Attempts to call save() on each NSManagedObjectContext in a given array. Rolls back saved changes and throws the provided error if a save fails.

SaveContexts.swift

/* Attempts to call save() on each NSManagedObjectContext in a given array. Rolls back saved changes and throws the provided error if a save fails.  */

func saveContexts(contexts: [NSManagedObjectContext]) throws {
    var successfulContextSaves = [NSManagedObjectContext]()
    
    for context in contexts where context.hasChanges {
        do {
            try context.save()
            successfulContextSaves += [context]
        } catch {
            for savedContext in successfulContextSaves {
                // roll back all changes made since the last save operation
                savedContext.rollback()
            }
            
            throw error
        }
    }
}