iOS应用开发视频教程笔记（十四）Core Data Demo

这节课的主要内容是Core Data的线程安全、Core DataTable View，以及大Demo。

Core Data Thread Safety

NSManagedObjectContext不是线程安全的，只能在创建NSManagedObjectContext的那个线程里访问它。一个数据库有多个UIManagedDocument和context，它们可以在不同的线程里创建，只要能管理好它们之间的关系就没问题。

线程安全的意思是，程序可能会崩溃，如果多路访问同一个NSManagedObjectContext，或在非创建实例的线程里访问实例，app就会崩溃。对此要怎么做呢？NSManagedObjectContext有个方法叫performBlock可以解决这个问题：

[context performBlock:^{   //or performBlockAndWait:    // do stuff with context}];

它会自动确保block里的东西都在正确的context线程里执行，但这不一定就意味着使用了多线程。事实上，如果在主线程下创建的context，那么这个block会回到主线程来，而不是在其他线程里运行，这个performBlock只是确保block运行在正确的线程里。

NSManagedObjectContext，包括所有使用SQL的Core Data，都有一个parentContext，这就像是另一个NSManagedObjectContext，在真正写入数据库之前要写入到这里。可以获取到parentContext，可以让parentContext调用performBlock来做一些事，这总是在另一个线程里进行。parentContext和创建的NSManagedObjectContext不在一个线程里运行，可以通过performBlock在那个线程里执行你要做的事。记住，如果改变了parentContext，必须保存，然后重新获取child context。如果你想在非主线程载入很多内容，那么就全部放入数据库，然后在主线程去获取，这个效率非常快。

Core Data and Table View

ios里有一个非常重要的类NSFetchedResultsController，它不是一个viewController，而是控制fetchedResults与tableView通信方式的controller，所以这个NSFetchedResultsController是一个NSObject。

它的作用就是用来连接NSFetchedResultsController和UITableViewController的，而且连接的方法很强大。首先，它可以回答所有UITableView DataSource、protocol的问题，唯一不能回答的是cellForRowAtIndexPath。这是个NSFetchedResultsController的例子：

- (NSUInteger)numberOfSectionsInTableView:(UITableView *)sender{     return [[self.fetchedResultsController sections] count];}- (NSUInteger)tableView:(UITableView *)sender numberOfRowsInSection:(NSUInteger)section{     return [[[self.fetchedResultsController sections] objectAtIndex:section] numberOfObjects];}

tableView有个property是NSFetchedResultsController，就是这个self.fetchedResultsController。

cellForRowAtIndexPath也是很容易实现的，因为NSFetchedResultsController有objectAtIndexPath这个方法，你给出一个index，它会返回给你该row所显示的NSManagedObject：

- (NSManagedObject *)objectAtIndexPath:(NSIndexPath *)indexPath;

如果要实现cellForRowAtIndexPath，可以像下面这样做：

- (UITableViewCell *)tableView:(UITableView *)sender         cellForRowAtIndexPath:(NSIndexPath *)indexPath{     UITableViewCell *cell = ...;      NSManagedObject *managedObject = [self.fetchedResultsController objectAtIndexPath:indexPath];     // load up the cell based on the properties of the managedObject     // of course, if you had a custom subclass, you’d be using dot notation to get them     return cell;}

有了managedObject，可以用valueForKey或者子类它，用.号来获取它的property。在一个Core Data驱动的tableView里，每一行都是通过数据库里的对象来表示的，一对一的关系。

如何创建这个NSFetchedResultsController呢？这是个如何alloc/init的例子：

NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@“Photo”]; NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@“title” ...]; request.sortDescriptors = [NSArray arrayWithObject:sortDescriptor]; request.predicate = [NSPredicate predicateWithFormat:@“whoTook.name = %@”, photogName];NSFetchedResultsController *frc = [[NSFetchedResultsController alloc]     initWithFetchRequest:(NSFetchRequest *)request     managedObjectContext:(NSManagedObjectContext *)context      sectionNameKeyPath:(NSString *)keyThatSaysWhichSectionEachManagedObjectIsIn                cacheName:@“MyPhotoCache”; // careful!

