Delay App Progression Until Firebase Data Retrieval Is Finished - swift

Question

In my app, I store data on the Firebase database (Firestore AND Storage) in the form of "Files" (what the user sees). When the user goes to their "Files" tab and selects a certain file (example: "Smith vs Wesson"), the app downloads data on the server (from BOTH Firestore and Storage) related to that file. Here's my problem: the app moves forward before the data has even finished returning and processing (sorted/stored into variables). I don't want the app to progress and take the user to the next screen until this is totally complete. The next three screens show the data retrieved from the server, so if it's moving forward before the data has even retrieved and sorted... well... you see the problem with that.
I tried using something like DispatchQueque.main.asyncAfter to add a three second delay, but the problem with this is that if the user's internet connection is poor, it may take longer than three seconds to retrieve the data. Likewise, if their internet connection is booming, it may take only a second to retrieve the data, but they're still stuck waiting on an unnecessary three-second delay. I only want the delay to last as long as it takes for the retrieval/sorting/storing function to complete its tasks... no matter how short or long.
I'm still learning and am mostly self-taught, so forgive my ignorance. From what I understand from the reading I've been doing, tasks are based off of "threads." The main thread is what the user sees, while there are threads that tasks can be done in the background to keep the user from experiencing longer wait times, etc (such as data retrieval from a server). I know typically you don't want to do tasks on the main thread, but in this case where I don't want the user to be able to progress, I need to find a way to pause the main thread until the other thread has completed the data retrieval and sorting/storing process.
I stumbled across something called "CountDownLatch." I read about it and kind of understand the concept of it... but not the code at all, to be honest. I don't know if CountDownLatch is the correct method to use here or not, but if it is, could someone please show me how I could use either CountDownLatch, or some other delay, to pause the progression of the app until the data is retrieved, sorted, and stored into the variables?
My data retrieval/sorting/storing function is called "getAndAppendClaimData." I handle all of those steps in this function, and it works perfectly. Like I said, I just need to provide some delay until it's finished, so that the code underneath this function isn't executed thereby segueing to the next screen.
So something kinda like this:
while getAndAppendClaimData is still processing {
showLoadingAnimation
}
once getAndAppendClaimData has finished **ALL** of its tasks {
performSegue to next screen
}
NOTE: I DON'T use listeners in my app because I don't need to update the user's screen in realtime... like with a messaging app or something. I just use the .getDocuments and documents.forEach functionalities for my data retrieval.
Please explain your answers or provide links to content explanations. You remember how it was when you were still learning..
Also, before some of you call this post a duplicate.. the other threads are outdated and most of them deal with apps that have listeners for realtime updates – which is different from my circumstance. Another thing, I'm doing a lot of research and learning... so please don't drop the whole "go do your research" bomb. Sometimes you need help tailoring things to your specific situation.
Thanks, I really appreciate the help!

Answer 1

Okay so after more research, I found that one way to keep your app from progressing while server data is downloading and sorting is with DispatchGroup.
First of all, create a DispatchGroup variable:
let dispatchGroup = DispatchGroup()
Then you can "enter" the group at the beginning of the call/function and "leave" the group at the end, once everything has finished processing (such as in completion in a Firebase call). If you're utilizing a loop to sort your data, then make sure to enter dispatchGroup
dispatchGroup.enter()
every time you enter the loop and leave dispatchGroup
dispatchGroup.leave()
at the end of each loop iteration. Once you're finished entering and leaving dispatchGroup for good, then call:
dispatchGroup.notify(queue: .main) {
// Here write whatever you want to do after it's finished retrieving and processing the data...
// Such as performing a segue
}
You would call this .notify outside of your loop, of course.
In my situation, I had a two loops: one to gather/store server data and one to sort it. I didn't want it to start trying to sort until it had finished gathering/sorting, so I executed the second loop inside dispatchGroup.notify, then performed my segue after the second loop finished.
Watch these three tutorials, they helped me out big time!
DispatchQueues
DispatchGroups
Semaphore vs DispatchGroup