iOS .addConstraint only works once - ios

Question

I'm making a custom keyboard for iOS8 and on the Apple Developer docs it says that you can change the height of a custom keyboard anytime after the initial primary view draws on the screen. And it says to do this you should use the .addConstaint() method.
Here's a link:
https://developer.apple.com/library/prerelease/ios/documentation/General/Conceptual/ExtensibilityPG/Keyboard.html
I'm using Swift. The initial height of the keyboard is 215 pixels. I have a swipe up gesture that increases the height to 350 pixels. Which works as expected. And a swipe down that changes the height to 300 pixels.
That all works ok, but the problem is it only works once. I swipe up and the height increases, I swipe down and it decreases, but if I swipe up again nothing happens. If I swipe down again nothing happens.
So I'd be greatful if anyone could take a look at my two functions and tell me what I'm doing wrong.
Here's the code:
// IBActions
#IBAction func action1(sender: AnyObject) {
if topboxvisible == false {
topboxvisible = true
UIView.animateWithDuration(0.08, delay: 0, options: .CurveEaseIn, animations: {
self.topbox.frame.offset(dx: 0, dy: 40)
}, completion: nil)
}
let expandedHeight:CGFloat = 300
let heightConstraint = NSLayoutConstraint(item:self.view,
attribute: .Height,
relatedBy: .Equal,
toItem: nil,
attribute: .NotAnAttribute,
multiplier: 0.0,
constant: expandedHeight)
self.view.removeConstraint(heightConstraint)
self.view.addConstraint(heightConstraint)
}
#IBAction func action2(sender: AnyObject) {
if topboxvisible == true {
topboxvisible = false
UIView.animateWithDuration(0.08, delay: 0, options: .CurveEaseOut, animations: {
self.topbox.frame.offset(dx: 0, dy: -40)
}, completion: nil)
}
let expandedHeight:CGFloat = 350
let heightConstraint = NSLayoutConstraint(item:self.view,
attribute: .Height,
relatedBy: .Equal,
toItem: nil,
attribute: .NotAnAttribute,
multiplier: 0.0,
constant: expandedHeight)
self.view.removeConstraint(heightConstraint)
self.view.addConstraint(heightConstraint)
}

Answer 1

Your removeConstraint calls aren't doing anything, because the constraint you just created (in the previous line of code) isn't in the view. So what you're doing here is adding more constraints every time, from what I can see. I imagine this is leading to multiple conflicting constraints—do you see any NSConstraint warnings popping out in the console log while you're running the code?
Try this: create both your height constraints in advance, and store them in instance variables. In action1, remove the 350-high constraint, and add the 300. In action2, remove the 300, and add the 350.