NSLayoutConstraint breaks - ios

Question

I have a custom UITableViewCell. The cell has three labels. The left most 'Amount' Label has the following constraint.
On the right side of the cell I have another label, 'Label Dollar Amount'. It has the following constraints:
The third label is right below 'Label Dollar Amount' is the 'Label Max Amount'. The constraints are as follows:
Nothing is done on the code. The table row height is calculated using UITableViewAutomaticDimension. The estimatedRowHeight is set to 100.
When the view is in landscape and the tableview is scrolled down, this particular cell is being dequeued since its out of the visible view. When the orientation is changed to portrait for the same state, the cell is expected to be visible.
I receive the following error:
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:0x1607a770 H:[UILabel:0x160c1990'Amount'(150)],
NSLayoutConstraint:0x160c2b30 H:|-(18)-[UILabel:0x160c1990'Amount'] (Names: '|':UITableViewCellContentView:0x160c18c0 ),
NSLayoutConstraint:0x160c2b90 H:[UILabel:0x160c1990'Amount']-(0)-[UILabel:0x160c1c40'$45.61'],
NSLayoutConstraint:0x160c2bf0 UILabel:0x160c1ac0'(Max: $1,000.00)'.leading == UILabel:0x160c1c40'$45.61'.leading,
NSLayoutConstraint:0x160c2c80 H:[UILabel:0x160c1ac0'(Max: $1,000.00)']-(12)-| (Names: '|':UITableViewCellContentView:0x160c18c0 ),
NSLayoutConstraint:0x14e179e0 'UIView-Encapsulated-Layout-Width' H:[UITableViewCellContentView:0x160c18c0(72)]
)
Will attempt to recover by breaking constraint
NSLayoutConstraint:0x1607a770 H:[UILabel:0x160c1990'Amount'(150)]`
Note: Priorities are set to 1000 (Required) for all possible constraints seen here.
If I change the priority of the width constraint on Amount label to 999 it works. But the label disappears for longer text. Since zero spacing in the trailing constraint has a hight priority. Any suggestion on what could have gone wrong?

Answer 1

The table is setting a fixed width of 72 points on your cell view. This may just be temporary while it adjusts to the new orientation or layout, but you need to accommodate it:
NSLayoutConstraint:0x14e179e0 'UIView-Encapsulated-Layout-Width' H:[UITableViewCellContentView:0x160c18c0(72)]
So, you need to decide what should happen in your layout when its superview is only 72 points wide.
Currently, you have the following (edited a bit to make things clearer):
H:|-(18)-[UILabel:0x160c1990'Amount'] (Names: '|':UITableViewCellContentView:0x160c18c0 ),
H:[UILabel:0x160c1990'Amount'(150)],
H:[UILabel:0x160c1990'Amount']-(0)-[UILabel:0x160c1c40'$45.61'],
UILabel:0x160c1ac0'(Max: $1,000.00)'.leading == UILabel:0x160c1c40'$45.61'.leading,
H:[UILabel:0x160c1ac0'(Max: $1,000.00)']-(12)-| (Names: '|':UITableViewCellContentView:0x160c18c0 ),
Or:
H:|-(18)-[Amount(150)][Max]-(12)-|
So, even if every view which could be collapsed to 0 width were, you would still require 18 + 150 + 12 == 180 points of width between the edges of the superview. That conflicts with the 72 points available.
How would you like that to be resolved? One possibility is to reduce the priority of the width constraint on the Amount label, as you mention. Or you could even eliminate this constraint entirely and rely on the intrinsic width and the compression resistance priority. You say "the label disappears for longer text. Since zero spacing in the trailing constraint has a hight priority." I'm not entirely sure what you mean. Perhaps with longer text, the label is wrapping and words are on a second line which isn't visible? If so, you can set it to truncate (although that should be the default). If even truncation isn't acceptable, then what should happen?
You can try setting adjustsFontSizeToFitWidth and minimumScaleFactor to allow the text in the label to shrink, but eventually you still hit the limit (if the text is long enough and the cell width is narrow enough), so you still need to decide what you want to happen.
Another approach is to reduce the priority of the trailing constraint between Label Max Amount and the superview, so that that label moves off the right side. You may also need to reduce the priority of the trailing constraint of Label Dollar Amount, too, since it imposes the same requirement on the layout.
But, of course, that means that some of your content is off-screen. There's just no getting around the fact that you have to decide what should be lost when the width is too small to fit everything.
Once you decide what should happen, you set the priorities to achieve that. For example, if you want the Max Amount to truncate first, set its compression resistance lowest. If you want Dollar amount to truncate next, set its compression second lowest. If you want the Amount label to truncate after that, set its width constraint (or compression resistance, if you get rid of the width constraint) third lowest. And if you want things to be pushed off the right edge as a last resort, set the priority of the trailing-to-superview constraints to 999.