XCTestCase+Error.swift

import XCTest

extension XCTestCase {
    /// Asserts that an expression throws a specific error.
    ///
    /// - Parameters:
    ///   - expression: An expression that can throw an error.
    ///   - throws: The expected `Error`. Must conform to the `Equatable` protocol.
    ///   - file: The file in which failure occurred. Defaults to the file name of the test case in which this function was called.
    ///   - line: The line number on which failure occurred. Defaults to the line number on which this function was called.
    func assert<T, E: Error & Equatable>(_ expression: @autoclosure () throws -> T, throws expectedError: E, in file: StaticString = #file, line: UInt = #line ) {
        
        var thrownError: Error?
        
        XCTAssertThrowsError(try expression(), file: file, line: line) {
            thrownError = $0
        }
        
        guard let error = thrownError else { return }
        
        guard error is E else {
            XCTFail("Unexpected error type: \(type(of: error))", file: file, line: line)
            return
        }
        
        XCTAssertEqual(error as? E, expectedError, file: file, line: line)
    }
    
    /// Asserts that an expression throws a specific CocoaError.
    ///
    /// - Parameters:
    ///   - expression: An expression that can throw an error.
    ///   - errorCode: The expected `CocoaError.code`.
    ///   - file: The file in which failure occurred. Defaults to the file name of the test case in which this function was called.
    ///   - line: The line number on which failure occurred. Defaults to the line number on which this function was called.
    func assert<T>(_ expression: @autoclosure () throws -> T, throwsCocoaErrorCode errorCode: CocoaError.Code, in file: StaticString = #file, line: UInt = #line) {
        
        var thrownError: Error?
        
        XCTAssertThrowsError(try expression(), file: file, line: line) {
            thrownError = $0
        }
        
        guard let error = thrownError else { return }
        
        guard let cocoaError = error as? CocoaError else {
            XCTFail("Unexpected error type: \(type(of: error))", file: file, line: line)
            return
        }
        
        XCTAssertEqual(cocoaError.code, errorCode, file: file, line: line)
    }
}