Building reusable components with generics and protocols
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This document discusses building reusable components with generics and protocols in Swift. It begins by outlining goals of understanding how to design code using generics, simple generics and associated types, and protocols with associated types and generics. An example problem of wanting a generic cache is presented. The document then walks through steps to build such a generic cache, including defining generics, protocols with associated types, and assembling the pieces while ensuring type safety. It concludes by summarizing best practices like designing the API first, introducing abstractions like generics and protocols as needed, and using constraints for type safety.
Building reusable components with generics and protocols
- 2. ! ! #adventofswift One article a day in December until Christmas eve
- 3. Building reusable components with generics and protocols
- 4. Goals
- 5. Goals • Learn how to design code that uses Generics
- 6. Goals • Learn how to design code that uses Generics • Understand simple generics and associated types
- 7. Goals • Learn how to design code that uses Generics • Understand simple generics and associated types • Know how to use protocols with associated types with generics
- 8. Goals • Learn how to design code that uses Generics • Understand simple generics and associated types • Know how to use protocols with associated types with generics • To (not) blow your mind (too much)
- 9. “I want to have a cache. But it should be able to cache, like, anything.”
- 10. “I want to have a cache. But it should be able to cache, like, anything.” “And it should have a remote backup, so if the item isn’t available locally, I can take it from the server.”
- 11. “I want to have a cache. But it should be able to cache, like, anything.” “And it should have a remote backup, so if the item isn’t available locally, I can take it from the server.” “And when the data is taken from the server, the local cache should be updated with the remote version.”
- 12. “I want to have a cache. But it should be able to cache, like, anything.” “And it should have a remote backup, so if the item isn’t available locally, I can take it from the server.” “And when the data is taken from the server, the local cache should be updated with the remote version.” “And I don’t want to create special cache managers for every type of object or something. Maybe the underlying storage could be specific, but that’s it!”
- 13. –Me, in the morning. Before I realize what I’m about to get myself into. “I want to have a cache. But it should be able to cache, like, anything.” “And it should have a remote backup, so if the item isn’t available locally, I can take it from the server.” “And when the data is taken from the server, the local cache should be updated with the remote version.” “And I don’t want to create special cache managers for every type of object or something. Maybe the underlying storage could be specific, but that’s it!”
- 14. $ the appropriate response
- 15. $ the appropriate response
- 16. $ my initial response
- 17. $ my initial response
- 18. Designing the call site
- 19. Designing the call site • Figure out how I want to use whatever it is that I’m building
- 20. Designing the call site • Figure out how I want to use whatever it is that I’m building • This helps to validate your ideas
- 21. Designing the call site • Figure out how I want to use whatever it is that I’m building • This helps to validate your ideas • Don’t worry about any generics, type names or details yet
- 22. Designing the call site An easy to use, straightforward API that uses existing data stores and applies the caching logic internally.
- 24. Designing the implementation And a fairly simple fetch method that tries to find an item in a local store, and uses the remote as a back-up.
- 25. So far, so good
- 26. So far, so good But this isn’t the actual thing, is it?
- 27. So far, so good But this isn’t the actual thing, is it? Oh good, because we were promised generics and protocols and the possibility of getting our minds blown.
- 28. Step 1 - Generics🎓
- 29. Step 1 - Generics🎓
- 30. Step 1 - Generics Generic 🎓
- 31. Step 1 - Generics🎓
- 32. Step 1 - Generics Generic Placeholder 🎓
- 33. Step 1 - Generics Generic Placeholder <YourGeneric> 🎓
- 34. Step 1 - Generics🎓
- 35. Step 1 - Generics The Generic defined on the type and the Generic type of the function argument must always be the same type 🎓
- 36. Step 1 - Generics🎓
- 37. Step 1 - Generics🎓
- 38. Step 1 - Generics🎓
- 39. Step 1 - Generics🎓
- 40. Step 1 - Generics The compiler will check and enforce that you don’t have a type mismatch 🎓
- 41. A (kind of) real example%
- 42. A (kind of) real example% These view models are very similar
- 43. A (kind of) real example% Much nicer
- 44. A (kind of) real example% And easy to use!
- 45. Step 1 - Generics&
- 46. Step 1 - Generics Our cache backed data store will be generic over T and use a Result of T?, or Optional<T> &
- 47. Step 1 - Generics Our cache backed data store will be generic over T and use a Result of T?, or Optional<T> &
- 48. Step 2 - Protocols🎓
- 49. Step 2 - Protocols🎓
- 50. Step 2 - Protocols🎓
- 51. Step 2 - Protocols Protocols don’t have generics like types do. We can’t add T to the declaration and we also can’t use T as a type for methods. 🎓
- 52. Step 2 - Protocols🎓
- 53. Step 2 - Protocols🎓 Associated types can be used to define placeholder types.
