Make self weak in methods in Swift

Question

I have a Swift class that needs to store a table of its own methods. Unfortunately this is causing a reference cycle, because its table retains references to self

Answer 1

With Swift 5.2, callAsFunction allows for nice syntax for this, until argument labels come into play.

public struct WeakMethod {
  public init(
    reference: Reference?,
    method: @escaping Method
  ) {
    self.reference = reference
    self.method = method
  }

  public weak var reference: Reference?
  public var method: Method
}

public extension WeakMethod {
  struct ReferenceDeallocatedError: Error { }

  typealias Method = (Reference) -> (Input) -> Output

  /// - Throws: ReferenceDeallocatedError
  func callAsFunction(_ input: Input) throws -> Output {
    guard let reference = reference
    else { throw ReferenceDeallocatedError() }

    return method(reference)(input)
  }
}

public extension WeakMethod where Input == () {
  init(
    reference: Reference?,
    method: @escaping (Reference) -> () -> Output
  ) {
    self.reference = reference
    self.method = { reference in
      { _ in method(reference)() }
    }
  }

  /// - Throws: ReferenceDeallocatedError
  func callAsFunction() throws -> Output {
    try self( () )
  }
}

final class WeakMethodTestCase: XCTestCase {
  func test_method() throws {
    var reference: Reference? = Reference()

    let assign1234 = WeakMethod(reference: reference, method: Reference.assign1234)

    try assign1234()
    XCTAssertEqual(reference?.property, 1234)

    reference = nil
    XCTAssertThrowsError( try assign1234() ) {
      XCTAssert($0 is WeakMethod.ReferenceDeallocatedError)
    }
  }

  func test_closure_noParameters() throws {
    var reference: Reference? = Reference()

    let assign1234 = WeakMethod(reference: reference) {
      reference in { reference.property = 1234 }
    }

    try assign1234()
    XCTAssertEqual(reference?.property, 1234)

    reference = nil
    XCTAssertThrowsError( try assign1234() ) {
      XCTAssert($0 is WeakMethod.ReferenceDeallocatedError)
    }
  }

  func test_closure_1Parameter() throws {
    var reference: Reference? = Reference()

    let assign = WeakMethod(reference: reference) {
      reference in { reference.property = $0 }
    }

    try assign(1234)
    XCTAssertEqual(reference?.property, 1234)

    reference = nil
    XCTAssertThrowsError( try assign(1234) ) {
      XCTAssert($0 is WeakMethod.ReferenceDeallocatedError)
    }
  }
}

private final class Reference {
  var property = 1

  func assign1234() {
    property = 1234
  }
}

Unfortunately, as you keep adding parameters, you'll need to keep adding initializer+method pairs, like so:

init(
  reference: Reference?,
  method: @escaping (Reference) -> (Input0, Input1) -> Output
)
where Input == (Input0, Input1) {
  self.reference = reference
  self.method = { reference in
    { method(reference)($0.0, $0.1) }
  }
}

/// - Throws: ReferenceDeallocatedError
func callAsFunction(_ input0: Input0, _ input1: Input1) throws -> Output
where Input == (Input0, Input1) {
  try self( (input0, input1) )
}