Annotation Processing in a Kotlin World

Annotation processing is a powerful tool to extend your codebase at build time, and the advent of Kotlin in the android community introduces a dimension to this. Kapt works with existing Java processors, but it doesn’t come without its quirks at the interop layer. This talk will dive into how you can leverage Kotlin bytecode’s @Metadata annotations to better understand Kotlin code during annotation processing, covering anywhere from case studies in Dagger and Moshi to leveraging tools like KotlinPoet to even generate Kotlin code that seamlessly leverages language features.

Transcript

1. Annotation Processing
   in a Kotlin World
   Zac Sweers
   @pandanomic
   

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2. Annotation Processing
   in a Kotlin World
   

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3. https://goo.gl/rSHTn4
   

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4. View Slide

5. Annotation Processing
   in a Kotlin World
   

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6. Annotation Processing
   in a Kotlin World
   

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7. Annotation Processing
   in a Kotlin World
   

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8. Dagger
   @Inject
   @Qualifier
   

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9. @Provides
   fun provideList(): List {
   return emptyList()
   }a
   

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10. @Provides
    fun provideList(): List {
    return emptyList()
    }a
    @Inject lateinit var tacoList: List
    

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11. @Provides
    fun provideList(): List {
    return emptyList()
    }a
    @Inject lateinit var tacoList:
    List<@SuppressJvmWildcard Taco>
    

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12. @Provides
    fun provideList(): List {
    return emptyList()
    }a
    @Inject lateinit var tacoList:
    List<@SuppressJvmWildcard Taco>
    
    

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13. @Provides
    fun provideTaco(): Taco {
    return BreakfastTaco()
    }a
    @Inject
    lateinit var taco: Taco
    

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14. @Breakfast
    @Provides
    fun provideTaco(): Taco {
    return BreakfastTaco()
    }a
    @Breakfast
    @Inject
    lateinit var taco: Taco
    

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15. @Breakfast
    @Provides
    fun provideTaco(): Taco {
    return BreakfastTaco()
    }a
    @field:Breakfast
    @Inject
    lateinit var taco: Taco
    

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16. @Breakfast
    @Provides
    fun provideTaco(): Taco {
    return BreakfastTaco()
    }a
    @field:Breakfast
    @Inject
    lateinit var taco: Taco
    

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17. @Module
    class NetworkModule {
    @Provides
    static HttpClient provideHttpClient() {
    return HttpClient.create();
    }a
    }b
    

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18. @Module
    class NetworkModule {
    @Provides
    static HttpClient provideHttpClient() {
    return HttpClient.create();
    }a
    companion object {
    }c
    }b
    

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19. @Module
    class NetworkModule {
    companion object {
    @Provides
    fun provideHttpClient(): HttpClient {
    return HttpClient.create()
    }a
    }c
    }b
    

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20. @Module
    class NetworkModule {
    companion object {
    @JvmStatic
    @Provides
    fun provideHttpClient(): HttpClient {
    return HttpClient.create()
    }a
    }c
    }b
    

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21. @Module
    class NetworkModule {
    @Module
    companion object {
    @JvmStatic
    @Provides
    fun provideHttpClient(): HttpClient {
    return HttpClient.create()
    }a
    }c
    }b
    

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22. @Module
    class NetworkModule {
    @Module
    companion object {
    @JvmStatic
    @Provides
    fun provideHttpClient(): HttpClient {
    return HttpClient.create()
    }a
    }c
    }b
    

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23. @Module
    class NetworkModule {
    @Module
    companion object {
    @JvmStatic
    @Provides
    fun provideHttpClient(): HttpClient {
    return HttpClient.create()
    }a
    }c
    }b
    

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24. Quick refresher
    

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25. Quick refresher
    • Compile time
    

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26. Quick refresher
    • Compile time
    • Since Java 5ish (APIs in Java 6)
    

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27. Quick refresher
    • Compile time
    • Since Java 5ish (APIs in Java 6)
    • Reads Java (byte) code in intermediate
    format
    

