我们知道，tableView的heightForRowAtIndexPath方法是调用最多的，反复在里面进行大量的重复计算消耗额外性能是不必要也不合理的。UITableView+FDTemplateLayoutCell就是抓住这一点做了缓存的优化，我们来看下他是怎么处理的吧！

1. 调用

我们在Demo中可以看到，有三种模式：

#pragma mark - UITableViewDelegate
- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {
    FDSimulatedCacheMode mode = self.cacheModeSegmentControl.selectedSegmentIndex;
    switch (mode) {
        case FDSimulatedCacheModeNone:
            return [tableView fd_heightForCellWithIdentifier:@"FDFeedCell" configuration:^(FDFeedCell *cell) {
                [self configureCell:cell atIndexPath:indexPath];
            }];
        case FDSimulatedCacheModeCacheByIndexPath:
            return [tableView fd_heightForCellWithIdentifier:@"FDFeedCell" cacheByIndexPath:indexPath configuration:^(FDFeedCell *cell) {
                [self configureCell:cell atIndexPath:indexPath];
            }];
        case FDSimulatedCacheModeCacheByKey: {
            FDFeedEntity *entity = self.feedEntitySections[indexPath.section][indexPath.row];

            return [tableView fd_heightForCellWithIdentifier:@"FDFeedCell" cacheByKey:entity.identifier configuration:^(FDFeedCell *cell) {
                [self configureCell:cell atIndexPath:indexPath];
            }];
        };
        default:
            break;
    }
}

对应的方法说明：

// 返回计算好的高度(无缓存)
- (CGFloat)fd_heightForCellWithIdentifier:(NSString *)identifier configuration:(void (^)(id cell))configuration; 
 
// 返回计算好的高度，并根据 indexPath 内部创建与之相应的二维数组缓存高度
- (CGFloat)fd_heightForCellWithIdentifier:(NSString *)identifier cacheByIndexPath:(NSIndexPath *)indexPath configuration:(void (^)(id cell))configuration;
 
// 返回计算好的高度，内部创建一个字典缓存高度并由使用者指定 key
- (CGFloat)fd_heightForCellWithIdentifier:(NSString *)identifier cacheByKey:(id<NSCopying>)key configuration:(void (^)(id cell))configuration;

引用原作者的话，如果调用了fd_heightForCellWithIdentifier: cacheByIndexPath: configuration:; 那么会就：

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {
    return [tableView fd_heightForCellWithIdentifier:@"identifer" cacheByIndexPath:indexPath configuration:^(id cell) {
        // 配置 cell 的数据源，和 "cellForRow" 干的事一致，比如：
        cell.entity = self.feedEntities[indexPath.row];
    }];
}

• 和每个 UITableViewCell ReuseID 一一对应的 template layout cell
  这个 cell 只为了参加高度计算，不会真的显示到屏幕上；它通过 UITableView 的 -dequeueCellForReuseIdentifier: 方法 lazy 创建并保存，所以要求这个 ReuseID 必须已经被注册到了 UITableView 中，也就是说，要么是 Storyboard 中的原型 cell，要么就是使用了 UITableView 的 -registerClass:forCellReuseIdentifier: 或 -registerNib:forCellReuseIdentifier:其中之一的注册方法。
• 根据 autolayout 约束自动计算高度
  使用了系统在 iOS6 就提供的 API：-systemLayoutSizeFittingSize:
• 根据 index path 的一套高度缓存机制
  计算出的高度会自动进行缓存，所以滑动时每个 cell 真正的高度计算只会发生一次，后面的高度询问都会命中缓存，减少了非常可观的多余计算。
• 自动的缓存失效机制
  无须担心你数据源的变化引起的缓存失效，当调用如-reloadData，-deleteRowsAtIndexPaths:withRowAnimation:等任何一个触发 UITableView 刷新机制的方法时，已有的高度缓存将以最小的代价执行失效。如删除一个 indexPath 为 [0:5] 的 cell 时，[0:0] ~ [0:4] 的高度缓存不受影响，而 [0:5] 后面所有的缓存值都向前移动一个位置。自动缓存失效机制对 UITableView 的 9 个公有 API 都进行了分别的处理，以保证没有一次多余的高度计算。
• 预缓存机制
  预缓存机制将在 UITableView 没有滑动的空闲时刻执行，计算和缓存那些还没有显示到屏幕中的 cell，整个缓存过程完全没有感知，这使得完整列表的高度计算既没有发生在加载时，又没有发生在滑动时，同时保证了加载速度和滑动流畅性，下文会着重讲下这块的实现原理。

