Gradle Project Reference

Gradle is the build system for Android, and is the equivalent of SwiftPM for Darwin Swift development. When Skip builds your project, it bridges and/or transpiles your Swift code to Kotlin, and it also converts your Package.swift file into a build.gradle.kts file in the Skip plugin’s output directory. This conversion happens transitively on all your project’s Skip-enabled dependencies, including the Skip Core Frameworks modules. This results in a local inter-connected network of Gradle projects containing the entirety of your project’s source dependency tree.

The following sections detail Skip’s use of Gradle, including how to build with Gradle from the command line.

Skip App Projects

Consider a minimal Swift app project created by the commands:

% brew install skiptools/skip/skip
% skip init --show-tree --no-build --no-test --no-module-tests --free --icon-color '' --appid=skip.hello.App --version 1.0.0 hello-skip HelloSkip

.
├─ Android
│  ├─ app
│  │  ├─ build.gradle.kts
│  │  ├─ proguard-rules.pro
│  │  └─ src
│  │     └─ main
│  │        ├─ AndroidManifest.xml
│  │        └─ kotlin
│  │           └─ hello
│  │              └─ skip
│  │                 └─ Main.kt
│  ├─ gradle
│  │  └─ wrapper
│  │     └─ gradle-wrapper.properties
│  ├─ gradle.properties
│  └─ settings.gradle.kts
├─ Darwin
│  ├─ Assets.xcassets
│  │  ├─ AccentColor.colorset
│  │  │  └─ Contents.json
│  │  ├─ AppIcon.appiconset
│  │  │  └─ Contents.json
│  │  └─ Contents.json
│  ├─ Entitlements.plist
│  ├─ HelloSkip.xcconfig
│  ├─ HelloSkip.xcodeproj
│  │  └─ project.pbxproj
│  └─ Sources
│     └─ HelloSkipAppMain.swift
├─ LICENSE.LGPL
├─ Package.swift
├─ README.md
├─ Skip.env
└─ Sources
   └─ HelloSkip
      ├─ ContentView.swift
      ├─ HelloSkip.swift
      ├─ HelloSkipApp.swift
      ├─ Resources
      │  └─ Localizable.xcstrings
      └─ Skip
         └─ skip.yml

Running swift build in the hello-skip folder builds the module and performs the Android translation.

This derived project hierarchy will look like:

hello-skip % tree .build/plugins/outputs/hello-skip/HelloSkip/skipstone/

.build/plugins/outputs/hello-skip/HelloSkip/skipstone/
├── HelloSkip
│   ├── build.gradle.kts
│   ├── proguard-rules.pro
│   └── src
│       └── main
│           ├── kotlin
│           │   └── hello
│           │       └── skip
│           │           ├── ContentView.kt
│           │           ├── HelloSkip.kt
│           │           └── HelloSkipApp.kt
│           └── resources
│               └── hello
│                   └── skip
│                       └── Resources
│                           ├── Localizable.xcstrings -> /private/tmp/hello-skip/Sources/HelloSkip/Resources/Localizable.xcstrings
│                           └── resources.lst
├── HelloSkip.skipcode.json
├── SkipFoundation -> ../../../skip-foundation/SkipFoundation/skipstone/SkipFoundation
├── SkipLib -> ../../../skip-lib/SkipLib/skipstone/SkipLib
├── SkipModel -> ../../../skip-model/SkipModel/skipstone/SkipModel
├── SkipUI -> ../../../skip-ui/SkipUI/skipstone/SkipUI
├── SkipUnit -> ../../../skip-unit/SkipUnit/skipstone/SkipUnit
├── gradle
│   └── wrapper
│       └── gradle-wrapper.properties
├── gradle.properties
└── settings.gradle.kts

The resulting Gradle project hierarchy contains not only the links to the plugin output for the current project, but also links to the project output for each dependent module. These dependencies are automatically created as a result of the Skip plugin being run on the dependent projects.

Building Locally

Gradle projects can be run manually, either from Android Studio or using the gradle command-line tool. Skip app projects created with skip init have a root Android/ folder that contains a settings.gradle.kts file. This can be opened in Android Studio, or it can be run directly from the command line.

