The document discusses state management and unidirectional data flow in applications. It describes some issues with the traditional MVC pattern for state management. The main concepts of unidirectional data flow are then introduced, including the store, actions, and reducers. An example is provided of how application state can be updated in response to actions via a reducer function. Resources for further learning about unidirectional data flow approaches like Flux and Redux are also listed.

1. @nikolasburkState Management & Unidirectional Data Flow
State Management &
Unidirectional Data Flow
“Where is truth in your application?”
Andy Matuschak, WWDC 2014

2. Nikolas Burk
@nikolasburk
• Developer at Graphcool
• Experienced with iOS
• Passionate about programming
paradigms and architecture

3. 1. State Management
2. Unidirectional Data Flow & ReSwift
@nikolasburkState Management & Unidirectional Data Flow
Goals 🎯

4. State Management
…and why it matters to you

5. @nikolasburkState Management & Unidirectional Data Flow

6. @nikolasburkState Management & Unidirectional Data Flow
An Application is more than the
Happy Path
“In the context of software or
information modeling, a happy
path is a default scenario
featuring no exceptional or error
conditions.”
Wikipedia 😏

7. @nikolasburkState Management & Unidirectional Data Flow
Decision process
for
Slack Notifications

8. The application state represents the
information that is kept in memory of a
running application and is the basis for the
generation of a user interface.
Application State? 🤔
@nikolasburkState Management & Unidirectional Data Flow

9. The application state represents the
information that is kept in memory of a
running application and is the basis for the
generation of a user interface.
Application State? 🤔
@nikolasburkState Management & Unidirectional Data Flow

10. The application state represents the
information that is kept in memory of a
running application and is the basis for
the generation of a user interface.
Application State? 🤔
@nikolasburkState Management & Unidirectional Data Flow

11. @nikolasburkState Management & Unidirectional Data Flow
The main responsibility of an app is to
transform an application state into a user interface.
(AppState) -> UI

12. Example: Conference Planner App 🗓
@nikolasburkState Management & Unidirectional Data Flow
struct Conference {
let name: String
let city: String
let year: String
var attending: Bool
}

13. Application State? 🤔
@nikolasburkState Management & Unidirectional Data Flow
conferences = [
Conference(
name: "AppDevcon",
city: "Amsterdam",
year: "2017",
attending: true
),
Conference(
name: "UIKonf",
city: "Berlin",
year: "2017",
attending: false
),
Conference(
name: "WWDC",
city: "San Jose",
year: "2017",
attending: true
)
]

14. Application State? 🤔
@nikolasburkState Management & Unidirectional Data Flow
conferences = [
Conference(
name: "AppDevcon",
city: "Amsterdam",
year: "2017",
attending: true
),
Conference(
name: "UIKonf",
city: "Berlin",
year: "2017",
attending: false
),
Conference(
name: "WWDC",
city: "San Jose",
year: "2017",
attending: true
)
]
selectedIndex = 0

15. Application State? 🤔
@nikolasburkState Management & Unidirectional Data Flow
conferences = [
Conference(
name: "AppDevcon",
city: "Amsterdam",
year: "2017",
attending: true
),
Conference(
name: "UIKonf",
city: "Berlin",
year: "2017",
attending: false
),
Conference(
name: "WWDC",
city: "San Jose",
year: "2017",
attending: true
)
]
selectedIndex = nil

16. @nikolasburkState Management & Unidirectional Data Flow
1. Where should the state live?
2. Who is allowed to change the state?
3. How does the state get changed?
3 Golden Questions of
State Management 👑
An architectural pattern should be able to answer these questions!

17. @nikolasburkState Management & Unidirectional Data Flow
Model View Controller

18. @nikolasburkState Management & Unidirectional Data Flow
Where should state live?
1 👑

19. @nikolasburkState Management & Unidirectional Data Flow
Who is allowed to change the state?
2 👑

20. @nikolasburkState Management & Unidirectional Data Flow
How does the state get changed?
KVO & Property Observers
Delegates
Callbacks NSNotificationCenter
Target-Action
3 👑

21. @nikolasburkState Management & Unidirectional Data Flow
UIViewController
What’s wrong with MVC?
Persistence
Networking
Navigation
Domain Logic
User Interface
Concurrency

22. @nikolasburkState Management & Unidirectional Data Flow
State Management is
hard
and MVC is not a solution, thanks Apple 😑

23. Unidirectional Data Flow
… and controlling your application state with ReSwift

24. @nikolasburkState Management & Unidirectional Data Flow
•
3 Principles
• Single source of truth
• State is read-only
• Changes with pure functions
http://redux.js.org/docs/introduction/ThreePrinciples.html

25. @nikolasburkState Management & Unidirectional Data Flow
Main Concepts
Store:
… manages the application state
… informs subscribers about changes
Actions:
… describe an intent to change the state
Reducer:
… pure functions that generate a new
application state based on actions

26. Example Flow
@nikolasburkState Management & Unidirectional Data Flow

27. Application
State (Old)
AppDevcon
UIKonf
WWDC
Reducer
add Swift Summit
to list and return new
application state
Action
“Swift Summit”
“San Francisco”
“2017”
Application
State (New)
AppDevcon
UIKonf
WWDC
Swift Summit

28. @nikolasburkState Management & Unidirectional Data Flow
Helpful Resources 📚
• Unidirectional Data Flow in Swift: An Alternative to Massive View
Controllers (Video) (https://realm.io/news/benji-encz-
unidirectional-data-flow-swift/)
• Real World Flux Architecture on iOS (http://blog.benjamin-
encz.de/post/real-world-flux-ios/)
• Redux Documentation (http://redux.js.org/)
• Advanced iOS Application Architecture and Patterns, Andy
Matuschak WWDC 2014 (Video) (http://asciiwwdc.com/2014/
sessions/229)

29. DEMO
Conference Planner App

31. Thank you! 🤓
@nikolasburk