2. 文件结构

实现可分为4个模块：

• UITableView+FDTemplateLayoutCell //提供高度获取方法
• UITableView+FDIndexPathHeightCache //通过NSIndexPath进行缓存高度
• UITableView+FDKeyedHeightCache //通过key值进行缓存高度
• UITableView+FDTemplateLayoutCellDebug //提供Debug打印

3. 源码分析

首先我们来看这个类：UITableView (FDIndexPathHeightCacheInvalidation)：

+ (void)load {
    //对 9 个触发 UITableView 刷新机制的公有方法分别进行了处理，保证缓存数组的正确
    SEL selectors[] = {
        @selector(reloadData),
        @selector(insertSections:withRowAnimation:),
        @selector(deleteSections:withRowAnimation:),
        @selector(reloadSections:withRowAnimation:),
        @selector(moveSection:toSection:),
        @selector(insertRowsAtIndexPaths:withRowAnimation:),
        @selector(deleteRowsAtIndexPaths:withRowAnimation:),
        @selector(reloadRowsAtIndexPaths:withRowAnimation:),
        @selector(moveRowAtIndexPath:toIndexPath:)
    };
    //拿到SEL总字节数除以SEL的字节数(8个字节)得到方法数
    for (NSUInteger index = 0; index < sizeof(selectors) / sizeof(SEL); ++index) {
      //遍历交互对应的方法实现
        SEL originalSelector = selectors[index];
        SEL swizzledSelector = NSSelectorFromString([@"fd_" stringByAppendingString:NSStringFromSelector(originalSelector)]);
        Method originalMethod = class_getInstanceMethod(self, originalSelector);
        Method swizzledMethod = class_getInstanceMethod(self, swizzledSelector);
        method_exchangeImplementations(originalMethod, swizzledMethod);
    }
}

//类似的几个方法都是清除fd_indexPathHeightCache中指定的sections和cell
- (void)fd_reloadData {
    if (self.fd_indexPathHeightCache.automaticallyInvalidateEnabled) {
        [self.fd_indexPathHeightCache enumerateAllOrientationsUsingBlock:^(FDIndexPathHeightsBySection *heightsBySection) {
            [heightsBySection removeAllObjects];
        }];
    }
    FDPrimaryCall([self fd_reloadData];);
}

然后我们直接从fd_heightForCellWithIdentifier: cacheByIndexPath: configuration:; 该方法入手看他的调用：

- (CGFloat)fd_heightForCellWithIdentifier:(NSString *)identifier cacheByIndexPath:(NSIndexPath *)indexPath configuration:(void (^)(id cell))configuration {
    if (!identifier || !indexPath) {
        return 0;
    }
    
    // 1.检测缓存中是否存在,UITableView+FDIndexPathHeightCache会判断当前是横屏还是竖屏返回对应的缓存二维数组，用于存放[indexPath.section][indexPath.row],为-1则返回NO
    if ([self.fd_indexPathHeightCache existsHeightAtIndexPath:indexPath]) {
        [self fd_debugLog:[NSString stringWithFormat:@"hit cache by index path[%@:%@] - %@", @(indexPath.section), @(indexPath.row), @([self.fd_indexPathHeightCache heightForIndexPath:indexPath])]];
      //2.如果命中缓存，则拿到对应的self.heightsBySectionForCurrentOrientation[indexPath.section][indexPath.row]返回高度
        return [self.fd_indexPathHeightCache heightForIndexPath:indexPath];
    }
    //3.如果不存在，则根据identifier取到对应的cell并且使用systemLayoutSizeFittingSize方法计算出高度
    CGFloat height = [self fd_heightForCellWithIdentifier:identifier configuration:configuration];
  
