Enable tests to dispatch blocks and wait for their results on the main dispatch queue

[Lievesley]

Use performSelectorOnMainThread instead of dispatch_sync because using a dispatch method prevents further blocks from being processed on the main queue until the initial block in which the httpResponseOnMainThreadForMethod was invoked has completed. dispatch_sync prevents other blocks being processed in nested calls to NSRunLoop run methods within tests.

[calabash-ci]

Can one of the admins verify this patch?

[msftclas]

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. We will now review your pull request. Thanks, Microsoft Pull Request Bot

[jmoody]

Jenkins test this please

[jmoody]

@Lievesley Can you provide an example of what problem this change solves?

[Lievesley]

@jmoody Here is a simplified code example of a custom Calabash action that is currently broken as no blocks are processed on the main queue until the initial dispatch_sync block completes.

__block BOOL flag = NO;
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    flag = YES;    // Without the fix in this PR this line only gets called after the initial dispatch_sync completes
});
NSDate* loopUntil = [NSDate dateWithTimeIntervalSinceNow:10];
while ([loopUntil timeIntervalSinceNow] > 0 && !flag)
{
    [[NSRunLoop mainRunLoop] runMode:NSDefaultRunLoopMode beforeDate:loopUntil];  // Does not process blocks on main queue as main queue is blocked waiting for dispatch_sync to complete
}
NSAssert(flag == YES, @"Flag was not set because main queue was blocked");

[jmoody]

this line only gets called after the initial dispatch_sync completes

I think this is the behavior we want. Can you describe how this is blocking a test?

[Lievesley]

I think this is the behavior we want. Can you describe how this is blocking a test?

It's not the behaviour that we want since it means that calling: [[NSRunLoop mainRunLoop] runMode:NSDefaultRunLoopMode beforeDate:loopUntil]; does nothing useful. What we want to do is to process the main queue in this runMode method.

In our test scenarios we have backdoor methods that call asynchronous operations that dispatch events back to the main queue. We wait for these events to be processed on the main queue using: [[NSRunLoop mainRunLoop] runMode:NSDefaultRunLoopMode beforeDate:loopUntil]; The asynchronous events in question must complete before the backdoor method returns. This cannot happen if the main queue is blocked by the dispatch_sync call from the Calabash framework.

Using performSelectorOnMain thread enables the scenario I have just described while not breaking any other test case. If you disagree please provide an example of why it is useful for: [[NSRunLoop mainRunLoop] runMode:NSDefaultRunLoopMode beforeDate:loopUntil]; not to work inside the backdoor method.

[jmoody]

Thank you for the explanation.

[jmoody]

@Lievesley If you make me (@jmoody) a collaborator on your fork, I will add the backdoor test to the LPTestTarget app.

[Lievesley]

Thanks! I just added you as a collaborator.

[jmoody]

Jenkins test this please

[jmoody]

Tests are failing on Jenkins because of a problem with develop. Will resolve the issue in a subsequent commit on origin/develop.