XCTAssertThrowsError 自定义错误处理程序的奇怪行为

Question

在我的单元测试中，我有以下代码检查抛出的错误是否属于预期类型。它是用两个相同的语句完成的，其中一个不能编译：

    enum Error: ErrorType {
        case SomeExpectedError
        case SomeUnexpectedError
    }

    func functionThatThrows() throws {
        throw Error.SomeExpectedError
    }

    // 1) this compiles fine
    XCTAssertThrowsError(try functionThatThrows()) { (error) in
        switch error {
        case Error.SomeExpectedError: break
            //everything is fine
        case Error.SomeUnexpectedError: fallthrough
        default:
            XCTFail("Unexpected error thrown")
        }
    }

    // 2) doesn't compiles at all
    XCTAssertThrowsError(try functionThatThrows()) { (error) in
        XCTAssertEqual(error as? Error, Error.SomeExpectedError)
    }

第一个语句编译并运行良好，但第二个语句告诉我有两个错误： Errors thrown from here are not handled 和 Cannot convert value of type '(Error) -> Void' to expected argument type 'String'.

这段代码可能有什么问题？错误消息是什么意思？

我正在使用 Xcode 7.3 .

Answer 1

经过一些研究，我发现我的代码有 2 个问题：

1) 似乎我的闭包被认为是 "Impilictly returning" 因为它只包含一个可执行语句

2) 编译器对我的陈述感到困惑，因为 XCTAssertThows 接受了另一个接受闭包的参数 - 它是 @autoclosure _ message: () -> String

因此，编译器认为我传递了消息闭包（恕我直言，令人困惑的设计决定 - 将 @autoclosure 作为消息参数传递），而实际上我传递了 errorHandler 的闭包。

解决方法很简单：

    // explicitly show closure returning type Void so that compiler couldn't
    // use it as message argument (which closure must return String)
    XCTAssertThrowsError(try functionThatThrows()) { (error) -> Void in
        XCTAssertEqual(error as? Error, Error.SomeExpectedError)
    }

    // explicitly show closure's argument type
    XCTAssertThrowsError(try functionThatThrows()) { (error: ErrorType) in
        XCTAssertEqual(error as? Error, Error.SomeExpectedError)
    }

    // use "message" parameter to preserve argument order
    XCTAssertThrowsError(try functionThatThrows(), "some message") { (error) in
        XCTAssertEqual(error as? Error, Error.SomeExpectedError)
    }

    // wrap executable line in "do {}". I guess it works because closure stops
    // being "implicitly returning" and now compiler treat it as () -> Void
    // which is expected type of errorHandler argument
    XCTAssertThrowsError(try functionThatThrows()) { (error) in
        do {
            XCTAssertEqual(error as? Error, Error.SomeExpectedError)
        }
    }

Answer 2

我正在使用这个静态函数来测试特定异常：

func XCTAssertThrowsError<T, E: Error & Equatable>(
  _ expression: @autoclosure () throws -> T,
  error: E,
  in file: StaticString = #file,
  line: UInt = #line
  ) {
  var thrownError: Error?
  XCTAssertThrowsError(
    try expression(),
    file: file,
    line: line) {
      thrownError = [=10=]
  }

  XCTAssertTrue(
    thrownError is E,
    "Unexpected error type: \(type(of: thrownError))",
    file: file,
    line: line
  )

  XCTAssertEqual(
    thrownError as? E,
    error,
    file: file,
    line: line
  )
}

用法示例：

  func testEmptyMrz() {
    XCTAssertThrowsError(
      try passportMrzValidator.validate(for: ""),
      error: PassportMrzValidationError.invalidMrz
    )
  }

XCTAssertThrowsError 自定义错误处理程序的奇怪行为

XCTAssertThrowsError strange behavior with custom errorHandler

error-handling

closures

syntax-error

swift