Mastering API Visibility in Kotlin (Rheinwerk Konferenz für Kotlin 2021)

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.

Resources and more: https://zsmb.co/talks/mastering-api-visibility/

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 › Easier to maintain and change
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 public fun Krate.stringPref( key: String, isValid: (newValue: String) -> Boolean, )
30. 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, )
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. 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, )
34. 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, )
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.HIDDEN, 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
39. Minimal APIs › Easier to maintain and change › Easier to learn
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 › Harder to misuse
43. Planning
44. Planning
45. Planning
46. Planning
47. Visibility in Kotlin public
48. Visibility in Kotlin public private
49. Visibility in Kotlin public private internal
50. Visibility in Kotlin public private internal
51. Internal visibility public interface Service { fun createUser(): User } internal class NetworkService : Service { override fun createUser(): User { ... } }
52. Internal visibility public class NetworkClient { public var state: State = Disconnected internal set public fun connect() { state = Connected } }
53. Internal visibility internal fun String.asMention(): String = "@$this"
54. Internal visibility
55. Testing
56. Testing private var state: State
57. @Test fun verifyState() { assertEquals(expectedState, state) } Testing private var state: State
58. @Test fun verifyState() { assertEquals(expectedState, state) } Testing private var state: State
59. @Test fun verifyState() { assertEquals(expectedState, state) } Testing internal var state: State
60. @Test fun verifyState() { assertEquals(expectedState, state) } Testing @VisibleForTesting(otherwise = PRIVATE) internal var state: State
61. Java interop
62. Java interop class Repository { internal fun createEntity() { ... } }
63. Java interop void repositoryExample() { } class Repository { internal fun createEntity() { ... } }
64. Java interop void repositoryExample() { new Repository().createEntity$examplelibrary(); } class Repository { internal fun createEntity() { ... } }
65. Java interop void repositoryExample() { new Repository().createEntity$examplelibrary(); } class Repository { internal fun createEntity() { ... } }
66. class Repository { internal fun createEntity() { ... } } Java interop @JvmName("pleaseDoNotCallThisMethod") (); void repositoryExample() { new Repository(). } pleaseDoNotCallThisMethod
67. Java interop void repositoryExample() { new Repository(). } class Repository { internal fun createEntity() { ... } } Cannot resolve method 'createEntity' in 'Repository' @JvmSynthetic (); createEntity
68. Explicit API mode
69. Explicit API mode › Explicit visibility modifiers for all declarations
70. Explicit API mode › Explicit visibility modifiers for all declarations › Explicit types for public declarations
71. Explicit API mode kotlin { explicitApi() }
72. Explicit API mode kotlin { explicitApi() } tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all { kotlinOptions { freeCompilerArgs += [ '-Xexplicit-api=strict', ] } }
73. Explicit API mode kotlin { explicitApi() } tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all { kotlinOptions { freeCompilerArgs += [ '-Xexplicit-api=strict', '-progressive', ] } }
74. Explicit API mode kotlin { explicitApiWarning() } tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all { kotlinOptions { freeCompilerArgs += [ '-Xexplicit-api=warning', '-progressive', ] } }
75. Explicit API mode class User(var name: String, var age: Int) { fun sayHi() { println("Hi, I'm $name") } fun reset() { name = "" age = 0 } }
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. 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 } }
78. 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
79. 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 } }
80. 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 } }
81. Explicit API mode private val DEFAULT_CLIENT = Client() public interface ClientFactory { public fun client() = DEFAULT_CLIENT }
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 = Client() public interface ClientFactory { public fun client() = DEFAULT_CLIENT }
85. Explicit API mode private val DEFAULT_CLIENT = OfflineClient() public interface ClientFactory { public fun client() = DEFAULT_CLIENT }
86. Explicit API mode private val DEFAULT_CLIENT = OfflineClient() public interface ClientFactory { public fun client(): Client = DEFAULT_CLIENT }
