UITableView优化那点事

至于UITableView的瓶颈在哪里，我相信网上随便一搜就能了解的大概，我这里顺便提供下信息点:

//罪魁祸首
tableView:cellForRowAtIndexPath:
tableView:heightForRowAtIndexPath:

框架同样根据这两个痛点给出了解决方案:

高度计算

fd_heightForCellWithIdentifier:configuration方法

- (CGFloat)fd_heightForCellWithIdentifier:(NSString *)identifier configuration:(void (^)(id cell))configuration {
    if (!identifier) {
        return 0;
    }
    
    UITableViewCell *templateLayoutCell = [self fd_templateCellForReuseIdentifier:identifier];
    
    // Manually calls to ensure consistent behavior with actual cells. (that are displayed on screen)
    [templateLayoutCell prepareForReuse];
    
    // Customize and provide content for our template cell.
    if (configuration) {
        configuration(templateLayoutCell);
    }
    
    return [self fd_systemFittingHeightForConfiguratedCell:templateLayoutCell];
}

这里先是通过调用fd_templateCellForReuseIdentifier:从dequeueReusableCellWithIdentifier取出之后,如果需要做一些额外的计算,比如说计算cell高度, 可以手动调用 prepareForReuse方法，以确保与实际cell(显示在屏幕上)行为一致。接着执行configuration参数对Cell内容进行配置。最后通过调用fd_systemFittingHeightForConfiguratedCell:方法计算实际的高度并返回。

fd_systemFittingHeightForConfiguratedCell方法

- (CGFloat)fd_systemFittingHeightForConfiguratedCell:(UITableViewCell *)cell {
    CGFloat contentViewWidth = CGRectGetWidth(self.frame);
    
    // 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) {
        contentViewWidth -= 16 + CGRectGetWidth(cell.accessoryView.frame);
    } else {
        static const CGFloat systemAccessoryWidths[] = {
            [UITableViewCellAccessoryNone] = 0,
            [UITableViewCellAccessoryDisclosureIndicator] = 34,
            [UITableViewCellAccessoryDetailDisclosureButton] = 68,
            [UITableViewCellAccessoryCheckmark] = 40,
            [UITableViewCellAccessoryDetailButton] = 48
        };
        contentViewWidth -= systemAccessoryWidths[cell.accessoryType];
    }
    
    // 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;
    
    if (!cell.fd_enforceFrameLayout && contentViewWidth > 0) {
        // 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];
        [cell.contentView addConstraint:widthFenceConstraint];
        
        // Auto layout engine does its math
        fittingHeight = [cell.contentView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize].height;
        [cell.contentView removeConstraint:widthFenceConstraint];
        
        [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
        // Try '- sizeThatFits:' for frame layout.
        // 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.
    if (self.separatorStyle != UITableViewCellSeparatorStyleNone) {
        fittingHeight += 1.0 / [UIScreen mainScreen].scale;
    }
    
    return fittingHeight;
}

这里作者考虑到了如果Cell使用了accessory view或者使用了系统的accessory type，需要减掉相应的宽度。接着判断如果使用了AutoLayout，则使用iOS 6提供的systemLayoutSizeFittingSize方法获取高度。如果高度为0，则尝试使用Frame Layout的方式，调用重写的sizeThatFits方法进行获取。如果还是为0，则给出默认高度并返回。

Cell重用

fd_templateCellForReuseIdentifier方法

- (__kindof UITableViewCell *)fd_templateCellForReuseIdentifier:(NSString *)identifier {
    NSAssert(identifier.length > 0, @"Expect a valid identifier - %@", identifier);
    
    NSMutableDictionary<NSString *, UITableViewCell *> *templateCellsByIdentifiers = objc_getAssociatedObject(self, _cmd);
    if (!templateCellsByIdentifiers) {
        templateCellsByIdentifiers = @{}.mutableCopy;
        objc_setAssociatedObject(self, _cmd, templateCellsByIdentifiers, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    
    UITableViewCell *templateCell = templateCellsByIdentifiers[identifier];
    
    if (!templateCell) {
        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;
}

这里通过dequeueReusableCellWithIdentifier方法从队列中获取templateCell，并通过fd_isTemplateLayoutCell属性标识其只用来充当模板计算，并不真正进行呈现，最后通过关联对象的方式进行存取。
注意：这里通过dequeueReusableCellWithIdentifier进行获取，也就意味着你必须对指定的Identifier先进行注册，注册可以通过以下三中方法:

1.使用storyboard中的Cell原型
2.使用registerNib:forCellReuseIdentifier:
3.使用registerClass:forCellReuseIdentifier:

到这里最重要的几个方法已经讲完了，除此之外框架还针对获取的高度进行了缓存。缓存的方式分为两种 ：

1.根据IndexPath进行缓存(fd_heightForCellWithIdentifier:cacheByIndexPath:configuration)
2.根据实体的唯一标识符进行缓存(fd_heightForCellWithIdentifier:cacheByKey:configuration)

总结:

UITableView优化方案其实还有很多，不同的场景选用不同的方案，实现效果达到预期，这才是我么最终的目标。我这里简单介绍下其他的优化的细节:

1.复杂界面的时候，我们可以尝试异步手动进行绘制。
2.针对超出屏幕的Cell进行预缓存
3.存在大量图片的时候，只针对目标范围内的图片进行异步加载并缓存结果。
4.设置Views/Layers为不透明。