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28. Quick refresher
    • Compile time
    • Since Java 5ish (APIs in Java 6)
    • Reads Java (byte) code in intermediate
    format
    • Elements, Types, Filer, etc APIs
    

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29. Quick refresher
    • Compile time
    • Since Java 5ish (APIs in Java 6)
    • Reads Java (byte) code in intermediate
    format
    • Elements, Types, Filer, etc APIs
    • Cannot modify code, only generate more
    

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30. Quick refresher
    • Compile time
    • Since Java 5ish (APIs in Java 6)
    • Reads Java (byte) code in intermediate format
    • Elements, Types, Filer, etc APIs
    • Cannot modify code, only generate more
    • Multiple rounds
    

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31. Quick refresher
    • Compile time
    • Since Java 5ish (APIs in Java 6)
    • Reads Java (byte) code in intermediate format
    • Elements, Types, Filer, etc APIs
    • Cannot modify code, only generate more
    • Multiple rounds
    

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32. Quick refresher
    • Compile time
    • Since Java 5ish (APIs in Java 6)
    • Reads Java (byte) code in intermediate format
    • Elements, Types, Filer, etc APIs
    • Cannot modify code, only generate more
    • Multiple rounds
    

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33. class Person(val name: String)
    

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34. @kotlin.Metadata(
    mv = {1, 1, 11},
    bv = {1, 0, 2},
    k = 1,
    d1 =
    {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
    \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u00020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
    002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004\b\u0005\u0010\u0006\u00a8\u0006\u0007"},
    d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/String;)V", "getName", "()Ljava/lang/String;", "test-
    project"}
    )
    public final class Person {
    @org.jetbrains.annotations.NotNull()
    private final java.lang.String name = null;
    @org.jetbrains.annotations.NotNull()
    public final java.lang.String getName() {
    return null;
    }
    public Person(@org.jetbrains.annotations.NotNull() java.lang.String name) {
    super();
    }
    }
    

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35. public final class Person {
    @org.jetbrains.annotations.NotNull()
    private final java.lang.String name = null;
    @org.jetbrains.annotations.NotNull()
    public final java.lang.String getName() {
    return null;
    }
    public Person(@org.jetbrains.annotations.NotNull()
    java.lang.String name) {
    super();
    }
    }
    

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36. class Person(val name: String)
    

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37. public final class Person {
    @org.jetbrains.annotations.NotNull()
    private final java.lang.String name = null;
    @org.jetbrains.annotations.NotNull()
    public final java.lang.String getName() {
    return null;
    }c
    public Person(@org.jetbrains.annotations.NotNull()
    java.lang.String name) {
    super();
    }a
    }b
    

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38. public final class Person {
    @org.jetbrains.annotations.NotNull()
    private final java.lang.String name = null;
    @org.jetbrains.annotations.NotNull()
    public final java.lang.String getName() {
    return null;
    }c
    public Person(@org.jetbrains.annotations.NotNull()
    java.lang.String name) {
    super();
    }a
    }b
    

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39. public final class Person {
    @org.jetbrains.annotations.NotNull()
    private final java.lang.String name = null;
    @org.jetbrains.annotations.NotNull()
    public final java.lang.String getName() {
    return null;
    }c
    public Person(@org.jetbrains.annotations.NotNull()
    java.lang.String name) {
    super();
    }a
    }b
    

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40. class Person(val name: String = "Joe")
    

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41. public final class Person {
    @org.jetbrains.annotations.NotNull()
    private final java.lang.String name = null;
    @org.jetbrains.annotations.NotNull()
    public final java.lang.String getName() {
    return null;
    }c
    public Person(@org.jetbrains.annotations.NotNull()
    java.lang.String name) {
    super();
    }a
    }b
    

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42. public final class Person {
    @org.jetbrains.annotations.NotNull()
    private final java.lang.String name = null;
    @org.jetbrains.annotations.NotNull()
    public final java.lang.String getName() {
    return null;
    }c
    public Person(@org.jetbrains.annotations.NotNull()
    java.lang.String name) {
    super();
    }a
    public Person() {
    super();
    }d
    }b
    

