A primer for testing asynchronous calls with XCTest in Objective-C

AsyncTestingPrimer.m

#import <XCTest/XCTest.h>

@interface AsyncTestingPrimer : XCTestCase

@end

@implementation AsyncTestingPrimer

/*
 * This is how not to test async calls
 */
- (void) testBrokenAsync {
    
    __block BOOL isTouched = false;
    
    dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
        // Wait for half a second before doing anything, make sure we're pre-empted
        [NSThread sleepForTimeInterval:0.5];
        isTouched = true;
    });
    
    XCTAssert(isTouched, @"This fails because the async block has not completed yet.");
    
}

/*
 * Let's try again, using test expectations
 */
- (void) testAsyncWithExpectations {
    
    __block BOOL isTouchedWithExpectations = false;
    
    XCTestExpectation *myExpectation = [self expectationWithDescription:@"Expectation for the following async block"];
    dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
        [NSThread sleepForTimeInterval:0.5];
        isTouchedWithExpectations = true;
        
        // Let anyone who's waiting know we're done
        [myExpectation fulfill];
    });
    
    // Wait for all expectations to be fulfilled.
    [self waitForExpectationsWithTimeout:1 handler:nil];
    XCTAssert(isTouchedWithExpectations, @"This succeeds because we waited for the expectation fulfilment in the async block");
    
}

/*
 * But make sure all your expectations are fulfilled, otherwise you will time out.
 */
- (void) testAsyncWithTooManyExpectations {
    
    __block BOOL isTouchedWithExpectations = false;
    
    XCTestExpectation *myExpectation = [self expectationWithDescription:@"Expectation for the following async block"];
    XCTestExpectation *excessExpectation = [self expectationWithDescription:@"This expectation will never be fulfilled"];
    dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
        [NSThread sleepForTimeInterval:0.5];
        isTouchedWithExpectations = true;
        
        // Let anyone who's waiting know we're done
        [myExpectation fulfill];
    });
    
    // Wait for all expectations to be fulfilled.
    [self waitForExpectationsWithTimeout:1 handler:nil];
}

@end