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Automated migration of Android apps to Bazel build system Pavlo Stavytskyi

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About me ● Mobile infra at Lyft ● Grab, Singapore ● Google Developer Expert for Android ● 6 years of Android development 2

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Agenda ● What is Bazel? 3

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Agenda ● What is Bazel? ● Creating a simple Gradle plugin for migration of Android app to Bazel 4

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Agenda ● What is Bazel? ● Creating a simple Gradle plugin for migration of Android app to Bazel ● See a type-safe Kotlin DSL for generating Bazel scripts 5

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What is Bazel?

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What is Bazel? ● Build tool (similarly to Gradle) 7

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What is Bazel? ● Build tool (similarly to Gradle) ● Polyglot. Extensible 8

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What is Bazel? ● Build tool (similarly to Gradle) ● Polyglot. Extensible ● Granularity. Incremental builds 9

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What is Bazel? ● Build tool (similarly to Gradle) ● Polyglot. Extensible ● Granularity. Incremental builds ● Remote build cache/execution 10

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What is Bazel? ● Build tool (similarly to Gradle) ● Polyglot. Extensible ● Granularity. Incremental builds ● Remote build cache/execution ● Query language 11

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What is it like to work with Bazel?

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plugins { id 'com.android.application' id 'kotlin-android' } android { compileSdkVersion 30 defaultConfig { applicationId "com.morfly.sample" minSdkVersion 21 targetSdkVersion 30 } buildTypes { release { minifyEnabled false proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro' } } compileOptions { sourceCompatibility JavaVersion.VERSION_1_8 targetCompatibility JavaVersion.VERSION_1_8 } kotlinOptions { jvmTarget = '1.8' } } dependencies { implementation "org.jetbrains.kotlin:kotlin-stdlib:1.5.10" implementation 'androidx.core:core-ktx:1.6.0' implementation 'androidx.appcompat:appcompat:1.3.1' implementation 'com.google.android.material:material:1.4.0' implementation 'androidx.constraintlayout:constraintlayout:2.1.0' } Gradle script example 13

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load("@io_bazel_rules_kotlin./kotlin:kotlin.bzl", "kt_android_library") load("@rules_jvm_external./:defs.bzl", "artifact") kt_android_library( name = "my_lib", srcs = glob([ "src/main/java/**/*.kt", "src/main/kotlin/**/*.kt", ]), custom_package = "com.morfly.sample", manifest = "src/main/AndroidManifest.xml", resource_files = glob(["src/main/res/**"]), deps = [ artifact("androidx.core:core-ktx"), artifact("androidx.appcompat:appcompat"), artifact("com.google.android.material:material"), artifact("androidx.constraintlayout:constraintlayout"), ], ) Gradle script example 14

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load("@io_bazel_rules_kotlin./kotlin:kotlin.bzl", "kt_android_library") load("@rules_jvm_external./:defs.bzl", "artifact") kt_android_library( name = "my_lib", srcs = glob([ "src/main/java/**/*.kt", "src/main/kotlin/**/*.kt", ]), custom_package = "com.morfly.sample", manifest = "src/main/AndroidManifest.xml", resource_files = glob(["src/main/res/**"]), deps = [ artifact("androidx.core:core-ktx"), artifact("androidx.appcompat:appcompat"), artifact("com.google.android.material:material"), artifact("androidx.constraintlayout:constraintlayout"), ], ) Bazel script components 15

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load("@io_bazel_rules_kotlin./kotlin:kotlin.bzl", "kt_android_library") load("@rules_jvm_external./:defs.bzl", "artifact") kt_android_library( name = "my_lib", srcs = glob([ "src/main/java/**/*.kt", "src/main/kotlin/**/*.kt", ]), custom_package = "com.morfly.sample", manifest = "src/main/AndroidManifest.xml", resource_files = glob(["src/main/res/**"]), deps = [ artifact("androidx.core:core-ktx"), artifact("androidx.appcompat:appcompat"), artifact("com.google.android.material:material"), artifact("androidx.constraintlayout:constraintlayout"), ], ) Bazel script components 16