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43. public final class Person {
    @org.jetbrains.annotations.NotNull()
    private final java.lang.String name = null;
    @org.jetbrains.annotations.NotNull()
    public final java.lang.String getName() {
    return null;
    }c
    public Person(@org.jetbrains.annotations.NotNull()
    java.lang.String name) {
    super();
    }a
    public Person() {
    super();
    }d
    }b
    
    

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44. class Person(val names: List)
    

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45. class Person(val names: List)
    public Person(@NotNull List names)
    

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46. class Person(val names: MutableList)
    public Person(@NotNull List names)
    

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47. public final class Person {
    @org.jetbrains.annotations.NotNull()
    private final java.lang.String name = null;
    @org.jetbrains.annotations.NotNull()
    public final java.lang.String getName() {
    return null;
    }
    public Person(@org.jetbrains.annotations.NotNull()
    java.lang.String name) {
    super();
    }
    }
    

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48. @kotlin.Metadata(
    mv = {1, 1, 11},
    bv = {1, 0, 2},
    k = 1,
    d1 =
    {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
    \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u00020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
    002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004\b\u0005\u0010\u0006\u00a8\u0006\u0007"},
    d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/String;)V", "getName", "()Ljava/lang/String;", "test-
    project"}
    )a
    public final class Person {
    @org.jetbrains.annotations.NotNull()
    private final java.lang.String name = null;
    @org.jetbrains.annotations.NotNull()
    public final java.lang.String getName() {
    return null;
    }
    public Person(@org.jetbrains.annotations.NotNull() java.lang.String name) {
    super();
    }
    }
    

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49. @kotlin.Metadata(
    mv = {1, 1, 11},
    bv = {1, 0, 2},
    k = 1,
    d1 =
    {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
    \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u00020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
    002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004\b\u0005\u0010\u0006\u00a8\u0006\u0007"},
    d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/String;)V", "getName", "()Ljava/lang/String;", "test-
    project"}
    )a
    public final class Person {
    @org.jetbrains.annotations.NotNull()
    private final java.lang.String name = null;
    @org.jetbrains.annotations.NotNull()
    public final java.lang.String getName() {
    return null;
    }
    public Person(@org.jetbrains.annotations.NotNull() java.lang.String name) {
    super();
    }
    }
    

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50. annotation class Metadata(
    )a
    

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51. annotation class Metadata(
    val k: Int = 1
    )a
    

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52. annotation class Metadata(
    val k: Int = 1,
    val mv: IntArray = []
    )a
    

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53. annotation class Metadata(
    val k: Int = 1,
    val mv: IntArray = [],
    val bv: IntArray = []
    )a
    

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54. annotation class Metadata(
    val k: Int = 1,
    val mv: IntArray = [],
    val bv: IntArray = [],
    val d1: Array = []
    )a
    

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55. annotation class Metadata(
    val k: Int = 1,
    val mv: IntArray = [],
    val bv: IntArray = [],
    val d1: Array = [],
    val d2: Array = []
    )a
    

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56. internal annotation class Metadata(
    val k: Int = 1,
    val mv: IntArray = [],
    val bv: IntArray = [],
    val d1: Array = [],
    val d2: Array = []
    )a
    

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57. @kotlin.Metadata(
    mv = {1, 1, 11},
    bv = {1, 0, 2},
    k = 1,
    d1 =
    {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
    \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u00020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
    002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004\b\u0005\u0010\u0006\u00a8\u0006\u0007"},
    d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/String;)V", "getName", "()Ljava/lang/String;", "test-
    project"}
    )a
    public final class Person {
    @org.jetbrains.annotations.NotNull()
    private final java.lang.String name = null;
    @org.jetbrains.annotations.NotNull()
    public final java.lang.String getName() {
    return null;
    }
    public Person(@org.jetbrains.annotations.NotNull() java.lang.String name) {
    super();
    }
    }
    

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58. @kotlin.Metadata(
    mv = {1, 1, 11},
    bv = {1, 0, 2},
    k = 1,
    d1 =
    {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n
    \u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
    \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u0
    0020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
    002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004
    \b\u0005\u0010\u0006\u00a8\u0006\u0007"},
    d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/
    String;)V", "getName", "()Ljava/lang/String;", "test-
    project"}
    )a
    