- 54. Step 2 - Protocols🎓 We can use these associated types as if they are real types. Objects that conform to this protocol define the associated type.
- 55. Step 2 - Protocols🎓
- 56. Step 2 - Protocols🎓 The Swift compiler can infer the “real” type of the associated type when we use it in the conforming type.
- 57. Step 2 - Protocols🎓 Note that the same type must be used in all places where the associated type was used in the protocol
- 58. We need to rewind🛑
- 59. We need to rewind🛑 Currently anything can be our associated type
- 60. We need to rewind🛑 Currently anything can be our associated type
- 61. We need to rewind🛑 and that’s a problem
- 62. We need to rewind🛑 and that’s a problem Why would that be a problem? Wasn’t our goal to cache anything?
- 63. Step 2 - Protocols🎓
- 64. Step 2 - Protocols🎓 You can constrain associated types to certain protocols. The compiler will enforce this constraint.
- 65. Step 2 - Protocols&
- 66. Step 2 - Protocols&
- 67. Step 2 - Protocols&
- 68. Step 2 - Protocols&
- 69. Step 2 - Protocols&
- 70. Step 3 - Assemble!🎓
- 71. Step 3 - Assemble!🎓
- 72. Step 3 - Assemble!🎓 Very nice.
- 73. Step 3 - Assemble!🎓 Okay, maybe not so nice…
- 74. Step 3 - Assemble!🎓
- 75. Step 3 - Assemble!🎓 We can give names to generics
- 76. Step 3 - Assemble!🎓 And we can constrain them to protocols
- 77. Step 3 - Assemble!🎓 And we can now use the LocalStore’s associated type as the success type for our Result.
- 78. Step 3 - Assemble!&
- 79. Step 3 - Assemble!& We can now use the local store’s find method to get a Result<Local.StoredObject, Error>
- 80. Step 3 - Assemble!& If the object was found, the completion closure is called with the Local.StoredObject item that we’ve found.
- 81. Step 3 - Assemble!& Remote store is used as backup…
- 82. Step 3 - Assemble!& Think about this line for just a second.
- 83. Flashback🤔
- 84. Flashback🤔 LocalStore has a StoredObject
- 85. Flashback🤔 RemoteStore has a TargetObject
- 86. Flashback🤔 RemoteStore.TargetObject can’t be used as LocalStore.StoredObject
- 87. Step 3 - Assemble!🎓 This is a problem…
- 88. Step 3 - Assemble!&
- 89. Step 3 - Assemble!&
- 90. Step 3 - Assemble!& The compiler can now proof and enforce that Local.StoredObject and Remote.TargetObject are the same
- 91. Step 3 - Assemble!&
- 92. Step 3 - Assemble!& This is now okay
- 93. Step 3 - Assemble!& And so is this
- 94. Step 4 - Using the cache🚀
- 95. Step 4 - Using the cache🚀
- 96. Step 4 - Using the cache🚀 • Conforms to LocalStore • LocalStore.StoredObject => User
- 97. Step 4 - Using the cache🚀 • Conforms to RemoteStore • RemoteStore.TargetObject => User • User is Decodable
- 98. Step 4 - Using the cache🚀 • Local == ArrayBackedUserStore • Remote == RemoteUserStore • Compiler can infer
- 99. Step 4 - Using the cache🚀 • We can proof that this is a User
- 100. Step 4 - Using the cache🚀
- 101. Step 4 - Using the cache🚀
- 102. Step 4 - Using the cache🚀
- 103. Step 4 - Using the cache🚀
- 104. Step 4 - Using the cache🚀
- 105. Step 4 - Using the cache🚀
- 106. Step 4 - Using the cache🚀
- 107. When is all this useful to know? • Code duplication in Swift is often related to types. Generics can help you clean this up. • You’re most likely to encounter generics in your model/networking layer. • If you want to write frameworks or SDKs.
- 108. In summary
- 109. In summary • Figure out how you want to use your code first.
- 110. In summary • Figure out how you want to use your code first. • Then find out what that means for your implementation.
- 111. In summary • Figure out how you want to use your code first. • Then find out what that means for your implementation. • If needed, introduce generics and protocols as abstractions.
- 112. In summary • Figure out how you want to use your code first. • Then find out what that means for your implementation. • If needed, introduce generics and protocols as abstractions. • Constrain your generics and associated types where needed so the compiler can help you.
- 113. In summary • Figure out how you want to use your code first. • Then find out what that means for your implementation. • If needed, introduce generics and protocols as abstractions. • Constrain your generics and associated types where needed so the compiler can help you. • Generics can help you limit code duplication.
- 115. Thank you Questions are always welcome