Kotlin DSL in Gradle build scripts

Gradle is a default build tool for Android applications. It is experiencing a phase of rapid growth now and tries to adopt new build script language – Kotlin. We will try to discover what benefits and dangers the new language brings, how it makes build script magic more understandable and what pitfalls we should avoid.

Demo project: https://github.com/lampapos/kotand

Transcript

1. Gradle scripts in Kotlin is it cool? Michael Pustovit Android Dev @ Keypr
2. Plan for next ~35 minutes 1. Groovy Gradle DSL magic 2. Kotlin DSL remedy 3. Demo (groovy -> kotlin) 1 2 3 Domain-Specific Language - a language for a specific problem.
3. Gradle manages dependencies ``` and builds a project ```
4. Gradle is awesome!
5. It builds your project automagically
6. But it uses too much magic sometimes
7. • Syntactic sugar ``` • Dynamic nature of Groovy ``` Sources of magic
8. apply plugin: 'java’ version '1.0-SNAPSHOT' sourceCompatibility = 1.8 repositories { mavenCentral() } dependencies { testCompile group: 'junit', name: 'junit', version: '4.12' } task hello { println "Hello world!" }
9. Parentheses apply plugin: 'java’ apply(plugin: 'java’)
10. Named parameters testCompile group: 'junit', name: 'junit', version: '4.12' testCompile([group: 'junit', name: 'junit', version: '4.12'])
11. Lambda as last parameter configure(task1, task2) { doSomething() } configure(task1, task2, { doSomething() })
12. Abstract Syntax Tree (AST) transformation task hello { println "Hello world!" } task('hello', { println "Hello world!" })
13. apply(plugin: 'java') version('1.0-SNAPSHOT') sourceCompatibility = 1.8 repositories({ mavenCentral() }) dependencies({ testCompile([group: 'junit', name: 'junit', version: '4.12']) }) task('hello', { println "Hello world!" })
14. Groovy is dynamic ```
15. Gradle build stages 1. Initialization 2. Configuration 3. Execution 1 2 3
16. Init Config Exec Project object Tasks A B C >gradle B
17. Gradle is about plugins Gradle core can do barely nothing All functions from plugins
18. Plugins class GreetingPlugin implements Plugin<Project> { void apply(Project project) { project.task('hello') { doLast { println 'Hello from the GreetingPlugin' } } } }
19. Plugin configuration Plugins are configured by extensions
20. Plugins class GreetingPluginExtension { String message } class GreetingPlugin implements Plugin<Project> { void apply(Project prj) { def ext = prj.extensions.create('greeting', GreetingPluginExtension) project.task('hello') { doLast { println ext.message }} } }
21. Plugin extension apply plugin: 'org.example.GreetingPlugin' greeting { message "Hello world!" } This method doesn’t exist at “compile” time
22. class Foo { def methodMissing(String name, def args) { return '1' } } assert new Foo().someUnknownMethod(42) == '1' “methodMissing” method
23. Problems 1. Too much Groovy specific magic 2. Lack of a help from an IDE 3. Late error detection 1 2 3
24. • Static typing ``` • Full IDE support Kotlin DSL = Solution
25. Kotlin feature Kotlin is statically typed and was designed with DSL creation in mind
26. apply plugin: 'java’ version '1.0-SNAPSHOT' sourceCompatibility = 1.8 repositories { mavenCentral() } dependencies { testCompile group: 'junit', name: 'junit', version: '4.12' } task hello { println "Hello world!" } Groovy DSL
27. apply { plugin("java") } version = "1.0-SNAPSHOT" java.sourceCompatibility = JavaVersion.VERSION_1_8 repositories { mavenCentral() } dependencies { testCompile(group = "junit", name = "junit", version = "4.12") } tasks.create("hello", { println("Hello world!") }) Kotlin DSL
28. • Dynamic nature of plugins • “Old” plugins • Kotlin-Groovy intercommunication Why is it not so easy?
29. Plugin extension apply plugin: 'org.example.GreetingPlugin' greeting { message "Hello world!" } In Kotlin this method must exist
30. We can generate ``` extension handlers and add them to a build script
31. Plugins application buildscript { repositories { jcenter() } def ver = “1.0.0” dependencies { classpath "jfrog…:$ver" } } apply plugin: "com.jfrog…" Where to look for plugins What plugins do we need
32. How to use this workaround 1. Apply a plugin but not add an extension yet 2. Trigger handlers generator 3. Add the extension & configure plugins 1 2 3
33. Config Tasks Extensions Serialized snapshot Generated extension handlers
34. > gradle kotlinDslAccessorsSnapshot gradle ├── project-schema.json └── wrapper ├── gradle-wrapper.jar └── gradle-wrapper.properties Dynamic > Static workaround
35. "conventions": { "base": "org...BasePluginConvention", "java": "org...JavaPluginConvention", ... }, "configurations": [ "compile", "provided", ... ], "extensions": { "ext": "org...ExtraPropertiesExtension", "defaultArtifacts": "org.gradle.api.inter…”, ... } project-schema.json
36. val Project.`android`: AppExtension get() = extensions.getByName("android") as AppExtension fun Project.`android`(configure: AppExtension.() -> Unit): Unit = extensions.configure("android", configure) Generated code
37. Current (“old”) plugin system buildscript { repositories { jcenter() } def ver = “1.0.0” dependencies { classpath "jfrog…:$ver" } } apply plugin: "com.jfrog…" Dynamic block Can be placed anywhere
38. New plugin system pluginManagement { repositories { gradlePluginPortal() } } plugins { id "com.jfrog…" version "4.5.2" } Static block Gradle plugins repo
39. Config Extensions Generated extension handlers
40. Kotlin DSL + Android • Android plugin is not in the repo • Android plugin is tricky • Not supported plugins
41. Unsupportable plugins android { playAccountConfigs { defaultAccountConfig { serviceAccountEmail = 'a@b.com' pk12File = file('key.p12') } } defaultConfig { // ... } } Triple-T/gradle-play-publisher
42. DEMO
43. Kotlin extension configuration hack apply { plugin("com.android.application") } // android { configure <com.android.build.gradle.AppExtension> { buildToolsVersion("26.0.2") compileSdkVersion(26) } > gradle kotlinDslAccessorsSnapshot
44. • Tools are in alpha-beta state ``` • Usable with Android projects • Should try - it’s a future ``` Summary
45. Sources ❖ Kotlin DSL GitHub repo ❖ Using Gradle Script Kotlin for Android ❖ Chicken and egg problem with accessors.kt ❖ Gradle and Kotlin, a personal perspective ❖ Compile time metaprogramming in Groovy ❖ Gradle tip #2: understanding syntax
46. Thank you! Questions? Michael Pustovit Android Dev @ Keypr