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59. @kotlin.Metadata(
    mv = {1, 1, 11},
    bv = {1, 0, 2},
    k = 1,
    d1 =
    {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n
    \u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
    \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u0
    0020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
    002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004
    \b\u0005\u0010\u0006\u00a8\u0006\u0007"},
    d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/
    String;)V", "getName", "()Ljava/lang/String;", "test-
    project"}
    )a
    

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60. @kotlin.Metadata(
    mv = {1, 1, 11},
    bv = {1, 0, 2},
    k = 1,
    d1 =
    {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n
    \u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
    \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u0
    0020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
    002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004
    \b\u0005\u0010\u0006\u00a8\u0006\u0007"},
    d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/
    String;)V", "getName", "()Ljava/lang/String;", "test-
    project"}
    )a
    

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61. @kotlin.Metadata(
    mv = {1, 1, 11},
    bv = {1, 0, 2},
    k = 1,
    d1 =
    {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n
    \u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
    \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u0
    0020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
    002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004
    \b\u0005\u0010\u0006\u00a8\u0006\u0007"},
    d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/
    String;)V", "getName", "()Ljava/lang/String;", "test-
    project"}
    )a
    

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62. @kotlin.Metadata(
    mv = {1, 1, 11},
    bv = {1, 0, 2},
    k = 1,
    d1 =
    {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n
    \u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
    \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u0
    0020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
    002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004
    \b\u0005\u0010\u0006\u00a8\u0006\u0007"},
    d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/
    String;)V", "getName", "()Ljava/lang/String;", "test-
    project"}
    )a
    

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63. @kotlin.Metadata(
    mv = {1, 1, 11},
    bv = {1, 0, 2},
    k = 1,
    d1 =
    {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n
    \u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
    \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u0
    0020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
    002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004
    \b\u0005\u0010\u0006\u00a8\u0006\u0007"},
    d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/
    String;)V", "getName", "()Ljava/lang/String;", "test-
    project"}
    )a
    

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64. @kotlin.Metadata(
    mv = {1, 1, 11},
    bv = {1, 0, 2},
    k = 1,
    d1 =
    {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n
    \u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
    \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u0
    0020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
    002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004
    \b\u0005\u0010\u0006\u00a8\u0006\u0007"},
    d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/
    String;)V", "getName", "()Ljava/lang/String;", "test-
    project"}
    )a
    

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65. @kotlin.Metadata(
    mv = {1, 1, 11},
    bv = {1, 0, 2},
    k = 1,
    d1 =
    {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n
    \u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
    \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u0
    0020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
    002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004
    \b\u0005\u0010\u0006\u00a8\u0006\u0007"},
    d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/
    String;)V", "getName", "()Ljava/lang/String;", "test-
    project"}
    )a
    

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66. @kotlin.Metadata(
    mv = {1, 1, 11},
    bv = {1, 0, 2},
    k = 1,
    d1 =
    {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n
    \u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
    \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u0
    0020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
    002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004
    \b\u0005\u0010\u0006\u00a8\u0006\u0007"},
    d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/
    String;)V", "getName", "()Ljava/lang/String;", "test-
    project"}
    )a
    

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67. @kotlin.Metadata(
    mv = {1, 1, 11},
    bv = {1, 0, 2},
    k = 1,
    d1 =
    {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n
    \u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
    \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u0
    0020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
    002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004
    \b\u0005\u0010\u0006\u00a8\u0006\u0007"},
    d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/
    String;)V", "getName", "()Ljava/lang/String;", "test-
    project"}
    )a
    

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68. @kotlin.Metadata(
    mv = {1, 1, 11},
    bv = {1, 0, 2},
    k = 1,
    d1 =
    {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n
    \u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
    \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u0
    0020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
    002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004
    \b\u0005\u0010\u0006\u00a8\u0006\u0007"},
    d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/
    String;)V", "getName", "()Ljava/lang/String;", "test-
    project"}
    )a
    
    

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69. View Slide

70. View Slide

71. View Slide

72. View Slide

73. val metadata
    = typeElement.getAnnotation(Metadata::class.java)
    

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74. "Reading information from an annotation,
    you say?"
    