如果用这个fetch request来创建NSFetchedResultsController，那么table的每一行的照片的photographer名字都会对应这个字符串。cacheName就是缓存名，缓存速度很快，但有一点要小心：如果改变了fetch request的参数，比如改了predicate或者sortDescriptors,缓存会被破坏。如果真的要通过NSFetchedResultsController改fetch request，那么得删除缓存。NSFetchedResultsController里有个工厂方法可以删除缓存，或者可以就设为null，这样就不会有缓存，对于小型数据库，可以不用缓存。对于一个非常大型的数据库，如果要改request，得先删除缓存。另一个参数sectionNameKeyPath表示每个managedObject所在的section，这就是objects的一个attribute，这个例子里就是photo的一个attribute，如果问这个attribute关于photo的事，它会告诉我这个section的title，所以这里就是section的title。

如果context改变了，NSFetchedResultsController会自动更新table，只要把两个东西连接起来就行。唯一的前提是，这些改变都要发生在创建NSFetchedResultsController的那个context里。原理是，NSFetchedResultsController有个delegate，它会发很多消息，例如，添加了对象或者对象的一个属性改了等等，然后tableView就会去做相应的调整，事实上还是要在tableView里写些代码的。

Demo

这个demo会从Flickr上得到几百张照片，但table里显示的不是照片而是摄影师，点击摄影师就会显示他拍的所有照片的一个列表，点击photo才会显示图片。我们要实现从Flickr获取数据并存到Core Data数据库，然后把CoreDataTableView和fetch request连接起来。

在xcode新建project，从Single View Application开始，就叫它Photomania，在storyboard中从library拖出两个tableViewController，并将其中一个导入到导航控制器中，然后将开始小箭头移到导航控制器前面。新建两个UITableViewController的子类，分别为PhotographersTableViewController和PhotosByPhotographerTableViewController，到storyboard中将两个tableViewController的类分别设为这两个。

Photo+Flickr.h文件代码：

#import "Photo.h"@interface Photo (Flickr)+ (Photo *)photoWithFlickrInfo:(NSDictionary *)flickrInfo        inManagedObjectContext:(NSManagedObjectContext *)context;@end

Photo+Flickr.m文件代码：

#import "Photo+Flickr.h"#import "FlickrFetcher.h"#import "Photographer+Create.h"@implementation Photo (Flickr)// 9. Query the database to see if this Flickr dictionary's unique id is already there// 10. If error, handle it, else if not in database insert it, else just return the photo we found// 11. Create a category to Photographer to add a factory method and use it to set whoTook// (then back to PhotographersTableViewController)+ (Photo *)photoWithFlickrInfo:(NSDictionary *)flickrInfo        inManagedObjectContext:(NSManagedObjectContext *)context{    Photo *photo = nil;        NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Photo"];    request.predicate = [NSPredicate predicateWithFormat:@"unique = %@", [flickrInfo objectForKey:FLICKR_PHOTO_ID]];    NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"title" ascending:YES];    request.sortDescriptors = [NSArray arrayWithObject:sortDescriptor];        NSError *error = nil;    NSArray *matches = [context executeFetchRequest:request error:&error];        if (!matches || ([matches count] > 1)) {        // handle error    } else if ([matches count] == 0) {        photo = [NSEntityDescription insertNewObjectForEntityForName:@"Photo" inManagedObjectContext:context];        photo.unique = [flickrInfo objectForKey:FLICKR_PHOTO_ID];        photo.title = [flickrInfo objectForKey:FLICKR_PHOTO_TITLE];        photo.subtitle = [flickrInfo valueForKeyPath:FLICKR_PHOTO_DESCRIPTION];        photo.imageURL = [[FlickrFetcher urlForPhoto:flickrInfo format:FlickrPhotoFormatLarge] absoluteString];        photo.whoTook = [Photographer photographerWithName:[flickrInfo objectForKey:FLICKR_PHOTO_OWNER] inManagedObjectContext:context];    } else {        photo = [matches lastObject];    }        return photo;}@end

Photographer+Create.h文件代码：

#import "Photographer.h"@interface Photographer (Create)+ (Photographer *)photographerWithName:(NSString *)name                inManagedObjectContext:(NSManagedObjectContext *)context;@end

Photographer+Create.m文件代码：

#import "Photographer+Create.h"@implementation Photographer (Create)+ (Photographer *)photographerWithName:(NSString *)name                inManagedObjectContext:(NSManagedObjectContext *)context{    Photographer *photographer = nil;        NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Photographer"];    request.predicate = [NSPredicate predicateWithFormat:@"name = %@", name];    NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"name" ascending:YES];    request.sortDescriptors = [NSArray arrayWithObject:sortDescriptor];        NSError *error = nil;    NSArray *photographers = [context executeFetchRequest:request error:&error];        if (!photographers || ([photographers count] > 1)) {        // handle error    } else if (![photographers count]) {        photographer = [NSEntityDescription insertNewObjectForEntityForName:@"Photographer"                                                     inManagedObjectContext:context];        photographer.name = name;    } else {        photographer = [photographers lastObject];    }        return photographer;}@end

