Last updated on April 12, 2023
Combine is a powerful framework in Swift that helps you manage asynchronous data streams, providing a declarative way to handle complex asynchronous operations. It is an integral part of modern iOS development, and can be used in a variety of scenarios, such as network requests, user input, and more.
Here’s an A to Z guide on getting started with Combine in Swift, with code examples for those who have zero experience.
A is for Asynchronous Programming
Asynchronous programming is a programming technique that allows your application to perform multiple tasks concurrently. It is essential for building responsive and efficient iOS applications. Combine is designed to help you work with asynchronous data streams by providing a declarative API.
B is for Bind
The Bind operator is used to bind a value from one publisher to another. The bind operator can be used to update a property on a view whenever the underlying data changes. Here’s an example:
let usernamePublisher = NotificationCenter.default.publisher(for: .UITextFieldTextDidChange)
let passwordPublisher = NotificationCenter.default.publisher(for: .UITextFieldTextDidChange)
let combinedPublisher = Publishers.CombineLatest(usernamePublisher, passwordPublisher)
.map { (username, password) -> Bool in
!username.isEmpty && !password.isEmpty
}
combinedPublisher.assign(to: \.isEnabled, on: loginButton)
In the above example, we are binding the combinedPublisher to the isEnabled property of a UIButton. Whenever the combinedPublisher emits a new value, the isEnabled property of the UIButton will be updated.
C is for Combine
Combine is a framework in Swift that helps you manage asynchronous data streams. It provides a declarative way to handle complex asynchronous operations.
D is for Data Streams
A data stream is a sequence of values that can be emitted over time. Combine provides a way to handle data streams in a declarative manner.
E is for Error Handling
Combine provides several ways to handle errors that may occur in a data stream. You can use the catch operator to handle errors, or the replaceError operator to replace errors with a default value.
F is for Future
A Future is a publisher that emits a single value or an error when it completes. You can use the Future publisher to perform an asynchronous operation and return a single value or error.
func fetchUser(id: Int) -> Future<User, Error> {
return Future { promise in
// Perform asynchronous operation here
promise(.success(user))
}
}
let userPublisher = fetchUser(id: 1)In the above example, we are creating a Future publisher that fetches a user with a given ID. When the asynchronous operation completes, the Future publisher will emit either a User or an error.
G is for Grouping
Combine provides several operators that allow you to group and combine values emitted by data streams. The zipoperator can be used to combine values emitted by multiple publishers into a single value.
let publisher1 = PassthroughSubject<Int, Never>()
let publisher2 = PassthroughSubject<String, Never>()
let zippedPublisher = Publishers.Zip(publisher1, publisher2)
zippedPublisher.sink { (intValue, stringValue) in
print("intValue: \(intValue), stringValue: \(stringValue)")
}
publisher1.send(1)
publisher2.send("hello")
In the above example, we are creating two publishers that emit an Int and a String, respectively. We are then using the zip operator to combine the values emitted by these publishers into a single tuple.
H is for Hot and Cold Publishers
Combine publishers can be classified as either hot or cold. A hot publisher emits values regardless of whether there are any subscribers, while a cold publisher only emits values when there are active subscribers.
I is for Publishers
Publishers are a fundamental concept in Combine. A publisher is an object that emits a sequence of values over time. Publishers can be created from a variety of sources, such as an array, a Future, or a network request.
J is for Just
The Just publisher is a simple publisher that emits a single value and then completes. You can use the Just publisher to create a publisher that emits a single value.
let justPublisher = Just("Hello, world!")
justPublisher.sink { (value) in
print(value)
}In the above example, we are creating a Just publisher that emits the string “Hello, world!”.
K is for Key-Value Observing
Combine provides a way to observe changes to properties using the Key-Value Observing (KVO) mechanism. You can use the publisher(for:options:) method of an object to create a publisher that emits a value whenever the specified property changes.
class MyClass {
@objc dynamic var myProperty = ""
func setup() {
self.publisher(for: \.myProperty)
.sink { (value) in
print(value)
}
}
}In the above example, we are using the publisher(for:options:) method to create a publisher that emits a value whenever the myProperty property of the MyClass instance changes.
L is for CombineLatest
The CombineLatest operator is used to combine the latest values emitted by two or more publishers into a single tuple. The CombineLatest operator emits a new tuple whenever any of the input publishers emit a new value.
let publisher1 = PassthroughSubject<Int, Never>()
let publisher2 = PassthroughSubject<String, Never>()
let combinedPublisher = Publishers.CombineLatest(publisher1, publisher2)
combinedPublisher.sink { (intValue, stringValue) in
print("intValue: \(intValue), stringValue: \(stringValue)")
}
publisher1.send(1)
publisher2.send("hello")
In the above example, we are creating two publishers that emit an Int and a String, respectively. We are then using the CombineLatest operator to combine the latest values emitted by these publishers into a single tuple.
