How to programmatically change constraint constants? - swift

Question

I read related questions(with nearly same titles), but it's not my case. I have a MKMapView and in the bottom of the screen i have ScrollView and ImageView. They are hidden, but when i show them i want to change bottom constraint of my MapView. The problem is, when i update bottom constraint, my MapView ignores it - i made ScrollView and ImageView transparent to check it - and even scrolls up, i don't understand why. My code for updating constant is this:
bt.flyImg.isHidden = true
bt.mapBottom.constant -= bt.flyImg.height
bt.view.layoutIfNeeded()
bt.flyImg.isHidden = false
bt.mapBottom.constant += bt.flyImg.height
bt.view.layoutIfNeeded()
I checked the constraint, it's the constraint i need. I tried to write something like
func layout()
{
self.view.layoutIfNeeded()
}
because i thought it might happen because i tried to update constant from other class, but it didn't help. What am i doing wrong?

Answer 1

The first thing you should do is to check if the constraint is still connected to mapBottom from the Connections Inspector (if you are using Storyboard). If it is and it is still not working, then you can try the alternative approach where you update the MapView's bottom constraint from the constarint's superview.
For this to work you must be able to identify the MapView's bottom constraint by giving it an identifier. To do that, look through the MapView's constraints in the Storyboard's Document Outline, find the bottom constraint, go over to the Size Inspector and give the constraint an identifier, let's say "MapViewBottom".
Now you must look through the MapView's superview constraints, identify the correct one and change the constant.
for constraint in yourMapView.superview!.constraints {
if constraint.identifier == "MapViewBottom" {
constraint.constant = flyImg.isHidden ? -flyImg.height : flyImg.height
}
}
You can then call the view to recalculate constraints if needed
view.layoutIfNeeded()

Answer 2

I think you might have initially decrease the constant of mapBottom to the height of flyImg. Instead of bt.mapBottom.constant -= bt.flyImg.height, change it to its default constant value. If it is pinned to sides of screen than make it
bt.flyImg.isHidden = true
bt.mapBottom.constant = 0 //if default value is 0. Just check it on storyboard.
bt.view.layoutIfNeeded()
bt.flyImg.isHidden = false
bt.mapBottom.constant += bt.flyImg.height
bt.view.layoutIfNeeded()
If it is still not working, Please insert detail source code so that we could figure out what's the matter.