Moving views with constraints

Question

I have a couple views in my view controller that move up when an up swipe is detected then down when a down swipe is detected. I was forcing the views to move by adjusting the y origin using CGRectOffset. I've now applied constraints to my views with IB and I'm not sure whats the best way to move the views so that they end up in the right position on the iphone 5, 6, and 6+.

Currently I'm doing something like this:

[self.view layoutIfNeeded];

    self.panFrameVerticalConstraint.constant = self.panFrameVerticalConstraint.constant +338;

    [UIView animateWithDuration:5
                     animations:^{
                         [self.view layoutIfNeeded];
                     }];

Is it better to change the constants using ratios? So for the constraint above, instead of using 338, would it be better to do this:

    self.panFrameVerticalConstraint.constant = self.panFrameVerticalConstraint.constant + (self.panView.frame.size.height/1.680);

    //self.panView.frame.size.height = 568
    //(568/1.680) = 338

Solution

Yes, no issue when you change the constants. The thing here is that you have to set your constraint appropriately.

Let's consider an example.

I have a UIView in Storyboard and I would like to change its width. Its default width is 1024.

After some animation, We will change it's width to 900.

Follow steps below to achieve this:

1. Select UIView in which we want to update constraints. Here we need to update width, So we will add a constraint for width.

2. Now we can see new constraint added to UIView.

3. Click on that constraint. It will show constraint properties. Here now we want to decrease width from 1024 to 900, So that in constraint property change Relation property to Less Than or Equal. Similarly, if you like to increase width from 1024 then Relation will be Greater Than or Equal.

4. Now create an IBOutlet variable for NSLayoutConstraint and connect it with above width constraint.

5. Change width constant

Swift 5.0

@IBOutlet weak var viewWidthConstraint : NSLayoutConstraint!

func reduceWidth() {
    // Reduce width of view.
    UIView.animate(withDuration: 0.35, animations: { () -> Void in
        self.viewWidthConstraint.constant = 900
        self.view.layoutIfNeeded()
    })
}

func normalWidth() {
    // Change to the default width of view.
    UIView.animate(withDuration: 0.35, animations: { () -> Void in
        self.viewWidthConstraint.constant = 1024
        self.view.layoutIfNeeded()
    })
}

Objective C

@property (weak, nonatomic) IBOutlet NSLayoutConstraint     *viewWidthConstraint; 

- (void) reduceWidth {
    // Reduce width of view.
    [UIView animateWithDuration:0.35f animations:^{
        self.viewWidthConstraint.constant = 900;
        [self.view layoutIfNeeded];
    }];
}

- (void) normalWidth {
    // Change to default width of view.
    [UIView animateWithDuration:0.35f animations:^{
        self.viewWidthConstraint.constant = 1024;
        [self.view layoutIfNeeded];
    }];
}

Comments

[Brosef]

@Pangu I pretty much followed the steps in the answer below. I created an IBOutlet NSLayoutConstraint property and connected it to the constraint I wanted to adjust in interface builder. Then when I needed to move my view up, i changed my constant like this self.panFrameVerticalConstraint.constant = -(self.panedView.frame.size.height/1.68);. I called layoutIfNeeded before changing the constant. After changing the constant I called animateWithDuration and called layoutIfNeeded again in the block just like the answer below.

[Brosef]

@pangu In viewDidLoad you need to add a UISwipeGestureRecognizer and the method that gets called when a swipe is detected. UISwipeGestureRecognizer * swipeUpRec = [[UISwipeGestureRecognizer alloc] initWithTarget:self action:@selector(handleUpSwipe:)]; Then add the gesture recognizer to whatever frame is suppose to detect the swipe [self.someView addGestureRecognizer:swipeUpRec];

[Brosef]

This is very helpful. You say you want the width to change from 1024 to 900, but why do you change the constraint constant to 705?

[Kampai]

It's just example. In code I have requirement to decrease width to 705.

[viral]

Nice Explanation. I appreciate. +1

[Markus]

Great and correct answer. I just want to add that setting the constraint does not need to be done inside the UIView.animateWithDuration block. You can set the constraint before the animation block and just call layoutIfNeeded inside the block.

[Luchi Parejo Alcazar]

Does someone how to capture a constraint change? I mean, the width constraint is being changed from 1024 to 900 and viceversa. What if I want to capture the moment when the constraint is equals to 1000?