targets: [
    ...
    .plugin(
        name: "RunMockolo",
        capability: .buildTool(),
        dependencies: [.target(name: "mockolo")]
    ),
    .binaryTarget(
        name: "mockolo",
        url: "...",
        checksum: "..."
    ),

import PackagePlugin

@main struct RunMockoloPlugin: BuildToolPlugin {
    func createBuildCommands(context: PluginContext, target: Target) async throws -> [Command] {
        let generatedSourcePath = context.pluginWorkDirectory.appending("GeneratedMocks.swift")
        let packageRoot = context.package.directory

        return [
            .prebuildCommand(
                displayName: "Run mockolo",
                executable: try context.tool(named: "mockolo").path,
                arguments: [
                    "-s", packageRoot.appending("Sources", "MyModule").string,
                    "-d", generatedSourcePath,
                ],
                outputFilesDirectory: context.pluginWorkDirectory
            ),
        ]
    }
}

.target(
    name: "MyTarget",
    dependencies: [
        ...
    ],
    plugins: [
        .plugin(name: "RunMockolo"),
    ]
),

/// @mockable
public protocol Foo {
    var num: Int { get set }
    func bar(arg: Float) -> String
}

public class FooMock: Foo {
    init() {}
    init(num: Int = 0) {
        self.num = num
    }

    var numSetCallCount = 0
    var underlyingNum: Int = 0
    var num: Int {
        get {
            return underlyingNum
        }
        set {
            underlyingNum = newValue
            numSetCallCount += 1
        }
    }

    var barCallCount = 0
    var barHandler: ((Float) -> (String))?
    func bar(arg: Float) -> String {
        barCallCount += 1
        if let barHandler = barHandler {
            return barHandler(arg)
        }
        return ""
    }
}

func testMock() {
    let mock = FooMock(num: 5)
    XCTAssertEqual(mock.numSetCallCount, 1)
    mock.barHandler = { arg in
        return String(arg)
    }
    XCTAssertEqual(mock.barCallCount, 1)
}

/// @mockable(module: prefix = Bar)
public protocol Foo {
    ...
}

public class FooMock: Bar.Foo {
    ...
}

/// @mockable(typealias: T = AnyObject; U = StringProtocol)
public protocol Foo {
    associatedtype T
    associatedtype U: Collection where U.Element == T
    associatedtype W
    ...
}

public class FooMock: Foo {
    typealias T = AnyObject // overridden
    typealias U = StringProtocol // overridden
    typealias W = Any // default placeholder type for typealias
    ...
}

/// @mockable(rx: intStream = ReplaySubject; doubleStream = BehaviorSubject)
public protocol Foo {
    var intStream: Observable<Int> { get }
    var doubleStream: Observable<Double> { get }
}

public class FooMock: Foo {
    var intStreamSubject = ReplaySubject<Int>.create(bufferSize: 1)
    var intStream: Observable<Int> { /* use intStreamSubject */ }
    var doubleStreamSubject = BehaviorSubject<Int>(value: 0)
    var doubleStream: Observable<Int> { /* use doubleStreamSubject */ }
}

/// @mockable(history: fooFunc = true)
public protocol Foo {
    func fooFunc(val: Int)
    func barFunc(_ val: (a: String, Float))
    func bazFunc(val1: Int, val2: String)
}

public class FooMock: Foo {
    var fooFuncCallCount = 0
    var fooFuncArgValues = [Int]() // arguments captor
    var fooFuncHandler: ((Int) -> ())?
    func fooFunc(val: Int) {
        fooFuncCallCount += 1
        fooFuncArgValues.append(val)   // capture arguments

        if fooFuncHandler = fooFuncHandler {
            fooFuncHandler(val)
        }
    }

    ...
    var barFuncArgValues = [(a: String, Float)]() // tuple is also supported.
    ...

    ...
    var bazFuncArgValues = [(Int, String)]()
    ...
}

/// @mockable(combine: fooPublisher = PassthroughSubject; barPublisher = CurrentValueSubject)
public protocol Foo {
    var fooPublisher: AnyPublisher<String, Never> { get }
    var barPublisher: AnyPublisher<Int, CustomError> { get }
}

public class FooMock: Foo {
    public init() { }

    public var fooPublisher: AnyPublisher<String, Never> { return self.fooPublisherSubject.eraseToAnyPublisher() }
    public private(set) var fooPublisherSubject = PassthroughSubject<String, Never>()

    public var barPublisher: AnyPublisher<Int, CustomError> { return self.barPublisherSubject.eraseToAnyPublisher() }
    public private(set) var barPublisherSubject = CurrentValueSubject<Int, CustomError>(0)
}

/// @mockable(combine: fooPublisher = @Published foo)
public protocol Foo {
    var foo: String { get }
    var fooPublisher: AnyPublisher<String, Never> { get }
}

public class FooMock: Foo {
    public init() { }
    public init(foo: String = "") {
        self.foo = foo
    }

    public private(set) var fooSetCallCount = 0
    @Published public var foo: String = "" { didSet { fooSetCallCount += 1 } }

    public var fooPublisher: AnyPublisher<String, Never> { return self.$foo.setFailureType(to: Never.self).eraseToAnyPublisher() }
}

/// @mockable(override: name = FooMock)
public protocol FooProtocol { ... }

public class FooMock: FooProtocol { ... }