layoutIfNeeded

layoutIfNeeded is a method available on UIView in UIKit. It forces the view to update its layout immediately if a layout pass is pending, typically because constraints or other layout-affecting changes have occurred. When layoutIfNeeded is called, the system checks whether the view or its subviews require layout and, if so, invokes layoutSubviews to refresh frames and propagate the update through the view hierarchy.

In the Auto Layout workflow, changes to constraints often mark views as needing layout. Rather than waiting for the system’s next layout cycle, calling layoutIfNeeded flushes those pending updates right away. This behavior is especially useful when preparing for animations, as you can adjust constraints and then call layoutIfNeeded inside an animation block to animate the resulting layout changes smoothly.

Usage patterns include:

- Wrapping constraint updates in an animation block and calling layoutIfNeeded to animate the transition to the new layout.

- Forcing a layout pass before capturing final frames or performing calculations that depend on up-to-date layout information.

Notes and caveats:

- layoutIfNeeded operates on the main thread and only performs work if a layout pass is actually needed.

- It is distinct from setNeedsLayout, which merely marks the view as needing layout; layoutIfNeeded performs the update immediately.

- If constraints are conflicting or unsatisfiable, the layout pass may fail to produce valid frames, potentially triggering constraint warnings or ambiguous layouts.