Swift India Conf 2019: Hacking Rule Systems & State Machines with Apple's GameplayKit Framework

Speaker: Srikanth KV, Senior iOS Engineer, PhonePe
LinkedIn: https://www.linkedin.com/in/srikanthvkabadi/

Bio: Srikanth is a Senior iOS Engineer at PhonePe and has been building iOS apps since 2011. Started working on Swift right from its inception. A great admirer of Swift because of its power and intuitiveness. Very keen on learning and using new technologies. Has handson experience with building 3D apps with SceneKit, small 2D apps with SpriteKit, AR app with ARKit and VR app with Unity. Has worked extensively on cross-platform frameworks like React Native and JUCE.

Abstract: Don't go with the name. It has nothing to do with "Games". Solve problems which are as simple as Text Validator to as complex as Payments System which has "n" number of states and "m" number of rules to evaluate.

 Also provides better readability, scalability, and reusability.

Example Repo: https://github.com/SrikanthKabadi/GameplayKit

Transcript

1. Hacking 
 Rule Systems & State Machines 
 with GameplayKit - Nothing to do with "Games" Srikanth K V PhonePe Private Limited
2. What does it solve? Solve problems which is as simple as Text Validator to as complex as Payments System which has "n" number of states and "m" number of rules to evaluate.
 
