Using auto layout and systemLayoutSizeFittingSize - ios

Question

I have a really simple view that I cannot for the life of me figure out the problem. I have successfully used auto layout and systemLayoutSizeFittingSize for fairly complex custom UITableViewCells. This is my first time using it for a UIView (to be used in a UITableView.tableHeaderView) and I cannot get it to work.
I have a simple label in the view that I want to cover the entire view. I have added the Trailing, Leading, Top, and Bottom constraints to the label against the parent's corresponding edges. When I do this, systemLayoutSizeFittingSize computes the correct height, but I get the following error:
Unable to simultaneously satisfy constraints.
Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints)
(
"<NSLayoutConstraint:0x7fbb71d9a310 V:|-(5)-[UILabel:0x7fbb71df5120'Test Label'] (Names: '|':mailapp.MailTableHeaderView:0x7fbb71d2c490 )>",
"<NSLayoutConstraint:0x7fbb71d2ebf0 UILabel:0x7fbb71df5120'Test Label'.bottom == app.HeaderView:0x7fbb71d2c490.bottom - 5>",
"<NSLayoutConstraint:0x7fbb71de31d0 V:[app.HeaderView:0x7fbb71d2c490(0)]>"
)
Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x7fbb71d2ebf0 UILabel:0x7fbb71df5120'Test Label'.bottom == app.HeaderView:0x7fbb71d2c490.bottom - 5>
Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.
Now, if I remove the "bottom" constraint on the label, auto layout works correctly but now systemLayoutSizeFittingSize returns 0.
What is the right way to make the constraints satisfy the auto layout engine but also to have systemLayoutSizeFittingSize compute the correct size?
As I mentioned, the most baffling thing is that I have used this successfully for complex auto layouts, but I noticed all those were UITableViewCells. I'm not sure if there is a difference.

Answer 1

I figured out the problem, but I don't understand why. For the UITableViewHeader, I cannot do the typical auto layout thing for the view. That is, I have to set translatesAutoresizingMaskIntoConstraints to be true. Effectively, I cannot use auto layout for the header view, and I have to set the frame manually.
Within the header view, I can use auto layout for the sub views successfully.
So my algorithm goes like this for constructing the tableHeaderView:
Create the header view with any arbitrary frame
Compute the constraints (of the header view's subviews)
Set the frame.width of the header view to match the parent tableView.frame.width
Layout the header view
Use systemLayoutSizeFittingSize to compute the height
Update the header view's frame.height
This works for me. I was not able to get it to work if I created the header view with translatesAutoresizingMaskIntoConstraints set to false.

Answer 2

As per the Apple Visual Format Language the following appears to be setting a '0' height constraint on your HeaderView.
E.g.
"<NSLayoutConstraint:0x7fbb71de31d0 V:[app.HeaderView:0x7fbb71d2c490(0)]>"
I would start by investigating this, as having a height of 0 will likely mess up your other label constraints for top and bottom spacing.