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75. Compiler Plugins
    

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76. Compiler Plugins
    • Faster/incremental
    

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77. Compiler Plugins
    • Faster/incremental
    • No stubs generation
    

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78. Compiler Plugins
    • Faster/incremental
    • No stubs generation
    • No kapt
    

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79. Compiler Plugins
    • Faster/incremental
    • No stubs generation
    • No kapt
    

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80. BUT!
    

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81. BUT!
    

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82. BUT!
    • Undocumented and not stable
    

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83. BUT!
    • Undocumented and not stable
    • Only applies to Kotlin sources
    

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84. BUT!
    • Undocumented and not stable
    • Only applies to Kotlin sources
    • Tooling can be painful
    

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85. BUT!
    • Undocumented and not stable
    • Only applies to Kotlin sources
    • Tooling can be painful
    • github.com/hzsweers/redacted-compiler-
    plugin
    

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86. BUT!
    • Undocumented and not stable
    • Only applies to Kotlin sources
    • Tooling can be painful
    • github.com/hzsweers/redacted-compiler-
    plugin
    • The talk title was Annotation Processing in
    a Kotlin
    

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87. BUT!*
    • Undocumented and not stable
    • Only applies to Kotlin sources
    • Tooling can be painful
    • github.com/hzsweers/redacted-compiler-
    plugin
    • The talk title was Annotation Processing in
    a Kotlin
    

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88. "Reading information from an annotation,
    you say?"
    

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89. kotlin-metadata
    https://github.com/takhion/kotlin-metadata
    

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90. val metadata
    = typeElement.getAnnotation(Metadata::class.java)
    

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91. val Element.kotlinMetadata: KotlinMetadata?
    val metadata
    = typeElement.getAnnotation(Metadata::class.java)
    

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92. val Element.kotlinMetadata: KotlinMetadata?
    val metadata = typeElement.kotlinMetadata
    

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93. when (typeElement.kotlinMetadata) {
    is KotlinPackageMetadata ->
    is KotlinClassMetadata ->
    is KotlinFileMetadata ->
    is KotlinMultiFileClassPartMetadata ->
    is KotlinMultiFileClassFacadeMetadata ->
    is KotlinSyntheticClassMetadata ->
    is KotlinUnknownMetadata ->
    }a
    

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94. when (typeElement.kotlinMetadata) {
    is KotlinClassMetadata ->
    }a
    

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95. when (metadata) {
    is KotlinClassMetadata -> {
    val classData: ClassData = metadata.data
    }a
    }b
    

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96. when (metadata) {
    is KotlinClassMetadata -> {
    val classData: ClassData = metadata.data
    val proto: ProtoBuf.Class = classData.classProto
    }a
    }b
    

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97. View Slide

98. when (metadata) {
    is KotlinClassMetadata -> {
    val classData: ClassData = metadata.data
    val proto: ProtoBuf.Class = classData.classProto
    }a
    }b
    

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99. when (metadata) {
    is KotlinClassMetadata -> {
    val classData: ClassData = metadata.data
    val proto: ProtoBuf.Class = classData.classProto
    val nameResolver = classData.nameResolver
    }a
    }b
    

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100. @kotlin.Metadata(
     mv = {1, 1, 11},
     bv = {1, 0, 2},
     k = 1,
     d1 =
     {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n
     \u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
     \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u0
     0020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
     002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004
     \b\u0005\u0010\u0006\u00a8\u0006\u0007"},
     d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/
     String;)V", "getName", "()Ljava/lang/String;", "test-
     project"}
     )a
     

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101. @kotlin.Metadata(
     mv = {1, 1, 11},
     bv = {1, 0, 2},
     k = 1,
     d1 =
     {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n
     \u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
     \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u0
     0020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
     002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004
     \b\u0005\u0010\u0006\u00a8\u0006\u0007"},
     d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/
     String;)V", "getName", "()Ljava/lang/String;", "test-
     project"}
     )a
     

