Question

当我们滚动出视图时，我正在淡出我的uitableview的单元格，或者当它们滚动到视图中时将它们淡入。我面临的问题是，如果我滚动得非常快，有时完全可见的细胞仍然会变暗。以下是我的代码：

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    // Fades out top and bottom cells in table view as they leave the screen
    NSArray *visibleCells = [self.tableView visibleCells];

    if (visibleCells != nil  &&  [visibleCells count] != 0) {       // Don't do anything for empty table view

        /* Get top and bottom cells */
        UITableViewCell *topCell = [visibleCells objectAtIndex:0];
        UITableViewCell *bottomCell = [visibleCells lastObject];

        /* Make sure other cells stay opaque */
        // Avoids issues with skipped method calls during rapid scrolling
        for (UITableViewCell *cell in visibleCells) {
            cell.contentView.alpha = 1.0;
        }

        /* Set necessary constants */
        NSInteger cellHeight = topCell.frame.size.height - 1;   // -1 To allow for typical separator line height
        NSInteger tableViewTopPosition = self.tableView.frame.origin.y;
        NSInteger tableViewBottomPosition = self.tableView.frame.origin.y + self.tableView.frame.size.height;

        /* Get content offset to set opacity */
        CGRect topCellPositionInTableView = [self.tableView rectForRowAtIndexPath:[self.tableView indexPathForCell:topCell]];
        CGRect bottomCellPositionInTableView = [self.tableView rectForRowAtIndexPath:[self.tableView indexPathForCell:bottomCell]];
        CGFloat topCellPosition = [self.tableView convertRect:topCellPositionInTableView toView:[self.tableView superview]].origin.y;
        CGFloat bottomCellPosition = ([self.tableView convertRect:bottomCellPositionInTableView toView:[self.tableView superview]].origin.y + cellHeight);

        /* Set opacity based on amount of cell that is outside of view */
        CGFloat modifier = 1.2;     /* Increases the speed of fading (1.0 for fully transparent when the cell is entirely off the screen,
                                 2.0 for fully transparent when the cell is half off the screen, etc) */
        CGFloat topCellOpacity = (1.0f - ((tableViewTopPosition - topCellPosition) / cellHeight) * modifier);
        CGFloat bottomCellOpacity = (1.0f - ((bottomCellPosition - tableViewBottomPosition) / cellHeight) * modifier);

        /* Set cell opacity */
        if (topCell) {
            topCell.alpha = topCellOpacity;
        }
        if (bottomCell) {
            bottomCell.alpha = bottomCellOpacity;
        }  
    }
}

为什么当我在视线中快速滚动时，为什么有时会保持暗淡？

Answer 1

问题是滚动事件并不总是频繁发生，滚动到视图的单元格将从褪色的alpha变为alpha的1.0，因为它从顶部/底部滚动到中间。但是当你快速滚动时，scrollViewDidScroll的调用频率不足以确保这种情况。显然正在尝试重置alpha的循环正在更新错误视图的alpha（contentView而不是单元格本身。）

您可以通过替换现有的for循环来解决此问题：

for (UITableViewCell *cell in visibleCells) {
    cell.contentView.alpha = 1.0;
}

使用

for (UITableViewCell *cell in visibleCells) {
    cell.alpha = 1.0;
}

或者，从那里消除那个循环，然后替换设置顶部和底部单元格的alpha的行：

if (topCell) {
    topCell.alpha = topCellOpacity;
}
if (bottomCell) {
    bottomCell.alpha = bottomCellOpacity;
}

使用：

for (UITableViewCell *cell in self.tableView.visibleCells) {
    if (cell == topCell) {
        cell.alpha = topCellOpacity;
    } else if (cell == bottomCell) {
        cell.alpha = bottomCellOpacity;
    } else {
        cell.alpha = 1.0;
    }
}

顺便说一下，实现类似效果的另一种方法是将渐变蒙版应用于整个tableview并退出scrollViewDidScroll方法。这里唯一的技巧是你不能将渐变蒙版应用于表视图本身（或者梯度将使用表视图滚动），而是将tableview放在某个容器UIView内，然后应用蒙版对此：

- (void)viewDidAppear:(BOOL)animated
{
    CAGradientLayer *gradient = [CAGradientLayer layer];
    gradient.frame = self.containerView.bounds;
    gradient.colors = @[(id)[UIColor clearColor].CGColor,
                        (id)[UIColor whiteColor].CGColor,
                        (id)[UIColor whiteColor].CGColor,
                        (id)[UIColor clearColor].CGColor];
    gradient.locations = @[@0.0, @0.1, @0.9, @1.0];
    self.containerView.layer.mask = gradient;
}

这无疑是一种略有不同的效果，但有时它是可取的。这取决于你拍摄的内容。

Answer 2

这是您为Swift 4做的方式

extension UITableView {

    func fadeEdges(with modifier: CGFloat) {

        let visibleCells = self.visibleCells

        guard !visibleCells.isEmpty else { return }
        guard let topCell = visibleCells.first else { return }
        guard let bottomCell = visibleCells.last else { return }

        visibleCells.forEach {
            $0.contentView.alpha = 1
        }

        let cellHeight = topCell.frame.height - 1
        let tableViewTopPosition = self.frame.origin.y
        let tableViewBottomPosition = self.frame.maxY

        guard let topCellIndexpath = self.indexPath(for: topCell) else { return }
        let topCellPositionInTableView = self.rectForRow(at:topCellIndexpath)

        guard let bottomCellIndexpath = self.indexPath(for: bottomCell) else { return }
        let bottomCellPositionInTableView = self.rectForRow(at: bottomCellIndexpath)

        let topCellPosition = self.convert(topCellPositionInTableView, to: self.superview).origin.y
        let bottomCellPosition = self.convert(bottomCellPositionInTableView, to: self.superview).origin.y + cellHeight
        let topCellOpacity = (1.0 - ((tableViewTopPosition - topCellPosition) / cellHeight) * modifier)
        let bottomCellOpacity = (1.0 - ((bottomCellPosition - tableViewBottomPosition) / cellHeight) * modifier)

        topCell.contentView.alpha = topCellOpacity
        bottomCell.contentView.alpha = bottomCellOpacity
    }

}

当您滚动时，ios uitableview会淡化底部单元格和顶部单元格

2 个答案: