动态获取cell高度的方案

最近的项目中大面积使用了TableView 。这让我又一次对现有的TableView的性能产生了怀疑。本着认真负责的态度我找到了Sunny的这篇博文，细细研究后有一种相见恨晚的感觉。本文就贴出cell高度动态计算的方案

- (CGFloat)fd_systemFittingHeightForConfiguratedCell:(UITableViewCell *)cell {
    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];
    }
    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;
    
    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];

        // [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 *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];
        }
        
        [cell.contentView addConstraint:widthFenceConstraint];

        // Auto layout engine does its math
        fittingHeight = [cell.contentView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize].height;
        
        // Clean-ups
        [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
        // 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;
}

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