如何对依赖Objective-C中的易失变量的方法进行单元测试？

Question

Here's a simplified version of my class:

@interface RTMovieBuilder : NSObject
@property (atomic, getter = isCancelled) volatile BOOL cancelled;
@property (nonatomic, weak) id<BuilderDelegate>delegate;
- (void)moviesFromJSON:(id)JSON;
- (Movie *)movieFromDictionary:(NSDictionary *)dict;
- (void)cancel;
@end

@implementation RTMovieBuilder

- (void)moviesFromJSON:(id)JSON
{
    // Check for errors -> If good, then do...

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
        [self syncrouslyCreateMoviesFromJSON:JSON];
        });
}

- (void)syncrouslyCreateMoviesFromJSON:(id)JSON
{        
    NSMutableArray *movies = [NSMutableArray array];

    for (NSDictionary *dict in JSON)
    {
        if ([self isCancelled])
            return;
        else
            [movies addObject:[self movieFromDictionary:dict]];
    }

    [self notifyDelegateCreatedObjects:movies];
}

- (Movie *)movieFromDictionary:(NSDictionary *)dict
{
    Movie *movie = [[Movie alloc] init];
    // Set movie properties based on dictionary...
    return movie;
}

- (void)cancel
{
    [self setCancelled:YES];
}

// ... Other methods omitted for brevity's sake

@end

The property cancelled is atomic and volatile because it may be accessed by other threads (ie the main thread may call cancel method to stop the operation). (I believe these are needed, if not, please note why it's not in your answer.)

I am trying to write unit tests to make sure this will work before writing the view controller class.

How can I write a unit test that will simulate a call to cancel while RTMovieBuilder is in the middle of creating movies ?

Edit

Here's a unit test I have already written which tests to make sure that notifyDelegateCreatedObjects: isn't called if cancel is called first.

- (void)testIfCancelledDoesntNotifyDelegateOfSuccess
{
    // given
    RTMovieBuilder *builder = [[RTMovieBuilder alloc] init];
    builder.delegate = mockProtocol(@protocol(BuilderDelegate));

    // when
    [builder cancel];
    [builder notifyDelegateCreatedObjects:@[]];

    // then
    [verifyCount(builder.delegate, never()) builder:builder createdObjects:anything()];
}

I'm using OCHamcrest and OCMockito . This test passes.

Answer 1

I would avoid trying to simulate thread timing in unit tests and focus more on figuring out what all the possible end states could be regardless of where the timing falls, and write tests for code under those conditions. This avoids endless complexity in your tests, as bbum points out as well.

In your case it seems the condition you need to be testing for is if the call to notifyDelegateCreatedObjects happens after the action is canceled, because the cancel came too late. So instead just unit test the handling of that scenario downstream in your notifyDelegateCreatedObjects method, or whatever class is being notified of that aborted event because of the thread timing.

I know this is not a specific answer to your question but I think its a better approach to achieve the same unit testing goal.

Answer 2

There is no reason to use volatile if your property is atomic and you always go through the setter/getter.

As well, this is a bit of re-inventing the wheel, as noted in the comments.

In general trying to unit test cancellation with any hope of full coverage is very hard because you can't really effectively test all possible timing interactions.