How to write Gradle Plugins in Kotlin

Given at Droidcon Italy 2018

Every Android Developer is familiar with Gradle, right?
We know how to apply a plugin, how to setup the Android extension and how to declare our dependencies.

But from where does the Android extension come from?
Which settings are possible here?
How can a task use these information to run a specific action?

I'll explain all the Gradle magic by showing how to write a Gradle plugin.
The plugin will be pretty simple and straightforward but shows you what Gradle does behind the scenes.

Naturally we will write the plugin in Kotlin.
We will also cover a little bit about Gradle's Kotlin DSL.

Basically you will leave my talk knowing the basics of the Gradle plugin development:
* How can I declare my own task?
* How can I declare my own extension?
* How can I test my plugin?

Transcript

1. How to write Gradle plugins
   in Kotlin
   1 / 42
   

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2. plugins {
   id("person-creater") version "26"
   }
   person {
   create("stefan") {
   name = "Stefan"
   lastName = "May"
   country = "Germany"
   avatar = "https://goo.gl/BV2a67"
   jobTitle = "Android Developer"
   workingSince = "07/2014"
   company {
   name = "grandcentrix"
   location = "Cologne"
   2 / 42
   

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3. lastName = "May"
   country = "Germany"
   avatar = "https://goo.gl/BV2a67"
   jobTitle = "Android Developer"
   workingSince = "07/2014"
   company {
   name = "grandcentrix"
   location = "Cologne"
   remoteWorker = true
   }
   social {
   twitter = "@StefMa91"
   github = "StefMa"
   medium = "@StefMa"
   }
   }
   }
   2 / 42
   

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4. • Why?
   • The Project: Gloc (Gradle Lines Of Code)
   • How to write a Plugin
   3 / 42
   

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5. • Android Developer since July 2014
   • Android Studio stable December 2014 together with the Gradle Plugin
   Gradle… what is Gradle?
   • Copy & Paste from stackoverflow
   • The raise of build variants & product flavors
   • Raise of plugins…
   5 / 42
   

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6. • Android Developer since July 2014
   • Android Studio stable December 2014 together with the Gradle Plugin
   Gradle… what is Gradle?
   • Copy & Paste from stackoverflow
   • The raise of build variants & product flavors
   • Raise of plugins…
   WHAT IS GRADLE!!!111!1oneeleven
   5 / 42
   

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7. • Why?
   • The Project: Gloc (Gradle Lines Of Code)
   • How to write a Plugin
   6 / 42
   

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8. plugins {
   id("guru.stefma.gloc") version "0.0.1"
   }
   gloc {
   enabled = true
   dirs = arrayOf("src/main", "src/test")
   }
   7 / 42
   

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9. ./gradlew gloc --console=plain
   Hello Plugin \o/
   :glocInput
   :gloc
   Output can be found at /Users/stefan/Developer/DroidconItaly/Gloc/gloc/build/gloc/gloc.txt
   BUILD SUCCESSFUL in 0s
   2 actionable tasks: 2 executed
   8 / 42
   

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10. BUILD SUCCESSFUL in 0s
    2 actionable tasks: 2 executed
    ./gradlew gloc --console=plain
    Hello Plugin \o/
    :glocInput
    :gloc UP-TO-DATE
    BUILD SUCCESSFUL in 0s
    2 actionable tasks: 1 executed, 1 up-to-date
    8 / 42
    

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11. 2 actionable tasks: 1 executed, 1 up-to-date
    ./gradlew clean gloc --build-cache --console=plain
    Hello Plugin \o/
    :clean
    :glocInput
    :gloc
    Output can be found at /Users/stefan/Developer/DroidconItaly/Gloc/gloc/build/gloc/gloc.txt
    BUILD SUCCESSFUL in 0s
    3 actionable tasks: 3 executed
    8 / 42
    

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12. BUILD SUCCESSFUL in 0s
    3 actionable tasks: 3 executed
    ./gradlew clean gloc --build-cache --console=plain
    Hello Plugin \o/
    :clean
    :glocInput
    :gloc FROM-CACHE
    BUILD SUCCESSFUL in 0s
    3 actionable tasks: 2 executed, 1 from cache
    8 / 42
    

