Effective Networking with iOS 8 and Swift

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Effective Networking with Swift and iOS 8

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Ben Scheirman @subdigital

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ChaiOne

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Agenda • Old and Crusty NSURLConnection • New Hotness • Live Demos ! • HTTP • Caching • Bonus Round: API Tips

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NSURLConnection Invented for Safari ~2000 Made public in 2003 Poor separation of settings, conﬁg, cache

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NSURLConnection was replaced by NSURLSession in iOS 7

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NSURLSession

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Still use NSURLRequest Conﬁgurable Container (isolation w/ 3rd party libraries) Improved auth Rich Callbacks

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The New Family

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NSURLSessionConfiguration NSURLSession NSURLSessionTask NSURLSessionDelegate

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Start with NSURLSessionConfiguration

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Default Sessions + defaultSessionConfiguration

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Ephemeral Sessions + ephemeralSessionConfiguration

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Ephemeral Sessions + ephemeralSessionConfiguration ?

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"private browsing"

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Background Sessions + backgroundSessionConfiguration:

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Background Sessions + backgroundSessionConfiguration: • Takes an NSString identiﬁer • Causes uploads / downloads to occur in background

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Customize it further All of these class methods return a copy, tweak from there...

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Well, what can you do with it?

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Set a default header var config = NSURLSessionConfiguration.defaultSessionConfiguration() config.HTTPAdditionalHeaders = [ "auth_token" : "1234abcd" ]

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Mark Requests as low-priority config.discretionary = true • Note: This has beneﬁts, such as retrying when connection terminates, avoiding request if user is low on battery, or if Wi- Fi performance is not good enough.

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Disable Cellular config.allowsCellular = false

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Set custom caching behavior config.URLCache = MyCustomCache() config.requestCachePolicy = NSURLRequestReturnCacheDataElseLoad

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Inject your own custom protocols!

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ben://awww.yeah

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(actually quite useful for mocking requests)

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Next Step: build an NSURLSession to make reuqests

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var session = NSURLSession(configuration: config)

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var session = NSURLSession(configuration: config) or with a delegate... var delegate: NSURLSessionDelegate = self var session = NSURLSession(configuration: config, delegate: delegate, delegateQueue: NSOperationQueue.mainQueue())

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NSURLSession • Construct without the delegate if you want to use block callbacks • Construct with a delegate for advanced control (or background sessions) • If you pass completion handler blocks, delegate methods are not called

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NSURLSessionTask

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NSURLSessionDataTask • Represents GET, PUT, POST, DELETE • Handle Completion Callback • If error is present, request failed

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What Constitutes an Error? • Connection failed • Timeouts • Host invalid • Bad URL • Too many redirects • ... dozens more (check URL Loading System Error Codes)

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NSURLSessionDownloadTask • Downloading a file / resource • Streams to disk • Useful when size is large and can't fit in memory • Temp file path is provided in completion block • MUST move it somewhere if you want it to stick around

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Building a simple GET Request • Build an instance of NSURLSessionDataTask • (optionally) give it a block callback* • Call resume

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var config = NSURLSessionConfiguration.defaultSessionConfiguration() var session = NSURLSession(configuration: config)

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var config = NSURLSessionConfiguration.defaultSessionConfiguration() var session = NSURLSession(configuration: config) var url = NSURL(string: "https://www.nsscreencast.com/api/episodes.json")

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var config = NSURLSessionConfiguration.defaultSessionConfiguration() var session = NSURLSession(configuration: config) var url = NSURL(string: "https://www.nsscreencast.com/api/episodes.json") var task = session.dataTaskWithURL(url) { (let data, let response, let error) in // ... }

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NSURLSessionDelegate Provide a delegate if you need more advanced control over: • Download Progress • Authentication Challenges • Connection Failure

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NSURLSessionDelegate

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Case Study: Requesting Images

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Have you ever seen this? NSURL *imageUrl = [NSURL URLWithString:@”http://i.imgur.com/kwpjYwQ.jpg”]; NSData *imageData = [NSData dataWithContentsOfURL:imageUrl]; UIImage *image = [UIImage imageWithData:imageData]; [imageView setImage:image];

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What is wrong here? NSURL *imageUrl = [NSURL URLWithString:@”http://i.imgur.com/kwpjYwQ.jpg”]; NSData *imageData = [NSData dataWithContentsOfURL:imageUrl]; UIImage *image = [UIImage imageWithData:imageData]; [imageView setImage:image];

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These are blocking calls ! NSData *imageData = [NSData dataWithContentsOfURL:imageUrl]; ! UIImage *image = [UIImage imageWithData:imageData];

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Demo: Downloading Images

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• Images should probably be requested with download tasks • Block-based callbacks can be unwieldy (especially in Objective-C) • Use the delegate if you want to report download progress

