Weird autolayout centering issue - ios

Question

This has been dogging me a bit.
Basically, what it comes down to is that when I manually define autolayout constraints using layout anchors, and use the view controller in a popover, the center is in the wrong place. It's seeming to center around the boundary from the left of the popover pop-out arrow to the left and the right margin, which is not the center of the area of the popover.
This seems to work fine with other UIViewController presentation methods, like UIModalPresentationFormSheet.
Any ideas how I can fix this in a layout-friendly way without special-casing the margins?
PS. The exact same thing happens if you peg the layout to the center of the parent view, it's slightly off.
(note: Edited because I posted the source badly)
-(void)loadView {
// create root view.
UIView *view = [[UIView alloc] init];
[view setBackgroundColor:[UIColor greenColor]];
[view setTranslatesAutoresizingMaskIntoConstraints:NO];
self.view = view;
UIView *parentView = [[UIView alloc] init];
[view addSubview:parentView];
[parentView setTranslatesAutoresizingMaskIntoConstraints:NO];
[parentView setBackgroundColor:[UIColor redColor]];
[parentView.leadingAnchor constraintEqualToAnchor:view.leadingAnchor constant:20.0].active = YES;
[parentView.trailingAnchor constraintEqualToAnchor:view.trailingAnchor constant:-20.0].active = YES;
[parentView.topAnchor constraintEqualToAnchor:view.topAnchor constant:10.0].active = YES;
[parentView.bottomAnchor constraintEqualToAnchor:view.bottomAnchor constant:-10.0].active = YES;
}

Answer 1

The edges of the view are not where you think they are; the leading edge is where the point of the arrow is, not where the side of the visible rectangle is.
However, the margins are where you expect them. So pin to the layoutMarginsGuide instead, and all will be well. Indeed, this is one of the things margins are intended to cover.
In Swift:
parentView.leadingAnchor.constraint(
equalTo: self.view.layoutMarginsGuide.leadingAnchor, constant:0).isActive = true
parentView.trailingAnchor.constraint(
equalTo: self.view.layoutMarginsGuide.trailingAnchor, constant:0).isActive = true
(And change those constant numbers as you see fit.)