  //4.将高度保存到self.heightsBySectionForCurrentOrientation[indexPath.section][indexPath.row] = @(height);
    [self.fd_indexPathHeightCache cacheHeight:height byIndexPath:indexPath];
    [self fd_debugLog:[NSString stringWithFormat: @"cached by index path[%@:%@] - %@", @(indexPath.section), @(indexPath.row), @(height)]];
    
    return height;
}

//根据identifier和configuration 计算获得 cell 高度
- (CGFloat)fd_heightForCellWithIdentifier:(NSString *)identifier configuration:(void (^)(id cell))configuration {
    if (!identifier) {
        return 0;
    }
    
//    调用fd_templateCellForReuseIdentifier:方法获取UITableViewCell
    UITableViewCell *templateLayoutCell = [self fd_templateCellForReuseIdentifier:identifier];
    
    //手动调用prepareForReuse方法, 确保一致性。这个方法调用时机在cell滑出屏幕的时候进行调用
    [templateLayoutCell prepareForReuse];
    
    //执行回掉函数, 用以给cell设置显示的样式以及内容
    if (configuration) {
        configuration(templateLayoutCell);
    }
    
    //返回高度
    return [self fd_systemFittingHeightForConfiguratedCell:templateLayoutCell];
}

////    调用fd_templateCellForReuseIdentifier:方法获取UITableViewCell
- (__kindof UITableViewCell *)fd_templateCellForReuseIdentifier:(NSString *)identifier {
    NSAssert(identifier.length > 0, @"Expect a valid identifier - %@", identifier);
    
    //懒加载templateCellsByIdentifiers 字典, 该字典用于存储cell重用标识符与cell的键值对
    NSMutableDictionary<NSString *, UITableViewCell *> *templateCellsByIdentifiers = objc_getAssociatedObject(self, _cmd);
    if (!templateCellsByIdentifiers) {
        templateCellsByIdentifiers = @{}.mutableCopy;
        objc_setAssociatedObject(self, _cmd, templateCellsByIdentifiers, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    
    //通过identifier获取cell
    UITableViewCell *templateCell = templateCellsByIdentifiers[identifier];
    
    if (!templateCell) {
        //如果不存在，调用dequeueReusableCellWithIdentifier获取并存入字典中
        templateCell = [self dequeueReusableCellWithIdentifier:identifier];
        NSAssert(templateCell != nil, @"Cell must be registered to table view for identifier - %@", identifier);
        templateCell.fd_isTemplateLayoutCell = YES;
        templateCell.contentView.translatesAutoresizingMaskIntoConstraints = NO;
        templateCellsByIdentifiers[identifier] = templateCell;
        [self fd_debugLog:[NSString stringWithFormat:@"layout cell created - %@", identifier]];
    }
    
    return templateCell;
}

再来看下高度计算：

- (CGFloat)fd_systemFittingHeightForConfiguratedCell:(UITableViewCell *)cell {
    //    获取当前cell的宽度
    CGFloat contentViewWidth = CGRectGetWidth(self.frame);
    
    CGRect cellBounds = cell.bounds;
    cellBounds.size.width = contentViewWidth;
    cell.bounds = cellBounds;
    
    CGFloat accessroyWidth = 0;
    // If a cell has accessory view or system accessory type, its content view's width is smaller
    // than cell's by some fixed values.
    if (cell.accessoryView) {
        accessroyWidth = 16 + CGRectGetWidth(cell.accessoryView.frame);
    } else {
        static const CGFloat systemAccessoryWidths[] = {
            [UITableViewCellAccessoryNone] = 0,
            [UITableViewCellAccessoryDisclosureIndicator] = 34,
            [UITableViewCellAccessoryDetailDisclosureButton] = 68,
            [UITableViewCellAccessoryCheckmark] = 40,
            [UITableViewCellAccessoryDetailButton] = 48
        };
        accessroyWidth = systemAccessoryWidths[cell.accessoryType];
    }
    //根据accessoryType判断类型，并将宽度减去accessoryView的宽度
    contentViewWidth -= accessroyWidth;

