XCTAssertThrows与自定义errorHandler的奇怪行为

Question

在我的单元测试中,我有以下代码来检查抛出的错误是否是预期类型.它由两个相同的语句完成,其中一个语句不能编译:

    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)
    }

第一个语句编译并正常工作,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

经过一番研究后,我发现我的代码有两个问题:

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

2)编译器被我的语句搞糊涂了,因为XCTAssertThows接受了另一个接受闭包的参数 - 就是这样 @autoclosure _ message: () -> String

结果编译器认为我传递了消息的闭包(IMHO令人困惑的设计决策 - 传递@autoclosure作为消息参数),当我实际上传递了errorHandler的闭包.

解决方案很简单:

    // 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)
        }
    }