claude-skills/react-native-skills/rules/ui-measure-views.md

title, impact, impactDescription, tags

title	impact	impactDescription	tags
Measuring View Dimensions	MEDIUM	synchronous measurement, avoid unnecessary re-renders	layout, measurement, onLayout, useLayoutEffect

Measuring View Dimensions

Use both useLayoutEffect (synchronous) and onLayout (for updates). The sync measurement gives you the initial size immediately; onLayout keeps it current when the view changes. For non-primitive states, use a dispatch updater to compare values and avoid unnecessary re-renders.

Height only:

import { useLayoutEffect, useRef, useState } from 'react'
import { View, LayoutChangeEvent } from 'react-native'

function MeasuredBox({ children }: { children: React.ReactNode }) {
  const ref = useRef<View>(null)
  const [height, setHeight] = useState<number | undefined>(undefined)

  useLayoutEffect(() => {
    // Sync measurement on mount (RN 0.82+)
    const rect = ref.current?.getBoundingClientRect()
    if (rect) setHeight(rect.height)
    // Pre-0.82: ref.current?.measure((x, y, w, h) => setHeight(h))
  }, [])

  const onLayout = (e: LayoutChangeEvent) => {
    setHeight(e.nativeEvent.layout.height)
  }

  return (
    <View ref={ref} onLayout={onLayout}>
      {children}
    </View>
  )
}

Both dimensions:

import { useLayoutEffect, useRef, useState } from 'react'
import { View, LayoutChangeEvent } from 'react-native'

type Size = { width: number; height: number }

function MeasuredBox({ children }: { children: React.ReactNode }) {
  const ref = useRef<View>(null)
  const [size, setSize] = useState<Size | undefined>(undefined)

  useLayoutEffect(() => {
    const rect = ref.current?.getBoundingClientRect()
    if (rect) setSize({ width: rect.width, height: rect.height })
  }, [])

  const onLayout = (e: LayoutChangeEvent) => {
    const { width, height } = e.nativeEvent.layout
    setSize((prev) => {
      // for non-primitive states, compare values before firing a re-render
      if (prev?.width === width && prev?.height === height) return prev
      return { width, height }
    })
  }

  return (
    <View ref={ref} onLayout={onLayout}>
      {children}
    </View>
  )
}

Use functional setState to compare—don't read state directly in the callback.