Arnav Bansal

Posted on Aug 7, 2019

How do you detect what line it is in wrapped text elements?

#javascript #webdev

I'm implementing a custom editor that uses rows of contenteditable divs. I'm having trouble implementing the right behavior for the 'up' or 'down' key.

When the 'up' key is pressed, I need to detect whether the caret is on the first line of text, and move the caret to the div above. Likewise for the 'down' key and the last line of text.

Thanks!

Top comments (7)

Erik • Aug 7 '19

Hi Arnav. I'm not at a computer right now to see if this will work but I have an idea.

is the size of your div static? if so, maybe you can figure out how many characters your input gets to before it goes to the next line, and then check the string location of your cursor. for example, say you can type 80 characters per line you can say something like

if (user_input == KEY.up) {
  if (cursor_location > 80) {
    // go up a line
  else { 
    // go up a div
  }
}

I hope this helps

Erik • Aug 8 '19

hi Arnav, did you ever get this figured out?

Arnav Bansal • Aug 9 '19

Hey Erik! I did figure out a solution, for variable sized divs

getLinePosition = () => {
    let r = getSelection().getRangeAt(0);
    let s = document.createElement('span');
    r.insertNode(s);
    let top = ((s.offsetTop - s.parentNode.offsetTop) === 0);
    let bottom = ((s.offsetTop - s.parentElement.offsetTop + s.offsetHeight) >= s.parentElement.offsetHeight);
    s.parentNode.removeChild(s);
    return {top, bottom};
  }

This returns an object {top: Boolean, bottom: Boolean} which works just enough for my purpose.

It can be modified to return the exact line number too if needed. s.offsetTop-s.parentNode.offsetTop returns a number that's fontHeight*lineNumber, which when divided by font height returns the line number.

I'm sure there's a less messy way of doing this that doesn't require adding a span.

Apologies for not replying sooner!

Erik • Aug 9 '19

looks great! is this running live anywhere?

Arnav Bansal • Aug 9 '19

Just deployed it. You can try the functionality here: laughing-gates-6a6364.netlify.com

If you press the up/down arrows, you move between the paragraphs (two separate contenteditable divs) only on the first and last lines.

(It's not a completely usable editor yet; still needs much fixing)

Erik • Aug 9 '19

very nice. Is this mostly just an exercise or do you have plans for this kind of feature?

Arnav Bansal • Aug 10 '19

I'm developing a platform where I need an editor with some custom features. I felt the need to understand all the moving parts of an editor.

My platform: deeplink.space

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How do you detect what line it is in wrapped text elements?

Top comments (7)

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