Focus

Focus blocks of code. Dim the unfocused code. Ensure the focused code is visible when there's overflow.

Useful when you want to change a codeblock focus depending on the user's interaction.

content.md

```js
function lorem(ipsum, dolor = 1) {
  const sit = ipsum == null ? 0 : ipsum.sit
  dolor = sit - amet(dolor)
  return sit ? consectetur(ipsum) : []
}

function ipsum(ipsum, dolor = 1) {
  return dolor
}

// !focus(1:5)
function dolor(ipsum, dolor = 1) {
  const sit = ipsum == null ? 0 : ipsum.sit
  dolor = sit - amet(dolor)
  return sit ? consectetur(ipsum) : []
}
```

function lorem(ipsum, dolor = 1) {
  const sit = ipsum == null ? 0 : ipsum.sit
  dolor = sit - amet(dolor)
  return sit ? consectetur(ipsum) : []
}

function ipsum(ipsum, dolor = 1) {
  return dolor
}

function dolor(ipsum, dolor = 1) {
  const sit = ipsum == null ? 0 : ipsum.sit
  dolor = sit - amet(dolor)
  return sit ? consectetur(ipsum) : []
}

You can also change the focus annotations on a rendered codeblock:

Implementation

We need two things:

Set a data-focus={true} to the focused lines
Get a ref of the pre element, and scroll it if needed

focus.tsx

"use client"

import {
  AnnotationHandler,
  InnerPre,
  getPreRef,
  InnerLine,
} from "codehike/code"
import React, { useLayoutEffect, useRef } from "react"

export const focus: AnnotationHandler = {
  name: "focus",
  onlyIfAnnotated: true,
  PreWithRef: (props) => {
    const ref = getPreRef(props)
    useScrollToFocus(ref)
    return <InnerPre merge={props} />
  },
  Line: (props) => (
    <InnerLine
      merge={props}
      className=""
    />
  ),
  AnnotatedLine: ({ annotation, ...props }) => (
    <InnerLine merge={props} data-focus={true} className="" />
  ),
}

function useScrollToFocus(ref: React.RefObject<HTMLPreElement>) {
  const firstRender = useRef(true)
  useLayoutEffect(() => {
    if (ref.current) {
      // find all descendants whith data-focus="true"
      const focusedElements = ref.current.querySelectorAll(
        "[data-focus=true]",
      ) as NodeListOf<HTMLElement>

      // find top and bottom of the focused elements
      const containerRect = ref.current.getBoundingClientRect()
      let top = Infinity
      let bottom = -Infinity
      focusedElements.forEach((el) => {
        const rect = el.getBoundingClientRect()
        top = Math.min(top, rect.top - containerRect.top)
        bottom = Math.max(bottom, rect.bottom - containerRect.top)
      })

      // scroll to the focused elements if any part of them is not visible
      if (bottom > containerRect.height || top < 0) {
        ref.current.scrollTo({
          top: ref.current.scrollTop + top - 10,
          behavior: firstRender.current ? "instant" : "smooth",
        })
      }
      firstRender.current = false
    }
  })
}