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load("@io_bazel_rules_kotlin./kotlin:kotlin.bzl", "kt_android_library") load("@rules_jvm_external./:defs.bzl", "artifact") kt_android_library( name = "my_lib", srcs = glob([ "src/main/java/**/*.kt", "src/main/kotlin/**/*.kt", ]), custom_package = "com.morfly.sample", manifest = "src/main/AndroidManifest.xml", resource_files = glob(["src/main/res/**"]), deps = [ artifact("androidx.core:core-ktx"), artifact("androidx.appcompat:appcompat"), artifact("com.google.android.material:material"), artifact("androidx.constraintlayout:constraintlayout"), ], ) Bazel script components 17

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load("@io_bazel_rules_kotlin./kotlin:kotlin.bzl", "kt_android_library") load("@rules_jvm_external./:defs.bzl", "artifact") kt_android_library( name = "my_lib", srcs = glob([ "src/main/java/**/*.kt", "src/main/kotlin/**/*.kt", ]), custom_package = "com.morfly.sample", manifest = "src/main/AndroidManifest.xml", resource_files = glob(["src/main/res/**"]), deps = [ artifact("androidx.core:core-ktx"), artifact("androidx.appcompat:appcompat"), artifact("com.google.android.material:material"), artifact("androidx.constraintlayout:constraintlayout"), ], ) Bazel script components 18

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load("@io_bazel_rules_kotlin./kotlin:kotlin.bzl", "kt_android_library") load("@rules_jvm_external./:defs.bzl", "artifact") kt_android_library( name = "my_lib", srcs = glob([ "src/main/java/**/*.kt", "src/main/kotlin/**/*.kt", ]), custom_package = "com.morfly.sample", manifest = "src/main/AndroidManifest.xml", resource_files = glob(["src/main/res/**"]), deps = [ artifact("androidx.core:core-ktx"), artifact("androidx.appcompat:appcompat"), artifact("com.google.android.material:material"), artifact("androidx.constraintlayout:constraintlayout"), ], ) Bazel script components 19

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load("@io_bazel_rules_kotlin./kotlin:kotlin.bzl", "kt_android_library") load("@rules_jvm_external./:defs.bzl", "artifact") kt_android_library( name = "my_lib", srcs = glob([ "src/main/java/**/*.kt", "src/main/kotlin/**/*.kt", ]), custom_package = "com.morfly.sample", manifest = "src/main/AndroidManifest.xml", resource_files = glob(["src/main/res/**"]), deps = [ artifact("androidx.core:core-ktx"), artifact("androidx.appcompat:appcompat"), artifact("com.google.android.material:material"), artifact("androidx.constraintlayout:constraintlayout"), ], ) Bazel script components 20

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load("@io_bazel_rules_kotlin./kotlin:kotlin.bzl", "kt_android_library") load("@rules_jvm_external./:defs.bzl", "artifact") kt_android_library( name = "my_lib", srcs = glob([ "src/main/java/**/*.kt", "src/main/kotlin/**/*.kt", ]), custom_package = "com.morfly.sample", manifest = "src/main/AndroidManifest.xml", resource_files = glob(["src/main/res/**"]), deps = [ artifact("androidx.core:core-ktx"), artifact("androidx.appcompat:appcompat"), artifact("com.google.android.material:material"), artifact("androidx.constraintlayout:constraintlayout"), ], ) Bazel script components 21

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load("@io_bazel_rules_kotlin./kotlin:kotlin.bzl", "kt_android_library") load("@rules_jvm_external./:defs.bzl", "artifact") kt_android_library( name = "my_lib", srcs = glob([ "src/main/java/**/*.kt", "src/main/kotlin/**/*.kt", ]), custom_package = "com.morfly.sample", manifest = "src/main/AndroidManifest.xml", resource_files = glob(["src/main/res/**"]), deps = [ artifact("androidx.core:core-ktx"), artifact("androidx.appcompat:appcompat"), artifact("com.google.android.material:material"), artifact("androidx.constraintlayout:constraintlayout"), ], ) Bazel script components 22

