检查UITableViewCell是否完全可见的最佳方法

Question

我有一个UITableView与不同高度的单元格,我需要知道它们何时完全可见.

目前,我循环遍历可见单元格列表中的每个单元格,以检查每次滚动视图时它是否完全可见.这是最好的方法吗？

这是我的代码:

- (void)scrollViewDidScroll:(UIScrollView *)aScrollView {

    CGPoint offset = aScrollView.contentOffset;
    CGRect bounds = aScrollView.bounds;    
    NSArray* cells = myTableView.visibleCells;

    for (MyCustomUITableViewCell* cell in cells) {

        if (cell.frame.origin.y > offset.y &&
            cell.frame.origin.y + cell.frame.size.height < offset.y + bounds.size.height) {

            [cell notifyCompletelyVisible];
        }
        else {

            [cell notifyNotCompletelyVisible];
        }
    }
}

编辑:

请注意* - (NSArray )visibleCells返回可见单元格,这些单元格完全可见且部分可见.

编辑2:

这是在结合lnafziger和Vadim Yelagin的解决方案后的修订代码:

- (void)scrollViewDidScroll:(UIScrollView *)aScrollView {
    NSArray* cells = myTableView.visibleCells;
    NSArray* indexPaths = myTableView.indexPathsForVisibleRows;

    NSUInteger cellCount = [cells count];

    if (cellCount == 0) return;

    // Check the visibility of the first cell
    [self checkVisibilityOfCell:[cells objectAtIndex:0] forIndexPath:[indexPaths objectAtIndex:0]];

    if (cellCount == 1) return;

    // Check the visibility of the last cell
    [self checkVisibilityOfCell:[cells lastObject] forIndexPath:[indexPaths lastObject]];

    if (cellCount == 2) return;

    // All of the rest of the cells are visible: Loop through the 2nd through n-1 cells
    for (NSUInteger i = 1; i < cellCount - 1; i++)
        [[cells objectAtIndex:i] notifyCellVisibleWithIsCompletelyVisible:YES];
}

- (void)checkVisibilityOfCell:(MultiQuestionTableViewCell *)cell forIndexPath:(NSIndexPath *)indexPath {
    CGRect cellRect = [myTableView rectForRowAtIndexPath:indexPath];
    cellRect = [myTableView convertRect:cellRect toView:myTableView.superview];
    BOOL completelyVisible = CGRectContainsRect(myTableView.frame, cellRect);

    [cell notifyCellVisibleWithIsCompletelyVisible:completelyVisible];
}

Answer 1

您可以使用rectForRowAtIndexPath:方法获取单元格的矩形,并使用CGRectContainsRect函数将其与tableview的bounds rect进行比较.

请注意,如果单元格不可见,则不会实例化该单元格,因此会非常快.

迅速

let cellRect = tableView.rectForRowAtIndexPath(indexPath)
let completelyVisible = tableView.bounds.contains(cellRect)

OBJ-C

CGRect cellRect = [tableView rectForRowAtIndexPath:indexPath];
BOOL completelyVisible = CGRectContainsRect(tableView.bounds, cellRect);

当然,这不会将表视图视为由superview剪切或被其他视图遮盖.

Answer 2

我会改变它:

- (void)checkVisibilityOfCell:(MyCustomUITableViewCell *)cell inScrollView:(UIScrollView *)aScrollView {
    CGRect cellRect = [aScrollView convertRect:cell.frame toView:aScrollView.superview];

    if (CGRectContainsRect(aScrollView.frame, cellRect))
        [cell notifyCompletelyVisible];
    else
        [cell notifyNotCompletelyVisible];
}

- (void)scrollViewDidScroll:(UIScrollView *)aScrollView { 
    NSArray* cells = myTableView.visibleCells;

    NSUInteger cellCount = [cells count];
    if (cellCount == 0)
        return;

    // Check the visibility of the first cell
    [self checkVisibilityOfCell:[cells firstObject] inScrollView:aScrollView];
    if (cellCount == 1)
        return;

    // Check the visibility of the last cell
    [self checkVisibilityOfCell:[cells lastObject] inScrollView:aScrollView];
    if (cellCount == 2)
        return;

    // All of the rest of the cells are visible: Loop through the 2nd through n-1 cells
    for (NSUInteger i = 1; i < cellCount - 1; i++)
        [[cells objectAtIndex:i] notifyCompletelyVisible];
}

Answer 3

您可以尝试这样的事情来查看可见的百分比:

-(void)scrollViewDidScroll:(UIScrollView *)sender
{
    [self checkWhichVideoToEnable];
}

-(void)checkWhichVideoToEnable
{
    for(UITableViewCell *cell in [tblMessages visibleCells])
    {
        if([cell isKindOfClass:[VideoMessageCell class]])
        {
            NSIndexPath *indexPath = [tblMessages indexPathForCell:cell];
            CGRect cellRect = [tblMessages rectForRowAtIndexPath:indexPath];
            UIView *superview = tblMessages.superview;

            CGRect convertedRect=[tblMessages convertRect:cellRect toView:superview];
            CGRect intersect = CGRectIntersection(tblMessages.frame, convertedRect);
            float visibleHeight = CGRectGetHeight(intersect);

            if(visibleHeight>VIDEO_CELL_SIZE*0.6) // only if 60% of the cell is visible
            {
                // unmute the video if we can see at least half of the cell
                [((VideoMessageCell*)cell) muteVideo:!btnMuteVideos.selected];
            }
            else
            {
                // mute the other video cells that are not visible
                [((VideoMessageCell*)cell) muteVideo:YES];
            }
        }
    }
}

Answer 4

如果您还想考虑 contentInset，并且不想依赖超级视图（超级视图中的表视图框架可能不是 0,0），这是我的解决方案：

extension UITableView {

    public var boundsWithoutInset: CGRect {
        var boundsWithoutInset = bounds
        boundsWithoutInset.origin.y += contentInset.top
        boundsWithoutInset.size.height -= contentInset.top + contentInset.bottom
        return boundsWithoutInset
    }

    public func isRowCompletelyVisible(at indexPath: IndexPath) -> Bool {
        let rect = rectForRow(at: indexPath)
        return boundsWithoutInset.contains(rect)
    }
}

Answer 5

来自文档:

visibleCells返回接收器中可见的表格单元格.

- (NSArray *)visibleCells

返回值包含UITableViewCell对象的数组,每个对象表示接收表视图中的可见单元格.

可用性适用于iOS 2.0及更高版本.

另请参阅 - indexPathsForVisibleRows