基于Xcode8.x的CoreData的使用

    首先说说对于CoreData的理解，相似于FMDB。他能将数据库中的列表转换成模型实体对象，方便开发者使用。同时，也能将一个将实体对象转换成数据库中的表，存储到数据库中。这就是CoreData,数据库与实体模型之间的转换工具。

    打开的沙盒的Library路径，你会发现，以下的文件目录：（前提是你的工程里面使用了CoreData）

由此能够证实，CoreData相似FMDB，都是基于sqlite进行数据存储的。

    因为Xcode升级到8.x，CoreData的使用与以往之间会有所不一样。这里先记下本身轨迹。

    首先在建立工程的时候，选中Use CoreData。

    这时候打开工程，会发现Appdelegate里面会多了如下的东西：

@property (readonly, strong) NSPersistentContainer *persistentContainer;

- (void)saveContext;

#pragma mark - Core Data stack

@synthesize persistentContainer = _persistentContainer;

- (NSPersistentContainer *)persistentContainer {
    @synchronized (self) {
        if (_persistentContainer == nil) {
            _persistentContainer = [[NSPersistentContainer alloc] initWithName:@"SmartOffice"];
            [_persistentContainer loadPersistentStoresWithCompletionHandler:^(NSPersistentStoreDescription *storeDescription, NSError *error) {
                if (error != nil) {
                    NSLog(@"Unresolved error %@, %@", error, error.userInfo);
                    abort();
                }
            }];
        }
    }
    return _persistentContainer;
}

#pragma mark - Core Data Saving support

- (void)saveContext {
    NSManagedObjectContext *context = self.persistentContainer.viewContext;
    NSError *error = nil;
    if ([context hasChanges] && ![context save:&error]) {
        NSLog(@"Unresolved error %@, %@", error, error.userInfo);
        abort();
    }
}

    而以前的版本会多出来这么写东西：

@property (readonly, strong, nonatomic) NSManagedObjectContext *managedObjectContext;
@property (readonly, strong, nonatomic) NSManagedObjectModel *managedObjectModel;
@property (readonly, strong, nonatomic) NSPersistentStoreCoordinator *persistentStoreCoordinator;

- (void)saveContext;
- (NSURL *)applicationDocumentsDirectory;

#pragma mark - Core Data stack

@synthesize managedObjectContext = _managedObjectContext;
@synthesize managedObjectModel = _managedObjectModel;
@synthesize persistentStoreCoordinator = _persistentStoreCoordinator;

- (NSURL *)applicationDocumentsDirectory {
    // The directory the application uses to store the Core Data store file. This code uses a directory named "com.try.SmartHome" in the application's documents directory.
    return [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
}

- (NSManagedObjectModel *)managedObjectModel {
    // The managed object model for the application. It is a fatal error for the application not to be able to find and load its model.
    if (_managedObjectModel != nil) {
        return _managedObjectModel;
    }
    NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"SmartHome" withExtension:@"momd"];
    _managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
    return _managedObjectModel;
}

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {
    // The persistent store coordinator for the application. This implementation creates and returns a coordinator, having added the store for the application to it.
    if (_persistentStoreCoordinator != nil) {
        return _persistentStoreCoordinator;
    }
    
    // Create the coordinator and store
    
    _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
    NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"SmartHome.sqlite"];
    NSError *error = nil;
    NSString *failureReason = @"There was an error creating or loading the application's saved data.";
    if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error]) {
        // Report any error we got.
        NSMutableDictionary *dict = [NSMutableDictionary dictionary];
        dict[NSLocalizedDescriptionKey] = @"Failed to initialize the application's saved data";
        dict[NSLocalizedFailureReasonErrorKey] = failureReason;
        dict[NSUnderlyingErrorKey] = error;
        error = [NSError errorWithDomain:@"YOUR_ERROR_DOMAIN" code:9999 userInfo:dict];
        // Replace this with code to handle the error appropriately.
        // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }
    
    return _persistentStoreCoordinator;
}


- (NSManagedObjectContext *)managedObjectContext {
    // Returns the managed object context for the application (which is already bound to the persistent store coordinator for the application.)
    if (_managedObjectContext != nil) {
        return _managedObjectContext;
    }
    
    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
    if (!coordinator) {
        return nil;
    }
    _managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
    [_managedObjectContext setPersistentStoreCoordinator:coordinator];
    return _managedObjectContext;
}

#pragma mark - Core Data Saving support

- (void)saveContext {
    NSManagedObjectContext *managedObjectContext = self.managedObjectContext;
    if (managedObjectContext != nil) {
        NSError *error = nil;
        if ([managedObjectContext hasChanges] && ![managedObjectContext save:&error]) {
            // Replace this implementation with code to handle the error appropriately.
            // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
            NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
            abort();
        }
    }
}

    很明显的一点就是Xcode升级到8.x的时候对CoreData又进行了进一步的优化，精简了不少。

    接下来就是建立Entity，建立属性，这些都没有什么变化。

    在选中Editor->Create NSManagerObject Subclass...的时候，出来了两个类，四个文件。这四个文件以下：（好比我是建立的User表的）

User+CoreDataClass.h
User+CoreDataClass.m
User+CoreDataProperties.h
User+CoreDataProperties.m

而以前生成的文件是这样的：

SHUser.h
SHUser.m
SHUserInfo.h
SHUserInfo.m

当你将这些文件导出完毕以后，进行编译。会发现编译出错，link command failed。

在我排除了未添加CoreData依赖裤这个错误的时候，我尝试了一下这个办法：

   

    就是将每个实体类的Codegen都设置成Manual/None，而后保存，从新又导出了一遍，再此编译。但是又出现了错误:File not found。看到这个错误，我忽然感受轻松了些许。既然文件找不到，我就来看看哪一个文件丢了。具体如图：

这明明是系统本身导出来的，为何还会报错了？？差了不少资料，最终给本身的解释就是，这是Xcode8.x的一个Bug。直接将报错的这句话注视了就能够了，或者将 #import "House.h" 改为#import "House+CoreDataClass.h"便可。修改完毕，再次编译一边。果真，很顺利的就跑通了。

接下来就是如何使用CoreData了，操做数据库不外乎这四种:

增：

删：

查：

改：

建议，最后建一个Manager，管理着数据库的增删查改。

若有不足，欢迎指正。