$ skip init --appid=com.xyz.HelloSkip hello-skip HelloSkip

Initializing Skip library hello-skip
[✓] Create project hello-skip (0.73s)
[✓] Resolve dependencies (5.89s)
[✓] Build hello-skip (15.2s)
[✓] Created module HelloSkip in ~/Desktop/hello-skip/HelloSkip.xcodeproj

$ cd hello-skip

$ swift build --build-tests

Fetched https://source.skip.tools/skip.git (0.92s)
…
[294/294] Linking libHelloSkip.dylib
Build complete! (23.13s)

$ skip gradle -p .build/plugins/outputs/hello-skip/HelloSkip/skipstone/HelloSkip test

GRADLE> > Task :HelloSkip:preBuild UP-TO-DATE
…
GRADLE> > Task :HelloSkip:testDebugUnitTest
GRADLE> > Task :HelloSkip:test
GRADLE>
GRADLE> BUILD SUCCESSFUL in 20s
GRADLE> 204 actionable tasks: 204 executed
note: Gradle SUCCESSFUL

$ skip gradle -p Android/ assemble

GRADLE> > Task :app:preBuild UP-TO-DATE
GRADLE> > Task :app:preDebugBuild UP-TO-DATE
GRADLE> > Task :app:mergeDebugNativeDebugMetadata NO-SOURCE
GRADLE> > Task :app:generateDebugBuildConfig
GRADLE> > Task :HelloSkip:preBuild UP-TO-DATE
…
GRADLE> > Task :SkipUI:bundleReleaseAar
GRADLE> > Task :HelloSkip:compileDebugJavaWithJavac NO-SOURCE
…
GRADLE> > Task :app:processReleaseJavaRes
GRADLE> > Task :HelloSkip:extractDebugAnnotations
GRADLE> > Task :HelloSkip:mergeDebugGeneratedProguardFiles
GRADLE> > Task :HelloSkip:mergeDebugConsumerProguardFiles
GRADLE> > Task :SkipUI:assembleRelease
GRADLE> > Task :SkipUI:assemble
GRADLE> > Task :HelloSkip:mergeDebugJavaResource
GRADLE> > Task :app:compileDebugKotlin
GRADLE> > Task :HelloSkip:syncDebugLibJars
GRADLE> > Task :app:compileDebugJavaWithJavac
GRADLE> > Task :app:mergeReleaseJavaResource
GRADLE> > Task :app:dexBuilderDebug
GRADLE> > Task :app:processDebugJavaRes
GRADLE> > Task :app:mergeDebugGlobalSynthetics
GRADLE> > Task :app:mergeProjectDexDebug
GRADLE> > Task :app:mergeLibDexDebug
GRADLE> > Task :HelloSkip:bundleDebugAar
GRADLE> > Task :HelloSkip:assembleDebug
GRADLE> > Task :HelloSkip:assemble
GRADLE> > Task :app:mergeDebugJavaResource
GRADLE> > Task :app:minifyReleaseWithR8
GRADLE> > Task :app:shrinkReleaseRes
GRADLE> > Task :app:packageRelease
GRADLE> > Task :app:createReleaseApkListingFileRedirect
GRADLE> > Task :app:assembleRelease
GRADLE> > Task :app:assemble
GRADLE>
GRADLE> BUILD SUCCESSFUL in 53s
GRADLE> 403 actionable tasks: 403 executed

The .apk from the build will be placed in the .build/Android/app/outputs/apk/ folder:

$ ls -lah .build/Android/app/outputs/apk/*/*.apk

57M .build/Android/app/outputs/apk/debug/app-debug.apk
13M .build/Android/app/outputs/apk/release/app-release-unsigned.apk

Skip Frameworks

A valid Skip project must contain a Skip/skip.yml file in each Skip-enabled module. This file is a YAML document that describes the module’s representation in the Android/Gradle world. During the build process, this file is combined with all the dependency project’s skip.yml files and used to affect how the Gradle project is created. Use skip.yml to customize the Gradle build process, or to add dependencies on external Java or Kotlin libraries that you can then use directly from Skip code.

Gradle projects use the files settings.gradle.kts and build.gradle.kts for configuration, much like a Swift Package Manager project uses Package.swift. These are kotlin-script files that can configure all aspects of the build process, from compilation arguments to output post-processing. More can be read on the topic at https://docs.gradle.org/current/userguide/writing_build_scripts.html ↗.

Skip abstracts away the details of the Gradle project configuration for Framework projects by automatically deriving a valid Gradle project from the SwiftPM Package.swift dependencies combined with the contents of each project’s Skip/skip.yml file. The Skip build plugin outputs a Gradle project folder hierarchy as part of its output process.

Framework Build Outputs

Framework projects differ from app projects in that while app projects surface the various Gradle app-level build files (Android/settings.gradle.kts and Android/app/build.gradle.kts), framework projects generate and maintain the build structures for the generated Android project. Skip considers the generated Gradle project, along with any generated Kotlin code, to be ephemeral. Cleaning the build may delete the files, and re-running the transpiler will override any changes. Unless you (manually) export or archive the Gradle project, it may be overwritten at any time.

When building from the command-line using swift build or skip test, the gradle projects will be output to the .build/plugins/outputs/ folder, each in its own package-named directory. When building from Xcode, the outputs are instead placed in ~/Library/Developer/Xcode/DerivedData/ProjectName-identifier/SourcePackages/plugins/.

