PreconditionFailable protocol

PreconditionFailable.swift

/* PRODUCTION */

protocol PreconditionFailable {
    associatedtype PreconditionFailure
}

extension PreconditionFailable {
    static func precondition(_ condition: Bool, _ message: PreconditionFailure) {
        guard !condition else { return }
        PreconditionFailureHandler.trap(String(describing: message))
        RunLoop.spin()
    }

    static func preconditionFailure(_ message: PreconditionFailure) -> Never {
        PreconditionFailureHandler.trap(String(describing: message))
        RunLoop.spin()
    }
}

struct PreconditionFailureHandler {
    static var trap: (String) -> Void = { message in
        fatalError(message)
    }
}

/* TESTING */

protocol PreconditionFailableTesting {
    associatedtype PreconditionFailableType: PreconditionFailable
}

extension PreconditionFailableTesting where Self: XCTestCase {
    func expectPreconditionFailure(
        _ expectedFailure: PreconditionFailableType.PreconditionFailure,
        file: StaticString = #file,
        line: UInt = #line,
        trigger: @escaping () -> Void
        ) {
        let failureExpectation = expectation(description: "Precondition failure testing expectation")
        let originalTrap = PreconditionFailureHandler.trap
        PreconditionFailureHandler.trap = { message in
            XCTAssertEqual(message, String(describing: expectedFailure), file: file, line: line)
            failureExpectation.fulfill()
            RunLoop.spin()
        }
        DispatchQueue(label: "Precondition failure testing queue").async(execute: trigger)
        waitForExpectations(timeout: 1) { error in
            if error != nil {
                let failureDescription = String(describing: expectedFailure)
                XCTFail("Precondition failure: " + failureDescription, file: file, line: line)
            }
        }
        PreconditionFailureHandler.trap = originalTrap
    }
}

/* UTILITIES */

extension RunLoop {
    static func spin() -> Never {
        repeat {
            RunLoop.current.run()
        } while (true)
    }
}