基於Xcode8.x的CoreData的使用

    首先說說對於CoreData的理解，相似於FMDB。他能將數據庫中的列表轉換成模型實體對象，方便開發者使用。同時，也能將一個將實體對象轉換成數據庫中的表，存儲到數據庫中。這就是CoreData,數據庫與實體模型之間的轉換工具。

    打開的沙盒的Library路徑，你會發現，以下的文件目錄：（前提是你的工程裏面使用了CoreData）

由此能夠證實，CoreData相似FMDB，都是基於sqlite進行數據存儲的。

    因爲Xcode升級到8.x，CoreData的使用與以往之間會有所不一樣。這裏先記下本身軌跡。

    首先在建立工程的時候，選中Use CoreData。

    這時候打開工程，會發現Appdelegate裏面會多瞭如下的東西：

@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();
    }
}

    而以前的版本會多出來這麼寫東西：

@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();
        }
    }
}

    很明顯的一點就是Xcode升級到8.x的時候對CoreData又進行了進一步的優化，精簡了不少。

    接下來就是建立Entity，建立屬性，這些都沒有什麼變化。

    在選中Editor->Create NSManagerObject Subclass...的時候，出來了兩個類，四個文件。這四個文件以下：（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這樣的：

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

當你將這些文件導出完畢以後，進行編譯。會發現編譯出錯，link command failed。

在我排除了未添加CoreData依賴褲這個錯誤的時候，我嘗試了一下這個辦法：

   

    就是將每個實體類的Codegen都設置成Manual/None，而後保存，從新又導出了一遍，再此編譯。但是又出現了錯誤:File not found。看到這個錯誤，我忽然感受輕鬆了些許。既然文件找不到，我就來看看哪一個文件丟了。具體如圖：

這明明是系統本身導出來的，爲何還會報錯了？？差了不少資料，最終給本身的解釋就是，這是Xcode8.x的一個Bug。直接將報錯的這句話注視了就能夠了，或者將 #import "House.h" 改爲#import "House+CoreDataClass.h"便可。修改完畢，再次編譯一邊。果真，很順利的就跑通了。

接下來就是如何使用CoreData了，操做數據庫不外乎這四種:

增：

刪：

查：

改：

建議，最後建一個Manager，管理着數據庫的增刪查改。

若有不足，歡迎指正。