The Skip plugin is run in a restrictive sandbox that limits the output folders that the plugin can write to. For this reason, each separate dependency is output to its own separate folder, and each depending project references it by making a symbolic link to the local output for that project. For example, the HelloSkip app’s local dependencies on the Skip Core frameworks can be seen:

$ tree -L 5 .build/plugins/outputs

.build/plugins/outputs
├── hello-skip
│   └── HelloSkip
│       └── skipstone
│           ├── HelloSkip
│           │   ├── build.gradle.kts
│           │   ├── proguard-rules.pro
│           │   └── src
│           ├── HelloSkip.skipcode.json
│           ├── SkipFoundation -> ../../../skip-foundation/SkipFoundation/skipstone/SkipFoundation
│           ├── SkipLib -> ../../../skip-lib/SkipLib/skipstone/SkipLib
│           ├── SkipModel -> ../../../skip-model/SkipModel/skipstone/SkipModel
│           ├── SkipUI -> ../../../skip-ui/SkipUI/skipstone/SkipUI
│           ├── SkipUnit -> ../../../skip-unit/SkipUnit/skipstone/SkipUnit
│           ├── gradle
│           │   └── wrapper
│           ├── gradle.properties
│           └── settings.gradle.kts
├── skip-foundation
│   └── SkipFoundation
│       └── skipstone
│           ├── SkipFoundation
│           │   ├── build.gradle.kts
│           │   ├── proguard-rules.pro
│           │   └── src
│           ├── SkipFoundation.skipcode.json
│           ├── SkipLib -> ../../../skip-lib/SkipLib/skipstone/SkipLib
│           ├── SkipUnit -> ../../../skip-unit/SkipUnit/skipstone/SkipUnit
│           ├── gradle
│           │   └── wrapper
│           ├── gradle.properties
│           └── settings.gradle.kts
├── skip-lib
│   └── SkipLib
│       └── skipstone
│           ├── SkipLib
│           │   ├── build.gradle.kts
│           │   ├── proguard-rules.pro
│           │   └── src
│           ├── SkipLib.skipcode.json
│           ├── SkipUnit -> ../../../skip-unit/SkipUnit/skipstone/SkipUnit
│           ├── gradle
│           │   └── wrapper
│           ├── gradle.properties
│           └── settings.gradle.kts
├── skip-model
│   └── SkipModel
│       └── skipstone
│           ├── SkipLib -> ../../../skip-lib/SkipLib/skipstone/SkipLib
│           ├── SkipModel
│           │   ├── build.gradle.kts
│           │   ├── proguard-rules.pro
│           │   └── src
│           ├── SkipModel.skipcode.json
│           ├── SkipUnit -> ../../../skip-unit/SkipUnit/skipstone/SkipUnit
│           ├── gradle
│           │   └── wrapper
│           ├── gradle.properties
│           └── settings.gradle.kts
├── skip-ui
│   └── SkipUI
│       └── skipstone
│           ├── SkipFoundation -> ../../../skip-foundation/SkipFoundation/skipstone/SkipFoundation
│           ├── SkipLib -> ../../../skip-lib/SkipLib/skipstone/SkipLib
│           ├── SkipModel -> ../../../skip-model/SkipModel/skipstone/SkipModel
│           ├── SkipUI
│           │   ├── build.gradle.kts
│           │   ├── proguard-rules.pro
│           │   └── src
│           ├── SkipUI.skipcode.json
│           ├── SkipUnit -> ../../../skip-unit/SkipUnit/skipstone/SkipUnit
│           ├── gradle
│           │   └── wrapper
│           ├── gradle.properties
│           └── settings.gradle.kts
└── skip-unit
    └── SkipUnit
        └── skipstone
            ├── SkipUnit
            │   ├── build.gradle.kts
            │   ├── proguard-rules.pro
            │   └── src
            ├── SkipUnit.skipcode.json
            ├── gradle
            │   └── wrapper
            ├── gradle.properties
            └── settings.gradle.kts

Building an app project APK

Building a framework project AAR

For more information on the published output format Publishing a project as module ↗ and Android Gradle Plugin Upload your library ↗.

Gradle Dependencies

Every Skip App project depends on the Skip Core Frameworks, which contain the five core modules for any application, and whose skip.yml files can serve as useful examples: SkipUI ↗, SkipModel ↗, SkipFoundation ↗, SkipLib ↗, and SkipUnit ↗. Each of these projects are automatically transpiled in turn, resulting in a web of project links that constitute a buildable Gradle project. As these are dependencies that are locally checked out, their transpiled output can be referenced from Gradle through the relative symbolic links that are created as part of a module’s transpilation.

Read the Dependencies documentation to learn more about using skip.yml to add Java/Kotlin dependencies.