layoutSubtreeIfNeeded

layoutSubtreeIfNeeded is a method in AppKit's NSView that forces a layout pass for the receiving view and its entire subview hierarchy if a layout update is required. The method checks whether the view or any part of its subtree is marked as needing layout and, if so, runs the layout cycle to update frames and positions. If no layout updates are pending, the call does nothing.

This method is commonly used after programmatically changing constraints, geometry, or the view hierarchy when you need to read accurate measurements or prepare for drawing or animation. Because layout updates can be deferred, layoutSubtreeIfNeeded ensures that all subviews have valid bounds before performing geometry-dependent operations. It behaves as a no-op when the layout is already up to date.

Relation to other layout methods: layoutSubtreeIfNeeded is the AppKit counterpart to similar layout utilities in other environments. In UIKit, the public counterpart is layoutIfNeeded on a UIView, which flushes pending layout for the receiver and its subviews as needed. The distinction is that layoutSubtreeIfNeeded explicitly targets the entire subview tree, making sure every descendant has a current layout before proceeding.

Caveats and best practices: Call this on the main thread as part of UI updates. Avoid forcing layout during active rendering cycles or in tight animation loops unless you specifically need an up-to-date layout prior to measurements or transitions. Use sparingly to minimize layout work, and rely on Auto Layout’s intrinsic behavior where possible.