 Also provides better readability, scalability and reusability.
3. Introduction - Rule System (GKRuleSystem) - State Machine (GKStateMachine) - Combining Rule System with State Machine - Usecases
4. GKRuleSystem
5. GKRuleSystem - Rules (GKRule)
6. GKRuleSystem - Rules (GKRule) - Input data (Dictionary)
7. GKRuleSystem - Rules (GKRule) - Input data (Dictionary) - Fact (Conclusion)
8. GKRuleSystem - Rules (GKRule) - Input data (Dictionary) - Fact (Conclusion) - Evaluate
9. Example enum PasswordError: Error { case passwordsDontMatch case notLongEnough case uppercaseMissing case lowerCaseMissing case noNumbers case spacesNotAllowed } enum ValidationResult { case valid case invalid(error: PasswordError) }
10. Example enum PasswordError: Error { case passwordsDontMatch case notLongEnough case uppercaseMissing case lowerCaseMissing case noNumbers case spacesNotAllowed } enum ValidationResult { case valid case invalid(error: PasswordError) }
11. Wrapper class Fact: NSObject { typealias EvaluationBlock = (_ system: GKRuleSystem) -> Bool let error: PasswordError let evaluationBlock: EvaluationBlock init(error: PasswordError, evaluationBlock: @escaping EvaluationBlock) { self.error = error self.evaluationBlock = evaluationBlock } }
12. Creating Rule class FactRule: GKRule { let fact: Fact init(fact: Fact) { self.fact = fact super.init() } override func evaluatePredicate(in system: GKRuleSystem) -> Bool { return fact.evaluationBlock(system) } override func performAction(in system: GKRuleSystem) { system.assertFact(fact) } }
13. class PasswordValidator { } private let lengthFact = Fact(error: .notLongEnough) { (system: GKRuleSystem) in guard let password = system.state["password"] as? String, password.count >= 8 else { return false } return true }
14. private let passwordsMatchFact = Fact(error: .passwordsDontMatch) { system in guard let password = system.state["password"] as? String, let confirmedPass = system.state["confirmedPassword"] as? String, password == confirmedPass else { return false } return true }
15. Password Validator
16. func validatePasswords(password: String, confirmedPassword: String) -> ValidationResult { // create the facts let facts = [lengthFact, uppercaseLetterFact, lowercaseLetterFact, numberFact, noWhiteSpaceFact, passwordsMatchFact] Password Validator
17. func validatePasswords(password: String, confirmedPassword: String) -> ValidationResult { // create the facts let facts = [lengthFact, uppercaseLetterFact, lowercaseLetterFact, numberFact, noWhiteSpaceFact, passwordsMatchFact] /* create rule system and provide all the facts which we want to evaluate */ let ruleSystem = GKRuleSystem() ruleSystem.add(facts.map(FactRule.init)) Password Validator
18. func validatePasswords(password: String, confirmedPassword: String) -> ValidationResult { // create the facts let facts = [lengthFact, uppercaseLetterFact, lowercaseLetterFact, numberFact, noWhiteSpaceFact, passwordsMatchFact] /* create rule system and provide all the facts which we want to evaluate */ let ruleSystem = GKRuleSystem() ruleSystem.add(facts.map(FactRule.init)) // add the state to the rule system ruleSystem.state["password"] = password ruleSystem.state["confirmedPassword"] = confirmedPassword // evaluate ruleSystem.evaluate() } Password Validator
19. // extract the errors from the facts after // evaluation let erroredFacts = facts.filter { ruleSystem.grade(forFact: $0) == 0 } let errors = erroredFacts.map { $0.error } if !errors.isEmpty { return .invalid(errors: errors) } else { return .valid } Extracting Result
20. // extract the errors from the facts after // evaluation let erroredFacts = facts.filter { ruleSystem.grade(forFact: $0) == 0 } let errors = erroredFacts.map { $0.error } if !errors.isEmpty { return .invalid(errors: errors) } else { return .valid } Extracting Result
21. GKStateMachine
22. GKStateMachine - States (GKState)
23. GKStateMachine - States (GKState) - Life Cycle
24. State Machine Example - Red - Yellow - Green
25. State Machine Example - Red - Yellow - Green
26. class Red: GKState { override func isValidNextState(_ stateClass: AnyClass) -> Bool { return stateClass is Yellow.Type } override func didEnter(from previousState: GKState?) { } override func willExit(to nextState: GKState) { } }
27. class Red: GKState { override func isValidNextState(_ stateClass: AnyClass) -> Bool { return stateClass is Yellow.Type } override func didEnter(from previousState: GKState?) { } override func willExit(to nextState: GKState) { } }
28. class Green: GKState { override func isValidNextState(_ stateClass: AnyClass) -> Bool { return stateClass is Yellow.Type } override func didEnter(from previousState: GKState?) { } override func willExit(to nextState: GKState) { } }
29. class Green: GKState { override func isValidNextState(_ stateClass: AnyClass) -> Bool { return stateClass is Yellow.Type } override func didEnter(from previousState: GKState?) { } override func willExit(to nextState: GKState) { } }
30. class Yellow: GKState { override func isValidNextState(_ stateClass: AnyClass) -> Bool { return stateClass is Red.Type || stateClass is Green.Type } override func didEnter(from previousState: GKState?) { } override func willExit(to nextState: GKState) { } }
31. class Yellow: GKState { override func isValidNextState(_ stateClass: AnyClass) -> Bool { return stateClass is Red.Type || stateClass is Green.Type } override func didEnter(from previousState: GKState?) { } override func willExit(to nextState: GKState) { } }
32. Combo
33. GKStateMachine Combo
34. GKStateMachine Combo State 1 State 2 State 3
35. GKStateMachine Combo State 1 State 2 State 3 Rule 1 Rule 2 GKRuleSystem 1
36. GKStateMachine Combo State 1 State 2 State 3 Rule 1 Rule 2 GKRuleSystem 1 Rule 1 Rule 2 GKRuleSystem 2
37. GKStateMachine Combo State 1 State 2 State 3 Rule 1 Rule 2 GKRuleSystem 1 Rule 1 Rule 2 GKRuleSystem 2
38. GKStateMachine Combo State 1 State 2 State 3 Rule 1 Rule 2 GKRuleSystem 1 Rule 1 Rule 2 GKRuleSystem 2
39. GKStateMachine Combo State 1 State 2 State 3 Rule 1 Rule 2 GKRuleSystem 1 Rule 1 Rule 2 GKRuleSystem 2
40. Usecase
41. Usecase Init 21/07/2019
42. Usecase Init 21/07/2019
43. Usecase Init 21/07/2019
44. Usecase Init 21/07/2019
45. Init State + Fraud Check System
46. Init Init State + Fraud Check System
47. Init Amount Threshold Check Transaction Frequency Check Device
 Check Location
 Check Fraud Check System Init State + Fraud Check System
48. Init Amount Threshold Check Transaction Frequency Check Device
 Check Location
 Check Fraud Check System Pending Init State + Fraud Check System
49. Init Amount Threshold Check Transaction Frequency Check Device
 Check Location
 Check Fraud Check System Pending Failed Init State + Fraud Check System
50. Init Amount Threshold Check Transaction Frequency Check Device
 Check Location
 Check Fraud Check System Pending Failed Yes Init State + Fraud Check System
51. Init Amount Threshold Check Transaction Frequency Check Device
 Check Location
 Check Fraud Check System Pending Failed Yes No Init State + Fraud Check System
52. class InitState: GKState { override func isValidNextState(_ stateClass: AnyClass) -> Bool { return stateClass == PendingState.Type || stateClass == FailedState.Type } override func didEnter(from previousState: GKState?) { } } Init Pending Failed Yes No
53. override func didEnter(from previousState: GKState?) { let facts = [locationFact, deviceVerificationFact, amountThresholdFact, transactionRateFact] let ruleSystem = GKRuleSystem() ruleSystem.add(facts.map(FactRule.init)) ruleSystem.state["locationInfo"] = data.locationInfo ruleSystem.state["deviceInfo"] = data.deviceInfo ruleSystem.state["amountInfo"] = data.amountInfo ruleSystem.state["transactionRate"] = data.transactionRate ruleSystem.evaluate() } Amount Threshold Check Transaction Frequency Check Device
 Check Location
 Check Fraud Check System
54. let erroredFraudFacts = facts.filter { ruleSystem.grade(forFact: $0) == 0 } if erroredFraudFacts.count > 0 { stateMachine?.enter(FailedState.self) } else { stateMachine?.enter(PendingState.self) } Reading the Result
55. - Offer Rule System - Check for offer window - Minimum Amount Check - Other set of rules - Order Completion - Book a cab - Order food Other Usecases
56. Advantages - Adding new rule - Adding new state - Reusing the same rule in multiple usecases/ states - App transition is always under control.
57. Thank You Credits
 
 Abhijit K G
 Aditya Rohan