RunLoop.main.perform vs DispatchQueue.main.async

RunLoopPerformVsDispatchQueueAsyncTests.swift

class RunLoopPerformVsDispatchQueueAsyncTests: XCTestCase {
    
    func testRunLoopPerformTakesPriorityOverDispatchAsync() {
        
        enum Event {
            case dispatchQueueAsync
            case runLoopPerform
        }
        
        var events: [Event] = []
        
        DispatchQueue.main.async {
            events.append(.dispatchQueueAsync)
        }
        XCTAssertEqual(events.count, 0)

        let mainRunLoop = RunLoop.main
        
        mainRunLoop.perform {
            events.append(.runLoopPerform)
        }
        XCTAssertEqual(events.count, 0)
        
        repeat {
            mainRunLoop.run(mode: .default, before: .distantPast)
        } while events.count < 2
        
        XCTAssertEqual(events, [.runLoopPerform, .dispatchQueueAsync])
    }
}