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‎Lessons/Combine-Pt.1/README.md‎

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<!-- Run this slideshow via the following command: -->
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<!-- reveal-md README.md -w -->
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# Combine
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## [Slides](https://make-school-courses.github.io/MOB-2.4-Advanced-Architectural-Patterns-in-iOS/Slides/Combine-Pt.1/README.html ':ignore')
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▶️ Watch this before the lesson:
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[Combine - Video introduction](https://developer.apple.com/videos/play/wwdc2019/722/)
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## Learning Objectives
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By the end of this lesson, you will be able to:
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Describe:
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- Reactive programming
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- Marble diagrams
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- Combine as a framework and it's main components
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- Basic operators
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- Lifecycle of a stream
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## Reactive Programming
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Reactive Programming can be thought of as the practice of programming with **asynchronous** data streams, or __*event streams.*__
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### Event Streams
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An event stream is a **sequence of events** happening over time.
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An **asynchronous data stream** is a stream of data where values are *emitted*, one after another, with a delay between them, and without blocking program flow to wait for results.
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And because the stream is asynchronous, the data emitted can appear anywhere in time.
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### Modeling Event Streams with Marble Diagrams
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The common way of modeling asynchronous streams is to place the emitted values on a time axis in what is called a **Marble Diagram**
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These are interactive diagrams that show how operators work with sequences over time.
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Here, our diagram shows a simple description of a hypothetical event stream, with events represented by colored bubbles drawn at intermittent time intervals:
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![Marble_diagram_breakdown](assets/Marble_diagram.png) </br>
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In a Marble Diagram, the left-to-right arrow represents time, and the numbered circles represent elements of a sequence, which are just values plotted on the timeline.
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In the last diagram, pay particular attention to these symbols:
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- The **Error** symbol
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- The **Event Stream Complete** symbol
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## Combine
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A framework that provides a declarative Swift API for processing values over time.
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These values can represent many kinds of asynchronous events.
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[Docs](https://developer.apple.com/documentation/combine)
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## Core Concepts
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- [Publisher](https://developer.apple.com/documentation/combine/publisher) and [Subscriber](https://developer.apple.com/documentation/combine/subscriber)
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- Operators
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- [Subjects](https://developer.apple.com/documentation/combine/subject)
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## Publisher
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- Described as a protocol
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- **Provides data when available and upon request**
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- A publisher that has not had any subscription requests will not provide any data
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- Has two associated types: one for **Output** and one for **Failure**
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- Value types
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## Subscriber
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- Responsible for **requesting data and accepting the data** (and possible failures) provided by a publisher
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- Has two associated types: one for **Input** and one for **Failure**
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- Reference types (usually act and mutate state upon receipt of values)
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<img src="https://heckj.github.io/swiftui-notes/images/diagrams/input_output.svg">
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Publishers and subscribers are meant to be connected, and make up the core of Combine.
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<aside class="notes">
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When you connect a subscriber to a publisher, both types must match: Output to Input, and Failure to Failure.
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</aside>
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## Operator
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- An object that acts both like a subscriber and a publisher
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- Classes that adopt both the Subscriber protocol and Publisher protocol
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- They support subscribing to a publisher, and sending results to any subscribers
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- Can be chained to process, transform data
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<img src="https://heckj.github.io/swiftui-notes/images/diagrams/pipeline.svg">
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```swift
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let _ = Just(8)
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.map { value -> String in
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// do something with the incoming value here
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return "\(value) as a string"
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}
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.sink { receivedValue in
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// sink is the subscriber and terminates the pipeline
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print("The end result was \(receivedValue)")
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}
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```
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## In Class Activity
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Instructions [here]()
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## After Class
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## Additional Resources
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- [Using Combine](https://heckj.github.io/swiftui-notes/#coreconcepts-publisher-subscriber)
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- Book: Practical Combine by Donny Walls

‎README.md‎

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| 8 | Mon, Feb 15 | [Functional Programming Pt.2] |
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| 9 | Wed, Feb 17 | Lab - Sharing Progress |
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| 10 | Mon, Feb 22 | [Reactive Programming Pt.1] |
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| 11 | Wed, Feb 24 | [Reactive Programming Pt.2]|
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| 11 | Wed, Feb 24 | [Reactive Programming Pt.2] |
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| 12 | Mon, Mar 1 | [Reactive Programming Pt.3] |
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| 13 | Wed, Mar 3 | Final Exam |
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