setNeedsLayout vs. setNeedsUpdateConstraints and layoutIfNeeded vs updateConstraintsIfNeeded

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I know that the auto layout chain consists in basically 3 different process.

  1. updating constraints
  2. layout views (here is where we get calculation of frames)
  3. display

What’s is not totally clear to me is the inner difference between -setNeedsLayout and -setNeedsUpdateConstraints. From Apple Docs:

setNeedsLayout

Call this method on your application’s main thread when you want to
adjust the layout of a view’s subviews. This method makes a note of
the request and returns immediately. Because this method does not
force an immediate update, but instead waits for the next update
cycle, you can use it to invalidate the layout of multiple views
before any of those views are updated. This behavior allows you to
consolidate all of your layout updates to one update cycle, which is
usually better for performance.

setNeedsUpdateConstraints

When a property of your custom view changes in a way that would impact
constraints, you can call this method to indicate that the constraints
need to be updated at some point in the future. The system will then
call updateConstraints as part of its normal layout pass. Updating
constraints all at once just before they are needed ensures that you
don’t needlessly recalculate constraints when multiple changes are
made to your view in between layout passes.

When I want to animate a view after modifying a constraint and animate the changes I usually call for instance:

[UIView animateWithDuration:1.0f delay:0.0f usingSpringWithDamping:0.5f initialSpringVelocity:1 options:UIViewAnimationOptionCurveEaseInOut animations:^{
        [self.modifConstrView setNeedsUpdateConstraints];
        [self.modifConstrView layoutIfNeeded];
    } completion:NULL];

I’ve found out that if I use -setNeedsLayout instead of -setNeedsUpdateConstraints everything work as expected, but if I change -layoutIfNeeded with -updateConstraintsIfNeeded, the animation won’t happen.
I’ve tried to make my own conclusion:

  • -updateConstraintsIfNeeded only update constraints but doesn’t force the layout to come into the process, thus original frames are still preserved
  • -setNeedsLayout calls also -updateContraints method

So when is ok to use one instead of the other? and about the layout methods, do I need to call them on the view that has a change in a constraint or on the parent view?

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