使用NSExpression组合实体并返回一个NSArray或NSArray元素

Question

我希望NSFetchRequest为我的UITableViewController根据特定的属性对相似的记录（实体）进行分组。我目前分两步进行，但我相信有可能会使用+ (NSExpression *)expressionForAggregate:(NSArray *)collection。

有人可以帮助使用适当的代码吗？

下面是返回一个数组的数组，我的两个步骤的代码：

+(NSArray *)getTopQforDogByProgram2:(Dog *)dog 
        inProgram:(RunProgramTypes)programType 
       inManagedContext:(NSManagedObjectContext *)context { 
    NSString *searchString; 

    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Run"]; 
    request.predicate = [NSPredicate predicateWithFormat:[NSString stringWithFormat:@"dog.callName = '%@'",dog.callName]]; 
    NSSortDescriptor *classSortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"runClass" ascending:NO]; 
    request.sortDescriptors = [NSArray arrayWithObject:classSortDescriptor]; 

    NSError *error = nil; 
    NSArray *dataArray = [context executeFetchRequest:request error:&error]; 

    NSMutableArray *returnArray = [[NSMutableArray alloc] init]; 
    if ([dataArray count] > 0) { 
     NSMutableArray *pointArray = [[NSMutableArray alloc] init]; 
     for (Run *run in dataArray) { 
      if (! [returnArray count]) { 
       [pointArray addObject:run]; 
       [returnArray addObject:pointArray]; 
      } else { 
       BOOL wasSame = FALSE; 
       for (NSMutableArray *cmpArray in returnArray) { 
        Run *cmpRun = [cmpArray lastObject]; 
        if (cmpRun.runClass == run.runClass) { 
         [cmpArray addObject:run]; 
         wasSame = TRUE; 
         break; 
        } 
       } 
       if (! wasSame) { 
        pointArray = [[NSMutableArray alloc] init]; 
        [pointArray addObject:run]; 
        [returnArray addObject:pointArray]; 

       } 
      } 
     } 
    } 
    return returnArray; 
} 

Answer 1

你可以使用一个获取结果控制器做切片你是这样的：

NSFetchedResultsController* controller = [[NSFetchedResultsController alloc] initWithFetchRequest:request 
                     managedObjectContext:context 
                      sectionNameKeyPath:@"runClass" 
                        cacheName:nil]; 
NSMutableArray* sections = [[NSMutableArray alloc] initWithCapacity:[controller sections] count]; 
for (id<NSFetchedResultsSectionInfo> section in [controller sections]) { 
    NSMutableArray* sectionCopy = [NSMutableArray arrayWithArray:[section objects]]; 
    [sections addObject:sectionCopy]; 
} 

或者自己做：（给定的结果按runClass排序）

NSMutableArray* sections = [NSMutableArray new]; 
NSMutableArray* currentSection = [NSMutableArray new]; 
for (Run* run in dataArray) { 
    Run* lastObject = (Run*)[currentSection lastObject]; 
    if (lastObject && (run.runClass == lastObject.runClass)) { 
     currentSection = [NSMutableArray new]; 
     [sections addObject:currentSection]; 
    } 
    [currentSection addObject:run]; 
}