Lessons In Swift Error Handling And Resilience

Transcript

1. LESSONS IN SWIFT ERROR HANDLING AND RESILIENCE CHRISTOPHER ROGERS
2. LESSONS IN SWIFT ERROR HANDLING MOTIVATION ▸ Stable, correctly behaving code ▸ A codebase that allows us to continue to move at a fast pace ▸ Graceful recovery from “unexpected” errors while minimizing user impact ▸ No “denial of service” crashing ▸ Avoid asking the user to try reinstalling the app ▸ ҆ఆత͔ͭਖ਼ৗʹಈ࡞͢Δίʔυ ▸ ։ൃεϐʔυΛམͱͣ͞ʹਐΊΒΕΔ ▸ ͍ΘΏΔ༧ظ͠ͳ͍ෆ۩߹͔Β෮ؼ͠, Ϣʔβʔ΁ͷӨڹൣғΛ࠷খݶʹ ▸ DoS తʹམͪͳ͍Α͏ʹ஫ҙ͢Δ ▸ ΞϓϦͷ࠶ΠϯετʔϧΛͳΔ΂͘
 ͤ͞ͳ͍Α͏ʹ എܠ
4. CORRECT CODE INPUT
5. CORRECT CODE INPUT ▸ Explicit ▸ Function parameters ▸ self ▸ Implicit ▸ a.k.a. “state” ▸ ໌ࣔత ▸ ؔ਺ͷҾ਺ ▸ self ▸ ҉໧త ▸ ঢ়ଶ ೖྗ
6. CORRECT CODE struct Foo { func bar(id: String) -> String { return settings[id] ?? "" } }
7. CORRECT CODE STATE ▸ Global variables ▸ Singletons ▸ άϩʔόϧม਺ ▸ Singleton ঢ়ଶ
8. CORRECT CODE STATE ▸ Temporal ▸ The code being executed at any given time ▸ Mostly defined by order of imperative code ▸ Usually implicit, but can be made more explicit ▸ Comments ▸ Not being compilable in incorrect order ▸ ࣌ؒత ▸ ͋Δ࣌ؒͰ࣮ߦ͞Ε͍ͯΔίʔυΛࣔ͢ ▸ ໋ྩܕίʔυͷॱ൪ʹΑͬͯఆٛ͞ΕΔ ▸ େ֓͸҉໧త͕ͩɺΑΓ໌ࣔతʹ͢Δ͜ͱ ΋Մೳ ▸ ίϝϯτ ▸ ؒҧͬͨॱ൪ͰίϯύΠϧͰ͖ͳ͍Α ͏ʹ͢Δ͜ͱ ঢ়ଶ
9. CORRECT CODE let queue = DispatchQueue.global() queue.async { print("1") } queue.async { print("2") } print("3")
10. CORRECT CODE 1 2 3 1 3 2 2 1 3 2 3 1 3 1 2 3 2 1
11. CORRECT CODE func f() -> Int { let a = foo() let b = bar(“b”) doSomething() let c = bar(a) return b + c }
12. CORRECT CODE STATE ▸ State from the “real world” ▸ Filesystem & network ▸ Previous executions/versions of your code ▸ Previous buggy versions ▸ Data from long lost & forgotten specs ▸ ϦΞϧੈքͷঢ়ଶ ▸ ϑΝΠϧγεςϜ΍ωοτϫʔΫ ▸ Ҏલͷίʔυͷ࣮ߦग़ྗ ▸ όά͍ͬͯͨίʔυ ▸ ๨ΕΒΕͨ࢓༷ ঢ়ଶ
13. EXAMPLE
14. CORRECT CODE import Foundation final class ThemeSubsystem { init() { let theme = UserDefaults.standard.string(forKey: "theme") // … } } DATA FORMAT MIGRATION σʔλܗࣜͷҠߦॲཧ
15. CORRECT CODE: EXAMPLE DATA FORMAT MIGRATION ▸ The value in stored in user defaults. ▸ It should be migrated on first launch after upgrading the app. ▸ User defaults ʹ֨ೲ͞Εͨ஋͕ର৅ɻ ▸ ΞϓϦͷΞοϓσʔτޙͷॳظىಈ࣌ ʹҠߦ͢Δɻ σʔλܗࣜͷҠߦॲཧ
16. CORRECT CODE: EXAMPLE DATA FORMAT MIGRATION ▸ However, the value was used prior to migration. ▸ Shared state + ambiguous dependency ▸ How do we go about fixing it? ▸ How do we prevent similar mistakes? ▸ Ҡߦ͢Δલʹར༻͞Εͯ͠·ͬͨɻ ▸ ڞ༗ঢ়ଶͱᐆດͳґଘؔ܎ ▸ मਖ਼ํ๏͸ʁ ▸ ࠶ൃ๷ࢭ͸ʁ σʔλܗࣜͷҠߦॲཧ
