@peerless提供的解决方案是一个很好的开始，但是它仅在拖动开始时才启动动画，而无需考虑滚动速度。这比您在Facebook应用程序中获得的体验更为混乱。为了匹配Facebook的行为，我们需要：
- 隐藏/显示导航栏，其速度与拖动速度成比例
- 如果在部分隐藏栏时滚动停止，则启动动画以完全隐藏栏
- 随着导航栏缩小，淡化导航栏的项目。
首先，您需要以下属性：
@property (nonatomic) CGFloat previousScrollViewYOffset;
这里是UIScrollViewDelegate方法：
- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
CGRect frame = self.navigationController.navigationBar.frame;
CGFloat size = frame.size.height - 21;
CGFloat framePercentageHidden = ((20 - frame.origin.y) / (frame.size.height - 1));
CGFloat scrollOffset = scrollView.contentOffset.y;
CGFloat scrollDiff = scrollOffset - self.previousScrollViewYOffset;
CGFloat scrollHeight = scrollView.frame.size.height;
CGFloat scrollContentSizeHeight = scrollView.contentSize.height + scrollView.contentInset.bottom;
if (scrollOffset <= -scrollView.contentInset.top) {
frame.origin.y = 20;
} else if ((scrollOffset + scrollHeight) >= scrollContentSizeHeight) {
frame.origin.y = -size;
} else {
frame.origin.y = MIN(20, MAX(-size, frame.origin.y - scrollDiff));
}
[self.navigationController.navigationBar setFrame:frame];
[self updateBarButtonItems:(1 - framePercentageHidden)];
self.previousScrollViewYOffset = scrollOffset;
}
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
[self stoppedScrolling];
}
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView
willDecelerate:(BOOL)decelerate
{
if (!decelerate) {
[self stoppedScrolling];
}
}
您还需要以下辅助方法：
- (void)stoppedScrolling
{
CGRect frame = self.navigationController.navigationBar.frame;
if (frame.origin.y < 20) {
[self animateNavBarTo:-(frame.size.height - 21)];
}
}
- (void)updateBarButtonItems:(CGFloat)alpha
{
[self.navigationItem.leftBarButtonItems enumerateObjectsUsingBlock:^(UIBarButtonItem* item, NSUInteger i, BOOL *stop) {
item.customView.alpha = alpha;
}];
[self.navigationItem.rightBarButtonItems enumerateObjectsUsingBlock:^(UIBarButtonItem* item, NSUInteger i, BOOL *stop) {
item.customView.alpha = alpha;
}];
self.navigationItem.titleView.alpha = alpha;
self.navigationController.navigationBar.tintColor = [self.navigationController.navigationBar.tintColor colorWithAlphaComponent:alpha];
}
- (void)animateNavBarTo:(CGFloat)y
{
[UIView animateWithDuration:0.2 animations:^{
CGRect frame = self.navigationController.navigationBar.frame;
CGFloat alpha = (frame.origin.y >= y ? 0 : 1);
frame.origin.y = y;
[self.navigationController.navigationBar setFrame:frame];
[self updateBarButtonItems:alpha];
}];
}
对于稍有不同的行为，请使用以下内容替换滚动时重新else放置条的行（中的块scrollViewDidScroll）：
frame.origin.y = MIN(20,
MAX(-size, frame.origin.y -
(frame.size.height * (scrollDiff / scrollHeight))));
这将根据最后一个滚动百分比而不是绝对值来定位条形，这会导致变慢。原始行为更像Facebook，但我也喜欢这一行为。
注意：此解决方案仅适用于iOS 7+。如果您支持旧版本的iOS，请确保添加必要的检查。