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load("@io_bazel_rules_kotlin./kotlin:kotlin.bzl", "kt_android_library") load("@rules_jvm_external./:defs.bzl", "artifact") kt_android_library( name = "my_lib", srcs = glob([ "src/main/java/**/*.kt", "src/main/kotlin/**/*.kt", ]), custom_package = "com.morfly.sample", manifest = "src/main/AndroidManifest.xml", resource_files = glob(["src/main/res/**"]), deps = [ artifact("androidx.core:core-ktx"), artifact("androidx.appcompat:appcompat"), artifact("com.google.android.material:material"), artifact("androidx.constraintlayout:constraintlayout"), ], ) Bazel script components 23

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load("@io_bazel_rules_kotlin./kotlin:kotlin.bzl", "kt_android_library") load("@rules_jvm_external./:defs.bzl", "artifact") kt_android_library( name = "my_lib", srcs = glob([ "src/main/java/**/*.kt", "src/main/kotlin/**/*.kt", ]), custom_package = "com.morfly.sample", manifest = "src/main/AndroidManifest.xml", resource_files = glob(["src/main/res/**"]), deps = [ artifact("androidx.core:core-ktx"), artifact("androidx.appcompat:appcompat"), artifact("com.google.android.material:material"), artifact("androidx.constraintlayout:constraintlayout"), ], ) Bazel script components 24

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What does the migration mean?

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What does the migration mean? build.gradle 26 build.gradle.kts BUILD.bazel

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What does the migration mean? 27 ./gradlew app:assembleDebug bazel build app

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Writing custom Gradle plugin for automated migration to Bazel

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Creating Gradle plugin Gradle plugin 29

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Creating Gradle plugin Gradle plugin Gradle task 30

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./gradlew migrateToBazel Gradle task 31

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Creating Gradle plugin Gradle plugin Gradle task Bazel file templates 32

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Gradle task purpose 33 For each project module…

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Gradle task purpose 34 For each project module… Get module dependencies

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Gradle task purpose 35 For each project module… Get module dependencies Determine Bazel module type

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Bazel module types ● Kotlin Android module — kt_android_library 36

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Bazel module types ● Kotlin Android module ● Pure Java Android module — android_library 37

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Bazel module types ● Kotlin Android module ● Pure Java Android module ● Kotlin library module — kt_jvm_library 38

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Bazel module types ● Kotlin Android module ● Pure Java Android module ● Kotlin library module ● Pure Java library module — java_library ... 39

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Gradle task purpose 40 For each project module… Get module dependencies Determine Bazel module type Generate Bazel file from a template

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Coding time...

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Android application 42

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import org.gradle.api.Plugin import org.gradle.api.Project import org.gradle.kotlin.dsl.register class MigrationPlugin : Plugin { override fun apply(target: Project) { target.tasks.register("migrateToBazel") } } Creating Gradle plugin 43

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import org.gradle.api.Plugin import org.gradle.api.Project import org.gradle.kotlin.dsl.register class MigrationPlugin : Plugin { override fun apply(target: Project) { target.tasks.register("migrateToBazel") } } Creating Gradle plugin 44

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import org.gradle.api.Plugin import org.gradle.api.Project import org.gradle.kotlin.dsl.register class MigrationPlugin : Plugin { override fun apply(target: Project) { target.tasks.register("migrateToBazel") } } Registering Gradle task 45

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import org.gradle.api.Plugin import org.gradle.api.Project import org.gradle.kotlin.dsl.register class MigrationPlugin : Plugin { override fun apply(target: Project) { target.tasks.register("migrateToBazel") } } Registering Gradle task 46

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import org.gradle.api.Plugin import org.gradle.api.Project import org.gradle.kotlin.dsl.register class MigrationPlugin : Plugin { override fun apply(target: Project) { target.tasks.register("migrateToBazel") } } Registering Gradle task 47

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./gradlew migrateToBazel Registering Gradle task 48

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import org.gradle.api.DefaultTask import org.gradle.api.tasks.TaskAction open class MigrateToBazelTask : DefaultTask() { @TaskAction fun migrateToBazel() { ... } } Writing Gradle task 49

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Demo

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Migration complications

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Migration complications ● Iterative process 52

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Migration complications ● Iterative process ● Gradle plugins 53

