iOS State Preservation and Restoration

Covers iOS app state preservation and restoration with extra emphasis on NSCoding. WWCD 2012 session 208 glosses over some of the fine details of how NSCoding works. Understanding this foundation better helps in understanding state preservation and restoration.

Transcript

1. iOS State Preservation and Restoration Robert Brown Twitter: @robby_brown app.net: @robert_brown
2. What is State Restoration? Restores your app where the user left it Makes your apps appear as if they never terminated Lets users seamlessly get back to what they want
3. NSCoding Similar to writing to a plist Any object that conforms to NSCoding can be encoded
4. plist NSCoding NSNumber Y Y NSString Y Y NSDictionary Y Y NSArray Y Y NSData Y Y NSDate Y Y NSSet N Y Blocks N N Anything Else N Y
5. NSCoding -initWithCoder: -decodeObjectForKey: -encodeWithCoder: -encodeObject:forKey:
6. NSSecureCoding Irrelevant to state restoration, but nice to know Securely persists data Many Cocoa classes conform to NSSecureCoding
7. Restoration Identifiers Each view controller you want to restore must define a restoration identifier Restoration identifiers make up a restoration ID path Restoration paths must be unique Example: /TabBarController/AboutViewController
8. Restoration Identifiers /A /A/C /A/B /A/B /A/C/B
9. Preservation Flow -application:shouldSaveApplicationState: -application:willEncodeRestorableStateWithCoder: Each window on main screen Save view controller Save view 1 2 3 4 5
10. Preservation Flow - (void)encodeRestorableStateWithCoder:(NSCoder *)coder { [super encodeRestorableStateWithCoder:coder]; // Assumes Person is NSCoding-compliant [coder encodeObject:self.person forKey:@“person”]; }
11. Restoration Flow -application:willFinishLaunchingWithOptions: -application:shouldRestoreApplicationState: -application:didFinishLaunchingWithOptions: State Restoration Yes No -application:didDecodeRestorableState:withCoder:
12. Restoration Flow Each view controller +viewControllerWithRestorationIdentifierPath:coder: Found view controllers -decodeRestorableStateWithCoder: Restore view controller state Restore view 1 3 4 5 2
13. Restoration Flow +viewControllerWithRestorationIdentifierPath:(NSArray *) components coder:(NSCoder *)coder { // Restore only what you absolutely need // You may not be able to restore other view controllers return [self new]; }
14. Restoration Flow - (void)decodeRestorableStateWithCoder:(NSCoder *)coder { [super decodeRestorableStateWithCoder:coder]; self.person = [coder decodeObjectForKey:@“person”]; }
15. UIDataSourceModelAssociation Many views will automatically restore their state UITableView and UICollectionView need some extra help to restore your visible and selected cells UIDataSourceModel association provides a mapping of cells to model objects This is NOT intended to be used to persist data
16. UIDataSourceModelAssociation -modelIdentifierForElementAtIndexPath:inView: Converts from model object to ID -indexPathForElementWithModelIdentifier:inView: Converts from ID to model object
17. UIDataSourceModelAssociation - (NSString*)modelIdentifierForElementAtIndexPath:(NSIndexPath *)idx inView:(UIView *)view { // Uses Core Data Person * person = self.persons[idx.row]; return person.objectID.URIRepresentation.absoluteString; }
18. UIDataSourceModelAssociation - (NSIndexPath *)indexPathForElementWithModelIdentifier:(NSString *)identifier inView:(UIView *)view { NSURL * uri = [NSURL urlWithString:identifier]; NSPersistentStoreCoordinator * coordinator = // Your coordinator NSManagedObjectContext * context = // Your context NSManagedObjectID * objectID = [coordinator managedObjectIDForURIRepresentation:uri]; Person * person = [context existingObjectWithID:objectID error:NULL]; NSInteger index = [self.persons indexOfObject:person]; return [NSIndexPath indexPathForRow:index inSection:0]; }
19. Demo
20. Must use proper UIViewController containment Restoration identifier paths must be unique For a view controller to be restored, it must have a path to the root view controller Manually terminating the app will delete the state Gotchas
21. Gotchas Restore only minimal state in +viewControllerWithRestorationIdentifierPath: If a view controller that was persisted is not restored, the app delegate is given a chance to restore it Testing state restoration isn’t straightforward Press home button, then have Xcode kill the app
22. Gotchas Restoration classes must conform to the UIViewControllerRestoration protocol Restoration classes aren’t necessary when using UIStoryboard
23. Gotchas If you change your view controller hierarchy, make sure you don’t restore from an older state If your app crashes during restoration, the state data is thrown out
24. Want to Learn More? WWDC Session 208 Archives and Serializations Programming Guide iOS App Programming Guide
25. Questions?