title: setNeedsLayout or layoutIfNeeded necessary after changing constraint constant?
body: <p>On iOS, is it necessary to make any API calls (such as <code>setNeedsLayout</code> or <code>layoutIfNeeded</code>) after changing a constraint's constant programmatically?  In my use case, I'm not interested in animating the change.  I believe there are some bugs (e.g.: <a href="https://stackoverflow.com/questions/25033653/ios-updating-auto-layout-constraint-constant-stops-working-after-pushing-anoth">this one</a>) where calling those functions is a workaround.  But other than that edge case I haven't needed to call them in my code.  Does this just happen to work by chance or is there a definitive source that mentions what to do when changing constraint constants?</p>
  
  <p>A couple of SO answers (<a href="https://stackoverflow.com/a/26513276/3022390">here</a> and <a href="https://stackoverflow.com/a/23963530/3022390">here</a>) say that setting the constant should be enough, but without any supporing reference.</p>
  
  <p>Apple's iOS Auto Layout Guide doesn't mention anything about it except in an <a href="https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/AutolayoutPG/AutoLayoutbyExample/AutoLayoutbyExample.html#//apple_ref/doc/uid/TP40010853-CH5-SW1" rel="noreferrer">animation block</a>.  The Cocoa <a href="https://developer.apple.com/library/mac/releasenotes/UserExperience/RNAutomaticLayout/#//apple_ref/doc/uid/TP40010631-CH1-SW14" rel="noreferrer">Auto Layout Release Notes</a> do say that calling <code>setNeedsLayout</code> is unnecessary unless you also override <code>layout</code> in your custom view.  However, that document seems rather OSX specific.</p>