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Migration complications ● Iterative process ● Gradle plugins ● Edge cases 54

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Type-safe DSL for Bazel script generation

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fun android_library_template( name: String, packageName: String, artifactDeps: String, moduleDeps: String ) = """ load("@io_bazel_rules_kotlin./kotlin:kotlin.bzl", "kt_android_library") load("@rules_jvm_external./:defs.bzl", "artifact") kt_android_library( name = "$name", srcs = glob(["src/main/java/**/*.kt"]), resource_files = glob(["src/main/res/**"]), custom_package = "$packageName", manifest = "src/main/AndroidManifest.xml", visibility = ["./visibility:public"], deps = [ "./:dagger", $moduleDeps, $artifactDeps ], ) """ Bazel file template 56

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fun android_library_build( name: String, packageName: String, artifactDeps: List, moduleDeps: List ) = BUILD.bazel { load("@io_bazel_rules_kotlin./kotlin:kotlin.bzl", "kt_android_library") load("@rules_jvm_external./:defs.bzl", "artifact") kt_android_library( name = name, srcs = glob("src/main/java/**/*.kt"), resource_files = glob("src/main/res/**"), custom_package = packageName, manifest = "src/main/AndroidManifest.xml", visibility = list["./visibility:public"], deps = moduleDeps + artifactDeps.map { artifact(it) } ) } Type-safe Bazel file template 57

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BUILD.bazel { ... } Entering Starlark DSL context 58

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Code snippets notation further Kotlin (Starlark DSL) Starlark (Bazel script) 59

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load("@io_bazel_rules_kotlin./kotlin:kotlin.bzl", "kt_android_library") Starlark DSL components load statement 60

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BUILD.bazel { load("@io_bazel_rules_kotlin./kotlin:kotlin.bzl", "kt_android_library") ... } Starlark DSL components load statement 61

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NAME = "app" Starlark DSL components Variable assignment 62

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NAME = "app" SRCS = ["MainActivity.java", "MainViewModel.java"] Starlark DSL components Variable assignment 63

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NAME = "app" SRCS = ["MainActivity.java", "MainViewModel.java"] MANIFEST_VALUES = {"minSdkVersion": "21"} Starlark DSL components Variable assignment 64

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BUILD.bazel { val NAME by "app" ... } Starlark DSL components Variable assignment 65

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BUILD.bazel { val NAME by "app" val SRCS by list["MainActivity.java", "MainViewModel.java"] ... } Starlark DSL components Variable assignment 66

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BUILD.bazel { val NAME by "app" val SRCS by list["MainActivity.java", "MainViewModel.java"] val MANIFEST_VALUES by dict { "minSdkVersion" to "21" } ... } Starlark DSL components Variable assignment 67

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SRCS = ["MainActivity.java"] + ["ViewModel.java"] Starlark DSL components Concatenation 68

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BUILD.bazel { val SRCS by list["MainActivity.java"] `+` list["MainViewModel.java"] ... } Starlark DSL components Concatenation 69

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BUILD.bazel { val SRCS by list["MainActivity.java"] `+` list["MainViewModel.java"] ... } Starlark DSL components Concatenation 70

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BUILD.bazel { val SRCS: List by list["MainActivity.java"] `+` "MainViewModel.java" ... } Starlark DSL components Concatenation 71

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BUILD.bazel { val SRCS: List by list["MainActivity.java"] `+` "MainViewModel.java" Compilation error: Required: List Found: String ... } Starlark DSL components Concatenation 72

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[i + i for i in LIST] Starlark DSL components List comprehensions 73

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[ for in ] Starlark DSL components List comprehension syntax 74

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BUILD.bazel { "i" `in` LIST take { it `+` it } ... } Starlark DSL components List comprehensions 75

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BUILD.bazel { "i" `in` LIST take { it `+` it } ... } Starlark DSL components List comprehensions 76

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BUILD.bazel { "i" `in` LIST take { it `+` it } ... } Starlark DSL components List comprehensions 77

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BUILD.bazel { "i" `in` LIST take { it `+` it } ... } Starlark DSL components List comprehensions 78

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[i + i for i in LIST] Starlark DSL components List comprehensions 79

