Constraint constant changes but layout doesn't get updated? - swift

Question

I have a strange problem: in my table view I want to update a single cell when its UISlider gets modified (value changed control event). At the moment the value of the slider changes, I want to animate a button into the cell (sliding in from the right). I set the constant of the button's constraint to -50 to make sure it's not visible by default, and when the slider's value gets changed, a method gets called which updates the table view, so cellForRowAtIndexPath gets called again for all cells in my table. Simplified it looks something like this:
func valueChanged(sender:UISlider) {
// Reload table view
self.myTableView.reloadData()
// Let table view know the value of the slider got modified
self.didChangeValueOfSlider = true
}
And in cellForRowAtIndexPath I'm keeping track of which cell's slider got changed by using a custom selectedCellIndexPath variable. When the table view gets to the cell that got modified: it runs the following code:
// Check if cell is selected
if indexPath == self.selectedCellIndexPath {
// Check if value of slider was changed
if self.didChangeValueOfSlider == true {
// Value was changed: set constraint back to default
saveButtonTrailingConstraint.constant = CGFloat(15)
self.view.setNeedsUpdateConstraints()
self.view.updateConstraintsIfNeeded()
// Reset slider update status
self.didChangeValueOfSlider = false
}
}
Calling those setNeedsUpdateConstraints() and updateConstraintsIfNeeded() might be overkill or unnecessary, but please note this is my 15th attempt or so to actually get the view to display the updated layout. I used breakpoints to confirm the constant actually changes, after the code above is finished running and everything works perfectly fine. The only thing I can't get working is the updating part. I've tried UIView.animateWithDuration to animate the change, and I've tried methods like layoutIfNeeded() and beginUpdates() and endUpdates of the table view: nothing works. What could be the reason the layout doesn't get updated? And what am I supposed to be calling the layout and update methods on? I've been calling them on self.view but I'm not even sure if you're supposed to be calling it on the view if you're trying to update the layout of a table view cell. Any help would be appreciated.

Answer 1

I figured it out. The problem turned out to be the constraint itself. as I was accessing the wrong one. I defined it as saveButtonTrailingConstraint but I was actually modifying was the saveButton's width constraint (which apparently can't be animated). Instead of accessing saveButton.constraints[indexOfTrailingConstraint] I should have defined saveButton.superview!.constraints[indexOfTrailingConstraint] as the constraint belongs to the superview and not to the button itself. I guess the best thing to take away from this clumsy mistake is to always double-check if you're modifying the correct constraint if you're doing it programmatically, because it doesn't always show on the first eye.