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13. cat /Users/stefan/Developer/DroidconItaly/Gloc/gloc/build/gloc/gloc.txt
    Directory 'main':
    'kt' has '145' LOC in sum
    Directory 'test':
    'kt' has '413' LOC in sum
    9 / 42
    

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14. • Why?
    • The Project: Gloc
    • How to write a Plugin
    10 / 42
    

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15. gradle
    wrapper
    gradle-wrapper.jar
    gradle-wrapper.properties
    gradlew
    gradlew.bat
    
    build.gradle.kts
    settings.gradle.kts
    
    src
    main
    kotlin
    guru.stefma.gloc
    test
    11 / 42
    

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16. gradle
    wrapper
    gradle-wrapper.jar
    gradle-wrapper.properties
    gradlew
    gradlew.bat
    
    build.gradle.kts
    settings.gradle.kts
    
    src
    main
    kotlin
    guru.stefma.gloc
    test
    kotlin
    guru.stefma.gloc
    11 / 42
    

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17. plugins {
    kotlin("jvm") version ("1.2.30")
    id("java-gradle-plugin")
    id("maven-publish")
    }
    12 / 42
    

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18. lugins {
    kotlin("jvm") version ("1.2.30")
    id("java-gradle-plugin")
    id("maven-publish")
    • applies java plugin
    • gradleApi() dependency
    • gradleTestKit() dependency
    13 / 42
    

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19. plugins {
    kotlin("jvm") version ("1.2.30")
    id("java-gradle-plugin")
    id("maven-publish")
    }
    repositories { 14 / 42
    

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20. }
    repositories {
    jcenter()
    }
    dependencies {
    implementation(kotlin("stdlib-jdk8"))
    testImplementation("org.assertj:assertj-core:3.9.0")
    testImplementation("org.junit.jupiter:junit-jupiter-api:5.1.0")
    testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine:5.1.0")
    }
    14 / 42
    

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21. testImplementation("org.junit.jupiter:junit-jupiter-api:5.1.0")
    testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine:5.1.0")
    }
    (tasks.findByName("test") as Test).useJUnitPlatform()
    group = "guru.stefma.gloc"
    version = "0.0.1"
    gradlePlugin {
    plugins {
    create("gloc") {
    id = "guru.stefma.gloc"
    implementationClass = "guru.stefma.gloc.GlocPlugin"
    }
    }
    }
    14 / 42
    

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22. open class GlocPlugin : Plugin {
    override fun apply(project: Project) {
    println("Hello Plugin \\o/")
    }
    }
    15 / 42
    

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23. val buildResult = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .withArguments("tasks")
    .build()
    16 / 42
    

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24. File(tempDir, "build.gradle").run {
    writeText("""
    plugins {
    id "guru.stefma.gloc"
    }
    """)
    }
    val buildResult = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .build()
    assertThat(buildResult.output).contains("Hello Plugin \\o/")
    17 / 42
    

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25. open class GlocExtension {
    var enabled: Boolean = true
    var dirs: Array = emptyArray()
    }
    18 / 42
    

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26. override fun apply(project: Project) {
    println("Hello Plugin \\o/")
    val extension = project.extensions.run {
    create("gloc", GlocExtension::class.java)
    }
    }
    19 / 42
    

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27. override fun apply(project: Project) {
    val extension = project.extensions.run {
    create("gloc", GlocExtension::class.java)
    }
    project.afterEvaluate {
    if (!extension.enabled) return@afterEvaluate
    println("Hello Plugin \\o/")
    }
    }
    20 / 42
    

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28. fun `apply and enabled false should not print hello plugin`(tempDir: File) {
    File(tempDir, "build.gradle").run {
    writeText("""
    plugins {
    id "guru.stefma.gloc"
    }
    gloc {
    enabled = false
    }
    """)
    }
    val buildResult = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    21 / 42
    

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29. File(tempDir, "build.gradle").run {
    writeText("""
    plugins {
    id "guru.stefma.gloc"
    }
    gloc {
    enabled = false
    }
    """)
    }
    val buildResult = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .build()
    assertThat(buildResult.output).doesNotContain("Hello Plugin \\o/")
    }
    21 / 42
    

