"API error: (null) 0 width returned, assuming UIViewNoIntrinsicMetric" in Xcode 9 beta

@rmaddy is correctly linked from another SO answer, however there are other possibilities related to auto-layout methods.

The message can also be related to auto-layout implementations and unique constraints defined in your project. It seems that this message does not usually appear when using a real device.

UIKit framework -> UIView API Global Variable UIViewNoIntrinsicMetric

UIViewNoIntrinsicMetric

No built-in metric for a given numeric property.

UIView Implementation Practice

Each UIView subclass must implement intrinsicContentSize and return a size that it thinks is appropriate for it. For example:

• If you know the width of the UIView, you use return CGSizeMake (200, 50).
  
• If you don't know how wide the view is, you should use UIViewNoIntrinsicMetric
  
  instead of width.return CGSizeMake(UIViewNoIntrinsicMetric, 50)
  
  

The UIView database implementation updateConstraints

will call intrinsicContentSize and use the content size returned from it to add constraints to the UIView.

UIView API Property intrinsicContentSize

intrinsicContentSize

The natural size for the receiving view, considering only the properties of the view itself.

Custom views typically contain content rendered on the screen that the layout system is not aware of. Setting this property allows the custom view to communicate with the layout system what size it would like to use on its content. This intrinsic size should be independent of the content frame because there is no way to dynamically link the resized width to a layout based on the resized height, for example. If the custom view does not have its own dimension for a given dimension, it can use UIViewNoIntrinsicMetric

that dimension.

Conclusion

Xcode error:

returns 0 width, assuming UIViewNoIntrinsicMetric

Therefore, we can get an idea of ​​Xcode taking a UIViewNoIntrinsicMetric value since a width size of 0 was returned by some UIView in your project.

If you still get the message when deploying to a real device, I give extra attention to your layout constraints.