以编程方式创建的Animate NSLayoutConstraint动画

Question

Something special has to be done to animate a constraint instantiated programmatically?

I instantiate a constraint programmatically as a lazy variable:

private lazy var topConstraint: NSLayoutConstraint = {
    let constraint = view.topAnchor.constraint(equalTo: otherView.topAnchor, constant: otherView)
    return constraint
}()

Then, I animate a change in the constant of this constraint:

topConstraint.constant = newValue
view.setNeedsLayout()
UIView.animate(withDuration: 1) {
    self.view.layoutIfNeeded()
}

It is not working... I also have tried putting the constant setting into the block:

UIView.animate(withDuration: 1) {
    self.topConstraint.constant = newValue
    self.view.setNeedsLayout()
    self.view.layoutIfNeeded()
}

But I achieved nothing at all, I've tried almost all the combinations. The constraint constant is properly changed, but always without animation.

PD: I also gave a shot to UIViewPropertyAnimator .

Answer 1

No need for setNeedsLayout() .

self.topConstraint.constant = 0.0
UIView.animate(withDuration: 1.0) {[weak self] in
    self?.topConstraint.constant = 10.0 
    self?.view.layoutIfNeeded()
}