android – React Native: Determine number of lines of Text component-ThrowExceptions

Exception or error:

As the title says, I’ve been trying to find a way to determine the number of lines the text component AFTER it has been given text. Look at my example below.

<Text>
    Lorem ipsum dolor sit amet, consectetur adipiscing elit. Morbi semper ut ipsum in ultrices. Vivamus fringilla lacinia odio in accumsan. Proin sit amet pellentesque tortor. Nam mollis sit amet ligula id convallis. Etiam in semper diam. Cras id elit consectetur, interdum ante id, tincidunt nisi. Integer non elit placerat, dignissim nibh at, faucibus sem. Curabitur nec posuere turpis. Vivamus rhoncus nulla vitae mi imperdiet, elementum eleifend mi laoreet. Vestibulum molestie turpis non nibh elementum, sed ornare magna tristique. Aliquam erat volutpat. Phasellus volutpat mi vel tempor finibus.
</Text>

At runtime, how can I determine how many lines this Text component has rendered. This number will vary depending on device (eg. iPhone 5 will need to render more lines vs iPhone 6+ as it has a smaller screen size). I’ve checked the source code for the Text component but there doesn’t seem to be anything I’m looking for.

I am using React Native 0.24.

Any ideas?

Cheers.

How to solve:

I want to provide a modern solution. There is now a onTextLayout event that includes an array of lines which can be determined what number of lines are being rendered. There’s other details in the lines array like actual height and width of every line which can be further used to determine if the text is being truncated.

const NUM_OF_LINES = 5;
const SOME_LONG_TEXT_BLOCK = 'Lorem ipsum ...';

function SomeComponent () { 
  const [ showMore, setShowMore ] = useState(false);
  const onTextLayout = useCallback(e =>
    setShowMore(e.nativeEvent.lines.length >= NUM_OF_LINES);
  }, []);

  return (
    <Text numberOfLines={NUM_OF_LINES} onTextLayout={onTextLayout}>
      {SOME_LONG_TEXT_BLOCK}
    </Text>
  );
}

###

It looks like React Native 0.24 implements an onLayout function
http://facebook.github.io/react-native/docs/text.html#onlayout

onLayout function
Invoked on mount and layout changes with
{nativeEvent: {layout: {x, y, width, height}}}

So it looks like you could pass an onLayout callback function, get the height of the Text component and then do some calculations using the line height to get to the number of lines

###

You can use this formula:

CPL = Width / (font-size / font-constant)

font-constant = a constant specified to each font.
CPL = number of characters per line

Here’s some fonts with their constant:

 - Serif Fonts:
 American Typewriter — 2.14
 Baskerville — 2.14
 Georgia — 1.91
 Times New Roman — 2.21

 - Sans-serif Fonts:
 Arial — 1.91
 Calibri — 2.1
 Helvetica Neue — 1.9
 Lucida Grande — 1.91
 Tahoma — 1.91
 Trebuchet MS — 2.11
 Verdana — 1.73

 - Monospace Font:
 Courier New — 1.64

for example:

function getNumberOfLines(text, fontSize, fontConstant, containerWidth){

    let cpl = Math.floor(containerWidth / (fontSize / fontConstant) );
    const words = text.split(' ');
    const elements = [];
    let line = '';

    while(words.length > 0){
        if(line.length + words[0].length + 1 <= cpl || line.length === 0 && words[0].length + 1 >= cpl){
            let word = words.splice(0,1);
            if(line.length === 0){
                line = word;
            }else {
                line = line + " " + word;
            }
            if(words.length === 0){
                elements.push(line);
            }
        }
        else {
            elements.push(line);
            line = "";
        }
    }
    return elements.length;
}

###

The solution Garrett McCullough have provided seem working for me, and I just want to add some code example:

import React from 'react';
import { StyleSheet, Text, View, TouchableHighlight } from 'react-native';

const styles = StyleSheet.create({
    text: {
      fontSize: 24,
      lineHeight: 30,
    }
});

export default class App extends React.Component {

    onLayout = e => {
        const { height } = e.nativeEvent.layout;
        this.count = Math.floor(height / styles.text.lineHeight)
    }

    render() {
        return (
            <View style={styles.page}>
              <Text onLayout={this.onLayout} style={styles.text}>
                Random text. Random text. Random text. Random text. Random text. Random text. Random text.
                Random text. Random text. Random text. Random text. Random text. Random text. Random text.
                Random text. Random text. Random text. Random text. Random text. Random text. Random text.
                Random text. Random text. Random text. Random text. Random text. Random text.
              </Text>

              <TouchableHighlight onPress={() => alert(`text lines count is ${this.count}`)}>
                <Text style={{ fontSize: 50 }}>touch me!</Text>
              </TouchableHighlight>
            </View>
        );
    }
}

https://snack.expo.io/@devastr1/text-lines-count-example

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