iOS 11 Files App扩展：文件标签元数据无法持久化问题咨询

Hey there, let's tackle this metadata persistence issue with your iOS 11 Files App extension—this is a common gotcha when working with the File Provider framework, so I’ve got some concrete steps to help you fix those disappearing tags and folder favorites.

Key Reasons Your Metadata Isn’t Sticking #

From what you’ve described, the problem likely stems from how you’re syncing metadata changes with the system and maintaining the working set. Let’s break down the fixes:

1. Always Submit Metadata Updates via NSFileProviderManager #

It’s not enough to just update your local copy of the NSFileProviderItem—you need to explicitly tell the Files App system about the change using update(_:completionHandler:). This triggers the system to persist the metadata and reflect it across its UI.

Here’s a quick example for updating a file’s tag:

// Fetch or create the updated item (make sure it has valid identifiers!)
var updatedItem: NSFileProviderItem = getLocalItem(for: itemIdentifier)

// Serialize the color tag (use NSKeyedArchiver for UIColor)
if let redTagData = try? NSKeyedArchiver.archivedData(withRootObject: UIColor.red, requiringSecureCoding: false) {
    updatedItem.tagData = redTagData
}

// Mark the item as a favorite (use a non-nil favoriteRank for favorites)
updatedItem.favoriteRank = 0 // Lower numbers = higher priority in favorites

// Submit the update to the system
NSFileProviderManager.default.update([updatedItem]) { error in
    if let error = error {
        print("Metadata update failed: \(error.localizedDescription)")
        return
    }
    // We’ll handle working set updates next!
}

2. Maintain and Persist the Working Set #

The working set (NSFileProviderWorkingSet) is the system’s way of tracking items the user cares about (like favorited folders or tagged files). If you don’t add modified items to the working set—and persist that set across extension restarts—the system won’t retain their metadata.

• When modifying metadata: Add the item’s itemIdentifier to your working set, then submit it to the system with setWorkingSet(_:completionHandler:).
• On extension launch: Load your persisted working set (store it in Core Data, UserDefaults, or your backend) and re-submit it to the system.

Example of updating the working set:

// After successfully updating the item...
func updateWorkingSet(with itemID: NSFileProviderItemIdentifier) {
    // Load your existing working set from persistent storage
    var workingSet = loadPersistedWorkingSet() ?? Set<NSFileProviderItemIdentifier>()
    workingSet.insert(itemID)
    
    // Persist the updated set (save to Core Data/UserDefaults here)
    saveWorkingSet(workingSet)
    
    // Submit to the system
    NSFileProviderManager.default.setWorkingSet(workingSet) { error in
        if let error = error {
            print("Working set update failed: \(error.localizedDescription)")
        }
    }
}

3. Ensure Consistent Item Identifiers #

The system uses itemIdentifier and parentItemIdentifier to uniquely track items. If these values change between extension launches, the system won’t associate your updated metadata with the correct file/folder.

• Make sure these identifiers are stable (e.g., use a unique ID from your backend, not a temporary local ID).
• Double-check that item(for:) and enumerator(for:) always return items with the same identifiers as when they were first created.

4. Avoid Stale Metadata in Enumeration Calls #

The "briefly shows in Recent Items" clue tells me your extension returns correct metadata initially, but fails to do so when the system re-queries. Make sure:

• enumerator(for:) returns the latest version of each item (don’t cache old metadata).
• item(for:) fetches the most up-to-date metadata from your local storage or backend every time it’s called.

5. Validate Tag Data Serialization #

tagData expects a serialized representation of the tags. Using NSKeyedArchiver for UIColor works, but make sure you’re unarchiving it correctly when returning items to the system. If the data is corrupted, the system won’t display the tag.

Quick Recap of WWDC '17 Best Practices #

From the WWDC '17 sessions, the core takeaway is that the File Provider framework relies on a two-way sync:

• Your extension tells the system about changes via update(_:) and setWorkingSet(_:).
• The system persists this metadata and requests updates when needed (via your extension’s enumeration methods).
• Never assume local changes will automatically sync to the system—you have to explicitly trigger the update.

内容的提问来源于stack exchange，提问作者Guna