在我的iPhone项目中,我想编写一个函数,检查Core Data ManagedObjectContext中是否存在某个特定值的对象,如some_property
。通过核心数据中的属性获取对象
如果已经有一个对象some_property == 12
,我想让该函数返回该对象,否则,我想创建该对象,或者至少返回nil
。
我该怎么做?
在我的iPhone项目中,我想编写一个函数,检查Core Data ManagedObjectContext中是否存在某个特定值的对象,如some_property
。通过核心数据中的属性获取对象
如果已经有一个对象some_property == 12
,我想让该函数返回该对象,否则,我想创建该对象,或者至少返回nil
。
我该怎么做?
以下片段显示如何检索匹配特定谓词的对象。如果没有这样的对象,代码片段展示了如何创建一个新的对象,保存并返回它。
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"YourEntityName" inManagedObjectContext:managedObjectContext];
[request setEntity:entity];
// retrive the objects with a given value for a certain property
NSPredicate *predicate = [NSPredicate predicateWithFormat: @"property == %@", value];
[request setPredicate:predicate];
// Edit the sort key as appropriate.
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"yourSortKey" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
// Edit the section name key path and cache name if appropriate.
// nil for section name key path means "no sections".
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"];
aFetchedResultsController.delegate = self;
NSError *error = nil;
NSArray *result = [managedObjectContext executeFetchRequest:request error:&error];
[request release];
[sortDescriptor release];
[sortDescriptors release];
if ((result != nil) && ([result count]) && (error == nil)){
return [NSMutableArray arrayWithArray:result];
}
else{
YourEntityName *object = (YourEntityName *) [NSEntityDescription insertNewObjectForEntityForName:@"YourEntityName" inManagedObjectContext:self.managedObjectContext];
// setup your object attributes, for instance set its name
object.name = @"name"
// save object
NSError *error;
if (![[self managedObjectContext] save:&error]) {
// Handle error
NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
}
return object;
}
如果您想检查本地数据的某些属性,最好不要多次抓取。只需使用预先填充的数组执行一个获取请求,然后迭代或过滤结果。
这是从核心数据编程指南代码段“实现查找 - 或 - 创建高效”:
// get the names to parse in sorted order
NSArray *employeeIDs = [[listOfIDsAsString componentsSeparatedByString:@"\n"]
sortedArrayUsingSelector: @selector(compare:)];
// create the fetch request to get all Employees matching the IDs
NSFetchRequest *fetchRequest = [[[NSFetchRequest alloc] init] autorelease];
[fetchRequest setEntity:
[NSEntityDescription entityForName:@"Employee" inManagedObjectContext:aMOC]];
[fetchRequest setPredicate: [NSPredicate predicateWithFormat: @"(employeeID IN %@)", employeeIDs]];
// make sure the results are sorted as well
[fetchRequest setSortDescriptors: [NSArray arrayWithObject:
[[[NSSortDescriptor alloc] initWithKey: @"employeeID"
ascending:YES] autorelease]]];
// Execute the fetch
NSError *error;
NSArray *employeesMatchingNames = [aMOC
executeFetchRequest:fetchRequest error:&error];
哇,那是快!让我试试这个...... – winsmith 2009-09-17 13:21:26
'aFetchedResultsController'有什么意义?我错误地认为你创造它,但从来没有用它做任何事情? – ArtOfWarfare 2013-06-30 04:30:58
你是对的,在这个特殊的例子中,NSFetchedResultsController没有被使用,但它应该在真实应用程序的上下文中(它简化了许多其他的事情,并提供了一个很好的缓存机制)。 – 2013-06-30 08:32:51