NSScrollView clips document view instead of allowing it to scroll - objective-c

Question

I have custom, layer-backed NSView as the document view of a NSScrollView. The scroll view is inside a NSSplitView, which itself uses constraints to ensure that it fills the entire window.
The bounds of the custom view are never explicitly set. At one point, however, the bounds of the backing layer are set to encompass every child layer. Setting the bounds explicitly doesn't help.
When I resize the window, the scroll view clips my custom view and I find myself unable to scroll:
When I debug my view, I find that resizing the window also resizes the custom view's frame. I don't really know if this is normal. My autoresize mask is .ViewMinYMargin, and translatesAutoresizingMaskIntoConstraints if off (but it doesn't appear to change anything if I turn it on).
Considering that there are basically no instructions on what to do to make scrolling work in the Apple guide, I must be missing something fairly simple. Does anyone have any idea?

Answer 1

This happened because of a misunderstanding of autolayout constraints. Each edge of the custom view was constrained to be at a distance of 0 to the edge of the clip view. This effectively set the size of the view to whatever the size of the clip view was, preventing scrolling because it made it look like the clipped size was the custom view's natural size.
The solution was to constrain only the left, top and right edges, and leave the bottom edge free. Instead, I made a height constraint on the custom view, and I programmatically set its constant value to the size of my view's contents (which is dynamically generated).
Sample code for LayerBackedView:
class LayerBackedView: NSView {
required init?(coder: NSCoder) {
super.init(coder: coder)
self.wantsLayer = true
}
override var flipped: Bool {
// hug top of scroll view if not high enough to fill it entirely
return true
}
private var heightConstraint: NSLayoutConstraint {
let firstConstraint = constraints[0] as! NSLayoutConstraint
assert(firstConstraint.firstAttribute == .Height)
return firstConstraint
}
override func awakeFromNib() {
let red = CALayer()
red.anchorPoint = CGPointMake(0, 0)
red.backgroundColor = CGColorCreateGenericRGB(1, 0, 0, 1)
red.borderWidth = 2
red.borderColor = CGColorCreateGenericGray(0, 1)
layer!.addSublayer(red)
// set height constraint to whatever you need it to be
let desiredHeight: CGFloat = 200
heightConstraint.constant = desiredHeight
red.frame = CGRectMake(0, 0, bounds.width, desiredHeight)
}
}
Constraints on the LayerBackedView (the height value doesn't matter because we change it programmatically, the constraint just needs to exist):
Size constraints are added to the view itself, but spacing constraints are added to the superview. This means that out of the four constraints, the height constraint will be the only one inside the view (and this is why we can do constraints[0] as! NSLayoutConstraint). If this is not your case, you can make an #IBOutlet to it instead.