UITableView-FDTemplateLayoutCell源码分析

高度的计算

通过从可重用队列里出列一个cell,并将该cell配置好要展示的数据后(ps:在使用时如果有一些会影响cell高度的代码没有包括在配置block里将导致计算不准),开始计算cell的高度:

1. 给cell添加约束:
   NSLayoutConstraint *widthFenceConstraint = [NSLayoutConstraint constraintWithItem:cell.contentView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:contentViewWidth];
   对于10.2以上系统还需添加cell上下左右的约束:

   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];
           }

2. 调用系统APIsystemLayoutSizeFittingSize计算出高度:fittingHeight = [cell.contentView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize].height;

3. 移除之前添加的约束:

   [cell.contentView removeConstraint:widthFenceConstraint];
   if (isSystemVersionEqualOrGreaterThen10_2) {
       [cell removeConstraints:edgeConstraints];
   }

最终得到cell的高度.

高度缓存的实现

通过给UITableView增加一个类别(FDIndexPathHeightCache):

@interface UITableView (FDIndexPathHeightCache)
/// Height cache by index path. Generally, you don't need to use it directly.
@property (nonatomic, strong, readonly) FDIndexPathHeightCache *fd_indexPathHeightCache;
@end

类别里有一个FDIndexPathHeightCache类型的属性fd_indexPathHeightCache是只读的,用于操作缓存:设置缓存,取出缓存,清除缓存,查询某个缓存是否存在等.FDIndexPathHeightCache内部扩展定义了一个用于保存缓存的二维数组.以后取缓存时是从该二维数组取得.默认高度是-1,即该行cell还未缓存高度.

缓存高度的清除时机

定义了一个类别FDIndexPathHeightCacheInvalidation,该类别通过method swizzle交换了一些系统方法的实现.

+ (void)load {
    // All methods that trigger height cache's invalidation
    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:)
    };
    
    for (NSUInteger index = 0; index < sizeof(selectors) / sizeof(SEL); ++index) {
        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);
    }
}

- (void)fd_reloadData {
    if (self.fd_indexPathHeightCache.automaticallyInvalidateEnabled) {
        [self.fd_indexPathHeightCache enumerateAllOrientationsUsingBlock:^(FDIndexPathHeightsBySection *heightsBySection) {
            [heightsBySection removeAllObjects];
        }];
    }
    FDPrimaryCall([self fd_reloadData];);
}

当调用这些可能会影响cell高度的方法时,就会将缓存清除.

bug

在iOS 10.2以上可能会出现崩溃,崩溃在systemLayoutSizeFittingSize:处.一种解决办法:设置cell中label的preferredMaxLayoutWidth属性.