优化TableView索引排序的性能

app中的一个TableView，使用原生的UILocalizedIndexedCollation进行索引排序（A-Z），实测速度很慢，400多条数据需要3秒多才能显示出来

定位后发现，瓶颈不在数据库访问和UI渲染上，就是索引排序太慢。优化前有性能问题的代码如下：

// slow method
-(void) assembleMembers:(NSArray*)origin
{
    [members removeAllObjects];
    
    UILocalizedIndexedCollation *collation = [UILocalizedIndexedCollation currentCollation];
    
    // slow point 1: takes 1.5 seconds when 400 records
    for (Member *member in origin) {
        NSInteger sect = [collation sectionForObject:member collationStringSelector:@selector(name)];
        member.sectionNumber = sect;
    }
    
    NSInteger highSection = [[collation sectionTitles] count];
    NSMutableArray *sectionArrays = [NSMutableArray arrayWithCapacity:highSection];
    for (int i = 0; i < highSection; i++) {
        NSMutableArray *sectionArray = [NSMutableArray arrayWithCapacity:1];
        [sectionArrays addObject:sectionArray];
    }
    
    for (Member *member in origin) {
        [(NSMutableArray*)[sectionArrays objectAtIndex:member.sectionNumber] addObject:member];
    }
    
    // slow point 2: takes 1.3 seconds when 400 records
    for (NSMutableArray *sectionArray in sectionArrays) {
        NSArray *sortedSection = [collation sortedArrayFromArray:sectionArray collationStringSelector:@selector(name)];
        [members addObject:sortedSection];
    }
}

有2段很慢，第一段是给Member分配sectionNumber，第二段是对26个子数组进行深度排序

发现了瓶颈，就针对瓶颈进行优化

分配sectionNumber的逻辑，放到每次插入数据库时。在表中增加section_number字段，保存这个值。这样每次从数据库取到的数据，就不需要在运行时排序了

深度排序耗时长的问题，改成首字母排序，速度也快了很多

最后的代码如下：

-(void) assembleMembers:(NSArray*)origin
{
    [members removeAllObjects];
    
    UILocalizedIndexedCollation *collation = [UILocalizedIndexedCollation currentCollation];
    
    NSInteger highSection = [[collation sectionTitles] count];
    NSMutableArray *sectionArrays = [NSMutableArray arrayWithCapacity:highSection];
    for (int i = 0; i < highSection; i++) {
        NSMutableArray *sectionArray = [NSMutableArray arrayWithCapacity:1];
        [sectionArrays addObject:sectionArray];
    }
    
    for (Member *member in origin) {
        [(NSMutableArray*)[sectionArrays objectAtIndex:member.sectionNumber] addObject:member];
    }
    
    for (NSMutableArray *sectionArray in sectionArrays) {
        
        [sectionArray sortUsingComparator:^NSComparisonResult(id obj1, id obj2) {
            Member *m1 = (Member*) obj1;
            Member *m2 = (Member*) obj2;
            return [m2.name localizedCompare:m1.name];
        }];
        
        [members addObject:sectionArray];
    }
}

这样改完之后，400条数据的处理时间缩短到0.3秒，基本可以接受了