How to unit test throwing functions in Swift? - xcode

Question

How to test wether a function in Swift 2.0 throws or not? How to assert that the correct ErrorType is thrown?

Answer 1

EDIT: Updated the code for Swift 4.1 (still valid with Swift 5.2)
Here's the latest Swift version of Fyodor Volchyok's answer who used XCTAssertThrowsError:
enum MyError: Error {
case someExpectedError
case someUnexpectedError
}
func functionThatThrows() throws {
throw MyError.someExpectedError
}
func testFunctionThatThrows() {
XCTAssertThrowsError(try functionThatThrows()) { error in
XCTAssertEqual(error as! MyError, MyError.someExpectedError)
}
}
If your Error enum has associated values, you can either have your Error enum conform to Equatable, or use the if case statement:
enum MyError: Error, Equatable {
case someExpectedError
case someUnexpectedError
case associatedValueError(value: Int)
}
func functionThatThrows() throws {
throw MyError.associatedValueError(value: 10)
}
// Equatable pattern: simplest solution if you have a simple associated value that can be tested inside 1 XCTAssertEqual
func testFunctionThatThrows() {
XCTAssertThrowsError(try functionThatThrows()) { error in
XCTAssertEqual(error as! MyError, MyError.associatedValueError(value: 10))
}
}
// if case pattern: useful if you have one or more associated values more or less complex (struct, classes...)
func testFunctionThatThrows() {
XCTAssertThrowsError(try functionThatThrows()) { error in
guard case MyError.associatedValueError(let value) = error else {
return XCTFail()
}
XCTAssertEqual(value, 10)
// if you have several values or if they require more complex tests, you can do it here
}
}

Answer 2

At least of Xcode 7.3 (maybe earlier) you could use built-in XCTAssertThrowsError():
XCTAssertThrowsError(try methodThatThrows())
If nothing is thrown during test you'll see something like this:
If you want to check if thrown error is of some concrete type, you could use errorHandler parameter of XCTAssertThrowsError():
enum Error: ErrorType {
case SomeExpectedError
case SomeUnexpectedError
}
func functionThatThrows() throws {
throw Error.SomeExpectedError
}
XCTAssertThrowsError(try functionThatThrows(), "some message") { (error) in
XCTAssertEqual(error as? Error, Error.SomeExpectedError)
}

Answer 3

Given the following functions and declarations:
enum SomeError: ErrorType {
case FifthError
case FirstError
}
func throwingFunction(x: Int) throws {
switch x {
case 1:
throw SomeError.FirstError
case 5:
throw SomeError.FifthError
default:
return
}
}
This function will throw a FifthError if 5 is given to the function and FirstError if 1 is given.
To test, that a function successfully runs the unit test could look as follows:
func testNotError() {
guard let _ = try? throwingFunction(2) else {
XCTFail("Error thrown")
return
}
}
The let _ may also be replaced by any other name, so you can further test the output.
To assert that a function throws, no matter what ErrorType the unit test could look like this:
func testError() {
if let _ = try? throwingFunction(5) {
XCTFail("No error thrown")
return
}
}
If you want to test for a specific ErrorType it's done with a do-catch-statement. This is not the best way compared to other languages.
You have to make sure that you...
return in the catch for the correct ErrorType
XCTFail() and return for all other catch
XCTFail() if no catch is executed
Given this requirements a test case could look like this:
func testFifthError() {
do {
try throwingFunction(5)
} catch SomeError.FifthError {
return
} catch {
XCTFail("Wrong error thrown")
return
}
XCTFail("No error thrown")
}

Answer 4

Swift 4.1 Error throwing Test for associated values
enum ParseError: Error, Equatable {
case unexpectedArgument(String)
}
func testWithNoSchemaButWithOneArgument() {
XCTAssertThrowsError(try Args(withSchema: "", andArguments: ["-x"])) { error in
XCTAssertEqual(error as? ParseError, ParseError.unexpectedArgument("Argument(s) -x unexpected."))
}
}

Answer 5

You can use this function:
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 = $0
}
XCTAssertTrue(
thrownError is E,
"Unexpected error type: \(type(of: thrownError))",
file: file,
line: line
)
XCTAssertEqual(
thrownError as? E,
error,
file: file,
line: line
)
}
Example:
XCTAssertThrowsError(try funcThatThrowsSpecificError(), error: SpecificErrorEnum.someError)