# Swift Library

version 9.8.0-milestone-2

The Swift Library Plugin provides the tasks, configurations and conventions for building a Swift library. In particular, a Swift library provides functionality that can be used by consumers (i.e., other projects using this plugin or the Swift Application Plugin).

## Usage

build.gradle.kts

    plugins {
        `swift-library`
    }

build.gradle

    plugins {
        id 'swift-library'
    }

## Build variants

The Swift Library Plugin understands the following dimensions. Read the introduction to build variants for more information.

Build types - always set to debug and release

  The build type controls the debuggability as well as the optimization of the generated binaries.

  - `debug` - Generate debug symbols and don’t optimize the binary

  - `release` - Generate debug symbols and optimize, but extract the debug symbols from the binary

Linkages - default to shared

  The linkage expresses whether a shared library or static library should be created. Libraries can produce a shared library, a static library or both.

The linkage can be configured as follows:

build.gradle.kts

    library {
        linkage = listOf(Linkage.STATIC, Linkage.SHARED)
    }

build.gradle

    library {
        linkage = [Linkage.STATIC, Linkage.SHARED]
    }

Target machines - defaults to the build host

  The target machine expresses which machines the application expects to run. A target machine is identified by its operating system and architecture. Gradle uses the target machine to decide which tool chain to choose based on availability on the host machine.

The target machine can be configured as follows:

build.gradle.kts

    library {
        targetMachines = listOf(machines.linux.x86_64, machines.macOS.x86_64)
    }

build.gradle

    library {
        targetMachines = [
            machines.linux.x86_64,
            machines.macOS.x86_64
        ]
    }

## Tasks

The following diagram shows the relationships between tasks added by this plugin.

Note the default linkage of a Swift library is shared linkage as shown in the diagram.

Figure 1. Swift Library Plugin default task graph

With static linkage, the diagram changes to the following:

Figure 2. Swift Library Plugin static library only task graph

### Variant-dependent Tasks

The Swift Library Plugin creates tasks based on variants of the library component. Read the introduction to build variants for more information. The following diagrams show the relationship between variant-dependent tasks.

Figure 3. Swift Library Plugin variant-dependent task graph

| Depending on the linkage property

`compileVariantSwift` (e.g. `compileDebugSwift` and `compileReleaseSwift`) - SwiftCompile

  Depends on: All tasks that contribute source files to the compilation :: Compiles Swift source files using the selected compiler.

`linkVariant` (e.g. `linkDebug` and `linkRelease`) - LinkSharedLibrary (shared linkage)

  Depends on: All tasks which contribute to the link libraries, including `linkVariant` and `createVariant` tasks from projects that are resolved via project dependencies :: Links shared library from compiled object files using the selected linker.

`createVariant` (e.g. `createDebug` and `createRelease`) - CreateStaticLibrary (static linkage)

  Creates static library from compiled object files using selected archiver

`assembleVariant` (e.g. `assembleDebug` and `assembleRelease`) - Task (lifecycle)

  Depends on: `linkVariant` (shared linkage) or `createVariant` (static linkage) :: Aggregates tasks that assemble the specific variant of this library.

### Lifecycle Tasks

The Swift Library Plugin attaches some of its tasks to the standard lifecycle tasks documented in the Base Plugin chapter — which the Swift Library Plugin applies automatically:

`assemble` - Task (lifecycle)

  Depends on: `linkDebug` when linkage includes `shared` or `createDebug` otherwise. :: Aggregate task that assembles the debug variant of the shared library (if available) for the current host (if present) in the project. This task is added by the Base Plugin.

`check` - Task (lifecycle)

  Aggregate task that performs verification tasks, such as running the tests. Some plugins add their own verification task to `check`. For example, the XCTest Plugin attaches its test task to this lifecycle task. This task is added by the Base Plugin.

`build` - Task (lifecycle)

  Depends on: `check`, `assemble` :: Aggregate task that performs a full build of the project. This task is added by the Base Plugin.

`clean` - Delete

  Deletes the build directory and everything in it, i.e. the path specified by the `layout.buildDirectory` project property. This task is added by the Base Plugin.

## Dependency management

Just like the tasks created by the Swift Library Plugin, multiple configurations are created based on the variants of the library component. Read the introduction to build variants for more information. The following graph describes the configurations added by the Swift Library Plugin:

Figure 4. Swift Library Plugin configurations

- The configurations in white are the ones a user should use to declare dependencies

- The configurations in pink, also known as consumable denoted by (C), are the ones used when a component compiles, links, or runs against the library

- The configurations in blue, also known as resolvable denoted by (R), are internal to the component, for its own use

The following configurations are used to declare dependencies:

`api`

  Used for declaring API dependencies (see API vs implementation section). This is where you should declare dependencies which are transitively exported to consumers, for compile and link.

`implementation` extends `api`

  Used for declaring implementation dependencies for all variants of the main component (see API vs implementation section). This is where you should declare dependencies which are purely internal and not meant to be exposed to consumers of any variants.

