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Whilst not perfect, the new Swift Package Manager Plugins provide most of the facilities that my Builder experiment was aimed at. Therefore, this tool will be archived.


The Builder executable built by this package is intended to be used to build other Swift Package Manager packages.

Please leave comments and suggestions on the Swift forums, or as issues in github.


The Swift Package Manager is a capable package manager, but at the moment it's quite basic as a build system. It uses llbuild under the hood, but currently it doesn't expose much beyond the ability to build/run/test a package.

In particular, it currently doesn't support running scripts or other tools as part of the build, or have a way to specify configuration settings in bulk and apply them uniformly, other than specifying each one on the command line.

This project is a proof-of-concept implementation of a meta-builder for the Swift Package Manager. It illustrates one way that settings configuration and custom build phases could be added.

It was originally inspired by a couple of discussions in the Swift forums (here and here), but has grown legs a bit since then.

The approach taken was deliberately chosen to work with the current abilities of SwiftPM, so that this could be a standalone tool that sits on top of SwiftPM and uses it. This also means that you can bootstrap the build process by using SwiftPM to build this tool.

Since I created this project, the Swift team have announced their own solution for extensible build tools, which will probably make this tool obsolete once it exists.

In the meantime though, it's an interesting experiment, and still has some other capabilities that are currently missing from SwiftPM.


An example package can be found at BuilderExample.

To see builder in action:

git clone ""
cd BuilderExample
swift run builder run

What this does is clone the example, then build & run builder (which is a dependency).

Running builder then builds and runs the Example target.

Wasn't that fun?


Builder is invoked as follows: builder <action>. If not supplied, action defaults to build (more on actions later).

Builder works by looking for a special target called Configure in the package that it's trying to build. You define this target in the Package.swift file just like any other target. It should contain a main.swift file, so that builder can run it.

Assuming that it finds the Configure target, builder uses swift run to build and run it, and capture its output.

All being well, this output should be a valid JSON dictionary, which builder parses to obtain a configuration.

This configuration consists of a settings section, and one or more action definitions.

Builder looks for an action definition matching the name that was supplied on the command line (or build if none was supplied). If present, this action definition describes a set of commands to execute in order to perform the action.

These commands can consist of:

  • build: build a product using swift build, applying the build settings from the configuration
  • test: test a product using swift test, applying the build settings from the configuration
  • run: run a product using swift run, applying the build settings from the configuration
  • action: perform another nested action
  • anything else: treat the named command as a dependency; build and run it with swift run, passing the arguments specified in the configuration

The idea behind this approach is that:

  • by moving the code for calculating the configuration into its own swift executable, it should be possible to eliminate any non-deterministic swift from the manifest
  • dependent packages required to determine the configuration can be listed in the manifest as dependencies of the Configure target, and will be fetched and built in the normal way
  • any tools required for custom build steps can also be listed in the manifest as dependencies
  • although the data format output by the configuration tool needs to be fixed, the actual implementation of it is decoupled from Swift itself, making it easier to support multiple different systems

By adding Builder as a configuration to a project, the entire chain of build dependencies, including Builder itself and additional tools (such as protobuf), can be fetched and built locally by the package manager itself.

This is intended to result in a stable build environment, which doesn't rely on having to pre-install tools globally.

It also makes it simple for people to share re-usable build tools, since they're just Swift packages - hopefully this will encourage people to write cross-platform Swift tools (or wrap other tools in Swift), and result in a range of off-the-shelf packages to suit most needs.

Of course, if you do have special requirements, such as tools that need to be installed or run with brew/apt-get/npm/whatever, then that's no problem either. You just build a Swift tool to perform the install, and do anything that you need to.

Dynamic Configuration

In this demo, the Configure target from the example project looks like this:

import BuilderConfiguration

let settings = Settings(specs: [
        values: [
          .setting("definition", "example")
        inherits: [
            .spec(name: "mac", filter: ["macOS"]),
            .spec(name: "debug", filter: ["debug"])
        name: "mac",
        values: [
          .setting("minimum-target", "macosx10.12"),
        name: "debug",
        values: [
          .setting("optimisation", "none")

let configuration = Configuration(
    settings: settings,
    actions: [
        .action(name:"build", phases:[
            .toolPhase(name:"Preparing", tool: "BuilderToolExample"),
            .buildPhase(name:"Building", target:"BuilderExample"),
            .toolPhase(name:"Packaging", tool: "BuilderToolExample", arguments:["--show-environment", "--show-arguments"]),
        .action(name:"test", phases:[
            .testPhase(name:"Testing", target:"BuilderExample"),
        .action(name:"run", phases:[
            .actionPhase(name:"Building", action: "build"),
            .toolPhase(name:"Running", tool: "run", arguments:["BuilderExample"]),


The job of the Configure target is to output a JSON description of the configuration.

