Networking and Multithreading
Many iOS application are clients of a REST API. This guide gives an overview of common ways to accomplish tasks associated with making HTTP requests and handling responses. Low level socket programming, although possible to do in iOS is out of the scope of this guide.
Overview of popular network programming methods
There is a wide variety of ways to make HTTP requests in iOS with which you might at least want to be familiar.
NSURLConnection is a lower level mechanism to make URL requests and is part of Apple’s Foundation Framework.
- simplest way to make URL request
- provides synchronous and asynchronous requests via with completion handler blocks and delegates
- does not have much support for authentication or a session concept
- does not have an “operation” or “task” concept associated with requests so there’s no convenient way to handle queue of requests or to pause/resume
- does not handle parsing of common content types
- not much built in support for HTTP error codes / request parameters
NSURLSession a higher level library that is part of Apple’s Foundation Framework.
- built on top of NSURLConnection
- better support for authentication and has a session concept
- concept of “task” enables pausing/resuming requests
- can perform requests while your app is in the background
- does not handle parsing of common content types
- not much built in support for HTTP error codes / request parameters
AFNetworking is the most popular library for and is the de facto gold standard for networking tasks in the iOS world. Chances are you will want to use this library if accessing an API and making network requests is a key part of your application.
- built on top of NSURLSession
- built-in support for parsing common content-types
- great support for common HTTP operations including handling of request params, headers, error codes
- great integration with UIKit components making complex behavior like loading remote images asynchronously very easy
AlamoFire is another networking library by the same author as AFNetworking. It is written in Swift.
- Swift only
- many of the same features as AFNetworking
- easy to use/read syntax for making common requests
- no integration with UIKit
A note about threading and network requests
In iOS much of the code that runs in your application is triggered by an
event on the main event loop. The main event loop is
responsible for executing code to respond to things like user
interaction (e.g triggering an @IBAction) or events in a view
controller’s lifecycle (e.g. viewDidLoad). Code executed from the
main event loop is run on the main thread. This is convenient for us
because any updates to an application’s UI elements must happen on the
main thread. We’ll want to keep this rule in mind when working with
network requests.
iOS provides a couple of higher level libraries for concurrent programming: Grand Central Dispatch and NSOperationQueue. You’ll be able use either to ensure you that a piece of code does or does not run on the main thread.
You should never make a synchronous network request on the main thread since this will block thread and UI will appear frozen while our request is pending. You’ll rarely run into instances where you’ll need to make synchronous requests
When we make an asynchronous request, any of the above libraries will execute the request on a background (i.e. not the main) thread. Some methods will allow us to specify the dispatch queue on which we want the response handler to run, others will provide no guarantees. If we need to update the UI in our response handler we must ensure that the code that manipulates the UI is run on the main thread. This can be tricky because we may call into a method that calls into another method that after a long stack of calls eventually updates a UI element.
One simple way to ensure a block of code is run on the main thread using Grand Central Dispatch is as follows
dispatch_async(dispatch_get_main_queue(), {
// this code will be executed on the main thread
})
Making a GET and parsing a JSON response
In order to provide you with the flavor of each of the major ways of making network requests we discussed above, we’ll go through an example of each one.
NSURLConnection (deprecated in iOS 9)
Notice that we are forced to specify a operation queue on which the completion handler will run.
import UIKit
private let apiKey = "53eb9541b4374660d6f3c0001d6249ca:19:70900879"
private let resourceUrl = NSURL(string: "http://api.nytimes.com/svc/topstories/v1/home.json?api-key=\(apiKey)")!
class Story {
var headline: String?
var thumbnailUrl: String?
init(jsonResult: NSDictionary) {
...
}
class func fetchStories(successCallback: ([Story]) -> Void, error: ((NSError?) -> Void)?) {
let request = NSURLRequest(URL: resourceUrl)
NSURLConnection.sendAsynchronousRequest(request, queue: NSOperationQueue.mainQueue()) { (response, data, requestError) -> Void in
if let requestError = requestError? {
error?(requestError)
} else {
if let data = data? {
let json = NSJSONSerialization.JSONObjectWithData(data, options: nil, error: nil) as NSDictionary
if let results = json["results"] as? NSArray {
var stories: [Story] = []
for result in results as [NSDictionary] {
stories.append(Story(jsonResult: result))
}
successCallback(stories)
}
} else {
// unexpected error happened
error?(nil)
}
}
}
}
}
NSURLSession
NSURLSession is now the preferred built-in method of performing network requests on iOS.
Swift
class Post {
// ...
class func fetchPosts(successCallback: (NSDictionary) -> Void, errorCallback: ((NSError?) -> Void)?) {
let clientId = "Put_Your_Client_Id_Here"
let url = NSURL(string:"https://api.instagram.com/v1/media/popular?client_id=\(clientId)")
let request = NSURLRequest(URL: url!)
let session = NSURLSession(
configuration: NSURLSessionConfiguration.defaultSessionConfiguration(),
delegate:nil,
delegateQueue:NSOperationQueue.mainQueue()
)
let task : NSURLSessionDataTask = session.dataTaskWithRequest(request,
completionHandler: { (dataOrNil, responseOrNil, errorOrNil) in
if let requestError = errorOrNil {
errorCallback?(requestError)
} else {
if let data = dataOrNil {
if let responseDictionary = try! NSJSONSerialization.JSONObjectWithData(
data, options:[]) as? NSDictionary {
NSLog("response: \(responseDictionary)")
successCallback(responseDictionary)
}
}
}
});
task.resume()
}
}
Objective-C
NSString *clientId = @"Put_Your_Client_Id_Here";
NSString *urlString =
[@"https://api.instagram.com/v1/media/popular?client_id=" stringByAppendingString:clientId];
NSURL *url = [NSURL URLWithString:urlString];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
NSURLSession *session =
[NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]
delegate:nil
delegateQueue:[NSOperationQueue mainQueue]];
NSURLSessionDataTask *task = [session dataTaskWithRequest:request
completionHandler:^(NSData * _Nullable data,
NSURLResponse * _Nullable response,
NSError * _Nullable error) {
if (!error) {
NSError *jsonError = nil;
NSDictionary *responseDictionary =
[NSJSONSerialization JSONObjectWithData:data
options:kNilOptions
error:&jsonError];
NSLog(@"Response: %@", responseDictionary);
} else {
NSLog(@"An error occurred: %@", error.description);
}
}];
[task resume];
AFNetworking
This code starts to look a little cleaner with AFNetworking. AFNetworking does some error handling for us and gives us a way to provide a failure handler. Also note that we no longer have to parse the JSON result ourselves since AFNetworking does this automatically based on the content type. Finally note that we were able to supply our GET parameters as a Swift dictionary. This is not particularly useful here, but becomes very nice to have if there is a large number of parameters.
private let params = ["api-key": "53eb9541b4374660d6f3c0001d6249ca:19:70900879"]
private let resourceUrl = "http://api.nytimes.com/svc/topstories/v1/home.json"
class Story {
var headline: String?
var thumbnailUrl: String?
init(jsonResult: NSDictionary) {
...
}
class func fetchStories(successCallback: ([Story]) -> Void, error: ((NSError?) -> Void)?) {
let manager = AFHTTPRequestOperationManager()
manager.GET(resourceUrl, parameters: params, success: { (operation ,responseObject) -> Void in
if let results = responseObject["results"] as? NSArray {
var stories: [Story] = []
for result in results as [NSDictionary] {
stories.append(Story(jsonResult: result))
}
successCallback(stories)
}
}, failure: { (operation, requestError) -> Void in
if let errorCallback = error? {
errorCallback(requestError)
}
})
}
}