M is for Map
The Map operator is used to transform the values emitted by a publisher. The Map operator applies a transformation function to each value emitted by the publisher, and emits the transformed value.
let publisher = Just(5)
let mappedPublisher = publisher.map { value in
value * 2
}
mappedPublisher.sink { (value) in
print(value)
}
In the above example, we are using the Map operator to transform the value emitted by the publisher to a value that is twice the original value.
N is for Never
The Never type is used in Combine to indicate that a publisher will never emit a value or complete. You can use the eraseToAnyPublisher() method to erase the type of a publisher to AnyPublisher<Never, Never>.
O is for Operators
Operators are functions that can be used to modify the behavior of a publisher. Combine provides a variety of operators that can be used to filter, transform, combine, and modify publishers.
P is for PassthroughSubject
The PassthroughSubject is a publisher that allows you to manually send values to its subscribers. You can use the send(_:) method to send a value to the subscribers.
let publisher = PassthroughSubject<Int, Never>()
publisher.sink { (value) in
print(value)
}
publisher.send(5)
In the above example, we are creating a PassthroughSubject publisher that emits an Int value of 5 to its subscriber.
Q is for Quality of Service
Quality of Service (QoS) is a property that can be used to prioritize tasks in a system. Combine allows you to set the QoS of a publisher to control its priority.
R is for ReplaceError
The ReplaceError operator is used to replace errors emitted by a publisher with a default value. The ReplaceErroroperator emits the default value when an error is encountered.
let publisher = Fail<Int, Error>(error: NSError(domain: "example.com", code: 1, userInfo: nil))
let replacedPublisher = publisher.replaceError(with: 5)
replacedPublisher.sink { (value) in
print(value)
}
In the above example, we are using the ReplaceError operator to replace the error emitted by the publisher with a default value of 5.
S is for Sink
The Sink operator is used to subscribe to a publisher and receive its values. The Sink operator takes two closures as arguments, one to handle the values emitted by the publisher, and one to handle any errors that may occur.
let publisher = Just("Hello, world!")
publisher.sink { (value) in
print(value)
}In the above example, we are using the Sink operator to subscribe to the publisher and print the value it emits.
T is for Throttle
The Throttle operator is used to limit the rate at which a publisher emits values. The Throttle operator drops any values emitted by the publisher within the specified time interval.
let publisher = PassthroughSubject<Int, Never>()
let throttledPublisher = publisher.throttle(for: .seconds(1), scheduler: DispatchQueue.main, latest: true)
throttledPublisher.sink { (value) in
print(value)
}
publisher.send(1)
publisher.send(2)
publisher.send(3)
In the above example, we are using the Throttle operator to limit the rate at which the publisher emits values. The throttledPublisher will only emit the latest value that was received within a time interval of one second.
U is for URL Session
Combine provides extensions to the URLSession class to create publishers that emit data, responses, and errors from network requests. You can use the dataTaskPublisher(for:) method of URLSession to create a publisher that emits the data, response, and error from a network request.
let url = URL(string: "https://example.com/api/data")!
let request = URLRequest(url: url)
URLSession.shared.dataTaskPublisher(for: request)
.map { data, response in
// Process data and response here
}
.sink { completion in
// Handle completion here
} receiveValue: { value in
// Handle data and response here
}
In the above example, we are using the dataTaskPublisher(for:) method of URLSession to create a publisher that emits the data, response, and error from a network request.
V is for Value
The Value type is used to represent the values emitted by a publisher. The Value type is generic and can be any type.
W is for Weak
When creating a subscription to a publisher, it is important to use a weak reference to avoid creating a strong reference cycle. You can use the sink(receiveCompletion:receiveValue:) method to create a subscription with a weak reference.
class MyClass {
var cancellables = Set<AnyCancellable>()
func setup() {
let publisher = Just("Hello, world!")
publisher
.sink(receiveCompletion: { [weak self] completion in
// Handle completion here
}, receiveValue: { [weak self] value in
// Handle value here
})
.store(in: &cancellables)
}
}In the above example, we are creating a subscription to the `publisher` using a weak reference to avoid creating a strong reference cycle.
X is for Xcode
Xcode is the integrated development environment (IDE) used for developing iOS applications in Swift. Xcode includes support for Combine, making it easy to work with asynchronous data streams.
Y is for Yield
Yield is a term used to describe the act of emitting a value from a publisher. When a publisher yields a value, it emits that value to its subscribers.
Z is for Zip
The `Zip` operator is used to combine the values emitted by two or more publishers into a single tuple. The `Zip` operator emits a new tuple only when all of the input publishers have emitted a new value.
let publisher1 = Just(1)
let publisher2 = Just("hello")
let zippedPublisher = Publishers.Zip(publisher1, publisher2)
zippedPublisher.sink { (value) in
print(value)
}
In the above example, we are using the Zipoperator to combine the values emitted by two publishers into a single tuple. The zippedPublisher will only emit a new tuple when both publishers have emitted a new value.
In conclusion, Combine is a powerful framework that allows you to manage asynchronous data streams in a declarative manner. By understanding the fundamental concepts of publishers, operators, and subscriptions, you can take advantage of Combine to build responsive and efficient iOS applications.