(Initially) getting duplicate results from FetchedObjects

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Summary

A developer reported seeing duplicate entries (24 instead of 12) when using SwiftUI’s FetchedObjects to display Core Data entities. The issue only occurred on the initial view load and resolved after performing a search operation. The root cause was two concurrent fetch operations initiated by the view’s lifecycle, caused by triggering a data fetch function twice and a race condition in the view rendering logic. Additionally, the id: \.self usage in ForEach exposed the underlying SwiftUI view identity instability.

Root Cause

The core issue was a race condition triggered by the view’s appearance logic combined with duplicate execution of the data loading function.

  • Duplicate Function Execution: The data loading function (the REST call) was being called twice in rapid succession (likely via onAppear or similar triggers). Although the second call cancelled the first, the rapid execution triggered multiple Core Data context updates.
  • Race Condition in View Rendering: FetchedObjects likely relies on a @FetchRequest property wrapper internally. When the view appeared, the data loading function modified the persistent store. Simultaneously, the view attempted to render based on the initial state. This created a race: did the fetch happen before or after the data load completed? The initial “24” count suggests the view rendered with “stale” or duplicated data during the transition state.
  • ForEach Identity Issue: The use of ForEach(dishes, id: \.self) is problematic. In SwiftUI, \.self relies on the Hashable implementation of the Dish object. If the Dish object changes or if the fetch returns an array with mixed object references (e.g., one from the main context, one from a background context), SwiftUI may treat them as distinct items even if they represent the same database row, leading to visual duplication.

Why This Happens in Real Systems

In complex iOS applications, especially those involving network requests and Core Data, this scenario is common.

  • Unintended Triggers: View lifecycle methods (like onAppear) can be triggered multiple times due to navigation re-renders or state changes, causing network requests to fire redundantly.
  • Context Merging Latency: When a background thread saves data to Core Data, the main thread’s context doesn’t see the changes immediately. If a view fetches data during or immediately after a background save, it can fetch a partial state, leading to inconsistent results until the context fully merges.
  • SwiftUI View Identity: SwiftUI relies on stable identifiers to manage the view hierarchy. Using \.self on mutable data or data without stable IDs causes SwiftUI to destroy and recreate views, which appears as flickering or duplication.

Real-World Impact

  • Data Integrity Perception: Users see incorrect data (duplicates), leading to mistrust in the application’s reliability.
  • Performance Degradation: Unnecessary network calls and Core Data fetches waste battery and bandwidth.
  • UI Jitter: Rapid re-rendering due to race conditions causes the UI to stutter or jump during the loading phase.
  • Debugging Nightmare: The issue is transient and state-dependent (fixes itself after interaction), making it notoriously difficult to reproduce and fix without deep knowledge of SwiftUI and Core Data internals.

Example or Code

The original code contained the following pattern which contributed to the issue:

// The problematic view structure
NavigationView {
    FetchedObjects(predicate: buildPredicate(), sortDescriptors: buildSortDescriptors()) { (dishes: [Dish]) in
        List {
            Text(String(dishes.count) + " dishes")
            // Using .self as ID is risky if the object isn't stable or Hashable correctly
            ForEach(dishes, id: \.self) { dish in
                DisplayDish(dish)
                    .onTapGesture {
                        self.showAlert.toggle()
                    }
            }
        }
    }
}
.searchable(text: $searchText, prompt: "Search items")

// The trigger likely looked like this (pseudo-code)
.onAppear {
    loadDataFunction() // Likely called twice due to navigation or parent view logic
}

How Senior Engineers Fix It

To resolve this, a senior engineer focuses on immutability, unique identifiers, and controlled data flow.

  1. Implement Stable Identifiers: Replace id: \.self with a stable key path, ideally the object’s ID property (e.g., id: \.id or id: \.objectID). This ensures SwiftUI can correctly track the object lifecycle.
    • Fix: ForEach(dishes, id: \.id) { ... }
  2. Debounce and Deduplicate Network Calls: Ensure the data loading function is idempotent or protected by a lock/state flag to prevent simultaneous executions.
    • Fix: Use an isLoading boolean flag or a Task cancellation mechanism (Swift Concurrency) to ensure the fetch only runs once per view appearance.
  3. Lifecycle Management: Move data loading to an appropriate place, such as a ViewModel initialized in the view’s initializer or using the .task modifier, which handles cancellation automatically when the view disappears.
  4. Explicit Fetch Request: Instead of relying on FetchedObjects (which might be a wrapper or custom component), use the standard @FetchRequest property wrapper. It automatically reacts to context changes and handles the view updates more predictably.

Why Juniors Miss It

  • Focus on Logic, Not Timing: Junior developers often look for logical errors (e.g., “am I appending twice?”) rather than timing errors (e.g., “is this running twice concurrently?”).
  • Misunderstanding \.self: The use of id: \.self is often a copy-paste pattern without understanding that self must be stable and unique. If the object is a Core Data NSManagedObject, \.self returns the object pointer, which might behave unpredictably if the context changes.
  • SwiftUI State Reactivity: Juniors often struggle to visualize the render cycle. They don’t anticipate that onAppear might fire multiple times or that a view might re-render while a network request is in flight, leading to “flash” of old/duplicate data.
  • Lack of Debugging Tools: Without using tools like the Xcode View Hierarchy Debugger or adding print statements to track the lifecycle of the dishes array count, it’s hard to see when the array becomes 24 items versus 12.