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30. plugins {
    id "guru.stefma.gloc"
    }
    gloc {
    enabled = false
    enabled = true
    }
    """)
    }
    val buildResult = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .build()
    assertThat(buildResult.output).doesNotContain("Hello Plugin \\o/")
    assertThat(buildResult.output).contain(„Hello Plugin \\o/")
    }
    22 / 42
    

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31. plugins {
    id("guru.stefma.gloc") version "0.0.1"
    }
    gloc {
    enabled = true
    dirs = arrayOf("src/main", "src/test")
    }
    23 / 42
    

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32. open class GlocTask : DefaultTask() {
    @TaskAction
    fun action() {
    println("Hello from Task")
    }
    }
    24 / 42
    

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33. override fun apply(project: Project) {
    ...
    val glocTask = with(project.tasks) {
    create("gloc", GlocTask::class.java) {
    it.group = "Development"
    it.description = "Get the lines of code for files"
    }
    }
    }
    25 / 42
    

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34. File(tempDir, "build.gradle").run {
    writeText(
    """
    plugins {
    id("guru.stefma.gloc")
    }
    """
    )
    }
    val buildResult = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .withArguments("gloc")
    .build()
    26 / 42
    

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35. """
    plugins {
    id("guru.stefma.gloc")
    }
    """
    )
    }
    val buildResult = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .withArguments("gloc")
    .build()
    assertThat(buildResult.output).contains("Hello from Task")
    assertThat(buildResult.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.SUCCESS)
    26 / 42
    

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36. val buildResult = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .withArguments("gloc")
    .build()
    val buildResult2 = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .withArguments("gloc")
    .build()
    assertThat(buildResult.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.SUCCESS)
    assertThat(buildResult2.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.UP_TO_DATE)
    27 / 42
    

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37. val buildResult = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .withArguments("gloc")
    .build()
    val buildResult2 = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .withArguments("gloc")
    .build()
    assertThat(buildResult.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.SUCCESS)
    assertThat(buildResult2.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.UP_TO_DATE)
    ❌
    27 / 42
    

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38. open class GlocTask : DefaultTask() {
    @OutputFile
    var output = project.file("${project.buildDir}/gloc/gloc.txt")
    @TaskAction
    fun action() {
    output.createNewFile()
    output.writeText("Hello World")
    }
    }
    28 / 42
    

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39. val buildResult = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .withArguments("gloc")
    .build()
    val buildResult2 = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .withArguments("gloc")
    .build()
    assertThat(buildResult.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.SUCCESS)
    assertThat(buildResult2.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.UP_TO_DATE)
    29 / 42
    

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40. val buildResult = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .withArguments("gloc")
    .build()
    val buildResult2 = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .withArguments("gloc")
    .build()
    assertThat(buildResult.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.SUCCESS)
    assertThat(buildResult2.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.UP_TO_DATE)
    ✅
    29 / 42
    

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41. open class GlocTask : DefaultTask() {
    @OutputFile
    var output = project.file("${project.buildDir}/gloc/gloc.txt")
    @TaskAction
    fun action() {
    val extension = project.extensions.run {
    findByName("gloc") as GlocExtension
    }
    if (extension.enabled) {
    createPrettyOutputFile(extensions.dirs)
    println("Output can be found at ${output.absolutePath}")
    }
    30 / 42
    

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42. open class GlocTask : DefaultTask() {
    @OutputFile
    var output = project.file("${project.buildDir}/gloc/gloc.txt")
    @TaskAction
    fun action() {
    val extension = project.extensions.run {
    findByName("gloc") as GlocExtension
    }
    if (extension.enabled) {
    createPrettyOutputFile(extensions.dirs)
    println("Output can be found at ${output.absolutePath}")
    }
    }
    }
    30 / 42
    

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43. fun `task should read test xml file and should write loc in it`(tempDir: File) {
    File(tempDir, "build.gradle").run {
    writeText(
    """
    plugins {
    id "guru.stefma.gloc"
    }
    gloc {
    enabled = true
    dirs = [projectDir.path + "/source"]
    }
    """
    )
    }
    31 / 42
    

