Core Data

• An external persistent store that contains saved records.

• A persistent object store that maps between records in the store and objects in your application.

• A persistent store coordinator that aggregates all the stores.

• A managed object model that describes the entities in the stores.

• A managed object context that provides a scratch pad for managed objects.

  
  

Managed object

Managed object context

Persistent store coordinator

Fetch request

Managed object model

Getting a Managed Object Context

managed object context 之下设置NSPersistentStoreCoordinator,NSPersistentStoreCoordinator 关联NSManagedObjectModel。

NSPersistentStoreCoordinator *psc = <#Get the coordinator#>;

NSManagedObjectContext *newContext = [[NSManagedObjectContext alloc] init];

[newContext setPersistentStoreCoordinator:psc];

NSManagedObjectContext *context = <#Get the context#>;

NSPersistentStoreCoordinator *psc = [context persistentStoreCoordinator];

NSManagedObjectModel *model = [psc managedObjectModel];

NSEntityDescription *entity = [[model entitiesByName] objectForKey:@"<#Entity name#>"];

Adding a Persistent Store

NSPersistentStoreCoordinator *psc = <#Get the coordinator#>;

NSURL *storeUrl = [NSURL fileURLWithPath:@"<#Path to store#>"];

NSString *storeType = <#Store type#>; // A store type, such as NSSQLiteStoreType

NSError *error;

if (![psc addPersistentStoreWithType:storeType configuration:nil

URL:storeUrl options:nil error:&error]) {

// Handle the error.

}

Fetch with Sorting and a Predicate

NSManagedObjectContext *context = <#Get the context#>;

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

NSEntityDescription *entity = [NSEntityDescription entityForName:@"<#Entity name#>" inManagedObjectContext:context];

[fetchRequest setEntity:entity];

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"<#Sort key#>" ascending:YES];

NSArray *sortDescriptors = @[sortDescriptor];

[fetchRequest setSortDescriptors:sortDescriptors];

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"<#Predicate string#>",

<#Predicate arguments#>];

[fetchRequest setPredicate:predicate];

NSError *error;

NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];

if (fetchedObjects == nil) {

// Handle the error.

}

Fetching Distinct Values 不重复的值

NSManagedObjectContext *context = <#Get the context#>;

NSEntityDescription *entity = [NSEntityDescription entityForName:@"<#Entity name#>" inManagedObjectContext:context];

NSFetchRequest *request = [[NSFetchRequest alloc] init];

[request setEntity:entity];

[request setResultType:NSDictionaryResultType];

[request setReturnsDistinctResults:YES];

[request setPropertiesToFetch:@[@"<#Attribute name#>"]];

// Execute the fetch.

NSError *error;

id requestedValue = nil;

NSArray *objects = [context executeFetchRequest:request error:&error];

if (objects == nil) {

// Handle the error.

}

Fetching Attribute Values that Satisfy a Given Function

NSManagedObjectContext *context = <#Get the context#>;

NSFetchRequest *request = [[NSFetchRequest alloc] init];

NSEntityDescription *entity = [NSEntityDescription entityForName:@"<#Entity name#>" inManagedObjectContext:context];

[request setEntity:entity];

// Specify that the request should return dictionaries.

[request setResultType:NSDictionaryResultType];

// Create an expression for the key path.

NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"<#Key-path for the property#>"];

// Create an expression to represent the function you want to apply

NSExpression *expression = [NSExpression expressionForFunction:@"<#Function name#>"

arguments:@[keyPathExpression]];

// Create an expression description using the minExpression and returning a date.

NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];

// The name is the key that will be used in the dictionary for the return value.

[expressionDescription setName:@"<#Dictionary key#>"];

[expressionDescription setExpression:expression];

[expressionDescription setExpressionResultType:<#Result type#>]; // For example, NSDateAttributeType

// Set the request's properties to fetch just the property represented by the expressions.

[request setPropertiesToFetch:@[expressionDescription]];

// Execute the fetch.

NSError *error;

id requestedValue = nil;

NSArray *objects = [context executeFetchRequest:request error:&error];

if (objects == nil) {

// Handle the error.

}

else {

if ([objects count] > 0) {

requestedValue = [[objects objectAtIndex:0] valueForKey:@"<#Dictionary key#>"];

}

}

Creating a Managed Object

NSManagedObjectContext *context = <#Get the context#>;

<#Managed Object Class#> *newObject = [NSEntityDescription

insertNewObjectForEntityForName:@"<#Entity name#>"

inManagedObjectContext:context];

NSManagedObjectContext *context = <#Get the context#>;

NSError *error;

if (![context save:&error]) {

// Handle the error.

}

Deleting a Managed Object

NSManagedObject *aManagedObject = <#Get the managed object#>;

NSManagedObjectContext *context = [aManagedObject managedObjectContext];

[context deleteObject:aManagedObject];

NSError *error;

if (![context save:&error]) {

// Handle the error.

}