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SRCS = ["MainActivity.java"] android_binary( name = "app", srcs = SRCS + ["utils/StringUtils.java"] ) Starlark DSL components Build rules and functions 80

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BUILD.bazel { val SRCS by list["MainActivity.java"] android_binary( name = "app", srcs = SRCS `+` list["utils/StringUtils.java"] ) ... } Starlark DSL components Build rules and functions 81

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VIEW_MODELS_WITH_RES_IMPORTS = ["MainViewModel.kt", ...] [ genrule( name = "modify_imports_in_" + file[0:-3], srcs = [file], outs = [file[0:-3] + "_synthetic.kt"], cmd = """ cat $(SRCS) | sed 's/import com.morfly.R/import com.morfly.viewmodels.R/g' > $(OUTS) """, ) for file in VIEW_MODELS_WITH_RES_IMPORTS ] APP_FILES = [ "modify_imports_in_" + file[0:-3] for file in VIEW_MODELS_WITH_RES_IMPORTS ] Starlark DSL components Putting it all together… 82

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VIEW_MODELS_WITH_RES_IMPORTS = ["MainViewModel.kt", ...] [ genrule( name = "modify_imports_in_" + file[0:-3], srcs = [file], outs = [file[0:-3] + "_synthetic.kt"], cmd = """ cat $(SRCS) | sed 's/import com.morfly.R/import com.morfly.viewmodels.R/g' > $(OUTS) """, ) for file in VIEW_MODELS_WITH_RES_IMPORTS ] APP_FILES = [ "modify_imports_in_" + file[0:-3] for file in VIEW_MODELS_WITH_RES_IMPORTS ] Starlark DSL components Putting it all together… 83

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VIEW_MODELS_WITH_RES_IMPORTS = ["MainViewModel.kt", ...] [ genrule( name = "modify_imports_in_" + file[0:-3], srcs = [file], outs = [file[0:-3] + "_synthetic.kt"], cmd = """ cat $(SRCS) | sed 's/import com.morfly.R/import com.morfly.viewmodels.R/g' > $(OUTS) """, ) for file in VIEW_MODELS_WITH_RES_IMPORTS ] APP_FILES = [ "modify_imports_in_" + file[0:-3] for file in VIEW_MODELS_WITH_RES_IMPORTS ] Starlark DSL components Putting it all together… 84

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VIEW_MODELS_WITH_RES_IMPORTS = ["MainViewModel.kt", ...] [ genrule( name = "modify_imports_in_" + file[0:-3], srcs = [file], outs = [file[0:-3] + "_synthetic.kt"], cmd = """ cat $(SRCS) | sed 's/import com.morfly.R/import com.morfly.viewmodels.R/g' > $(OUTS) """, ) for file in VIEW_MODELS_WITH_RES_IMPORTS ] APP_FILES = [ "modify_imports_in_" + file[0:-3] for file in VIEW_MODELS_WITH_RES_IMPORTS ] Starlark DSL components Putting it all together… 85

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VIEW_MODELS_WITH_RES_IMPORTS = ["MainViewModel.kt", ...] [ genrule( name = "modify_imports_in_" + file[0:-3], srcs = [file], outs = [file[0:-3] + "_synthetic.kt"], cmd = """ cat $(SRCS) | sed 's/import com.morfly.R/import com.morfly.viewmodels.R/g' > $(OUTS) """, ) for file in VIEW_MODELS_WITH_RES_IMPORTS ] APP_FILES = [ "modify_imports_in_" + file[0:-3] for file in VIEW_MODELS_WITH_RES_IMPORTS ] Starlark DSL components Putting it all together… 86

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VIEW_MODELS_WITH_RES_IMPORTS = ["MainViewModel.kt", ...] [ genrule( name = "modify_imports_in_" + file[0:-3], srcs = [file], outs = [file[0:-3] + "_synthetic.kt"], cmd = """ cat $(SRCS) | sed 's/import com.morfly.R/import com.morfly.viewmodels.R/g' > $(OUTS) """, ) for file in VIEW_MODELS_WITH_RES_IMPORTS ] APP_FILES = [ "modify_imports_in_" + file[0:-3] for file in VIEW_MODELS_WITH_RES_IMPORTS ] Starlark DSL components Putting it all together… 87