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44. enabled = true
    dirs = [projectDir.path + "/source"]
    }
    """
    )
    }
    File(tempDir, "source/test.xml").apply {
    parentFile.mkdirs()
    createNewFile()
    writeText("This\nis\ndroidcon\nitaly\nturin")
    }
    GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .withArguments("gloc")
    .build()
    31 / 42
    

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45. )
    }
    File(tempDir, "source/test.xml").apply {
    parentFile.mkdirs()
    createNewFile()
    writeText("This\nis\ndroidcon\nitaly\nturin")
    }
    GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .withArguments("gloc")
    .build()
    val glocFileText = File(tempDir, "build/gloc/gloc.txt")
    assertThat(glocFileText.readText()).contains("5")
    }
    31 / 42
    

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46. @Test
    fun `task run twice with different dirs should be run twice`(tempDir: File) {
    runTest(tempDir, "source") {
    assertThat(it.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.SUCCESS)
    }
    runTest(tempDir, "anotherSource") {
    assertThat(it.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.SUCCESS)
    }
    }
    32 / 42
    

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47. @Test
    fun `task run twice with different dirs should be run twice`(tempDir: File) {
    runTest(tempDir, "source") {
    assertThat(it.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.SUCCESS)
    }
    runTest(tempDir, "anotherSource") {
    assertThat(it.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.SUCCESS)
    }
    }
    ❌
    32 / 42
    

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48. @Test
    fun `task run twice with different dirs should be run twice`(tempDir: File) {
    runTest(tempDir, "source") {
    assertThat(it.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.SUCCESS)
    }
    runTest(tempDir, "anotherSource") {
    assertThat(it.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.SUCCESS)
    TaskOutcome.UP_TO_DATE
    }
    }
    #
    33 / 42
    

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49. open class GlocTask : DefaultTask() {
    @OutputFile
    var output = project.file("${project.buildDir}/gloc/gloc.txt")
    @Input
    var inputDirs = (project.extensions.findByName("gloc") as GlocExtension)
    .dirs.run { map { it }.toString() }
    ...
    }
    34 / 42
    

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50. open class GlocTask : DefaultTask() {
    ...
    @Input
    var inputDirs = (project.extensions.findByName("gloc") as GlocExtension)
    .dirs.run { map { it }.toString() }
    @InputFile
    var inputDirs = project.file("${project.buildDir}/gloc/inputdirs.txt")
    ...
    }
    35 / 42
    

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51. open class GlocInputTask : DefaultTask() {
    @TaskAction
    fun writeInput() {
    val filePath = "{project.buildDir}/gloc/inputdirs.txt"
    val inputFile = project.file("$filePath").apply {
    parentFile.mkdirs()
    createNewFile()
    writeText("")
    }
    (project.extensions.findByName("gloc") as GlocExtension)
    .dirs.forEach { inputFile.appendText(it) }
    }
    }
    36 / 42
    

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52. override fun apply(project: Project) {
    val inputTask = project.tasks.run {
    create("glocInput", GlocInputTask::class.java)
    }
    with(project.tasks) {
    create("gloc", GlocTask::class.java) {
    it.group = "Development"
    it.description = "Get the lines of code for files"
    it.dependsOn(inputTask)
    }
    }
    }
    37 / 42
    

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53. @Test
    fun `task run twice with different dirs should be run twice`(tempDir: File) {
    runTest(tempDir, "source") {
    assertThat(it.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.SUCCESS)
    }
    runTest(tempDir, "anotherSource") {
    assertThat(it.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.SUCCESS)
    }
    }
    38 / 42
    

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54. @Test
    fun `task run twice with different dirs should be run twice`(tempDir: File) {
    runTest(tempDir, "source") {
    assertThat(it.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.SUCCESS)
    }
    runTest(tempDir, "anotherSource") {
    assertThat(it.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.SUCCESS)
    }
    }
    ✅
    38 / 42
    

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55. open class GlocTask : DefaultTask() {
    @OutputFile
    ...
    @InputFile
    ...
    @TaskAction
    ...
    }
    39 / 42
    