17. CORRECT CODE: EXAMPLE EXPLICIT DEPENDENCIES WITH TYPES ▸ If you don't have an instance of a type, you can't call a function that requires it as input. ▸ Create the type to represent the transition into a certain state. ▸ Requiring an instance of that type as a function parameter allows you to specify the state that the type represents as a prerequisite. ▸ ܕͷΠϯελϯε͕ͳ͚Ε͹ͦΕΛҾ਺ͱ͢ Δؔ਺Λݺͼग़͢͜ͱ͕Ͱ͖ͳ͍ɻ ▸ ಛఆͷঢ়ଶ΁ͷભҠΛද͢ܕΛ࡞Δɻ ▸ ͦͷܕͷΠϯελϯεΛҾ਺ͱͯ͠ड͚औΔ ͜ͱʹΑͬͯͦͷܕ͕ද͢ঢ়ଶΛલఏ৚݅ͱ ͯ͠ఆٛ͢Δ͜ͱ͕Ͱ͖Δɻ ܕʹΑΔ໌ࣔతͳґଘؔ܎
18. CORRECT CODE import Foundation final class ThemeSubsystem { init(_: SetupComplete) { let theme = UserDefaults.standard.string(forKey: "theme") // … } } EXPLICIT DEPENDENCIES WITH TYPES ܕʹΑΔ໌ࣔతͳґଘؔ܎
20. OPTIONAL VS. ERROR
21. OPTIONAL VS. ERROR WHEN TO USE OPTIONAL ▸ “Right or wrong”, “present or not present” semantics as output ▸ As input, “optional” semantics ▸ Swift’s syntax makes it easy to safely handle. ▸ The “go to” choice ▸ ग़ྗͷ஋ͱͯ͠͸ʮਖ਼͍͔͠ਖ਼͘͠ͳ͍ʯ ΍ʮ༗Γ͔ແ͠ʯͷҙຯ ▸ ೖྗͷ஋ͱͯ͠͸ʮ೚ҙʯͷҙຯ ▸ ҆શʹѻ͏͜ͱ͕༰қʹͰ͖Δ ▸ େ఍ͷ৔߹͸ Optional ܕ͕ਖ਼ղ
22. OPTIONAL VS. ERROR WHEN TO USE ERROR ▸ Similar “wrong”, “unable to complete successfully” semantics ▸ Often used for “fatal errors” ▸ Syntax lets you pretend nothing will go wrong at the call site of throwing functions. ▸ Best suited for handling low- probability, high-impact errors. ▸ ಉ͡Α͏ͳʮෆਖ਼ղʯ΍ʮਖ਼ৗʹॲཧ Ͱ͖ͳ͔ͬͨʯҙຯ ▸ க໋తͳ໰୊ʹ࢖ΘΕΔ͜ͱ͕ଟ͍ ▸ εϩʔ͢Δؔ਺ͷݺͼग़͠ଆ͸ਖ਼ৗʹ ॲཧ͞ΕΔৼΔ෣͍͕Ͱ͖Δ ▸ ൃੜ཰͕௿͘Өڹൣғ͕޿͍Τϥʔʹ ར༻͢Δͷ͕ద੾ ERROR ͷ࢖͍ಓ
23. WHEN TO USE ERROR OPTIONAL VS. ERROR struct ChatID { let type: ChatIDType let stringValue: String init?(_ string: String) { } } ERROR ͷ࢖͍ಓ
24. WHEN TO USE ERROR OPTIONAL VS. ERROR struct ChatID { let type: ChatIDType let stringValue: String init(_ string: String) throws { } } ERROR ͷ࢖͍ಓ
25. LESSONS IN SWIFT ERROR HANDLING AND RESILIENCE RECAP ▸ Being cognizant of input/output flow leads to more robust code. ▸ Be aware of code critically important. ▸ Use Optional for lightweight, low- context, somewhat likely errors. ▸ Use Error for heavyweight, high- context, and unlikely errors. ▸ ೖग़ྗͷྲྀΕΛҙ͍ࣝͯ͠Ε͹͋ΒΏ ΔΤϥʔঢ়ଶΛରॲͰ͖ΔΑ͏ʹίʔ υ͕ͳ͍ͬͯ͘ɻ ▸ ඇৗʹॏཁͳίʔυʹؾΛ͚ͭΔ͜ͱ ▸ Optional͸৘ใྔ͕গͳ͍ൃੜස౓ ͕΍΍ى͜Γ͕ͪͳΤϥʔʹ ▸ Error ͸৘ใྔ͕ଟ͍كͳΤϥʔʹ ·ͱΊ
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