There are lots of ways to accomplish this goal: just writing code to print the JSON directly, making a dictionary and converting it to JSON then printing that, loading it as text from a file and printing that, etc.

In this example we make use of some utility code defined in the BuilderConfiguration module1, which lets us write Settings and Configuration definitions in a similar way to SwiftPM's Package descriptions.

In the Settings part we supply a basic settings scheme which always adds a single Swift setting: "definition" : "example".

We also define a couple of other settings schemes which are optionally mixed in, depending on filters. If the platform is macOS, we mix in a target setting. If the configuration is debug we mix in an optimiser setting.

In the Configuration part we describe three build schemes: "build", "test" and "run". These list the build phases to execute when builder is invoked with the build, test or run actions2.

In the "build" phase, we see an example of calling out to an external tool: BuilderToolExample. This tool is itself another external dependency.

Hopefully this example illustrates a few things:

  • the configuration can be generated dynamically (by running code, including external dependencies) and therefore could change based on the environment it's run in
  • the code that generates the configuration can make use of dependencies
  • we can also fetch, compile and run arbitrary tools as part of the overall build process

Mapping Settings

In the description above, we skirted over the details of settings.

How does a setting such as "minimum-target" : "macosx10.12" get passed along to an actual tool?

The answer is that there's a translation layer, and it's a per-tool translation.

For the swift compiler, the setting above will be translated to: -Xswiftc, -target, -Xswiftc, x86_64-apple-macosx10.12.

There are other tools, however, which might also want to know about the minimum target.

A tool that generates an xcconfig file could translate the same setting to: MACOSX_DEPLOYMENT_TARGET = 10.12.

An application bundling tool could take the value 10.12 and insert it into the LSMinimumSystemVersion key of the Info.plist file.

And so on.

So where do these settings maps live? Right now, they're very minimal, and are hard-coded into the builder binary. In future versions of builder they will be external, and extensible, so that builder can be taught about new settings and new tools, in a flexible manner.


Caveat Emptor

I hacked this together as a quick demo, although it's evolved a bit since then.

It builds for me on MacOS and Linux - your mileage may vary.

Lots of things have been glossed over, including:

  • passing in useful environment to the helper tools (Configure and Tool)
  • niceties such as error checking, help, etc, etc...
  • generating a fully-functional xcode project with
    • dependencies and sub-projects to build the tools
    • an xcconfig file built from the settings, applied to the products
    • build phases to run the tools as part of the build (this is tricky, but by no means impossible)
  • the configuration and tool items are defined as targets, but built/run as products. This seems to work but is probably unsupported behaviour.

Declarative Shmarative

A stated aim of the package format is to create a declarative model of the package, which SwiftPM can use to build it.

What Builder is doing arguably defeats this.

I can see why a fully declarative model is attractive, but by definition it requires a rich enough language to be able to describe all the ways that any product can be built.

Since this is effectively infinite in scope, it's a pretty hard problem to solve - there are a lot of weird requirements and custom build steps out there.

The fact that the package format includes an optional section of additional code seems to acknowledge this flaw itself, and in my view this is a necessary evil currently, and probably always will be.

As such, I actually think that Builder provides a cleaner way to execute arbitrary swift code during the build that the optional section does. At least with Builder the custom code lives outside the Package.swift file, and in fact is neatly packaged up into its own package(s) which can safely be re-usable, have dependencies, and be arbitrarily complex.

Dependency Checking

To quote the original community proposal for SwiftPM:

Support for Other Build Systems

We are considering supporting hooks for the Swift Package Manager to call out to other build systems, and/or to invoke shell scripts.

Adding this feature would further improve the process of adapting existing libraries for use in Swift code.

We intend to tread cautiously here, as incorporating shell scripts and other external build processes significantly limits the ability of the Swift Package Manager to understand and analyze the build process.

