How to unit test asynchronous APIs?

Question

I have installed Google Toolbox for Mac into Xcode and followed the instructions to set up unit testing found here.

It all works great, and I can test my synchronous methods on all my objects absolutely fine. However, most of the complex APIs I actually want to test return results asynchronously via calling a method on a delegate - for example a call to a file download and update system will return immediately and then run a -fileDownloadDidComplete: method when the file finishes downloading.

How would I test this as a unit test?

It seems like I'd want to the testDownload function, or at least the test framework to 'wait' for fileDownloadDidComplete: method to run.

EDIT: I've now switched to using the XCode built-in XCTest system and have found that TVRSMonitor on Github provides a dead easy way to use semaphores to wait for async operations to complete.

For example:

- (void)testLogin {   TRVSMonitor *monitor = [TRVSMonitor monitor];   __block NSString *theToken;    [[Server instance] loginWithUsername:@"foo" password:@"bar"                                success:^(NSString *token) {                                    theToken = token;                                    [monitor signal];                                }                                 failure:^(NSError *error) {                                    [monitor signal];                                }];    [monitor wait];    XCTAssert(theToken, @"Getting token"); } 

Answer 1

I ran into the same question and found a different solution that works for me.

I use the "old school" approach for turning async operations into a sync flow by using a semaphore as follows:

// create the object that will perform an async operation MyConnection *conn = [MyConnection new]; STAssertNotNil (conn, @"MyConnection init failed");  // create the semaphore and lock it once before we start // the async operation NSConditionLock *tl = [NSConditionLock new]; self.theLock = tl; [tl release];      // start the async operation self.testState = 0; [conn doItAsyncWithDelegate:self];  // now lock the semaphore - which will block this thread until // [self.theLock unlockWithCondition:1] gets invoked [self.theLock lockWhenCondition:1];  // make sure the async callback did in fact happen by // checking whether it modified a variable STAssertTrue (self.testState != 0, @"delegate did not get called");  // we're done [self.theLock release]; self.theLock = nil; [conn release]; 

Make sure to invoke

[self.theLock unlockWithCondition:1]; 

In the delegate(s) then.