coredata学习总结(六)

Connecting the Model to Views

在os x中，core data被设计为通过cocoa绑定同user interface来交互。但是，cocoa 绑定并不是user interface中的一部分。在ios中，你使用NSFetchedResultsController来将model（core data）和views（storyboards）来连接。

NSFetchedResultsController 提供了core data和UITableView 对象之间的接口。因为table views是最常见的展示数据的方式之一，uitableview被设计为处理数据的展示。

Creating a Fetched Results Controller

典型的，一个NSFetchedResultsController实例会被UITableViewController实例来初始化。这个初始化可以在viewDidLoad或者viewWillAppear方法中被实现。或者在view controller的其它地方被初始化。这个例子展示了NSFetchedResultsController的初始化方法。

1. @property (nonatomic,strong)NSFetchedResultsController*fetchedResultsController;
2. 
   
3. - (void)initializeFetchedResultsController
4. {
5. NSFetchRequest *request = [NSFetchRequestfetchRequestWithEntityName:@"Person"];
6. 
   
7. NSSortDescriptor *lastNameSort = [NSSortDescriptor sortDescriptorWithKey:@"lastName"ascending:YES];
8. 
   
9. [requestsetSortDescriptors:@[lastNameSort]];
10. 
    
11. NSManagedObjectContext *moc = …;//Retrieve the main queue NSManagedObjectContext
12. 
    
13. [self setFetchedResultsController:[[NSFetchedResultsControlleralloc]initWithFetchRequest:requestmanagedObjectContext:mocsectionNameKeyPath:nil cacheName:nil]];
14. [[selffetchedResultsController]setDelegate:self];
15. 
    
16. NSError *error=nil;
17. if (![[selffetchedResultsController]performFetch:&error]){
18. NSLog(@"Failed to initialize FetchedResultsController: %@\n%@",[errorlocalizedDescription],[erroruserInfo]);
19. abort();
20. }
21. }

1. var fetchedResultsController:NSFetchedResultsController!
2. 
   
3. func initializeFetchedResultsController() {
4. let request =NSFetchRequest(entityName:"Person")
5. let departmentSort =NSSortDescriptor(key:"department.name", ascending: true)
6. let lastNameSort =NSSortDescriptor(key:"lastName", ascending: true)
7. request.sortDescriptors = [departmentSort,lastNameSort]
8. 
   
9. let moc =dataController.managedObjectContext
10. fetchedResultsController = NSFetchedResultsController(fetchRequest: request, managedObjectContext: moc,sectionNameKeyPath:"department.name",cacheName: "rootCache")
11. fetchedResultsController.delegate =self
12. 
    
13. do {
14. try fetchedResultsController.performFetch()
15. } catch {
16. fatalError("Failed to initialize FetchedResultsController:\(error)")
17. }
18. }

首先要构造一个fetch请求，NSFetchRequest，这位于NSFetchedResultsController的重要位置。注意fetch请求包含一个排序描述，NSSortDescriptor。NSFetchedResultsController至少需要一个排序descriptor来控制将要呈现数据的显示顺序。

一旦fetch请求被初始化了，你就可以初始化NSFetchedResultsController实例了。fetched results controller会要求你传给他一个NSFetchRequest实例和NSFetchRequest对象引用。sectionNameKeyPath和cacheName属性都是可选的。

一旦fetched results controller被初始化了，你就分配一个 i 饿delegate。这个delegate会在有数据变化时通知table view controller。重要的是，table view controller也是fetched results controller的delegate，因此当有数据变化时，他可以被回调。

然后，你通过performFetch:方法来启动NSFetchedResultsController。

Integrating the Fetched Results Controller with the Table View Data Source

在你初始化完成并且有数据需要展示在table view的时候，你需要将fetched results controller同table view data source UITableViewDataSource来归整为一。

objc

1. #pragma mark - UITableViewDataSource
2. 
   
3. - (void)configureCell:(id)cellatIndexPath:(NSIndexPath*)indexPath
4. {
5. id object=[[self fetchedResultsController]objectAtIndexPath:indexPath];
6. 
   
7. // Populate cell from the NSManagedObject instance
8. }
9. 
   
10. - (UITableViewCell*)tableView:(UITableView*)tableViewcellForRowAtIndexPath:(NSIndexPath*)indexPath
11. {
12. id cell=[tableViewdequeueReusableCellWithIdentifier:CellReuseIdentifier];
13. // Set up the cell
14. [self configureCell:cell atIndexPath:indexPath];
15. return cell;
16. }
17. 
    
