Mastering API Visibility in Kotlin (Android Worldwide 2021 April)

When designing a library, minimizing your API surface - the types, methods, properties, and functions you expose to the outside world - is a great idea. This doesn't apply to just libraries: it's a consideration you should make for every module in a multi-module project. In this talk, we'll look at all the ways that Kotlin lets you get your visibility just right.

This talk is adapted from the article with the same title: https://zsmb.co/mastering-api-visibility-in-kotlin/

Transcript

1. Márton Braun zsmb.co zsmb13 Mastering API Visibility in Kotlin
2. API surface
3. API surface
4. API surface
5. API surface
6. API surface
7. API surface
8. API surface
9. API surface
10. API surface
11. API surface
12. API surface
13. API surface
14. API surface
15. API surface
16. API surface
17. API surface
18. API surface
19. Minimal APIs
20. Minimal APIs Item 15: Minimize the accessibility of classes and members
21. Minimal APIs Item 15: Minimize the accessibility of classes and members Chapter 4: Classes and Interfaces
22. Minimal APIs Item 15: Minimize the accessibility of classes and members Chapter 4: Classes and Interfaces
23. Minimal APIs › Easier to maintain and change
24. Minimal APIs › Easier to maintain and change
25. Minimal APIs › Easier to maintain and change
26. Minimal APIs › Easier to maintain and change
27. Minimal APIs › Easier to maintain and change
28. Minimal APIs › Easier to maintain and change
29. Minimal APIs › Easier to maintain and change
30. Minimal APIs › Easier to maintain and change public fun Krate.stringPref( key: String, isValid: (newValue: String) -> Boolean, )
31. replaceWith = ReplaceWith( "this.stringPref(key).validate(isValid)", ), Minimal APIs › Easier to maintain and change @Deprecated( message = "Use a chained .validate {} call instead", level = DeprecationLevel.WARNING, ) public fun Krate.stringPref( key: String, isValid: (newValue: String) -> Boolean, )
32. replaceWith = ReplaceWith( "this.stringPref(key).validate(isValid)", ), Minimal APIs › Easier to maintain and change @Deprecated( message = "Use a chained .validate {} call instead", level = DeprecationLevel.WARNING, ) public fun Krate.stringPref( key: String, isValid: (newValue: String) -> Boolean, )
33. replaceWith = ReplaceWith( "this.stringPref(key).validate(isValid)", ), Minimal APIs › Easier to maintain and change @Deprecated( message = "Use a chained .validate {} call instead", level = DeprecationLevel.WARNING, ) public fun Krate.stringPref( key: String, isValid: (newValue: String) -> Boolean, )
34. Minimal APIs › Easier to maintain and change @Deprecated( message = "Use a chained .validate {} call instead", level = DeprecationLevel.WARNING, replaceWith = ReplaceWith( "this.stringPref(key).validate(isValid)", imports = arrayOf("com.example.validate"), ), ) public fun Krate.stringPref( key: String, isValid: (newValue: String) -> Boolean, )
35. Minimal APIs › Easier to maintain and change @Deprecated( message = "Use a chained .validate {} call instead", level = DeprecationLevel.ERROR, replaceWith = ReplaceWith( "this.stringPref(key).validate(isValid)", imports = arrayOf("com.example.validate"), ), ) public fun Krate.stringPref( key: String, isValid: (newValue: String) -> Boolean, )
36. Minimal APIs › Easier to maintain and change @Deprecated( message = "Use a chained .validate {} call instead", level = DeprecationLevel.ERROR, replaceWith = ReplaceWith( "this.stringPref(key).validate(isValid)", imports = arrayOf("com.example.validate"), ), ) public fun Krate.stringPref( key: String, isValid: (newValue: String) -> Boolean, )
37. Minimal APIs › Easier to maintain and change @Deprecated( message = "Use a chained .validate {} call instead", level = DeprecationLevel.HIDDEN, replaceWith = ReplaceWith( "this.stringPref(key).validate(isValid)", imports = arrayOf("com.example.validate"), ), ) public fun Krate.stringPref( key: String, isValid: (newValue: String) -> Boolean, )
38. Minimal APIs › Easier to maintain and change @Deprecated( message = "Use a chained .validate {} call instead", level = DeprecationLevel.HIDDEN, replaceWith = ReplaceWith( "this.stringPref(key).validate(isValid)", imports = arrayOf("com.example.validate"), ), ) public fun Krate.stringPref( key: String, isValid: (newValue: String) -> Boolean, )
39. Minimal APIs › Easier to maintain and change
40. Minimal APIs › Easier to maintain and change › Easier to learn
41. Minimal APIs › Easier to maintain and change › Easier to learn
42. Minimal APIs › Easier to maintain and change › Easier to learn
43. Minimal APIs › Easier to maintain and change › Easier to learn › Harder to misuse
44. Planning
45. Planning
46. Planning
47. Planning
48. Visibility in Kotlin public
49. Visibility in Kotlin public private
50. Visibility in Kotlin public private internal
51. Visibility in Kotlin public private internal
52. Internal visibility public interface Service { fun createUser(): User } internal class NetworkService : Service { override fun createUser(): User { ... } }