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102. @kotlin.Metadata(
     mv = {1, 1, 11},
     bv = {1, 0, 2},
     k = 1,
     d1 =
     {"\u0000\u0012\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n
     \u0000\n\u0002\u0010\u000e\n\u0002\b\u0004\b\u0007\u0018
     \u00002\u00020\u0001B\r\u0012\u0006\u0010\u0002\u001a\u0
     0020\u0003\u00a2\u0006\u0002\u0010\u0004R\u0011\u0010\u0
     002\u001a\u00020\u0003\u00a2\u0006\b\n\u0000\u001a\u0004
     \b\u0005\u0010\u0006\u00a8\u0006\u0007"},
     d2 = {"Ltest/Person;", "", "name", "", "(Ljava/lang/
     String;)V", "getName", "()Ljava/lang/String;", "test-
     project"}
     )a
     

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103. when (metadata) {
     is KotlinClassMetadata -> {
     val classData: ClassData = metadata.data
     val proto: ProtoBuf.Class = classData.classProto
     val nameResolver = classData.nameResolver
     }a
     }b
     

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104. val proto: ProtoBuf.Class = classData.classProto
     val constructors: List = proto.constructorList
     val properties: List = proto.propertyList
     val functions: List = proto.functionList
     

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105. class Person(val name: String = "Joe")
     

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106. class Person(val name: String = "Joe")
     val name = TODO()
     

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107. class Person(val name: String = "Joe")
     val name = TODO()
     val mutable = TODO()
     

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108. class Person(val name: String = "Joe")
     val name = TODO()
     val mutable = TODO()
     val nullable = TODO()
     

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109. class Person(val name: String = "Joe")
     val name = TODO()
     val mutable = TODO()
     val nullable = TODO()
     val hasDefault = TODO()
     

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110. class Person(val name: String = "Joe")
     val name = TODO()
     val mutable = TODO()
     val nullable = TODO()
     val hasDefault = TODO()
     

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111. class Person(val name: String = "Joe")
     val proto = classData.classProto
     val constr = proto.constructorList.find { it.isPrimary }
     val name = TODO()
     val mutable = TODO()
     val nullable = TODO()
     val hasDefault = TODO()
     

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112. it.isPrimary
     

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113. val Constructor.isPrimary: Boolean get() {
     return !Flags.IS_SECONDARY[flags]
     }
     

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114. class Person(val name: String = "Joe")
     val proto = classData.classProto
     val constr = proto.constructorList.find { it.isPrimary }
     val name = TODO()
     val mutable = TODO()
     val nullable = TODO()
     val hasDefault = TODO()
     

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115. class Person(val name: String = "Joe")
     val proto = classData.classProto
     val constr = proto.constructorList.find { it.isPrimary }
     val nameParam = constr.valueParameterList[0]
     val name = TODO()
     val mutable = TODO()
     val nullable = TODO()
     val hasDefault = TODO()
     

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116. class Person(val name: String = "Joe")
     val proto = classData.classProto
     val constr = proto.constructorList.find { it.isPrimary }
     val nameParam = constr.valueParameterList[0]
     val name = nameResolver.getString(nameParam.name)
     val mutable = TODO()
     val nullable = TODO()
     val hasDefault = TODO()
     

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117. class Person(val name: String = "Joe")
     val proto = classData.classProto
     val constr = proto.constructorList.find { it.isPrimary }
     val nameParam = constr.valueParameterList[0]
     val name = nameResolver.getString(nameParam.name)
     val mutable = TODO()
     val nullable = TODO()
     val hasDefault = nameParam.declaresDefaultValue
     

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118. class Person(val name: String = "Joe")
     val proto = classData.classProto
     val constr = proto.constructorList.find { it.isPrimary }
     val nameParam = constr.valueParameterList[0]
     val name = nameResolver.getString(nameParam.name)
     val mutable = TODO()
     val nullable = nameParam.type.nullable
     val hasDefault = nameParam.declaresDefaultValue
     