PhotographersTableViewController.h文件代码：

#import <UIKit/UIKit.h>#import "CoreDataTableViewController.h"// inherits from CoreDataTableViewController to get an NSFetchedResultsController @property// and to get all the copy/pasted code for the NSFetchedResultsController delegate from the documentation@interface PhotographersTableViewController : CoreDataTableViewController@property (nonatomic, strong) UIManagedDocument *photoDatabase;  // Model is a Core Data database of photos@end

PhotographersTableViewController.m文件代码：

#import "PhotographersTableViewController.h"#import "FlickrFetcher.h"#import "Photographer.h"#import "Photo+Flickr.h"@implementation PhotographersTableViewController@synthesize photoDatabase = _photoDatabase;// 4. Stub this out (we didn't implement it at first)// 13. Create an NSFetchRequest to get all Photographers and hook it up to our table via an NSFetchedResultsController// (we inherited the code to integrate with NSFRC from CoreDataTableViewController)- (void)setupFetchedResultsController // attaches an NSFetchRequest to this UITableViewController{    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Photographer"];    request.sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"name" ascending:YES selector:@selector(localizedCaseInsensitiveCompare:)]];    // no predicate because we want ALL the Photographers                                 self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request                                                                        managedObjectContext:self.photoDatabase.managedObjectContext                                                                          sectionNameKeyPath:nil                                                                                   cacheName:nil];}// 5. Create a Q to fetch Flickr photo information to seed the database// 6. Take a timeout from this and go create the database model (Photomania.xcdatamodeld)// 7. Create custom subclasses for Photo and Photographer// 8. Create a category on Photo (Photo+Flickr) to add a "factory" method to create a Photo// (go to Photo+Flickr for next step)// 12. Use the Photo+Flickr category method to add Photos to the database (table will auto update due to NSFRC)- (void)fetchFlickrDataIntoDocument:(UIManagedDocument *)document{    dispatch_queue_t fetchQ = dispatch_queue_create("Flickr fetcher", NULL);    dispatch_async(fetchQ, ^{        NSArray *photos = [FlickrFetcher recentGeoreferencedPhotos];        [document.managedObjectContext performBlock:^{ // perform in the NSMOC's safe thread (main thread)            for (NSDictionary *flickrInfo in photos) {                [Photo photoWithFlickrInfo:flickrInfo inManagedObjectContext:document.managedObjectContext];                // table will automatically update due to NSFetchedResultsController's observing of the NSMOC            }            // should probably saveToURL:forSaveOperation:(UIDocumentSaveForOverwriting)completionHandler: here!            // we could decide to rely on UIManagedDocument's autosaving, but explicit saving would be better            // because if we quit the app before autosave happens, then it'll come up blank next time we run            // this is what it would look like (ADDED AFTER LECTURE) ...            [document saveToURL:document.fileURL forSaveOperation:UIDocumentSaveForOverwriting completionHandler:NULL];            // note that we don't do anything in the completion handler this time        }];    });    dispatch_release(fetchQ);}// 3. Open or create the document here and call setupFetchedResultsController- (void)useDocument{    if (![[NSFileManager defaultManager] fileExistsAtPath:[self.photoDatabase.fileURL path]]) {        // does not exist on disk, so create it        [self.photoDatabase saveToURL:self.photoDatabase.fileURL forSaveOperation:UIDocumentSaveForCreating completionHandler:^(BOOL success) {            [self setupFetchedResultsController];            [self fetchFlickrDataIntoDocument:self.photoDatabase];                    }];    } else if (self.photoDatabase.documentState == UIDocumentStateClosed) {        // exists on disk, but we need to open it        [self.photoDatabase openWithCompletionHandler:^(BOOL success) {            [self setupFetchedResultsController];        }];    } else if (self.photoDatabase.documentState == UIDocumentStateNormal) {        // already open and ready to use        [self setupFetchedResultsController];    }}// 2. Make the photoDatabase's setter start using it- (void)setPhotoDatabase:(UIManagedDocument *)photoDatabase{    if (_photoDatabase != photoDatabase) {        _photoDatabase = photoDatabase;        [self useDocument];    }}// 0. Create full storyboard and drag in CDTVC.[mh], FlickrFetcher.[mh] and ImageViewController.[mh]// (0.5 would probably be "add a UIManagedDocument, photoDatabase, as this Controller's Model)// 1. Add code to viewWillAppear: to create a default document (for demo purposes)- (void)viewWillAppear:(BOOL)animated{    [super viewWillAppear:animated];        if (!self.photoDatabase) {  // for demo purposes, we'll create a default database if none is set        NSURL *url = [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];        url = [url URLByAppendingPathComponent:@"Default Photo Database"];        // url is now "<Documents Directory>/Default Photo Database"        self.photoDatabase = [[UIManagedDocument alloc] initWithFileURL:url]; // setter will create this for us on disk    }}// 14. Load up our cell using the NSManagedObject retrieved using NSFRC's objectAtIndexPath:// (go to PhotosByPhotographerViewController.h (header file) for next step)- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{    static NSString *CellIdentifier = @"Photographer Cell";        UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];    if (cell == nil) {        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];    }        // ask NSFetchedResultsController for the NSMO at the row in question    Photographer *photographer = [self.fetchedResultsController objectAtIndexPath:indexPath];    // Then configure the cell using it ...    cell.textLabel.text = photographer.name;    cell.detailTextLabel.text = [NSString stringWithFormat:@"%d photos", [photographer.photos count]];        return cell;}// 19. Support segueing from this table to any view controller that has a photographer @property.- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender{    NSIndexPath *indexPath = [self.tableView indexPathForCell:sender];    Photographer *photographer = [self.fetchedResultsController objectAtIndexPath:indexPath];    // be somewhat generic here (slightly advanced usage)    // we'll segue to ANY view controller that has a photographer @property    if ([segue.destinationViewController respondsToSelector:@selector(setPhotographer:)]) {        // use performSelector:withObject: to send without compiler checking        // (which is acceptable here because we used introspection to be sure this is okay)        [segue.destinationViewController performSelector:@selector(setPhotographer:) withObject:photographer];    }}@end

