📝 SwiftData — ModelContainer & ModelContext (Lifecycle & Concurrency)

📦 ModelContainer

• A container that manages your model schema and storage (backed by SQLite by default)

• Created once (usually in App struct)

• Can have multiple ModelContexts`
  
  let container = try ModelContainer(for: [User.self, Task.self])
  
  
  🧠 ModelContext

• Represents a scratch space for reading/writing model instances

• Think of it as an in-memory workspace

• Must call try context.save() to persist changes to disk

• Creating Contexts

• Main UI context (automatically injected in SwiftUI via @Environment(\.modelContext))

• Background context for heavy work
  
  @Environment(\.modelContext) var context   // UI context

  Task.detached {
   let backgroundContext = ModelContext(container)
   let user = User(name: "Async User")
   backgroundContext.insert(user)
   try? backgroundContext.save()
}
   

1. Multiple Contexts

• A ModelContainer can have multiple concurrent ModelContexts`

• Useful for parallel work or background tasks

• ---

2. Thread Safety

• ModelContext is not thread-safe

• Use each context only on the actor it was created on

• Always create a separate background context when working off the main actor

3. Merging Changes

• Contexts are isolated

• When one saves, others get notified and must refresh

• Use:
  
  try context.fetch(FetchDescriptor<User>())
context.refreshAllObjects()
   

• Similar to Core Data’s merge behavior
  4. Conflict Resolution

• If two contexts edit the same object → last writer wins

• To avoid data loss:

  • Refresh objects before saving

  • Use background contexts for isolated work

  • Consider versioning if needed

5. Background Work

• Use background contexts to do large inserts/updates without blocking UI

Example:
Task.detached {
   let ctx = ModelContext(container)
   for i in 0..<1000 {
       ctx.insert(User(name: "User \(i)"))
   }
   try? ctx.save()
}

6. Persistence & Crashes

• Unsaved changes are lost if the app terminates/crashes

• Always call save() after important changes

7. Multiple Containers

• You can create multiple ModelContainers in one app

• They are completely isolated (separate schemas and storage)
  
  ()) context.refreshAllObjects() Similar to Core Data’s merge behavior 4. Conflict Resolution If two contexts edit the same object → last writer wins To avoid data loss: Refresh objects before saving Use background contexts for isolated work Consider versioning if needed 5. Background Work Use background contexts to do large inserts/updates without blocking UI Example: Task.detached { let ctx = ModelContext(container) for i in 0..<1000 { ctx.insert(User(name: "User \(i)")) } try? ctx.save() } 6. Persistence & Crashes Unsaved changes are lost if the app terminates/crashes Always call save() after important changes 7. Multiple Containers You can create multiple ModelContainers in one app They are completely isolated (separate schemas and storage) 💡 Best Practices Always save() on critical points (onDisappear, backgrounding) Refresh contexts after background saves Use @MainActor for UI-bound contexts Use background contexts for long operations Keep ModelContainer as a singleton-like shared resource