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119. class Person(val name: String = "Joe")
     val proto = classData.classProto
     val constr = proto.constructorList.find { it.isPrimary }
     val nameParam = constr.valueParameterList[0]
     val name = nameResolver.getString(nameParam.name)
     val mutable = proto.propertyList[0].isVar
     val nullable = nameParam.type.nullable
     val hasDefault = nameParam.declaresDefaultValue
     

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120. class Person(val name: String = "Joe")
     val proto = classData.classProto
     val constr = proto.constructorList.find { it.isPrimary }
     val nameParam = constr.valueParameterList[0]
     val name = nameResolver.getString(nameParam.name)
     val mutable = proto.propertyList[0].isVar
     val nullable = nameParam.type.nullable
     val hasDefault = nameParam.declaresDefaultValue
     

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121. kotlin-metadata
     

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122. kotlinx-metadata
     

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123. kotlinx-metadata
     https://github.com/JetBrains/kotlin/tree/master/libraries/kotlinx-metadata/jvm
     

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124. kotlinx-metadata?
     https://github.com/JetBrains/kotlin/tree/master/libraries/kotlinx-metadata/jvm
     

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125. kotlinx-metadata?
     https://github.com/JetBrains/kotlin/tree/master/libraries/kotlinx-metadata/jvm
     https://github.com/square/moshi/pull/642
     

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126. Case Studies
     

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127. copydynamic
     https://github.com/hzsweers/copydynamic
     

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128. –Chris Banes, somewhere online once upon a time
     “I wish I could use copy but like,
     conditionally set properties”
     

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129. data class Taco(
     val seasoning: Set,
     val isBreakfast: Boolean
     )a
     

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130. data class Taco(
     val seasoning: Set,
     val isBreakfast: Boolean
     )a
     val taco = Taco(setOf("el paso"), true)
     

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131. data class Taco(
     val seasoning: Set,
     val isBreakfast: Boolean
     )a
     val taco = Taco(setOf("el paso"), true)
     val newTaco = taco.copyDynamic({
     if (isBreakfastTime) {
     isBreakfast = true
     }
     }
     

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132. class TacoDynamicBuilder constructor(
     private val source: Taco) {
     var seasoning: Set = source.seasoning
     var isBreakfast: Boolean = source.isBreakfast
     fun build(): Taco = source.copy(seasoning,
     isBreakfast)
     }a
     fun Taco.copyDynamic(
     copyBlock: TacoDynamicBuilder.() -> Unit): Taco {
     return TacoDynamicBuilder(this)
     .also { copyBlock(it) }
     .build()
     }b
     

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133. class TacoDynamicBuilder constructor(
     private val source: Taco) {
     var seasoning: Set = source.seasoning
     var isBreakfast: Boolean = source.isBreakfast
     fun build(): Taco = source.copy(seasoning,
     isBreakfast)
     }a
     fun Taco.copyDynamic(
     copyBlock: TacoDynamicBuilder.() -> Unit): Taco {
     return TacoDynamicBuilder(this)
     .also { copyBlock(it) }
     .build()
     }b
     

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134. class TacoDynamicBuilder constructor(
     private val source: Taco) {
     var seasoning: Set = source.seasoning
     var isBreakfast: Boolean = source.isBreakfast
     fun build(): Taco = source.copy(seasoning,
     isBreakfast)
     }a
     fun Taco.copyDynamic(
     copyBlock: TacoDynamicBuilder.() -> Unit): Taco {
     return TacoDynamicBuilder(this)
     .also { copyBlock(it) }
     .build()
     }b
     

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135. data class Taco(
     val seasoning: Set,
     val isBreakfast: Boolean
     )a
     

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136. @CopyDynamic
     data class Taco(
     val seasoning: Set,
     val isBreakfast: Boolean
     )aa
     

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137. @CopyDynamic
     data class Taco(
     val seasoning: Set,
     val isBreakfast: Boolean
     )aa
     •Read the proto off Taco
     •Ensure it's a data class
     •Find constructor
     •Get all params + matching properties
     •Generate with KotlinPoet
     

