核心数据优化提取请求 [英] Core Data Optimize Fetch Request

问题描述

此提取请求在iPhone上执行需要大约10秒钟。
目标是从每个类别中提取1个随机缩略图。
设置fetchlimit = 1大多数时间返回相同的缩略图，因此我必须从每个类别获取所有照片。

This Fetch Request takes almost 10 seconds to execute on iPhone. The goal is to fetch 1 random thumbnail from each category. setting fetchlimit = 1 most of the times return the same thumbnail so I have to fetch all photos from each category.

任何想法？

    [categoriesArray enumerateObjectsUsingBlock:^(id object, NSUInteger index, BOOL *stop) {

    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Photos"];
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"place.subcategory.category == %@", object];
    [request setPredicate:predicate];
    NSError *error;
    NSArray *photosManagedObjectsArray = [managedObjectContext executeFetchRequest:request error:&error];

    NSUInteger randomIndex = arc4random() % [photosManagedObjectsArray count];

    NSManagedObject *photoObject = [photosManagedObjectsArray objectAtIndex:randomIndex];
    UIImage *photoImage = [UIImage imageWithData:[photoObject valueForKey:@"thumbnail"]];   
    UIImage *resizedImage = [photoImage imageCroppedToFitSize:CGSizeMake(IMAGE_WIDTH, IMAGE_HEIGHT)];
    [imagesArray addObject:resizedImage];
    [objectsArray addObject:photoObject];

}];

非常感谢！
Bill。

many thanks! Bill.

推荐答案

您不需要将所有对象拉入数组。只需更改

You don't need to pull all your objects into an array. Just change

NSArray *photosManagedObjectsArray = [managedObjectContext executeFetchRequest:request error:&error];`
NSUInteger randomIndex = arc4random() % [photosManagedObjectsArray count];

到此

NSUInteger count = [managedObjectContext countForFetchRequest:request error:&error];
NSUInteger randomIndex = arc4random() % count;

现在使用 fetchOffset

[request setFetchOffset:randomIndex];
[request setFetchLimit:1];

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