activating constraints programmatically at runtime

Question

I want to change the positions of my view programmatically at runtime using constraints I have following arrangement of views

 |
viewA
 |
viewB
 |
viewC

say i have viewA with topAnchor constraints set to top of margin of parent view and viewB with topAnchorset to viewA and so on for viewC

And i want to change the position of these views on certain action at runtime

 |
viewA
 |
viewC
 |
viewB

i have store two different constraints for viewB one with top of viewA and another with top of viewC

viewBTopConstraints = viewB.topAnchor.constraint(equalTo: viewA.bottomAnchor, constant: TOP_SPACE)

newViewBTopConstraints = viewB.topAnchor.constraint(equalTo: viewC.bottomAnchor, constant: TOP_SPACE)

In my toggle action method i have done something like this

viewBTopConstraints?.isActive = false
newViewBTopConstraints?.isActive = true

This works on first run of action however it fails on second time, on further debugging in view debugger i found out that it creates duplicate constraints rather that changing the original one.

Is there a way to achieve this kind of behaviour? all i want is to toggle the position of these views based on certain action and since i have created my views programmatically using constraints, i am unable to figure out. — Fahadsk
– Fahadsk, Commented Nov 1, 2017 at 6:42

user7014451 · Accepted Answer · 2017-11-01 07:11:10Z

Your requirements sounds similar to an app where I have some constraints that change based on landscape or portrait. There I set up three groups of constraints:

Those that always are isActive = true
Those that are isActive = true in portrait
Those that are isActive = true in landscape

The trick is to put the latter two into arrays, and activate/deactive the proper array at the right time. Here's a snippet of what I mean:

var p = [NSLayoutConstraint]()
var l = [NSLayoutConstraint]()


////// portrait layout....

// pin info button above imageLayout, right justified

p.append(info.topAnchor.constraint(equalTo: margins.topAnchor, constant: 20.0))
p.append(info.trailingAnchor.constraint(equalTo: margins.trailingAnchor))


////// landscape layout....

// pin info button above buttons, right justified

l.append(info.topAnchor.constraint(equalTo: margins.topAnchor, constant: 20.0))
l.append(info.trailingAnchor.constraint(equalTo: margins.trailingAnchor))

Note that I'm not setting any isActive = true. I'm just appending things to the two arrays as needed. Now, in viewWillLayoutSubviews() - depending on your needs, viewDidLayoutSubviews() may be the better override - you *activate/deactivate the correct array:

NSLayoutConstraint.deactivate(l)
NSLayoutConstraint.deactivate(p)
if self.bounds.width > self.bounds.height {
    NSLayoutConstraint.activate(l)
} else {
    NSLayoutConstraint.activate(p)
}

I've found that working with individual constraints, setting isActive, greatly increases the risks of conflicts.

Collectives™ on Stack Overflow

activating constraints programmatically at runtime

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related