How to Programmatically Change an Auto-Layout constraint, set in IB?

Question

This question seems to come close to what I need, but not quite...

I have a UIView (UITableView, to be precise) that has an assigned set of AL constraints, usually from IB (It's supposed to be a reusable class, so that's why "usually").

I need to nudge the bottom when the keyboard pops up. I know how to get the keyboard events and the keyboard size, so getting the number and the execution thread isn't a problem.

Nudging the bottom is. Since this is a reusable class, and the controller's purview does not extend outside the UITableView (which is where the table's lower AL constraint lives), I can't do the "tie the constraint to the controller" thing that I'd otherwise do.

This means that I need to get the constraint programmatically. If there's no constraint, then I need to know that, so I can tell the view to hitch up its skirts the old-fashioned way.

If there is a layout constraint, then I need to get the one that anchors the bottom of the table view.

Since there could be all kinds of junk there (like people assigning multiple constraints), I can't just grab the first constraint in the anchor.

I have a feeling that there's no actual solution to this, and I'll end up extending the bottom of the table view's scrollable content size, and hitching the contentOffset. That will work, but it won't be as "cool." :)

Any ideas?

Have a look at this one. [move-view-with-keyboard-using-swift](https://stackoverflow.com/questions/26070242/move-view-with-keyboard-using-swift) — MwcsMac, Feb 20 '18 at 18:17
@MwcsMac Thanks! Yeah, that's pretty much what I'm doing now. I'm not actually scrolling (yet), mostly because getting the relative position of an item buried four levels deep inside a scrolled table is a real pain. However, increasing the contentSize of the UITableView allows the user to scroll the view up, even if it's at the bottom. I consider this to be only a partial solution. — Chris Marshall, Feb 20 '18 at 18:33
Well, it looks like you can't get there from here, but that's all right. The scroller manipulation did a great job without me having to hack. — Chris Marshall, Feb 20 '18 at 20:13

score 3 · Answer 1 · answered Feb 20 '18 at 18:27

3

You can link the constraint from IB to your code file just like you link your views.

open the document outline
Look for your tableView
expand its constraints
right-click drag from the constraint to your code file to create an outlet to the constraint

after that you can do this in code

constraintOutletName.constant = 100.0 //set the value of the constraint

answered Feb 20 '18 at 18:27

K.A.Q

469
4
13

Yeah, that's what I can't do (It's how I do it in other programs). That's because doing this actually violates the context controlled by the UITableViewController. Accessing it programmatically is also a bit nefarious, but I can "get away with it." I think the solution will really be to just say "Ya cain't git thar f'm here", and mess with the table view's scroller. – Chris Marshall Feb 20 '18 at 18:35
Well, if it is not too much work try to implement the tableViewController using a UIViewController that conforms to tableViewDelegate and tableViewDatasource. Thats more flexible and allows you to manipulate and change the view as much as you like. One other solution I might suggest although I am not sure if it will work is that you access the tableView's height by `tableView.frame.size.height = 100.0` – K.A.Q Feb 20 '18 at 18:56

score 1 · Accepted Answer · answered Feb 20 '18 at 18:35

override func viewDidLoad() {
    super.viewDidLoad()            
    NotificationCenter.default.addObserver(self, selector: #selector(ViewController.keyboardWillShow), name: NSNotification.Name.UIKeyboardWillShow, object: nil)
    NotificationCenter.default.addObserver(self, selector: #selector(ViewController.keyboardWillHide), name: NSNotification.Name.UIKeyboardWillHide, object: nil)    
}

@objc func keyboardWillShow(notification: NSNotification) {        
    if let keyboardSize = (notification.userInfo?[UIKeyboardFrameBeginUserInfoKey] as? NSValue)?.cgRectValue {

        self.tableViewBotCon.constant = -1 * keyboardSize.height

        self.view.layoutIfNeeded()

    }        
}

@objc func keyboardWillHide(notification: NSNotification) {

      self.tableViewBotCon.constant = 0

      self.view.layoutIfNeeded()
}

Also, make sure that you remove the observers in the deinit. — Chris Marshall, Feb 20 '18 at 23:57

Chris Marshall · Answer 3 · 2018-02-21T13:17:17.417

And the answer is...can I have the envelope, please?...

"Yew Cain't Git Thar F'm Here." -At least, not in a sane manner.

Mainly because of the chaotic number of constraints that can be attached to an anchor point (including none that fit the bill).

I'm solving the issue much more simply, using the technique I already was working on (and alluded by the answers above).

I'll greencheck the answer, even though it basically said to do what I'm already doing, since it was the correct way to address the issue.

[UPDATE] Well, this is annoying. Apparently, on the iPad, it already does this in the OS, so I’m going to need to add a dratted “idiom” check.

[UPDATE 2] It's not that simple. It appears as if the iPad describes a keyboard twice the height of the one on the screen.

[UPDATE 3] Well...lookee here what I found.

[UPDATE 4] Turns out the issue is about the opening animation. I needed to look a bit further at the keyboard record. I was looking at the Y origin, when I should have been looking at the height.

How to Programmatically Change an Auto-Layout constraint, set in IB?

3 Answers3