问题描述

在 此技术说明中，Apple 声明您可以通过向 UISCrollView 的超级视图添加约束来固定/浮动 UIScrollView 的子视图.我试过了，但我做错了，我不知道是什么问题.

In this technical Note Apple states that you can make a subview of UIScrollView fixed / floating by adding constraints to UISCrollView's superview. I tried that but I'm doing something wrong and I can't figure out whats the problem.

请注意，您可以通过在视图和滚动视图的子树之外的视图(例如滚动视图的超级视图)之间创建约束，使滚动视图的子视图看起来浮动(不滚动)在其他滚动内容之上.

Note that you can make a subview of the scroll view appear to float (not scroll) over the other scrolling content by creating constraints between the view and a view outside the scroll view’s subtree, such as the scroll view’s superview.

这就是我所做的.我已经设置了 UIScrollView 并尝试将固定视图添加到滚动视图的顶部，如下所示:

That's what I did. I have a UIScrollView already set up and try to add the fixed view to the top of the scrollview like the following:

_testOverlay = [[UIView alloc] init];
_testOverlay.backgroundColor = [UIColor blueColor];
_testOverlay.translatesAutoresizingMaskIntoConstraints = NO;
[self.scrollView addSubview:_testOverlay];

[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"|[_testOverlay]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(_testOverlay)]];
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[_testOverlay(64)]-(>=0)-|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(_testOverlay)]];

但是，这不起作用，添加的视图将与滚动视图一起移动并且不会浮动".有什么想法吗?

However, this does not work, the added view will move along with the scrollview and does not 'float'. Any ideas whats wrong here?

推荐答案

在视图和滚动视图子树之外的视图之间，例如滚动视图的超级视图.

这部分很关键.self.scrollView 是 _testOverlay 的超级视图.因此，在 @"|[_testOverlay]|" 中，竖线引用 self.scrollView.你必须用 _testOverlay 和(我想)self.view 之间的约束替换这个约束.我不确定视觉格式语言是否可行，但您当然可以使用 constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant 来实现.它会是这样的(我不会发布整个代码，因为它很长):

This part is critical. self.scrollView is a superview of _testOverlay. So, in @"|[_testOverlay]|" vertical bars reference self.scrollView. You have to replace this constraint with the constraint between _testOverlay and (I suppose) self.view. I'm not sure if it's possible with the visual format language, but you certainly can do it with constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant. It would go like this (I won't post the whole code, because it's looong):

[self.view addConstraint:[NSLayoutConstraint
                          constraintWithItem:self.view
                          attribute:NSLayoutAttributeLeft
                          relatedBy:NSLayoutRelationEqual
                          toItem:_testOverlay
                          attribute:NSLayoutAttributeLeft
                          multiplier:1.0
                          constant:0]];

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