Swift 4中Firebase操作应选用哪个GCD队列？最优队列选择咨询

Great question! Let's break this down clearly since Firebase's queue handling is actually pretty straightforward once you know the SDK's built-in behavior.

First off: Firebase's iOS SDK (covering Firestore, Realtime Database, Storage, Auth, etc.) already handles async execution under the hood. Most API calls will offload network/disk work to the SDK's own background queues automatically—you don't need to manually wrap Firebase calls in GCD unless you're adding extra heavy processing.

Here's the breakdown of optimal queue choices by scenario:

1. Basic Firebase API Calls: No Manual Queue Needed #

Whether you call Firebase methods from the main queue or a background queue (like Firestore.firestore().collection("users").getDocuments() or Auth.auth().signIn(withEmail:)), the SDK will take care of moving the time-consuming work to the background. It won't block your current queue.

Even better: By default, Firebase routes all completion closures/callbacks to the main queue. This is intentional—most callback logic involves updating UI, so you can skip writing DispatchQueue.main.async for UI updates right out of the box.

2. Operations with Heavy Pre/Post Processing: Use Background Queues #

If your workflow involves time-consuming tasks before calling Firebase (like encrypting large files, parsing complex local datasets) or after receiving results (like batch-parsing hundreds of documents, compressing images), these steps must not run on the main queue—they'll cause UI freezes.

In these cases, move the heavy work to a global background queue (e.g., DispatchQueue.global(qos: .userInitiated) or a custom background queue), then trigger the Firebase call, or process results in the background first before switching back to the main queue for UI updates. Example:

// Handle heavy parsing in background, then call Firebase
DispatchQueue.global(qos: .userInitiated).async {
    let processedUserData = self.parseLargeLocalUserDataset() // Time-consuming task
    Firestore.firestore().collection("users").addDocument(data: processedUserData) { error in
        // Default callback runs on main queue—safe to update UI directly
        if let error = error {
            print("Failed to add user: \(error)")
        } else {
            self.showSuccessAlert()
        }
    }
}

3. Custom Callback Queues (Advanced/Optional) #

If you want Firebase callbacks to default to a background queue (e.g., for post-processing that doesn't need UI access), you can override the SDK's default queue. For example, with Firestore:

let firestoreSettings = FirestoreSettings()
firestoreSettings.dispatchQueue = DispatchQueue.global(qos: .background)
Firestore.firestore().settings = firestoreSettings

This is a niche use case though—most apps don't need this, since the main queue default is convenient for UI workflows.

Quick Summary #

• Pure Firebase API calls: Call them from any queue—SDK handles background execution, callbacks default to main queue.
• Calls with heavy processing: Isolate the time-consuming work to a background queue, reserve the main queue only for UI updates.
• You don't need to force Firebase operations onto the main queue unless you're explicitly updating UI (and the SDK already does that for you).

内容的提问来源于stack exchange，提问作者Michael Hsu