Change Constraints constant on view load has no effect

Question

I have UIPageViewController that contains simple UIViewControllers. The View of the VC contains an UIImageView (aspect fit) at the top and a UITextView at the bottom. The LayoutConstraints for the ImageView are:

• Fixed Width: Which has an Outlet to the VCs-Class
• Fixed Height: Which has an Outlet to the VCs-Class
• Center X is equal to the superviews centerX
• Top is equal to Superviews MarinTop (+8)

The LayoutConstraints for the ImageView are:

• Top is equal to ImageView bottom
• Bottom is equal to superviews MarginBottom (+8)
• Width is equal superviews width (Multiplicator is 0.8)

This works as expected.

But when the VC is loaded and has the image to display, I want to change the size of the imageView. There is a minimum and a maximum size and if a image view is smaller or larger it needs to be resized. Otherwise the the imageview has to get the size of the image.

So I calculate the size for the actual image and set the constant for the two LayoutConstraints. The values are correct but the ImageView does not change.

I tried to set them in viewWillLayoutSubviews but no change.

What am I doing wrong?

Answer 1

Try update your views inside viewDidLayoutSubviews:

- (void) viewDidLayoutSubviews {
 // setup your constraints here
}

Hope this helps.

Answer 2

Try changing the vertical spacing between the ImageView and the textfield to be flexible (>=0). If that doesn't solve the issue, add:

[view setNeedsLayout];
[view layoutIfNeeded];

After you adjust the size of the imageview to size of the new image.

Answer 3

Write your change constraint code in the below provided block

dispatch_async(dispatch_get_main_queue(), {

})