tableView计算完高度后，把高度缓存起来，避免下次重复计算，以减少不必要的消耗

// declare cellHeightsDictionary
NSMutableDictionary *cellHeightsDictionary;
 
// initialize in code (thanks to @Gerharbo)
cellHeightsDictionary = @{}.mutableCopy;
 
// declare table dynamic row height and create correct constraints in cells
tableView.rowHeight = UITableViewAutomaticDimension;
 
// save height
- (void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath {
    [cellHeightsDictionary setObject:@(cell.frame.size.height) forKey:indexPath];
}
 
// give exact height value
- (CGFloat)tableView:(UITableView *)tableView estimatedHeightForRowAtIndexPath:(NSIndexPath *)indexPath {
    NSNumber *height = [cellHeightsDictionary objectForKey:indexPath];
    if (height) return height.doubleValue;
    return UITableViewAutomaticDimension;
}

iOS tableView高度缓存