18. - (NSInteger)numberOfSectionsInTableView:(UITableView*)tableView
19. {
20. return [[[selffetchedResultsController]sections] count];
21. }
22. 
    
23. - (NSInteger)tableView:(UITableView*)tableViewnumberOfRowsInSection:(NSInteger)section
24. {
25. id< NSFetchedResultsSectionInfo> sectionInfo=[[self fetchedResultsController]sections][section];
26. return [sectionInfonumberOfObjects];
27. }

swift

1. func configureCell(cell:UITableViewCell,indexPath: NSIndexPath) {
2. guard letselectedObject =fetchedResultsController.objectAtIndexPath(indexPath)as?AAAEmployeeMO else { fatalError("Unexpected Object in FetchedResultsController") }
3. // Populate cell from the NSManagedObject instance
4. print("Object for configuration:\(object)")
5. }
6. 
   
7. override functableView(tableView:UITableView, cellForRowAtIndexPathindexPath: NSIndexPath) -> UITableViewCell {
8. let cell =tableView.dequeueReusableCellWithIdentifier("cellIdentifier",forIndexPath:indexPath)
9. // Set up the cell
10. configureCell(cell,indexPath:indexPath)
11. return cell
12. }
13. 
    
14. override funcnumberOfSectionsInTableView(tableView:UITableView) ->Int {
15. return fetchedResultsController.sections!.count
16. }
17. 
    
18. override functableView(tableView:UITableView, numberOfRowsInSectionsection: Int) -> Int {
19. guard letsections =fetchedResultsController.sectionselse {
20. fatalError("No sections in fetchedResultsController")
21. }
22. let sectionInfo =sections[section]
23. return sectionInfo.numberOfObjects
24. }

正如在如上的UITableViewDataSource方法中所展示的，fetched results controller和table view的关联都设计为单一的方法调用来完成。

Communicating Data Changes to the Table View

NSFetchedResultsController还掌控了当有数据变化时，同UITableViewController的通讯。实现NSFetchedResultsControllerDelegate即可

objc

1. #pragma mark - NSFetchedResultsControllerDelegate
2. - (void)controllerWillChangeContent:(NSFetchedResultsController*)controller
3. {
4. [[selftableView]beginUpdates];
5. }
6. - (void)controller:(NSFetchedResultsController*)controllerdidChangeSection:(id<NSFetchedResultsSectionInfo>)sectionInfoatIndex:(NSUInteger)sectionIndexforChangeType:(NSFetchedResultsChangeType)type
7. {
8. switch(type){
9. case NSFetchedResultsChangeInsert:
10. [[selftableView]insertSections:[NSIndexSetindexSetWithIndex:sectionIndex]withRowAnimation:UITableViewRowAnimationFade];
11. break;
12. case NSFetchedResultsChangeDelete:
13. [[selftableView]deleteSections:[NSIndexSetindexSetWithIndex:sectionIndex]withRowAnimation:UITableViewRowAnimationFade];
14. break;
15. case NSFetchedResultsChangeMove:
16. case NSFetchedResultsChangeUpdate:
17. break;
18. }
19. }
20. - (void)controller:(NSFetchedResultsController*)controllerdidChangeObject:(id)anObjectatIndexPath:(NSIndexPath*)indexPathforChangeType:(NSFetchedResultsChangeType)typenewIndexPath:(NSIndexPath*)newIndexPath
21. {
22. switch(type){
23. case NSFetchedResultsChangeInsert:
24. [[selftableView]insertRowsAtIndexPaths:@[newIndexPath]withRowAnimation:UITableViewRowAnimationFade];
25. break;
26. case NSFetchedResultsChangeDelete:
27. [[selftableView]deleteRowsAtIndexPaths:@[indexPath]withRowAnimation:UITableViewRowAnimationFade];
28. break;
29. case NSFetchedResultsChangeUpdate:
30. [self configureCell:[[self tableView] cellForRowAtIndexPath:indexPath]atIndexPath:indexPath];
31. break;
32. case NSFetchedResultsChangeMove:
33. [[selftableView]deleteRowsAtIndexPaths:@[indexPath]withRowAnimation:UITableViewRowAnimationFade];
34. [[selftableView]insertRowsAtIndexPaths:@[newIndexPath]withRowAnimation:UITableViewRowAnimationFade];
35. break;
36. }
37. }
38. - (void)controllerDidChangeContent:(NSFetchedResultsController*)controller
39. {
40. [[selftableView]endUpdates];
41. }

swift

1. func controllerWillChangeContent(controller:NSFetchedResultsController) {
2. tableView.beginUpdates()
3. }
4. 
   
