SwiftIndia Combine Workshop –– 08 Feb 2020

Agenda

Session 1: Quick intro to Combine, part 1

• What is Combine
• Building blocks of Combine
• What is a Publisher
• Different types of Publishers supported by Combine
• What is a Subscriber
• Different types of Subscribers supported by Combine
• Lifecycle of publishers & subscribers
• How to cancel a subscription

Session 2: Quick intro to Combine, part 2

• What is an Operator
• Different types of Operators supported by Combine

Session 3: How to make custom publishers & subscribers?

• Publisher is a protocol
• Wrapping UserDefaults into a publisher
• Subscriber is a protocol
• Wrapping UserDefaults into a subscriber

Session 4: Combine meets UIKit

• Add TapPublisher in UIButton (UIControl in general)
• Convert UIScrollViewDelegate into a Combine like API

Session 5: Error handling in Combine

• What is Error in Combine
• How to model custom Error
• How does Error flow
• How to react to errors
• What is Fail publisher and how to use it
• What are the operators used to transform and handle errors (Catch, TryCatch, MapError, ReplaceError, SetFailureType)

Session 6: Networking using Combine

• Networking without Combine
• Networkin with Combine
• What is DataTaskPublisher
• How to make an API request
• How to handle response from server
• How to parse success response
• How to validate response to handle error
• How to parse and handle error

Session 7: Handling multiple async tasks using Combine

• Handling multiple async tasks without Combine (GCD & OperationQueue)
• Combine operators that enable handling multiple async tasks
• Merge
• Zip
• CombineLatest
• Debounce

Session 8: Handling threads using Combine

• Threading without Combine
• Threading with Combine
• SubscribeOn operator
• ReceiveOn operator

Session 1: Quick intro to Combine, part 1

• What is Combine

  • The Combine framework provides a declarative Swift API for processing values over time. These values can represent many kinds of asynchronous events.
  • Combine declares publishers to expose values that can change over time, and subscribers to receive those values from the publishers.
  • Customize handling of asynchronous events by combining event-processing operators.

• Building blocks of Combine

  • Publisher
  • Subscriber
  • Operator

• What is a Publisher

  • Declares that a type can transmit a sequence of values over time.
  • Source of values

• Different types of Publishers supported by Combine

  • Just
  • Fail
  • Future
  • Deferred
  • DataTaskPublisher
  • NotificationCenter.Publisher
  • NSObject.KeyValueObservingPublisher
  • @Published
  • PassthroughSubject
  • CurrentValueSubject
  • AnyPublisher
  • many more...

• What is a Subscriber

  • A Subscriber instance receives a stream of elements from a Publisher, along with life cycle events describing changes to their relationship.
  • Consumer of values

• Different types of framework supported Subscribers

  • Sink
  • Assign
  • AnySubscriber

• Lifecycle of publishers & subscribers

• How to cancel a subscription
  • Cancellable
  • AnyCancellable

Session 2: Quick intro to Combine, part 2

• What is an Operator

  • Operators are those publishers which can't generate value on their own but rather transform the value provided by a source publisher or a previous operator
  • They are used between a publisher and a subscriber
  • Roughly 3 kinds of operators
    • ones which modify the values
    • ones which modify the thread
    • ones which modify the stream itself

• Different types of Operators supported by Combine

  • Map
  • FlatMap
  • Filter
  • Catch
  • MapError
  • Merge
  • Zip

Session 3: How to make custom publishers & subscribers?

• Publisher is a protocol

public protocol Publisher {

    /// The kind of values published by this publisher.
    associatedtype Output

    /// The kind of errors this publisher might publish.
    associatedtype Failure : Error

    /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
    func receive<S>(subscriber: S) where S : Subscriber, Self.Failure == S.Failure, Self.Output == S.Input
}

// Self.Failure == S.Failure, Self.Output == S.Input

• Wrapping UserDefaults into a publisher

• Subscriber is a protocol

public protocol Subscriber : CustomCombineIdentifierConvertible {

    /// The kind of values this subscriber receives.
    associatedtype Input

    /// The kind of errors this subscriber might receive.
    associatedtype Failure : Error

    /// Tells the subscriber that it has successfully subscribed to the publisher and may request items.
    /// Use the received `Subscription` to request items from the publisher.
    func receive(subscription: Subscription)

    /// Tells the subscriber that the publisher has produced an element.
    func receive(_ input: Self.Input) -> Subscribers.Demand

    /// Tells the subscriber that the publisher has completed publishing, either normally or with an error.
    func receive(completion: Subscribers.Completion<Self.Failure>)
}

• Wrapping UserDefaults into a subscriber

Session 4: Combine meets UIKit

• Complete UIControl.Publisher

• Convert UITableView's scrollViewDidScroll method into a Publisher

Session 5: Error handling in Combine

• What is Error in Combine
  • Error is part of Publisher & Subscriber both
  • Any subscription can terminate in two ways, either via .finished or .failure(Error)
  • .failure(Error) is used to send errors
  • Just like value, we can also transform and observe errors

public protocol Publisher {

    /// The kind of errors this publisher might publish.
    /// Use `Never` if this `Publisher` does not publish errors.
    associatedtype Failure : Error
    ...
}

public protocol Subscriber {

    /// The kind of errors this subscriber might receive.
    /// Use `Never` if this `Subscriber` cannot receive errors.
    associatedtype Failure : Error
    ...
}

extension Subscribers {
    /// - finished: The publisher finished normally.
    /// - failure: The publisher stopped publishing due to the indicated error.
    public enum Completion<Failure> where Failure : Error {
	
        case finished
        case failure(Failure)
    }
}

• How to model custom Error

• How does Error flow

• How to react to errors

• A note about Never

/// Use `Never` as the return type when declaring a closure, function, or
/// method that unconditionally throws an error, traps, or otherwise does
/// not terminate.
///
///     func crashAndBurn() -> Never {
///         fatalError("Something very, very bad happened")
///     }

public enum Never {}

Session 6: Networking using Combine

• Networking without Combine
• Networkin with Combine
• What is DataTaskPublisher
• How to make an API request
• How to handle response from server
• How to parse success response
• How to validate response to handle error
• How to parse and handle error

Session 7: Handling multiple async tasks using Combine

• Handling multiple async tasks without Combine (GCD & OperationQueue)
• Combine operators that enable handling multiple async tasks
  • Merge
  • Zip
  • CombineLatest
  • Debounce

Session 8: Handling threads using Combine

• Threading without Combine
• Threading with Combine
• SubscribeOn operator
• ReceiveOn operator