Setting constraint "isActive=false" causes fatal error

Question

I have an imageView which has a top constraint IBOutlet called imageViewTopConstraint. In viewWillLayoutSubviews( ) I set the imageViewTopConstraint.constant value. The imageView displays properly for all iPhone models while in portrait mode, which is good.

When I switch to landscape mode, I want to disable the imageViewTopConstraint, so the viewWillLayoutSubviews( ) has the following code:

if UIDevice.current.orientation.isPortrait {
   imageViewTopConstraint.constant = (2 * screenHeight) / 3
   imageViewTopConstraint.isActive = true
}
else if UIDevice.current.orientation.isLandscape {
   imageViewTopConstraint.isActive = false
}

Issue

When I run the application it is initially in portrait mode. When I switch to landscape mode I receive the following fatal error at the line where I set "imageViewTopConstraint.isActive = false" to disable the constraint (line 6 above)

Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value

Any idea what is going on or what this means?

Interface Builder

5.4k

Posted by

uncletr

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Answer 1

The problem is that viewWilllayout is called several times.

Second time, you have set the constraint as inactive, which means it has been removed (it will be reinstalled when you set to true).

So one way is to test

if UIDevice.current.orientation.isPortrait {
   imageViewTopConstraint.constant = (2 * screenHeight) / 3
   imageViewTopConstraint.isActive = true
}
else if UIDevice.current.orientation.isLandscape {
   if imageViewTopConstraint != nil { imageViewTopConstraint.isActive = false }
}

Posted by

Claude31

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Answer 2

To be sure of what happens, please add some log:

if UIDevice.current.orientation.isPortrait {
   imageViewTopConstraint.constant = (2 * screenHeight) / 3
   print("isPortrait  imageViewTopConstraint",  imageViewTopConstraint)
   imageViewTopConstraint.isActive = true
}
else if UIDevice.current.orientation.isLandscape {
   print("isLandscape  imageViewTopConstraint before",  imageViewTopConstraint)
   imageViewTopConstraint.isActive = false
   print("isLandscape  imageViewTopConstraint after",  imageViewTopConstraint)
}

and tell what youi get when you turn landscape. Notably if this is called several times.

Probably, you have removed the constraint somewhere making it nil.

Posted by

Claude31

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Answer 3

hi uncletr,

this sounds familiar to me ... a constraint reference unexpectedly being nil when trying to tweak a layout between portrait and landscape.

please let us know how the constraint is defined: is it defined in IB and then referenced from your code, or is it defined and added directly in code, say in viewDidLoad()?

in my recent case, i had a reference to a constraint defined in IB; when i set it to active = false, the constraint reference immediately became nil, almost exactly as you describe.

looking forward to more info,

DMG

Posted by

DelawareMathGuy

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Answer 4

DelawareMathGuy,

I defined the constraint in IB (using the little tie-fighter icon), then highlighted the constraint and dragged it into the code and assigned an IBOutlet name to the constraint.

>>when i set it to active = false, the constraint reference immediately became nil

I believe this is exactly what is happening to me (please read below). This feels wrong to me.

Claude:

If I add the "print" statements like you requested (as below) ...

print("isPortrait imageViewTopConstraint", imageViewTopConstraint)

then I see the following warnings produced for that "print" line

Coercion of implicitly unwrappable value of type 'NSLayoutConstraint?' to 'Any' does not unwrap optional

Provide a default value to avoid this warning

Force-unwrap the value to avoid this warning

Explicitly cast to 'Any' with 'as Any' to silence this warning

That being said, since these are just warnings, I ran the code anyway.

Results

ViewWillLayoutSubviews