87. Explicit API mode › Explicit visibility modifiers for all declarations › Explicit types for public declarations
88. Explicit API mode › Explicit visibility modifiers for all declarations › Explicit types for public declarations › Configured per-module
89. Explicit API mode › Explicit visibility modifiers for all declarations › Explicit types for public declarations › Configured per-module › Strict or warning level
90. Published API
91. Published API public inline fun song() { secretFunction() } internal fun secretFunction() { println("through the mountains") }
92. Published API public inline fun song() { secretFunction() } internal fun secretFunction() { println("through the mountains") } fun clientCode() { song() }
93. 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()
94. Published API public inline fun song() { secretFunction() } internal fun secretFunction() { println("through the mountains") } fun clientCode() { } secretFunction()
95. Published API fun clientCode() { } secretFunction() public inline fun song() { secretFunction() } internal fun secretFunction() { println("through the mountains") }
96. Published API fun clientCode() { } song() @PublishedApi public inline fun song() { secretFunction() } internal fun secretFunction() { println("through the mountains") }
97. 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
98. Published API fun clientCode() { } song() @PublishedApi public inline fun song() { secretFunction() } internal fun secretFunction() { println("through the mountains") }
100. Opt-in APIs core
101. Opt-in APIs core addon
102. Opt-in APIs core addon
103. core addon Opt-in APIs
104. core addon Opt-in APIs
105. package com.example.lib.core public annotation class InternalMyLibraryApi core addon Opt-in APIs
106. Opt-in APIs package com.example.lib.core public annotation class InternalMyLibraryApi
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
108. 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
109. 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
110. 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', ] } }
111. 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', ] } }
112. 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', ] } }
113. 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()
114. 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
115. 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
116. core addon @InternalMyLibraryApi public fun coreApi() Opting in public fun addonFunction() { coreApi() }
117. Opting in @InternalMyLibraryApi public fun coreApi() @InternalMyLibraryApi public fun addonFunction() { coreApi() }
118. 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
119. Opting in @InternalMyLibraryApi public fun coreApi() public fun addonFunction() { coreApi() }
120. Opting in @InternalMyLibraryApi public fun coreApi() public fun addonFunction() { @OptIn(InternalMyLibraryApi::class) coreApi() }
121. 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', ] } }
122. 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', ] } }
123. 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', ] } }
124. 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', ] } }
125. 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', ] } }
126. Opt-in conventions InternalCoroutinesApi DelicateCoroutinesApi ExperimentalCoroutinesApi ObsoleteCoroutinesApi
127. Android time
128. Android time
129. Android time implementation 'androidx.annotation:annotation:1.1.0'
130. Android time implementation 'androidx.annotation:annotation-experimental:1.1.0'
131. core Validating API
132. core Validating API buildscript { dependencies { classpath 'org.jetbrains.kotlinx:binary-compatibility-validator:0.5.0' } } apply plugin: 'binary-compatibility-validator'
133. core apiValidation { ignoredPackages += [ 'com/getstream/sdk/chat/databinding', 'io/getstream/chat/android/ui/databinding', ] ignoredProjects += [ 'stream-chat-android-docs', 'stream-chat-android-sample', 'stream-chat-android-ui-components-sample', 'stream-chat-android-test', ] nonPublicMarkers += [ 'io.getstream.chat.android.core.internal.InternalStreamChatApi', ] } Validating API
134. core apiValidation { ignoredPackages += [ 'com/getstream/sdk/chat/databinding', 'io/getstream/chat/android/ui/databinding', ] ignoredProjects += [ 'stream-chat-android-docs', 'stream-chat-android-sample', 'stream-chat-android-ui-components-sample', 'stream-chat-android-test', ] nonPublicMarkers += [ 'io.getstream.chat.android.core.internal.InternalStreamChatApi', ] } Validating API