In effect what this tool supplies is those hooks, but instead of living inside SwiftPM, they exist on the outside.

An obvious flaw with our approach is the one alluded to in the final paragraph of the quote.

The SwiftPM build system itself is backed by llbuild, thus can efficiently re-build only the things that it has to. If one file changes, only things that depend on it need be rebuilt.

The custom phases that we execute live outside of the SwiftPM build system. It has no knowledge of them, and so we can't take advantage of this behaviour.

Let's say we add a custom build phase which runs protobuf and creates some Swift source files from a model. If we're lucky, SwiftPM may be smart enough to only recompile the generated source files that have actually changed.

Builder however will not be so smart and will run protobuf every time, regardless of whether the model has changed.

Clearly protobuf could implement its own dependency checking, as could every custom tool. Not ideal.

Possibly we could extend Builder's model in a way that allows custom tools to describe their dependencies, so that we could provide the dependency logic for all custom tools, maybe even backed by llbuild. Xcode's custom build phases attempts to do this, allowing you to describe a list of input and output files, but it's not very flexible and requires files to be listed individually. A decent system would require good pattern matching and the ability to write general rules...

If we do that then rather than just enhancing the SwiftPM build system, we're at least halfway towards re-implementing it.

What we really need is to be able to hook into SwiftPM's own use of llbuild at a level where it can invoke our custom phases for us, when it knows that it needs to.

The fact that we can't do that is not ideal - but it is part of the reason why this tool exists in the first place 😁.

Update: the Swift team's extensible build tools proposal aims to address this problem for SwiftPM itself.

Ideas And Improvements

Potential Tools

The tools that could be integrated into Builder are obviously infinite, but a few that I imagine being of fairly immediate use are those that replace the built-in abilities of Xcode, such as:

  • copy resources
  • build an application/framework bundle
  • expand Info.plist
  • code sign
  • expand values into source files
  • calculate the build no (eg from the git commit count)
  • archive build products
  • upload a build
  • release on github
  • post notifications


This is a prototype, so for ease of development I made it a standalone tool rather than trying to modify SwiftPM itself.

In theory though it could be integrated into the swift command.

It could replace the existing swift-build tool (with that being renamed to something else so that this tool could use it). Invoking swift build would then run this tool (if no Configuration target was present in the manifest, we could fall back to the previous swift build behaviour).

Clearly this would have performance implications.


This prototype is an overlay on SwiftPM, so makes no changes to the Package.swift format. Because of this, the configuration and tool targets are just listed in the manifest along with the targets from the package that we're building.

This is potentially confusing, a properly integrated implementation might change the format slightly to allow the special targets to be listed explicitly, like so:

let package = Package(
    name: "Example",
    products: [
            name: "Example",
            targets: ["Example"]),
    dependencies: [
        .package(url: "", from: "1.0.3"),
        .package(url: "", from: "1.1.0"),
        name: "Configure",
        dependencies: ["BuilderToolExample"]),
        name: "BuilderToolExample",
        dependencies: [])
    targets: [
        // Targets are the basic building blocks of a package. A target can define a module or a test suite.
        // Targets can depend on other targets in this package, and on products in packages which this package depends on.
            name: "Example",
            dependencies: []),
            name: "ExampleTests",
            dependencies: ["Example"]),

I suspect that this would be preferable, but I wanted to start with something that didn't require modifying SwiftPM itself.

Separate Package File

Rather than living in the main Package.swift file, the configuration and tool information could live in its own file which lived alongside the main one.

It could either have a standard name such as Configure.swift (which would require changes to SwiftPM), or it could live in a sub-folder, such as Configure/Package.swift.

Libraries Not Executables

It's a bit messy that the Configure target it built as an executable, and thus has to output the configuration as JSON and then have Builder convert it back into objects on the other side.

In theory it ought to be possible to build Configure as a dynamic library, then link to it and use it directly from Builder.

Similarly this could also be done for the standalone tools - although there is arguably more advantage with them being able to be invoked manually from the command line.


  1. This module is an external dependency, defined in a different git repo, and listed in our manifest in the normal way.

  2. These action names are actually arbitrary - we could call them anything we like. If builder fails to find a Configure target however, it will fall back to just calling swift, and will pass the action to it as a first parameter. For this reason, it makes sense to use names like "build" and "test" if they correspond to SwiftPM actions.