postlayout

Postlayout is a term used in user interface (UI) development to describe code or callbacks that execute after the rendering engine has completed a layout pass. During layout, the framework measures and assigns sizes and positions to UI elements. Postlayout code runs after these final metrics are known, allowing it to read actual dimensions or perform tasks that depend on the completed layout.

Different ecosystems provide mechanisms for postlayout actions. In Android, developers commonly attach a global layout listener (ViewTreeObserver.OnGlobalLayoutListener) to react once the layout pass has finished and the view tree has a valid size. In Flutter, the framework offers addPostFrameCallback, which schedules a function to run after the current frame is laid out and painted. In web browsers, postlayout work often uses requestAnimationFrame to run code on the next repaint after layout changes; MutationObserver or resize observers can also trigger postlayout logic when DOM geometry changes occur.

Common uses include measuring final element sizes, aligning overlays or popups to the actual positions, starting animations that require final geometry, and performing one-time calculations. Considerations include avoiding expensive work during postlayout to prevent layout thrashing, cleaning up listeners to avoid memory leaks, and understanding that multiple postlayout callbacks may fire during a single change, so guards are advisable.

See also: layout pass, measurement, reflow, paint.