前面我的一篇博文UITableViewCell 高度自适应扩展中说到了可以缓存Cell高度提高性能！下面为大家讲述如何添加缓存提高性能！先看两张用Time Profiler测试的缓存前后的耗时对比：

重点看函数heightForRowAtIndexPath

没加缓存前

timeProfile.gif

添加缓存后

recordTime2.gif

前后对比差距是巨大的,计算高度这里减少的时间大约是五到六倍

如何添加缓存？

• 新建一个缓存高度的类取名:ZHCellHeightCalculator

ZHCellHeightCalculator.h代码

@interface ZHCellHeightCalculator : NSObject

//系统计算高度后缓存进cache
-(void)setHeight:(CGFloat)height withCalculateheightModel:(ZHCalculateHeightModel *)model;

//根据model hash 获取cache中的高度,如过无则返回－1
-(CGFloat)heightForCalculateheightModel:(ZHCalculateHeightModel *)model;

//清空cache
-(void)clearCaches;

@end

ZHCellHeightCalculator.m代码

#import "ZHCellHeightCalculator.h"

@interface ZHCellHeightCalculator ()
@property (strong, nonatomic, readonly) NSCache *cache;
@end

@implementation ZHCellHeightCalculator

#pragma mark - Init
-(instancetype)init
{
    self = [super init];
    if (self) {
        [self defaultConfigure];
    }
    return self;
}

-(void)defaultConfigure
{
    NSCache *cache = [NSCache new];
    cache.name = @"ZHCellHeightCalculator.cache";
    cache.countLimit = 200;
    _cache = cache;

}

#pragma mark - NSObject

- (NSString *)description
{
    return [NSString stringWithFormat:@"<%@: cache=%@",
            [self class], self.cache];
}

#pragma mark - Publci Methods
-(void)clearCaches
{
    [self.cache removeAllObjects];
}


-(void)setHeight:(CGFloat)height withCalculateheightModel:(ZHCalculateHeightModel *)model
{
    NSAssert(model != nil, @"Cell Model can't  nil");
    NSAssert(height >= 0, @"cell height must greater than or equal to 0");
    
    [self.cache setObject:[NSNumber numberWithFloat:height] forKey:@(model.hash)];
}


-(CGFloat)heightForCalculateheightModel:(ZHCalculateHeightModel *)model
{
    NSNumber *cellHeightNumber = [self.cache objectForKey:@(model.hash)];
    if (cellHeightNumber) {
        return [cellHeightNumber floatValue];
    }else
        return -1;
    
}
@end

修改ZHCustomLayoutTableViewController.m中heightForRowAtIndexPath方法如下：

-(CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
   ZHCalculateHeightModel *model = model = [dataArray objectAtIndex:indexPath.row];
    
    CGFloat height = [heightCalculator heightForCalculateheightModel:model];
    if (height>0) {
        NSLog(@"cache height");
        return height;
    }else{
        NSLog(@"calculate height");
    }
    ZHCalculateTableViewCell *cell = self.prototypeCell;
    cell.contentView.translatesAutoresizingMaskIntoConstraints = NO;
    [self configureCell:cell atIndexPath:indexPath];//必须先对Cell中的数据进行配置使动态计算时能够知道根据Cell内容计算出合适的高度
    
    /*------------------------------重点这里必须加上contentView的宽度约束不然计算出来的高度不准确-------------------------------------*/
    CGFloat contentViewWidth = CGRectGetWidth(self.tableView.bounds);
    NSLayoutConstraint *widthFenceConstraint = [NSLayoutConstraint constraintWithItem:cell.contentView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:contentViewWidth];
    [cell.contentView addConstraint:widthFenceConstraint];
    // Auto layout engine does its math
    CGFloat fittingHeight = [cell.contentView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize].height;
    [cell.contentView removeConstraint:widthFenceConstraint];
    /*-------------------------------End------------------------------------*/
    
    CGFloat cellHeight = fittingHeight+2*1/[UIScreen mainScreen].scale;//必须加上上下分割线的高度
    [heightCalculator setHeight:cellHeight withCalculateheightModel:model];
    return cellHeight;
}

Demo 下载地址

GitHub