9.
Networking
Written by Florent Pillet
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As programmers, a lot of what we do revolves around networking. Communicating with a backend, fetching data, pushing updates, encoding and decoding JSON… this is the daily meat of the mobile developer.
Combine offers a few select APIs to help perform common tasks declaratively. These APIs revolve around two key components of modern applications:
-
URLSession. - JSON encoding and decoding through the
Codableprotocol.
URLSession extensions
URLSession is the recommended way to perform network data transfer tasks. It offers a modern asynchronous API with powerful configuration options and fully transparent backgrounding support. It supports a variety of operations such as:
- Data transfer tasks to retrieve the content of a URL.
- Download tasks to retrieve the content of a URL and save it to a file.
- Upload tasks to upload files and data to a URL.
- Stream tasks to stream data between two parties.
- Websocket tasks to connect to websockets.
Out of these, only the first one, data transfer tasks, exposes a Combine publisher. Combine handles these tasks using a single API with two variants, taking a URLRequest or just a URL.
Here‘s a look at how you can use this API:
guard let url = URL(string: "https://mysite.com/mydata.json") else {
return
}
// 1
let subscription = URLSession.shared
// 2
.dataTaskPublisher(for: url)
.sink(receiveCompletion: { completion in
// 3
if case .failure(let err) = completion {
print("Retrieving data failed with error \(err)")
}
}, receiveValue: { data, response in
// 4
print("Retrieved data of size \(data.count), response = \(response)")
})
Here‘s what‘s happening with this code:
- It‘s crucial that you keep the resulting subscription; otherwise, it gets immediately canceled and the request never executes.
- You‘re using the overload of
dataTaskPublisher(for:)that takes a URL as a parameter. - Make sure you always handle errors! Network connections are prone to failure.
- The result is a tuple with both a
Dataobject and aURLResponse.
As you can see, Combine provides a transparent bare-bones publisher abstraction on top of URLSession.dataTask, only exposing a publisher instead of a closure.
Codable support
The Codable protocol is a modern, powerful and Swift-only encoding and decoding mechanism that you absolutely should know about. If you don‘t, please do yourself a favor and learn about it from Apple‘s documentation and tutorials on raywenderlich.com!
let subscription = URLSession.shared
.dataTaskPublisher(for: url)
.tryMap { data, _ in
try JSONDecoder().decode(MyType.self, from: data)
}
.sink(receiveCompletion: { completion in
if case .failure(let err) = completion {
print("Retrieving data failed with error \(err)")
}
}, receiveValue: { object in
print("Retrieved object \(object)")
})
.map(\.data)
.decode(type: MyType.self, decoder: JSONDecoder())
Publishing network data to multiple subscribers
Every time you subscribe to a publisher, it starts doing work. In the case of network requests, this means sending the same request multiple times if multiple subscribers need the result.
let url = URL(string: "https://www.raywenderlich.com")!
let publisher = URLSession.shared
// 1
.dataTaskPublisher(for: url)
.map(\.data)
.multicast { PassthroughSubject<Data, URLError>() }
// 2
let subscription1 = publisher
.sink(receiveCompletion: { completion in
if case .failure(let err) = completion {
print("Sink1 Retrieving data failed with error \(err)")
}
}, receiveValue: { object in
print("Sink1 Retrieved object \(object)")
})
// 3
let subscription2 = publisher
.sink(receiveCompletion: { completion in
if case .failure(let err) = completion {
print("Sink2 Retrieving data failed with error \(err)")
}
}, receiveValue: { object in
print("Sink2 Retrieved object \(object)")
})
// 4
let subscription = publisher.connect()
Key points
- Combine offers a publisher-based abstraction for its
dataTask(with:completionHandler:)method calleddataTaskPublisher(for:). - You can decode
Codable-conforming models using the built-indecodeoperator on a publisher that emitsDatavalues. - While there‘s no operator to share a replay of a subscription with multiple subscribers, you can recreate this behavior using a
ConnectablePublisherand themulticastoperator.
Where to go from here?
Great job on going through this chapter!