iphone - 为 UITableView 创建节索引标题的性能问题

Question

我在表格 View 中显示联系人数组 ( [[ContactStore sharedStore]allContacts] ) 并将列表分成字母部分。我使用以下代码返回联系人首字母的数组，以及每个字母条目数的字典。

    //create an array of the first letters of the names in the sharedStore
nameIndex = [[NSMutableArray alloc] init];

//create a dictionary to save the number of names for each first letter
nameIndexCount = [[NSMutableDictionary alloc]init];

for (int i=0; i<[[[ContactStore sharedStore]allContacts]count]; i++){

//Get the first letter and the name of each person
    Contact *p = [[[ContactStore sharedStore]allContacts]objectAtIndex:i];
    NSString *lastName = [p lastName];
    NSString *alphabet = [lastName substringToIndex:1];


    //If that letter is absent from the dictionary then add it and set its value as 1 
    if ([nameIndexCount objectForKey:alphabet] == nil) {
        [nameIndex addObject:alphabet];
        [nameIndexCount setValue:@"1" forKey:alphabet];
    //If its already present add one to its value
    } else {

        NSString *newValue = [NSString stringWithFormat:@"%d", ([[nameIndexCount valueForKey:alphabet] intValue] + 1)];

        [nameIndexCount setValue:newValue forKey:alphabet];
    }
} 

这行得通，但是当数组很大时速度很慢，我相信有更好的方法可以做到这一点，但我对此很陌生，所以不确定如何做。是否有更好的方法来执行此操作的建议？

Answer 1

虽然 Bio Cho 有一个很好的观点，但您可能会通过调用看到性能的提高

[[ContactStore sharedStore]allContacts]

只有一次。例如:

nameIndex = [[NSMutableArray alloc] init];
nameIndexCount = [[NSMutableDictionary alloc] init];

/*
 Create our own copy of the contacts only once and reuse it
 */
NSArray* allContacts = [[ContactStore sharedStore] allContacts];

for (int i=0; i<[allContacts count]; i++){
    //Get the first letter and the name of each person
    Contact *p = allContacts[i];
    NSString *lastName = [p lastName];
    NSString *alphabet = [lastName substringToIndex:1];

    //If that letter is absent from the dictionary then add it and set its value as 1 
    if ([nameIndexCount objectForKey:alphabet] == nil) {
        [nameIndex addObject:alphabet];
        [nameIndexCount setValue:@"1" forKey:alphabet];
    //If its already present add one to its value
    } else {
        NSString *newValue = [NSString stringWithFormat:@"%d", ([[nameIndexCount 
            valueForKey:alphabet] intValue] + 1)];

        [nameIndexCount setValue:newValue forKey:alphabet];
    }
} 

虽然我不能肯定地说，但我猜重复访问您的共享商店是让您丧命的原因。也许只需访问一次就可以满足您的需求。