PhotosByPhotographerTableViewController.h文件代码：

#import <UIKit/UIKit.h>#import "Photographer.h"#import "CoreDataTableViewController.h"// inherits from CoreDataTableViewController to get an NSFetchedResultsController @property// and to get all the copy/pasted code for the NSFetchedResultsController delegate from the documentation@interface PhotosByPhotographerTableViewController : CoreDataTableViewController// 15. Added public Model (the photographer whose photos we want to show)@property (nonatomic, strong) Photographer *photographer;@end

PhotosByPhotographerTableViewController.m文件代码：

#import "PhotosByPhotographerTableViewController.h"#import "Photo.h"#import "ImageViewController.h"@implementation PhotosByPhotographerTableViewController@synthesize photographer = _photographer;// 17. Create a fetch request that looks for Photographers with the given name and hook it up through NSFRC// (we inherited the code to integrate with NSFRC from CoreDataTableViewController)- (void)setupFetchedResultsController // attaches an NSFetchRequest to this UITableViewController{    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Photo"];    request.sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"title"                                                                                     ascending:YES                                                                                      selector:@selector(localizedCaseInsensitiveCompare:)]];    request.predicate = [NSPredicate predicateWithFormat:@"whoTook.name = %@", self.photographer.name];        self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request                                                                        managedObjectContext:self.photographer.managedObjectContext                                                                          sectionNameKeyPath:nil                                                                                   cacheName:nil];}// 16. Update our title and set up our NSFRC when our Model is set- (void)setPhotographer:(Photographer *)photographer{    _photographer = photographer;    self.title = photographer.name;    [self setupFetchedResultsController];}// 18. Load up our cell using the NSManagedObject retrieved using NSFRC's objectAtIndexPath:// (back to PhotographersTableViewController.m for next step, segueing)- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{    static NSString *CellIdentifier = @"Photo Cell";        UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];    if (cell == nil) {        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];    }        // Configure the cell...    Photo *photo = [self.fetchedResultsController objectAtIndexPath:indexPath]; // ask NSFRC for the NSMO at the row in question    cell.textLabel.text = photo.title;    cell.detailTextLabel.text = photo.subtitle;        return cell;}// 20. Add segue to show the photo (ADDED AFTER LECTURE)- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender{    NSIndexPath *indexPath = [self.tableView indexPathForCell:sender];    Photo *photo = [self.fetchedResultsController objectAtIndexPath:indexPath]; // ask NSFRC for the NSMO at the row in question    if ([segue.identifier isEqualToString:@"Show Photo"]) {        [segue.destinationViewController setImageURL:[NSURL URLWithString:photo.imageURL]];        [segue.destinationViewController setTitle:photo.title];    }}@end