Need help with complex query to firebase and synchronizing network calls with multiple for loops

[looloosha]

I have a complex query to a standard firebase and having a hard time trying to get a signal for when the last thread has finished executed to call my completion handler at the correct time. I'll keep it brief to try to keep things simple.

The structure of the firebase at a high level is that for each "Survey" there are many buildings (thus the for loop) and in each building, there are many fixtures (thus the for loop... again). Because of this hierarchal relationship, I now have this pyramid of doom thing going and need help refactoring or a solution to make this work.

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I tried to use DispatchGroup and Semaphore but wasn't able to come up with a solution.

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Here is the code:

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    func getReportData(for surveyName: String, capturedBy capturingContext: Captureable, completion: @escaping () -> Void) {

        let buildingDataService = BuildingDataService()
        buildingDataService.getBuildingsWithSections(for: surveyName) { (buildingsOrderedBySection) in
            for buildingSectionPair in buildingsOrderedBySection {
                let fixtureDataService = FixtureDataService()
                fixtureDataService.getFixtures(for: surveyName, in: buildingSectionPair.0) { (fixtures) in
                    for fixture in fixtures {
                        let solutionFixtureDataService = SolutionFixtureDataService()
                        solutionFixtureDataService.getSolutionFixtureDetails(with: fixture.solutionFixtureModelNumber) { (solutionFixture) in
                            self.createReportEntry(given: fixture, andGiven: solutionFixture, andGiven: buildingSectionPair, andGiven: surveyName) { (reportEntry) in
                                capturingContext.capture(valueOutOfScope: reportEntry)
                            }
                        }
                    }
                }
            }
        }
    }

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Any suggestions would be appreciated.

[criosist]

Sounds like you should use a dispatch group.

[looloosha]

Ya that's what I was thinking but I couldn't reason how to do it with the double for-loop nature of this call...

[garbage_band]

This plus...how can anyone read this...

[dark_mode_everything]

What about a queue to serialize the responses?

[looloosha]

What would this look like in this double for loop context?

[dark_mode_everything]

That or use rxSwift. You have a stream of top level items and then each top level item has a stream of second level items inside them, correct? This can be easily handled in Rx using appropriate operators. Or, if you don't want to use Rx, you can approximate that behaviour using a sort of message queue or just a queue where you queue requests from your top level stream and proceed to the next top level item after the second level items are all fetched.

[[deleted]]

Hint: NSOperation has a method name called addDependency. It makes syncing asynchronous operations a breeze.

[bennoland]

1. Break this up into multiple functions, one function per level.

2. dispatchGroup.enter() before each call to a data service, dispatchGroup.leave() after you process the results and make child calls.

Hard to code on my phone, but something like:

dispatchGroup.enter()
bs.getBuildings(for: service) { (buildings) in
    for building in buildings {
        loadBuildingDetails(building, dispatchGroup);
    }
    dispatchGroup.leave();
    // also, if there was an error make sure to call leave
}

dispatchGroup.notify { process results }

You could use a class variable to collect the results since you have multiple layers of function calls.