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138. @CopyDynamic
     data class Taco(
     val seasoning: Set,
     val isBreakfast: Boolean
     )a
     •Read the proto off Taco
     •Ensure it's a data class
     •Find constructor
     •Get all params + matching properties
     •Generate with KotlinPoet
     

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139. @CopyDynamic
     data class Taco(
     val seasoning: Set,
     val isBreakfast: Boolean
     )a
     •Read the proto off Taco
     •Ensure it's a data class
     •Find constructor
     •Get all params + matching properties
     •Generate with KotlinPoet
     

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140. @CopyDynamic
     data class Taco(
     val seasoning: Set,
     val isBreakfast: Boolean
     )a
     •Read the proto off Taco
     •Ensure it's a data class
     •Find constructor
     •Get all params + matching properties
     •Generate with KotlinPoet
     

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141. @CopyDynamic
     data class Taco(
     val seasoning: Set,
     val isBreakfast: Boolean
     )a
     •Read the proto off Taco
     •Ensure it's a data class
     •Find constructor
     •Get all params + matching properties
     •Generate with KotlinPoet
     

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142. @CopyDynamic
     data class Taco(
     val seasoning: Set,
     val isBreakfast: Boolean
     )a
     •Read the proto off Taco
     •Ensure it's a data class
     •Find constructor
     •Get all params + matching properties
     •Generate with KotlinPoet
     

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143. @CopyDynamic
     data class Taco(
     val seasoning: Set,
     val isBreakfast: Boolean
     )a
     •Read the proto off Taco
     •Ensure it's a data class
     •Find constructor
     •Get all params + matching properties
     •Generate with KotlinPoet
     "kapt.kotlin.generated"
     

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144. Moshi
     https://github.com/square/moshi
     API overview blog post: https://goo.gl/dRjqFb
     

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145. Guidelines
     

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146. Metadata is
     added signal
     

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147. Abstract your
     renderer
     

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148. Be mindful of
     mixed source sets
     

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149. Lean on language
     features
     

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150. Be mindful of
     quirks and caveats
     

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151. Quirks & Caveats
     • Not public yet
     

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152. Quirks & Caveats
     • Not public yet
     • Not incremental
     

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153. Quirks & Caveats
     • Not public yet
     • Not incremental
     • No multiplatform
     

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154. Quirks & Caveats
     • Not public yet
     • Not incremental
     • No multiplatform
     • (Kapt) No multiple rounds*
     

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155. Quirks & Caveats
     • Not public yet
     • Not incremental
     • No multiplatform
     • (Kapt) No multiple rounds*
     • (Kapt) Varying build system support
     

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156. Quirks & Caveats
     • Not public yet
     • Not incremental
     • No multiplatform
     • (Kapt) No multiple rounds*
     • (Kapt) Varying build system support
     

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157. –you?
     “I have a processor and want to support
     kotlin today! What should I use?”
     

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158. “I have a processor and want to support
     kotlin today! What should I use?”
     “kotlin-metadata, probably”
     

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159. “I have a processor and want to support
     kotlin today! What should I use?”
     “kotlin-metadata, probably”
     “and KotlinPoet”
     

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160. Thanks!
     • kotlin-metadata: github.com/takhion/kotlin-metadata
     • kotlinx-metadata: https://github.com/JetBrains/kotlin/tree/master/
     libraries/kotlinx-metadata/jvm
     • Moshi: github.com/square/moshi
     • Km API prototype: github.com/square/moshi/pull/642
     • CopyDynamic: github.com/hzsweers/copydynamic
     • FR for value type API: youtrack.jetbrains.com/issue/KT-26602
     • Multiple rounds demo: github.com/Takhion/generate-kotlin-multiple-rounds
     • redacted compiler plugin: github.com/hzsweers/redacted-compiler-plugin
     • KotlinPoet: github.com/square/kotlinpoet
     

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161. Thanks!
     Special thanks to Eugenio Marletti (@workingkills)
     

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162. Thanks!
     Zac Sweers
     @pandanomic
     

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