135. core apiValidation { ignoredPackages += [ 'com/getstream/sdk/chat/databinding', 'io/getstream/chat/android/ui/databinding', ] ignoredProjects += [ 'stream-chat-android-docs', 'stream-chat-android-sample', 'stream-chat-android-ui-components-sample', 'stream-chat-android-test', ] nonPublicMarkers += [ 'io.getstream.chat.android.core.internal.InternalStreamChatApi', ] } Validating API
136. core apiValidation { ignoredPackages += [ 'com/getstream/sdk/chat/databinding', 'io/getstream/chat/android/ui/databinding', ] ignoredProjects += [ 'stream-chat-android-docs', 'stream-chat-android-sample', 'stream-chat-android-ui-components-sample', 'stream-chat-android-test', ] nonPublicMarkers += [ 'io.getstream.chat.android.core.internal.InternalStreamChatApi', ] } Validating API
137. core Validating API gradlew apiDump
138. core Validating API public abstract interface class io/getstream/chat/android/client/ChatEventListener { public abstract fun onEvent (Lio/getstream/chat/android/client/events/ChatEvent;)V } public final class io/getstream/chat/android/client/api/models/AutocompleteFilterObject : io/getstream/chat/android/client/api/models/FilterObject { public final fun component1 ()Ljava/lang/String; public final fun component2 ()Ljava/lang/String; public final fun copy (Ljava/lang/String;Ljava/lang/String;)Lio/getstream/chat/android/client/api/models/Autocompl eteFilterObject; public fun equals (Ljava/lang/Object;)Z public final fun getFieldName ()Ljava/lang/String; public final fun getValue ()Ljava/lang/String; public fun hashCode ()I public fun toString ()Ljava/lang/String; } gradlew apiDump
139. core Validating API gradlew apiCheck
140. core Validating API Execution failed for task ':stream-chat-android-client:apiCheck'. > API check failed for project stream-chat-android-client. @@ -218,9 +218,8 @@ public final class io/getstream/chat/android/client/api/models/AutocompleteFilterObject : io/getstream/chat/android/client/api/models/FilterObject { public final fun component1 ()Ljava/lang/String; - public final fun component2 ()Ljava/lang/String; - public final fun copy (Ljava/lang/String;Ljava/lang/String;)Lio/getstream/chat/android/client/api/models/Autocompl eteFilterObject; + public final fun copy (Ljava/lang/String;)Lio/getstream/chat/android/client/api/models/AutocompleteFilterObject; public fun equals (Ljava/lang/Object;)Z public final fun getFieldName ()Ljava/lang/String; public final fun getValue ()Ljava/lang/String; You can run :stream-chat-android-client:apiDump task to overwrite API declarations gradlew apiCheck
141. core Validating API Execution failed for task ':stream-chat-android-client:apiCheck'. > API check failed for project stream-chat-android-client. @@ -218,9 +218,8 @@ public final class io/getstream/chat/android/client/api/models/AutocompleteFilterObject : io/getstream/chat/android/client/api/models/FilterObject { public final fun component1 ()Ljava/lang/String; - public final fun component2 ()Ljava/lang/String; - public final fun copy (Ljava/lang/String;Ljava/lang/String;)Lio/getstream/chat/android/client/api/models/Autocompl eteFilterObject; + public final fun copy (Ljava/lang/String;)Lio/getstream/chat/android/client/api/models/AutocompleteFilterObject; public fun equals (Ljava/lang/Object;)Z public final fun getFieldName ()Ljava/lang/String; public final fun getValue ()Ljava/lang/String; You can run :stream-chat-android-client:apiDump task to overwrite API declarations gradlew apiCheck
142. core Validating API Execution failed for task ':stream-chat-android-client:apiCheck'. > API check failed for project stream-chat-android-client. @@ -218,9 +218,8 @@ public final class io/getstream/chat/android/client/api/models/AutocompleteFilterObject : io/getstream/chat/android/client/api/models/FilterObject { public final fun component1 ()Ljava/lang/String; - public final fun component2 ()Ljava/lang/String; - public final fun copy (Ljava/lang/String;Ljava/lang/String;)Lio/getstream/chat/android/client/api/models/Autocompl eteFilterObject; + public final fun copy (Ljava/lang/String;)Lio/getstream/chat/android/client/api/models/AutocompleteFilterObject; public fun equals (Ljava/lang/Object;)Z public final fun getFieldName ()Ljava/lang/String; public final fun getValue ()Ljava/lang/String; You can run :stream-chat-android-client:apiDump task to overwrite API declarations gradlew apiCheck
143. core Validating API
144. • 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/ • How to Build Awesome Android Libraries  https://zsmb.co/talks/how-to-build-awesome-android-libraries/ • Effective Java, 3rd Edition  https://www.amazon.com/Effective-Java-Joshua-Bloch/dp/0134685997 • Effective Kotlin  https://leanpub.com/effectivekotlin/ Resources
145. zsmb13 zsmb.co/talks
146. 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