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56. open class GlocTask : DefaultTask() {
    @OutputFile
    ...
    @InputFile
    ...
    @TaskAction
    ...
    }
    @CacheableTask
    39 / 42
    

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57. fun `task should read from build cache`(tempDir: File) {
    File(tempDir, "build.gradle").run {
    writeText(
    """
    plugins {
    id "guru.stefma.gloc"
    }
    gloc {
    enabled = true
    dirs = [projectDir.path + "/source"]
    }
    """
    )
    }
    40 / 42
    

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58. plugins {
    id "guru.stefma.gloc"
    }
    gloc {
    enabled = true
    dirs = [projectDir.path + "/source"]
    }
    """
    )
    }
    File(tempDir, "source/test.xml").apply {
    parentFile.mkdirs()
    createNewFile()
    writeText("This\nis\droidcon\nitaly\nturin")
    }
    40 / 42
    

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59. createNewFile()
    writeText("This\nis\droidcon\nitaly\nturin")
    }
    File(tempDir, "settings.gradle").run {
    writeText(
    """
    buildCache {
    local { directory = "$tempDir/cache" }
    }
    """
    )
    }
    val build = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .withArguments("gloc", "--build-cache")
    .build()
    40 / 42
    

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60. }
    val build = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .withArguments("gloc", "--build-cache")
    .build()
    assertThat(build.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.SUCCESS)
    // Clean build dir and run again - should be read from build cache
    File(tempDir, "build/").deleteRecursively()
    val build2 = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .withArguments("gloc", "--build-cache")
    .build()
    40 / 42
    

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61. .withPluginClasspath()
    .withArguments("gloc", "--build-cache")
    .build()
    assertThat(build.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.SUCCESS)
    // Clean build dir and run again - should be read from build cache
    File(tempDir, "build/").deleteRecursively()
    val build2 = GradleRunner.create()
    .withProjectDir(tempDir)
    .withPluginClasspath()
    .withArguments("gloc", "--build-cache")
    .build()
    assertThat(build2.task(":gloc")!!.outcome)
    .isEqualTo(TaskOutcome.FROM_CACHE)
    }
    40 / 42
    

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62. 41 / 42
    

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63. • Created the project
    42 / 42
    

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64. • Created the project
    • Created the gradle files and registered the plugin
    42 / 42
    

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65. • Created the project
    • Created the gradle files and registered the plugin
    gradlePlugin {
    plugins {
    create("gloc") {
    id = "guru.stefma.gloc"
    implementationClass = "guru.stefma.gloc.GlocPlugin"
    }
    }
    }
    42 / 42
    

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66. • Created the project
    • Created the plugin
    • Created the gradle files and registered the plugin
    42 / 42
    

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67. • Created the project
    • Created the plugin
    • Created the gradle files and registered the plugin
    open class GlocPlugin : Plugin {
    override fun apply(project: Project) {
    project.extensions.create(...)
    project.tasks.create(...)
    }
    }
    42 / 42
    

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68. • Created the project
    • Created the plugin
    • Created the extension
    • Created the gradle files and registered the plugin
    42 / 42
    

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69. • Created the project
    • Created the plugin
    • Created the extension
    • Created the gradle files and registered the plugin
    open class GlocExtension {
    var enabled: Boolean = true
    var dirs: Array = emptyArray()
    }
    42 / 42
    

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70. • Created the project
    • Created the plugin
    • Created the extension
    • Created the task
    • Created the gradle files and registered the plugin
    42 / 42
    

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71. • Created the project
    • Created the plugin
    • Created the extension
    • Created the task
    • Created the gradle files and registered the plugin
    @CacheableTask
    open class GlocTask : DefaultTask() {
    @OutputFile
    ...
    @InputFile
    ...
    @Action
    ...
    }
    42 / 42
    

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72. plugins {
    id("gradle-plugin-how-to") version "0.1-ALPHA"
    }
    about {
    name = "Stefan May“
    twitter = "https://twitter.com/@StefMa91"
    medium = "https://medium.com/@StefMa"
    }
    code {
    availableAt = "https://github.com/StefMa/Gloc"
    }
    43
    

    View Slide