    
    // If not using auto layout, you have to override "-sizeThatFits:" to provide a fitting size by yourself.
    // This is the same height calculation passes used in iOS8 self-sizing cell's implementation.
    //
    // 1. Try "- systemLayoutSizeFittingSize:" first. (skip this step if 'fd_enforceFrameLayout' set to YES.)
    // 2. Warning once if step 1 still returns 0 when using AutoLayout
    // 3. Try "- sizeThatFits:" if step 1 returns 0
    // 4. Use a valid height or default row height (44) if not exist one
    
    CGFloat fittingHeight = 0;
    
    //判断是FrameLayout还是autoLayout
    if (!cell.fd_enforceFrameLayout && contentViewWidth > 0) {
//        autoLayout
        
        // Add a hard width constraint to make dynamic content views (like labels) expand vertically instead
        // of growing horizontally, in a flow-layout manner.
        NSLayoutConstraint *widthFenceConstraint = [NSLayoutConstraint constraintWithItem:cell.contentView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:contentViewWidth];

        // [bug fix] after iOS 10.3, Auto Layout engine will add an additional 0 width constraint onto cell's content view, to avoid that, we add constraints to content view's left, right, top and bottom.
        static BOOL isSystemVersionEqualOrGreaterThen10_2 = NO;
        static dispatch_once_t onceToken;
        dispatch_once(&onceToken, ^{
            isSystemVersionEqualOrGreaterThen10_2 = [UIDevice.currentDevice.systemVersion compare:@"10.2" options:NSNumericSearch] != NSOrderedAscending;
        });
        
        NSArray<NSLayoutConstraint *> *edgeConstraints;
        if (isSystemVersionEqualOrGreaterThen10_2) {
            // To avoid confilicts, make width constraint softer than required (1000)
            widthFenceConstraint.priority = UILayoutPriorityRequired - 1;
            
            // Build edge constraints
            NSLayoutConstraint *leftConstraint = [NSLayoutConstraint constraintWithItem:cell.contentView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:cell attribute:NSLayoutAttributeLeft multiplier:1.0 constant:0];
            NSLayoutConstraint *rightConstraint = [NSLayoutConstraint constraintWithItem:cell.contentView attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:cell attribute:NSLayoutAttributeRight multiplier:1.0 constant:accessroyWidth];
            NSLayoutConstraint *topConstraint = [NSLayoutConstraint constraintWithItem:cell.contentView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:cell attribute:NSLayoutAttributeTop multiplier:1.0 constant:0];
            NSLayoutConstraint *bottomConstraint = [NSLayoutConstraint constraintWithItem:cell.contentView attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:cell attribute:NSLayoutAttributeBottom multiplier:1.0 constant:0];
            edgeConstraints = @[leftConstraint, rightConstraint, topConstraint, bottomConstraint];
            [cell addConstraints:edgeConstraints];
        }
        //为contentView添加一个宽度的约束(在自动约束中, label的换行要依赖superView的宽度和约束进行计算, 从而得到高度, 一般做法是给cell填充数据, 然后计算高度, 缓存; 而cell宽度, 在填充数据的时候, 是有问题的, 因为storyboard中建立的cell的宽度是不确定的, 但是如果写死, 又无法适配宽屏手机, 这就导致最后算的cell的高度不准确, 导致一些奇怪的UI布局, 解决办法正如源码中所展示的那样, 先为cell添加好宽度约束, 然后再计算高度, 再移除宽度约束, 这样一来, 就解决了问题)
        [cell.contentView addConstraint:widthFenceConstraint];

        //通过systemLayoutSizeFittingSize:方法获取cell的高度
        fittingHeight = [cell.contentView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize].height;
        
