问题描述

我想创建一个 UITableView 或 UIScrollView 的子类，当内容偏移量大于 0 时，它会在顶部有一些阴影，表示内容是可滚动的.(见附图)

I want to create a subclass of UITableView or UIScrollView that will have some shading at the top when the content offset is > 0 to indicate that the content is scrollable. (See image attached)

我现在实现它的方式是使用 UIViewController，它是 tableView 的委托.我只是在 tableView 顶部有一个 GradientView，然后截取 scrollViewDidScroll: 以动画化顶部渐变的可见性.

The way I'm implementing it right now is using the UIViewController that is the delegate of the tableView. I simply have a GradientView on top of the tableView, and I intercept scrollViewDidScroll: to animate the visibility of that top gradient.

我对这个实现的问题是它不是干净的".我希望我的 UIViewControllers 处理逻辑，而不是处理应用渐变和其他东西.我希望我可以删除一个 UITableView 的子类，它会为我做到这一点.

My problem with this implementation is that it's not "clean". I want my UIViewControllers to take care of logic, and not to deal with applying gradients and stuff. I wish I could just drop a subclass of UITableView that will do that for me.

我面临的挑战是我无法弄清楚 tableView 如何在可滚动内容之上添加一个固定内容.

The challenge for me is that I can't figure out how the tableView could add to itself a fixed content on top of the scrollable content.

另一个问题是我应该重写 UIScrollView 的什么方法来拦截滚动事件.显然我不希望 tableView 成为自己的代表......

Another question is what method/s of UIScrollView should I override to intercept the scrolling event. Obviously I don't want the tableView to be the delegate of itself...

有什么想法吗?

谢谢！

推荐答案

好的，我在 Apple 的 WWDC 2011 Session 104 video - Advanced Scroll View Techniques 中找到了解决方案.

Ok, so I found the solution on Apple's WWDC 2011 Session 104 video - Advanced Scroll View Techniques.

此视频中有一个完整的部分是关于滚动视图中的固定视图".根据 Apple 的说法，这里的方法是覆盖 layoutSubviews 并将所有代码放在那里以放置您想要的任何位置 - 任何您想要的位置.

There is a whole section in this video about "Stationary Views" inside a scroll view. According to Apple, the way to go here is to override layoutSubviews and put there all the code to position whatever you want - wherever you want.

我试过了，它实际上很简单，并且按预期工作.

I tried it and it's actually pretty easy and it's working as expected.

例如，如果我想在滚动内容时在表格顶部有一个阴影标题，这就是我应该编写的代码:

So for example if I would like a shadowed header on top of the table when the content is being scrolled, this is the code I should write:

-(void) layoutSubviews
{
    [super layoutSubviews];
    [self positionTopShadow];
}

-(void) positionTopShadow
{
    CGFloat yOffset = self.contentOffset.y;
    // I'm doing some limiting so that the maximum height of the shadow view will be 40 pixels
    yOffset = MIN(yOffset, 40);
    yOffset = MAX(0, yOffset);

    CGRect frame = self.topShadowView.frame;
    // The origin should be exactly like the content offset so it would look like
    // the shadow is at the top of the table (when it's actually just part of the content) 
    frame.origin = CGPointMake(0, self.contentOffset.y);
    frame.size.height = yOffset;
    frame.size.width = self.frame.size.width;
    self.topShadowView.frame = frame;

    if (self.topShadowView.superview == nil)
    {
        [self addSubview:self.topShadowView];
    }
    [self bringSubviewToFront:self.topShadowView];
}

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