53. Internal visibility public class NetworkClient { public var state: State = Disconnected internal set public fun connect() { state = Connected } }
54. Internal visibility internal fun String.asMention(): String = "@$this"
55. Internal visibility
56. Testing
57. Testing private var state: State
58. @Test fun verifyState() { assertEquals(expectedState, state) } Testing private var state: State
59. @Test fun verifyState() { assertEquals(expectedState, state) } Testing private var state: State
60. @Test fun verifyState() { assertEquals(expectedState, state) } Testing internal var state: State
61. @Test fun verifyState() { assertEquals(expectedState, state) } Testing @VisibleForTesting(otherwise = PRIVATE) internal var state: State
62. Java interop
63. Java interop class Repository { internal fun createEntity() { ... } }
64. Java interop void repositoryExample() { } class Repository { internal fun createEntity() { ... } }
65. Java interop void repositoryExample() { new Repository().createEntity$examplelibrary(); } class Repository { internal fun createEntity() { ... } }
66. Java interop void repositoryExample() { new Repository().createEntity$examplelibrary(); } class Repository { internal fun createEntity() { ... } }
67. class Repository { internal fun createEntity() { ... } } Java interop @JvmName("pleaseDoNotCallThisMethod") (); void repositoryExample() { new Repository(). } pleaseDoNotCallThisMethod
68. Java interop void repositoryExample() { new Repository(). } class Repository { internal fun createEntity() { ... } } Cannot resolve method 'createEntity' in 'Repository' @JvmSynthetic (); createEntity
69. Explicit API mode
70. Explicit API mode › Explicit visibility modifiers for all declarations
71. Explicit API mode › Explicit visibility modifiers for all declarations › Explicit types for public declarations
72. Explicit API mode kotlin { explicitApi() }
73. Explicit API mode kotlin { explicitApi() } tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all { kotlinOptions { freeCompilerArgs += [ '-Xexplicit-api=strict', ] } }
74. Explicit API mode kotlin { explicitApi() } tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all { kotlinOptions { freeCompilerArgs += [ '-Xexplicit-api=strict', '-progressive', ] } }
75. Explicit API mode kotlin { explicitApiWarning() } tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all { kotlinOptions { freeCompilerArgs += [ '-Xexplicit-api=warning', '-progressive', ] } }
76. Explicit API mode class User(var name: String, var age: Int) { fun sayHi() { println("Hi, I'm $name") } fun reset() { name = "" age = 0 } }
77. Explicit API mode class User(var name: String, var age: Int) { fun sayHi() { println("Hi, I'm $name") } fun reset() { name = "" age = 0 } }
78. public Explicit API mode public class User(var name: String, var age: Int) { fun sayHi() { println("Hi, I'm $name") } fun reset() { name = "" age = 0 } }
79. Explicit API mode public class User(public var name: String, var age: Int) { fun sayHi() { println("Hi, I'm $name") } fun reset() { name = "" age = 0 } } public
80. Explicit API mode public class User(public var name: String, public var age: Int) { public fun sayHi() { println("Hi, I'm $name") } fun reset() { name = "" age = 0 } }
81. Explicit API mode public class User(public var name: String, public var age: Int) { public fun sayHi() { println("Hi, I'm $name") } internal fun reset() { name = "" age = 0 } }
82. Explicit API mode private val DEFAULT_CLIENT = Client() public interface ClientFactory { public fun client() = DEFAULT_CLIENT }
83. Explicit API mode private val DEFAULT_CLIENT = Client() public interface ClientFactory { public fun client() = DEFAULT_CLIENT }
84. Explicit API mode private val DEFAULT_CLIENT = OfflineClient() public interface ClientFactory { public fun client() = DEFAULT_CLIENT }
85. Explicit API mode private val DEFAULT_CLIENT = OfflineClient() public interface ClientFactory { public fun client(): Client = DEFAULT_CLIENT }
86. Published API
87. Published API public inline fun song() { secretFunction() } internal fun secretFunction() { println("through the mountains") }
88. Published API public inline fun song() { secretFunction() } internal fun secretFunction() { println("through the mountains") } fun clientCode() { song() }
89. fun clientCode() { } secretFunction() Published API public inline fun song() { secretFunction() } internal fun secretFunction() { println("through the mountains") } e: Public-API inline function cannot access non-public-API song()
90. Published API public inline fun song() { secretFunction() } internal fun secretFunction() { println("through the mountains") } fun clientCode() { } secretFunction()
91. Published API fun clientCode() { } secretFunction() public inline fun song() { secretFunction() } internal fun secretFunction() { println("through the mountains") }
92. Published API fun clientCode() { } song() @PublishedApi public inline fun song() { secretFunction() } internal fun secretFunction() { println("through the mountains") }