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VIEW_MODELS_WITH_RES_IMPORTS = ["MainViewModel.kt", ...] [ genrule( name = "modify_imports_in_" + file[0:-3], srcs = [file], outs = [file[0:-3] + "_synthetic.kt"], cmd = """ cat $(SRCS) | sed 's/import com.morfly.R/import com.morfly.viewmodels.R/g' > $(OUTS) """, ) for file in VIEW_MODELS_WITH_RES_IMPORTS ] APP_FILES = [ "modify_imports_in_" + file[0:-3] for file in VIEW_MODELS_WITH_RES_IMPORTS ] Starlark DSL components Putting it all together… 88

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VIEW_MODELS_WITH_RES_IMPORTS = ["MainViewModel.kt", ...] [ genrule( name = "modify_imports_in_" + file[0:-3], srcs = [file], outs = [file[0:-3] + "_synthetic.kt"], cmd = """ cat $(SRCS) | sed 's/import com.morfly.R/import com.morfly.viewmodels.R/g' > $(OUTS) """, ) for file in VIEW_MODELS_WITH_RES_IMPORTS ] APP_FILES = [ "modify_imports_in_" + file[0:-3] for file in VIEW_MODELS_WITH_RES_IMPORTS ] Starlark DSL components Putting it all together… 89

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fun android_library_build( packageName: String, ... ) = BUILD { val VIEW_MODELS_WITH_RES_IMPORTS by list["MainViewModel.kt", ...] "file" `in` VIEW_MODELS_WITH_RES_IMPORTS take { file .> genrule( name = "modify_imports_in_" `+` file[0..-3], srcs = list[file], outs = list[file[0..-3] `+` "_synthetic.kt"], cmd = """ cat $(SRCS) | sed 's/import $packageName.R/import $packageName.viewmodels.R/g' > $(OUTS) """.trimIndent() ) } val APP_FILES by ("file" `in` VIEW_MODELS_WITH_RES_IMPORTS take { "modify_imports_in_" `+` it[0..-3] }) } Starlark DSL components Putting it all together… 90

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fun android_library_build( packageName: String, ... ) = BUILD { val VIEW_MODELS_WITH_RES_IMPORTS by list["MainViewModel.kt", ...] "file" `in` VIEW_MODELS_WITH_RES_IMPORTS take { file .> genrule( name = "modify_imports_in_" `+` file[0..-3], srcs = list[file], outs = list[file[0..-3] `+` "_synthetic.kt"], cmd = """ cat $(SRCS) | sed 's/import $packageName.R/import $packageName.viewmodels.R/g' > $(OUTS) """.trimIndent() ) } val APP_FILES by ("file" `in` VIEW_MODELS_WITH_RES_IMPORTS take { "modify_imports_in_" `+` it[0..-3] }) } Starlark DSL components Putting it all together… 91

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fun android_library_build( packageName: String, ... ) = BUILD { val VIEW_MODELS_WITH_RES_IMPORTS by list["MainViewModel.kt", ...] "file" `in` VIEW_MODELS_WITH_RES_IMPORTS take { file .> genrule( name = "modify_imports_in_" `+` file[0..-3], srcs = list[file], outs = list[file[0..-3] `+` "_synthetic.kt"], cmd = """ cat $(SRCS) | sed 's/import $packageName.R/import $packageName.viewmodels.R/g' > $(OUTS) """.trimIndent() ) } val APP_FILES by ("file" `in` VIEW_MODELS_WITH_RES_IMPORTS take { "modify_imports_in_" `+` it[0..-3] }) } Starlark DSL components Putting it all together… 92

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Conclusions

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Conclusions Auto-migration components 94 ● Gradle Plugin

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Conclusions Auto-migration components ● Gradle Plugin ● Type-safe DSL for Bazel script generation 95

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Source code 96 Sample project source code github.com/morfly/ android-bazel-migration-sample

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Source code 97 A tool for auto-migration to Bazel github.com/morfly/airin Sample project source code github.com/morfly/ android-bazel-migration-sample