        //移除宽度约束
        [cell.contentView removeConstraint:widthFenceConstraint];
        if (isSystemVersionEqualOrGreaterThen10_2) {
            [cell removeConstraints:edgeConstraints];
        }
        
        [self fd_debugLog:[NSString stringWithFormat:@"calculate using system fitting size (AutoLayout) - %@", @(fittingHeight)]];
    }
    
    if (fittingHeight == 0) {
#if DEBUG
        // Warn if using AutoLayout but get zero height.
        if (cell.contentView.constraints.count > 0) {
            if (!objc_getAssociatedObject(self, _cmd)) {
                NSLog(@"[FDTemplateLayoutCell] Warning once only: Cannot get a proper cell height (now 0) from '- systemFittingSize:'(AutoLayout). You should check how constraints are built in cell, making it into 'self-sizing' cell.");
                objc_setAssociatedObject(self, _cmd, @YES, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
            }
        }
#endif
        // 如果是 frame layout. 使用 '- sizeThatFits:' 方法
        // Note: fitting height should not include separator view.
        fittingHeight = [cell sizeThatFits:CGSizeMake(contentViewWidth, 0)].height;
        
        [self fd_debugLog:[NSString stringWithFormat:@"calculate using sizeThatFits - %@", @(fittingHeight)]];
    }
    
    // Still zero height after all above.
    if (fittingHeight == 0) {
        // Use default row height.
        fittingHeight = 44;
    }
    
    // Add 1px extra space for separator line if needed, simulating default UITableViewCell.
    //如果tableView的separatorStyle 不是UITableViewCellSeparatorStyleNone, 则加1px的高度, 这里是1px, 所以要除以scale
    if (self.separatorStyle != UITableViewCellSeparatorStyleNone) {
        fittingHeight += 1.0 / [UIScreen mainScreen].scale;
    }
    //返回高度
    return fittingHeight;
}

另：如果是通过Key缓存,原理是几近类似的，区别在于存储的方式是改用了NSMutableDictionary去把height和key一一对应。

 case FDSimulatedCacheModeCacheByKey: {
            FDFeedEntity *entity = self.feedEntitySections[indexPath.section][indexPath.row];

            return [tableView fd_heightForCellWithIdentifier:@"FDFeedCell" cacheByKey:entity.identifier configuration:^(FDFeedCell *cell) {
                [self configureCell:cell atIndexPath:indexPath];
            }];
        };

- (CGFloat)fd_heightForCellWithIdentifier:(NSString *)identifier cacheByKey:(id<NSCopying>)key configuration:(void (^)(id cell))configuration {
    if (!identifier || !key) {
        return 0;
    }
    
    // Hit cache
    if ([self.fd_keyedHeightCache existsHeightForKey:key]) {
        CGFloat cachedHeight = [self.fd_keyedHeightCache heightForKey:key];
        [self fd_debugLog:[NSString stringWithFormat:@"hit cache by key[%@] - %@", key, @(cachedHeight)]];
        return cachedHeight;
    }
    
    CGFloat height = [self fd_heightForCellWithIdentifier:identifier configuration:configuration];
    [self.fd_keyedHeightCache cacheHeight:height byKey:key];
    [self fd_debugLog:[NSString stringWithFormat:@"cached by key[%@] - %@", key, @(height)]];
    
    return height;
}

对于UITableView+FDTemplateLayoutCellDebug，使用了objc_setAssociatedObject添加属性并加了一个NSLog方法方便打印数据：

- (BOOL)fd_debugLogEnabled {
    return [objc_getAssociatedObject(self, _cmd) boolValue];
}

- (void)setFd_debugLogEnabled:(BOOL)debugLogEnabled {
    objc_setAssociatedObject(self, @selector(fd_debugLogEnabled), @(debugLogEnabled), OBJC_ASSOCIATION_RETAIN);
}

- (void)fd_debugLog:(NSString *)message {
    if (self.fd_debugLogEnabled) {
        NSLog(@"** FDTemplateLayoutCell ** %@", message);
    }
}

参考文献：

优化UITableViewCell高度计算的那些事