`mainVariantImplementation` (e.g. `mainDebugImplementation` and `mainReleaseImplementation`) extends `implementation`

  Used for declaring implementation dependencies for a specific variant of the main component (see API vs implementation section). This is where you should declare dependencies which are purely internal and not meant to be exposed to consumers of this specific variant.

The following configurations are used by consumers:

`variantSwiftApiElements` (e.g. `debugSwiftApiElements` and `releaseSwiftApiElements`) extends `mainVariantImplementation`

  Used for compiling against the library. This configuration is meant to be used by consumers, to retrieve all the elements necessary to compile against the library.

`variantLinkElements` (e.g. `debugLinkElements` and `releaseLinkElements`) extends `mainVariantImplementation`

  Used for linking against the library. This configuration is meant to be used by consumers, to retrieve all the elements necessary to link against the library.

`variantRuntimeElements` (e.g. `debugRuntimeElements` and `releaseRuntimeElements`) extends `mainVariantImplementation`

  Used for executing the library. This configuration is meant to be used by consumers, to retrieve all the elements necessary to run against the library.

The following configurations are used by the library itself:

`swiftCompileVariant` (e.g. `swiftCompileDebug` and `swiftCompileRelease`) extends `mainVariantImplementation`

  Used for compiling the library. This configuration contains the compile include roots of the library and is therefore used when invoking the Swift compiler to compile it.

`nativeLinkVariant` (e.g. `nativeLinkDebug` and `nativeLinkRelease`) extends `mainVariantImplementation`

  Used for linking the shared library only. This configuration contains the libraries of the library and is therefore used when invoking the Swift linker to link it.

`nativeRuntimeVariant` (e.g. `nativeRuntimeDebug` and `nativeRuntimeRelease`) extends `mainVariantImplementation`

  Used for executing the library. This configuration contains the runtime libraries of the library.

### API vs implementation

The plugin exposes two configurations that can be used to declare dependencies: `api` and `implementation`. The `api` configuration should be used to declare dependencies which are exported by the library API, whereas the `implementation` configuration should be used to declare dependencies which are internal to the component.

build.gradle.kts

    library {
        dependencies {
            // FIXME: Put real deps here.
            api("io.qt:core:5.1")
            implementation("io.qt:network:5.1")
        }
    }

build.gradle

    library {
        dependencies {
            // FIXME: Write better deps here.
            api "io.qt:core:5.1"
            implementation "io.qt:network:5.1"
        }
    }

Dependencies appearing in the `api` configurations will be transitively exposed to consumers of the library, and as such will appear on the compile include root and link libraries of consumers. Dependencies found in the `implementation` configuration will, on the other hand, not be exposed to consumers, and therefore not leak into the consumer’s compile include root and link libraries. This comes with several benefits:

- dependencies do not leak into the compile include roots and link libraries of consumers, so they can never accidentally depend on a transitive dependency

- faster compilation thanks to the reduced include roots and link libraries

- fewer recompilations when implementation dependencies change since the consumer would not need to be recompiled

## Conventions

The Swift Library Plugin adds conventions for sources and tasks, shown below.

### Project layout

The Swift Library Plugin assumes the project layout shown below. None of these directories need to exist or have anything in them. The Swift Library Plugin will compile whatever it finds and ignore anything missing.

`src/main/swift`

  Swift source with extension of `.swift`

You configure the project layout by configuring the `source` on the `library` script block.

### `compileVariantSwift` Task

The Swift Library Plugin adds a SwiftCompile instance for each variant of the library component to build (e.g. `compileDebugSwift` and `compileReleaseSwift`). Read the introduction to build variants for more information. Some of the most common configuration options are shown below.

compilerArgs | []
debuggable | `true`
modules | `configurations.swiftCompileVariant`
macros | []
objectFileDir | `layout.buildDirectory.dir("obj/main/$variant")`
optimized | `false` for debug build type or `true` otherwise
source | `library.swiftSource`
targetPlatform | derived from the `TargetMachine` of the binary
toolChain | automatically selected based on target machine

### `linkVariant` Task

The Swift Library Plugin adds a LinkSharedLibrary instance for each variant of the library containing shared linkage as a dimension - e.g. `linkDebug` and `linkRelease`. Read the introduction to build variants for more information. Some of the most common configuration options are shown below.

debuggable | `true`
libs | `configurations.nativeLinkVariant`
linkedFile | `layout.buildDirectory.dir("lib/main/$variant/libModule[.so|dylib]")`
linkerArgs | []
source | `compileVariantSwift.objects`
targetPlatform | derived from the `TargetMachine` of the binary
toolChain | automatically selected based on target machine

### `createVariant` Task

The Swift Library Plugin adds a CreateStaticLibrary instance for each variant of the library containing static linkage as a dimension - e.g. `createDebug` and `createRelease`. Read the introduction to build variants for more information. Some of the most common configuration options are shown below.

outputFile | `layout.buildDirectory.dir("lib/main/$variant/libModule.a")`
source | `compileVariantSwift.objects`
staticLibArgs | []
targetPlatform | derived from the `TargetMachine` of the binary
toolChain | automatically selected based on target machine