5. func controller(controller:NSFetchedResultsController,didChangeSectionsectionInfo: NSFetchedResultsSectionInfo,atIndex sectionIndex: Int, forChangeTypetype: NSFetchedResultsChangeType) {
6. switch type {
7. case .Insert:
8. tableView.insertSections(NSIndexSet(index:sectionIndex),withRowAnimation: .Fade)
9. case .Delete:
10. tableView.deleteSections(NSIndexSet(index:sectionIndex),withRowAnimation: .Fade)
11. case .Move:
12. break
13. case .Update:
14. break
15. }
16. }
17. 
    
18. func controller(controller:NSFetchedResultsController,didChangeObjectanObject: AnyObject, atIndexPath indexPath:NSIndexPath?,forChangeType type: NSFetchedResultsChangeType, newIndexPath: NSIndexPath?) {
19. switch type {
20. case .Insert:
21. tableView.insertRowsAtIndexPaths([newIndexPath!],withRowAnimation: .Fade)
22. case .Delete:
23. tableView.deleteRowsAtIndexPaths([indexPath!],withRowAnimation: .Fade)
24. case .Update:
25. configureCell(tableView.cellForRowAtIndexPath(indexPath!)!,indexPath:indexPath!)
26. case .Move:
27. tableView.moveRowAtIndexPath(indexPath!,toIndexPath:newIndexPath!)
28. }
29. }
30. 
    
31. func controllerDidChangeContent(controller:NSFetchedResultsController) {
32. tableView.endUpdates()
33. }

Adding Sections

目前为止，table view只有一个section，如果需要使用多个Employee对象，可以划分多个section。

objc

1. - (void)initializeFetchedResultsController
2. {
3. NSFetchRequest *request = [NSFetchRequestfetchRequestWithEntityName:@"Person"];
4. NSSortDescriptor *departmentSort = [NSSortDescriptor sortDescriptorWithKey:@"department.name"ascending:YES];
5. NSSortDescriptor *lastNameSort = [NSSortDescriptor sortDescriptorWithKey:@"lastName"ascending:YES];
6. [requestsetSortDescriptors:@[departmentSort,lastNameSort]];
7. NSManagedObjectContext *moc = [[selfdataController]managedObjectContext];
8. [self setFetchedResultsController:[[NSFetchedResultsControlleralloc]initWithFetchRequest:requestmanagedObjectContext:mocsectionNameKeyPath:@"department.name"cacheName:nil]];
9. [[selffetchedResultsController]setDelegate:self];

swift

1. func initializeFetchedResultsController() {
2. let request =NSFetchRequest(entityName:"Person")
3. let departmentSort =NSSortDescriptor(key:"department.name", ascending: true)
4. let lastNameSort =NSSortDescriptor(key:"lastName", ascending: true)
5. request.sortDescriptors = [departmentSort,lastNameSort]
6. let moc =dataController.managedObjectContext
7. fetchedResultsController = NSFetchedResultsController(fetchRequest: request, managedObjectContext: moc,sectionNameKeyPath:"department.name",cacheName: nil)
8. fetchedResultsController.delegate =self
9. do {
10. try fetchedResultsController.performFetch()
11. } catch {
12. fatalError("Failed to initialize FetchedResultsController:\(error)")
13. }
14. }

这个例子中你为NSFetchRequest实例添加了一个NSSortDescriptor实例。fetched results controller使用这个初始化排序controller来将数据分隔为不同的sections，因此需要keys来匹配。

这个变化使得fetched results controller将Person实例来分隔为多个sections。唯一的条件是：

• sectionNameKeyPath属性必须也是一个NSSortDescriptor实例。
  

• NSSortDescriptor必须是数组中的第一个descriptor被传递给fetch request。

Adding Caching for Performance

大多数情况下，table view会展示相对固定的数据。在整个程序的运行周期内，创建table view controller时的fetch request不会发生变化。使用NSFetchedResultsController添加缓存可以在程序再次启动的时候而且数据不发生改变，table view会即刻初始化。这个缓存对于大数据尤为有用。

1. [self setFetchedResultsController:[[NSFetchedResultsControlleralloc]initWithFetchRequest:requestmanagedObjectContext:mocsectionNameKeyPath:@"department.name"cacheName:@"rootCache"]];

1. fetchedResultsController = NSFetchedResultsController(fetchRequest: request, managedObjectContext: moc,sectionNameKeyPath:"department.name",cacheName: "rootCache")

如上这样后续的数据加载就可以一瞬间完成。

注意：如果数据改变了，那么你如果需要清除缓存就直接调用deleteCacheWithName:即可。