ios - 如何随着滚动改变 alpha 值

Question

我找不到这种棘手问题的任何地方。我的问题是当用户开始滚动时我需要更改 alpha 值。在滚动开始时 alpha 值应该是 1，然后在滚动中间 alpha 值应该是0.5，最后它必须是 0。这是我需要做的。我用谷歌搜索找不到。请帮助我

Answer 1

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    /* This is the offset at the bottom of the scroll view. */
    CGFloat totalScroll = scrollView.contentSize.height - scrollView.bounds.size.height;

    /* This is the current offset. */
    CGFloat offset = - scrollView.contentOffset.y;

    /* This is the percentage of the current offset / bottom offset. */
    CGFloat percentage = offset / totalScroll;

    /* When percentage = 0, the alpha should be 1 so we should flip the percentage. */
    scrollView.alpha = (1.f - percentage);
}