCache = RamCache<URL, NSData>()

let dataCache = ramCache + (diskCache + networkFetcher).reuseInFlight() 
let jsonCache = dataCache
            .mapValues(f: { (data) -> AnyObject in
                //convert NSData to json object
                return try JSONSerialization.jsonObject(with: data as Data, options: [.allowFragments]) as AnyObject
            }, fInv: { (object) -> NSData in
                //convert json object to NSData
                return try object, options: []) as NSData

    .subscribe(onNext: { jsonObject in
        //use jsonObject
    }).disposed(by: aDisposeBag)


Out of the box

We provide some singletons of caches that are ready for use.

  • Caches.sharedJSONCache takes URL and returns AnyObject which could either be an Array or a Dictionary depending on the endpoint’s response
  • Caches.sharedDataCache takes URL and returns NSData
  • Caches.sharedImageCache takes URL and returns UIImage

Compose caches

You can compose different types of caches, such as a composition of ram cache with a disk cache backed by a network fetcher.
To compose two or more caches, you can use .compose or + operator.


let ramCache = RamCache<URL, NSData>()
let diskCache = DiskCache<URL, NSData>()

//First hit ram cache and in case of failure, hit the disk cache
let ramDiskCache = ramCache.compose(other: diskCache)

//Same composition example using the the overloaded `+` operator
let ramDiskCache = ramCache + diskCache

Expirable caches

You can set expiry in each cache that supports it. DiskCache and RamCache both have out of the box support!
You can set an expiry in the following ways

// seconds from now
let ramCache = RamCache<URL, NSData>().expires(at: .seconds(30))

// or by explicitly setting a date 
let diskCache = DiskCache<URL, NSData>().expires(at: .date(Date(timeIntervalSince1970: 1516045540)))

Refreshing a resource after 5 minutes in both ram and disk cache.

let ramCache = RamCache<URL, NSData>().expires(at: .seconds(600))
let diskCache = DiskCache<URL, NSData>().expires(at: .seconds(600))

let dataCache = ramCache + diskCache + networkFetcher

Cancel Requests

The fact that Droste leverages RxSwift’s power, gives us some extra free functionality, such as canceling on-going requests.

Example with DisposeBag

var aDisposeBag = DisposeBag()

let diskCache = DiskCache<URL, NSData>()
let networkFetcher = NetworkFetcher<URL, NSData>()

let diskNetworkCache = diskCache + networkFetcher
    .get("a url")
    .subscribe(onNext: { response in

    }).disposed(by: aDisposeBag)

aDisposeBag = DisposeBag() // this line will cancel the on-going request

Example with flatMapLatest

var aDisposeBag = DisposeBag()

let diskCache = DiskCache<URL, NSData>()
let networkFetcher = NetworkFetcher<URL, NSData>()

let diskNetworkCache = diskCache + networkFetcher

//Each time the user input emits a new value, it cancels the previous request in cache
    .flatMapLatest{ userInput in
        let key = exampleKey(from: userInput)
        return diskNetworkCache.get(key)
    .subscribe(onNext: { response in

    }).disposed(by: aDisposeBag)

Map Values and Keys

Different Caches may know about different types of values and keys.
For example, you may have a RAM cache which uses a String type key and returns a UIImage type and a network fetcher (which is a type of cache) that uses URL as a key and responds with NSData.
If you would like to combine those two Caches you could not do it directly because their types do not match.
You can map the values and key of a cache using mapValues and mapKeys.


let networkFetcher = NetworkFetcher()      //Value = NSData, Key = URLRequest
let ramCache = RamCache<String, UIImage>() //Value = UIImage, Key = String

let imageNetworkFetcher = networkFetcher  //Value = UIImage, Key = String
                            .mapKeys { (urlString: String) -> URLRequest in
                                let url = URL(string: urlString)!
                                return URLRequest(url: url)
                                f: { (data) -> UIImage in
                                    return UIImage(data: data as Data)!
                                }) { (image) -> NSData in
                                    return UIImagePNGRepresentation(image) as! NSData

let imageRamNetworkCache = ramCache + imageNetworkFetcher //Value = UIImage, Key = String

    .subscribe(onNext: { image in
        //in a view controller context
        self.imageView.image = image
    }).disposed(by: disposeBag)

More examples here

Forward request

In a scenario that you have composed two Caches together and you want the right hand side cache to always hit the network, regardless if the first cache has succeded or not, you can use the forward request operator. By doing this, you are able to get the value from the first cache (if it exists) and followning the value from the second cache (if it exists). Also, note that the .forwardRequest operator, does not provide any guarantees on the order of the emissions to be aligned with the order of the composition.


Let’s assume that you have a screen which you want to load instantly and present a cached result for a short period of time, but always update it when the network request succeeds.

let screenCache = diskCache.forwardRequest() + networkFetcher

    .subscribe(onNext: {  screenViewModel
        self.viewModel = screenViewModel //if disk cache has a value this will be called twice

Switch cache

You can switch between Caches in runtime depending on the key used.


let cacheA = ...
let cacheB = ...
let cache = switchCache(cacheA: cacheA, cacheB: cacheB) { (key) -> CacheSwitchResult in
    if key == "Hello" {
        return .cacheA
    if key == "World" {
        return .cacheB

cache.get("Hello") //this will use cacheA
cache.get("World") //this will use cacheB

Reuse in flight

When you want to get an expensive resource from a cache/fetcher (e.g. NetworkFetcher), you can use .reuseInFlight operator to consolidate all requests that have the same key provided, when there is already an in-flight request for the same key. An example of this scenario is, a chat app that presents the same avatar multiple times on the screen and each avatar makes its own request from the cache. Using this operator means that only one request will be made and any subsequent requests will share the response of the first one.


let cache = memoryCache + (diskCache + networkFetcher).reuseInFlight()

    .subscribe(onNext: { image in
        avatar1.image = image

//a short while after, before the first request finished
    .subscribe(onNext: { image in
        avatar2.image = image //this will get the same response as the above without making a second request


  • Keys must be Hashable.
  • This operator is thread safe, meaning that you can dispatch the same request from multiple queues and it will work without any unexpected behaviors.

Skip while

With .skipWhile operator you can, at runtime, to skip the cache depending on a given condition.


let updateFromNetwork = true
let conditionedDiskCache = diskCache.skipWhile { key in
        let shouldSkip = (key == "a" && updateFromNetwork)
        return Observable(shouldSkip)

let cache = conditionedDiskCache + networkFetcher

    .subscribe(onNext: { response in


Salient get

We have two .get operators which have a slightly different behavior.

  • func get(_ key: Key) -> Observable<Value?> Optional return value

Using this operator, means that, if the cache does not have a value and has not thrown an error internally, then it will return a nil value. All thrown errors will propagate as expected.

  • func get(_ key: Key) -> Observable<Value> Non-optional return value

Using this operator, means that, if the cache does not have a value, then it will throw a CacheFetchError.valueNotFound error. This operator does not change the expected behavior of other thrown errors.


  • iOS 9.0+
  • Xcode 8.0+

Getting involved

  • If you want to contribute please feel free to submit PRs.
  • If you have a feature request please open an issue.
  • If you found a bug or need help please check older issues, FAQ and threads on StackOverflow (Tag ‘Droste’) before submitting an issue.

Before contributing check the CONTRIBUTING file for more info.

If you use Droste in your app we would love to hear about it! Drop us a line on Twitter.


Follow these 3 steps to run the example project:

  • Clone Droste repository
  • Open Example/Example.xcworkspace workspace
  • Run Example project

You can also experiment and learn the using the Droste Playground, contained in Example/Example.workspace under Droste project.




To install Droste, simply add the following line to your Podfile:

pod 'Droste', '~> 0.2.0'



To install Droste, simply add the following line to your Cartfile:

github "gtsifrikas/Droste" ~> 0.2.0

Tasks for version 1.0

  • [ ] 100% coverage
  • [ ] Support macOS
  • [ ] Support linux
  • [ ] Update README to include custom Cache creation.
  • [x] Come up with a api for TTL and support it in DiskCache and RamCache.




Latest podspec

    "name": "Droste",
    "version": "0.2.2",
    "summary": "Droste is a lightweight composable caching library which leverages RxSwift's Observable for its API.",
    "homepage": "",
    "license": {
        "type": "MIT",
        "file": "LICENSE"
    "authors": {
        "George Tsifrikas": "[email protected]"
    "source": {
        "git": "",
        "tag": "0.2.2"
    "social_media_url": "",
    "platforms": {
        "ios": "9.0"
    "requires_arc": true,
    "source_files": "Sources/**/*.{swift,h,m}",
    "module_name": "Droste",
    "pod_target_xcconfig": {
        "SWIFT_VERSION": "4.0"
    "dependencies": {
        "RxSwift": []
    "testspecs": [
            "name": "Tests",
            "test_type": "unit",
            "source_files": [
            "dependencies": {
                "RxSwift": [],
                "Nimble": [],
                "Quick": [],
                "RxTest": []

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