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如何在核心数据中获得一个超出请求数据范围的数据点?

linechart tiles ios core-data iphone

TinkerTank · 技术社区 · 14 年前

每个磁贴使用谓词检索与该磁贴相关的数据:

[请求集合谓词:谓词];

NSMutableArray*result=[[managedObjectContext executeFetchRequest:请求错误:&错误]mutableCopy];

我在绘制位于不同分幅中的数据点之间的线时遇到问题,因为绘制此线需要分幅范围之外的数据点。

1 回复 | 直到 14 年前

TinkerTank 14 年前

写下问题实际上解决了问题的经典案例之一。在这种情况下,不幸的是在实际发布几分钟后。对不起的。

解决方案很简单:我只需再添加两个谓词,一个用于在请求的数据区域之前检索数据点,另一个用于在请求的数据区域之后检索数据点,然后将数组合并在一起。

NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Event" inManagedObjectContext:managedObjectContext];
[request setEntity:entity];

// Get the the last point (closest to now) first.
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"creationDate" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
[request setFetchLimit:1]; // Keeps the application fast and the memory requirements low.
[sortDescriptors release];
[sortDescriptor release];
NSPredicate* predicate = [NSPredicate predicateWithFormat:@"creationDate > %@",realEndDate];
[request setPredicate:predicate];
NSMutableArray *mutableFetchResults = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy];

// Get the data within the requested range.
sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"creationDate" ascending:NO];
sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
[request setFetchLimit:200]; // Keeps the application fast and the memory requirements low.
[sortDescriptors release];
[sortDescriptor release];
predicate = [NSPredicate predicateWithFormat:@"creationDate > %@ AND creationDate < %@",beginDate, realEndDate];
[request setPredicate:predicate];
NSMutableArray *additionalFetchResults = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy];
[mutableFetchResults addObjectsFromArray:additionalFetchResults];
[additionalFetchResults release];

// Get the first point last
[request setFetchLimit:1];
predicate = [NSPredicate predicateWithFormat:@"creationDate < %@",beginDate];
[request setPredicate:predicate];
additionalFetchResults = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy];
[mutableFetchResults addObjectsFromArray: additionalFetchResults];
[additionalFetchResults release];