Saving segue passed managedObjectContext in Swift - swift

Question

say i have one view controller (VCA), which segues to another another view controller (VCB). while preparing to segue, VCA passes its managedObjectContext (i.e. following the "tell don't ask" convention). VCA also has a function saveManagedObjectContext() that performs the saving and error handling.
so, in VCA:
let managedObjectContext = (UIApplication.sharedApplication().delegate as! AppDelegate).managedObjectContext
...
func saveManagedObjectContext() {
if managedObjectContext.hasChanges {
do {
try managedObjectContext.save()
} catch let error as NSError {
print("Could not save \(error), \(error.userInfo)")
abort()
}
}
}
...
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
...
if let vcb = segue.destinationViewController as? VCB {
vcb.managedObjectContext = managedObjectContext
}
...
}
and in VCB:
var managedObjectContext: NSManagedObjectContext? = nil
say VCB adds some entity to the managedObjectContext that was passed to it from VCA
func createSomeEntity() {
let entity = NSEntityDescription.entityForName("SomeEntity", inManagedObjectContext: managedObjectContext!)
something = NSManagedObject(entity: entity!, insertIntoManagedObjectContext: managedObjectContext) as? SomeEntity
}
is there any (correct) way to call the saveManagedObjectContext() function from VCA rather than having to copy the function over to VCB?
thanks

Answer 1

An alternative is a singleton class.
Create a new Swift file, replace the predefined code with
class CoreDataManager: NSObject {
// MARK: - Shared Instance
class var sharedManager : CoreDataManager {
struct Singleton {
static let instance = CoreDataManager()
}
return Singleton.instance
}
// MARK: - Core Data stack
lazy var ...
}
and then replace lazy var ... with the entire Core Data stack from AppDelegate.
Now you can access Core Data from everywhere using
let managedObjectContext = CoreDataManager.sharedManager.managedObjectContext
or to call the save action
CoreDataManager.sharedManager.saveAction(self)

Answer 2

Good point about "tell don't ask". A lot of Apple sample code evangelizes this concept. However, in recent years, Apple has also provided sample code with a Core Data stack class that handles the object graph, practically abandoning the "tell don't ask" pattern.
Also, in many popular and acclaimed open source projects, this pattern is used. In most cases you reduce the code and still have a robust solution. For example, in more complex projects with nested background contexts it is often the only feasible setup.
Thus, I would recommend to create a CoreDataManager class that handles the core data stack, or for less complex apps (single, main thread context) use the app delegate.
Note that in Swift, you can really make this very concise with global variables (which you should use sparingly!). E.g., on top of the AppDelegate.swift you could write
let SharedAppDelegate = (UIApplication.sharedApplication().delegate as! AppDelegate)
(I like to capitalize my global singletons). You can use this as follows
do { try SharedAppDelegate.context.save() } catch {}