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Demo: Searching the iTunes Store

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HTTP

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Status Code Cheat Sheet 1xx - Informational / Transient 2xx - A-OK ! 3xx - It's over there " 4xx - You Messed up ✋ 5xx - We Messed Up $

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HTTP Caching

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HTTP Caching Techniques • If-Modiﬁed-Since • If-None-Match • Cache-Control

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If-Modified-Since • Client requests a resource • Server responds, includes a Date response header • Client Caches the data, including the date • Client sends request header If-Modified-Since • Server compares, can return 304 (Not Modiﬁed) with no body

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If-None-Match • Client requests a resource • Server responds, includes a E-Tag header • Client Caches the data, including the E-Tag • Client sends request header If-None-Match • Server compares, can return 304 (Not Modiﬁed) with no body

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Cache-Control • Server indicates when the resource expires • Client caches the data until that time • Client will immediately return local cache data if still fresh

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What does it look like on the client?

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What does it look like on the server?

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def show @band = Band.find(params[:id]) fresh_when(:etag => @band, :last_modified => @band, :public => true) expires_in 10.minutes, :public => true end

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Observations • Server still executes a query to compute E-Tag and Modiﬁed Date • No body is transfered for a matching E-Tag or Date • Client doesn't even make request if Cache-Control is used and content is still fresh

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Cache-Control sounds perfect • Not everything is inherently cacheable • Only helpful for a single client, after the initial request

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Reverse-Proxy Cache to the Rescue

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Pros / Cons of Cache-Control • Client doesn't wait for a network request • Server spends zero resources • Clients May Render Stale Data

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Pros / Cons of E-Tag & IMS • Server skips rendering • Miniscule Data Transfer • Can appear instantaneous* • Server still spending resources computing & comparing E- Tag & dates

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Reason About Your Data • List of US States • User’s Proﬁle • Customer’s Order • Activity Timeline • Yesterday’s stats

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Tradeoff between fresh data and user experience

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Caching with NSURLSession

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• Uses NSURLCache out of the box • Customize on the NSURLSessionConfiguration instance • Use ephemeralSessionConfiguration to disable caching.

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NSURLCache Gotchas

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NSURLCache Gotchas • will not cache on disk if max-age is less than 5 minutes

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NSURLCache Gotchas • will not cache on disk if max-age is less than 5 minutes • might not cache at all if Cache-Control isn't passed

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NSURLCache Gotchas • will not cache on disk if max-age is less than 5 minutes • might not cache at all if Cache-Control isn't passed • might choose an arbitrarily large max-age if none provided *

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Tuning the built-in cache let MB = 1024 * 1024 var cache = NSURLCache(memoryCapacity: 10 * MB, diskCapacity: 50 * MB, diskPath: nil) sessionConfiguration.URLCache = cache

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Default Cache Location • ~/Library/Caches/com.acme.app/Cache.db

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What do I have to do?

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Maybe nothing! • Server should return appropriate cache headers • Tweak caching behavior in willCacheResponse delegate method

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The Content Flicker Problem

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The Content Flicker Problem • Data is already cached and fresh with E-Tag / IMS info • Client must validate content is still fresh and wait for a 304 • Response is fast, but not fast to avoid drawing empty screen • ...ﬂicker

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Resolving the Content Flicker Problem

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1. Return cached data immediately (if we have it) 2. Proceed with Request 3. Update callback (again) with fresh data

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let url = NSURL(string: "http://cache-tester.herokuapp.com/contacts.json") let request = NSURLRequest(URL: url) var task = session.dataTaskWithRequest(request) task.resume()

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let url = NSURL(string: "http://cache-tester.herokuapp.com/contacts.json") let request = NSURLRequest(URL: url) if let cachedResponse = config.URLCache.cachedResponseForRequest(request) { // update UI processData(cachedResponse.data) } var task = session.dataTaskWithRequest(request) task.resume()

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New in iOS 8: getCachedResponseForTask: • Provides asynchronous cache fetch: let url = NSURL(string: "http://localhost:3000/contacts.json") let request = NSURLRequest(URL: url) var task = session.dataTaskWithRequest(request) config.URLCache.getCachedResponseForDataTask(task) { (let cachedResponse: NSCachedURLResponse?) in if cachedResponse != nil { self.processData(cachedResponse!.data) } task.resume() }

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Bonus Round API Tips

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Don't Expose your Internal Model

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Version Your API

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Send Device Info as Headers

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Turn on Gzip Compression

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Measure Response Times

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Page Unbounded Data Sets

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Thank You! [email protected] @subdigital • @nsscreencast benscheirman.com • nsscreencast.com speakerdeck.com/subdigital/ ios8-networking