Swift Unit testing with XCTAssertThrows analogue

Question

Is there any equivalent to check for throwing exceptions in swift language Unit tests?

For example I Have a class:

class Square : NSObject{

    let sideLength: Int

    init(sideLength: Int) {
        assert(sideLength >= 0, "Wrong initialization of Square class with below zero side length")
        self.sideLength = sideLength
        super.init()
    }
}

and Test to check it work. In objective C I can write test method like this:

- (void)testInitializationWithWrongSideLengthThrowsExceptions{
   XCTAssertThrows([[Shape alloc] initWithSideLength: -50], "Should throw exceptions on wrong side values initialisations");
}

What is Swift equal technic?

Answer 1

If you add the following three files to your tests:

//  ThrowsToBool.h
#import <Foundation/Foundation.h>

/// A 'pure' closure; has no arguments, returns nothing.
typedef void (^VoidBlock)(void);

/// Returns: true if the block throws an `NSException`, otherwise false
BOOL throwsToBool(VoidBlock block);


//  ThrowsToBool.m
#import "ThrowsToBool.h"

BOOL throwsToBool(VoidBlock const block) {
    @try {
        block();
    }
    @catch (NSException * const notUsed) {
        return YES;
    }
    return NO;
}


// xxxTests-Bridging-Header.h
#import "ThrowsToBool.h"

Then you can write:

XCTAssert(throwsToBool {
   // test code that throws an NSException
})

But it doesn't work for assert or precondition :(

PS I got the idea from: http://modocache.io/xctest-the-good-parts

Answer 2

I think the assert()-function should only be used for debug-purposes. Not only because of the following statement from Apple's Swift-Book (https://itun.es/de/jEUH0.l):

„Assertions cause your app to terminate and are not a substitute for designing your code in such a way that invalid conditions are unlikely to arise.“

Thats why I would solve this as follows:

import Cocoa
import XCTest

class Square
{
    let sideLength: Int

    init(_ sideLength: Int)
    {
        self.sideLength = sideLength >= 0 ? sideLength : 0
    }
}

class SquareTests: XCTestCase
{
    override func setUp() { super.setUp() }
    override func tearDown() { super.tearDown() }

    func testMySquareSideLength() {
        let square1 = Square(1);
        XCTAssert(square1.sideLength == 1, "Sidelength should be 1")

        let square2 = Square(-1);
        XCTAssert(square2.sideLength >= 0, "Sidelength should be not negative")
    }
}

let tester = SquareTests()
tester.testMySquareSideLength()