93. Published API fun clientCode() { song() secretFunction() } public inline fun song() { secretFunction() } @PublishedApi internal fun secretFunction() { println("through the mountains") } e: Cannot access 'secretFunction': it is internal
94. Published API fun clientCode() { } song() @PublishedApi public inline fun song() { secretFunction() } internal fun secretFunction() { println("through the mountains") }
96. Opt-in APIs core
97. Opt-in APIs core addon
98. Opt-in APIs core addon
99. core addon Opt-in APIs
100. core addon Opt-in APIs
101. package com.example.lib.core public annotation class InternalMyLibraryApi core addon Opt-in APIs
102. Opt-in APIs package com.example.lib.core public annotation class InternalMyLibraryApi
103. Opt-in APIs package com.example.lib.core @RequiresOptIn( level = RequiresOptIn.Level.ERROR, message = "This is internal API for my library, " + "please don't rely on it." ) public annotation class InternalMyLibraryApi
104. Opt-in APIs package com.example.lib.core @RequiresOptIn( level = RequiresOptIn.Level.WARNING, message = "This is internal API for my library, " + "please don't rely on it." ) public annotation class InternalMyLibraryApi
105. Opt-in APIs package com.example.lib.core @RequiresOptIn( level = RequiresOptIn.Level.ERROR, message = "This is internal API for my library, " + "please don't rely on it." ) public annotation class InternalMyLibraryApi
106. Opt-in APIs package com.example.lib.core @RequiresOptIn( level = RequiresOptIn.Level.ERROR, message = "This is internal API for my library, " + "please don't rely on it." ) public annotation class InternalMyLibraryApi tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all { kotlinOptions { freeCompilerArgs += [ '-Xopt-in=kotlin.RequiresOptIn', ] } }
107. Opt-in APIs package com.example.lib.core @RequiresOptIn( level = RequiresOptIn.Level.ERROR, message = "This is internal API for my library, " + "please don't rely on it." ) public annotation class InternalMyLibraryApi @InternalMyLibraryApi public fun coreApi()
108. Opt-in APIs package com.example.lib.core @RequiresOptIn( level = RequiresOptIn.Level.ERROR, message = "This is internal API for my library, " + "please don't rely on it." ) public annotation class InternalMyLibraryApi @InternalMyLibraryApi public fun coreApi()
109. package com.example.lib.core @RequiresOptIn( level = RequiresOptIn.Level.ERROR, message = "This is internal API for my library, " + "please don't rely on it." ) public annotation class InternalMyLibraryApi @InternalMyLibraryApi public fun coreApi() core addon Opt-in APIs
110. public fun addonFunction() { coreApi() } package com.example.lib.core @RequiresOptIn( level = RequiresOptIn.Level.ERROR, message = "This is internal API for my library, " + "please don't rely on it." ) public annotation class InternalMyLibraryApi @InternalMyLibraryApi public fun coreApi() core addon Opt-in APIs
111. core addon @InternalMyLibraryApi public fun coreApi() Opting in public fun addonFunction() { coreApi() }
112. Opting in @InternalMyLibraryApi public fun coreApi() @InternalMyLibraryApi public fun addonFunction() { coreApi() }
113. Opting in tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all { kotlinOptions { freeCompilerArgs += [ '-Xopt-in=com.example.lib.core.InternalMyLibraryApi', ] } } @InternalMyLibraryApi public fun coreApi() public fun addonFunction() { coreApi() } @InternalMyLibraryApi
114. Opting in @InternalMyLibraryApi public fun coreApi() public fun addonFunction() { coreApi() }
115. Opting in @InternalMyLibraryApi public fun coreApi() public fun addonFunction() { @OptIn(InternalMyLibraryApi::class) coreApi() }
116. Opting in @InternalMyLibraryApi public fun coreApi() public fun addonFunction() { @OptIn(InternalMyLibraryApi::class) coreApi() } tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all { kotlinOptions { freeCompilerArgs += [ '-Xopt-in=kotlin.RequiresOptIn', ] } }
117. Opting in @InternalMyLibraryApi public fun coreApi() @OptIn(InternalMyLibraryApi::class) public fun addonFunction() { coreApi() } tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all { kotlinOptions { freeCompilerArgs += [ '-Xopt-in=kotlin.RequiresOptIn', ] } }
118. Opting in @InternalMyLibraryApi @JvmSynthetic public fun coreApi() @OptIn(InternalMyLibraryApi::class) public fun addonFunction() { coreApi() } tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all { kotlinOptions { freeCompilerArgs += [ '-Xopt-in=kotlin.RequiresOptIn', ] } }
119. Resources • Mastering API Visibility in Kotlin  https://zsmb.co/mastering-api-visibility-in-kotlin/ • Maintaining Compatibility in Kotlin Libraries  https://zsmb.co/maintaining-compatibility-in-kotlin-libraries/ • Effective Java, 3rd Edition  https://www.amazon.com/Effective-Java-Joshua-Bloch/dp/0134685997 • Effective Kotlin  https://leanpub.com/effectivekotlin/
120. zsmb13 zsmb.co/talks
121. Mastering API Visibility in Kotlin zsmb.co/talks zsmb13 Márton Braun › Minimal API is nice › Internal visibility › Explicit API mode › Published API › Opt-in APIs