How to get the current number of lines in String of TextInput?

Question

After entering text in TextInput I want to know the currunt number of lines in TextInput. Or The current number of strings is also pos

Answer 1

For react-native version >= 0.46.1

You can use onContentSizeChange for a more accurate line track, for example with react hooks as following:

     /**
     * used to tracker content height and current lines
     */
    const [contentHeightTracker, setContentHeightTracker] = useState<{
        height: number,
        usedLines: number;
    }>({
        height: 0,
        usedLines: 0
    });

    useEffect(() => {
        // console.log('used line change : ' + lineAndHeightTracker.usedLines);
        // console.log('props.extremeLines : ' + props.extremeLines);
        if (contentHeightTracker.usedLines === props.extremeLines) {
            if (extremeHeight.current === undefined) {
                extremeHeight.current = contentHeightTracker.height;
            }
        }
        //callback height change
        if (contentHeightTracker.height !== 0) {
            props.heightChange && props.heightChange(contentHeightTracker.height,
                contentHeightTracker.usedLines >= props.extremeLines,
                extremeHeight.current);
        }
    }, [contentHeightTracker]);

    const _onContentHeightChange = (event: NativeSyntheticEvent<TextInputContentSizeChangeEventData>) => {
        // console.log('event height : ', event.nativeEvent.contentSize.height);
        // console.log('tracker height : ', lineAndHeightTracker.height);
        // the height increased therefore we also increase the usedLine counter
        if (contentHeightTracker.height < event.nativeEvent.contentSize.height) {
            setContentHeightTracker({
                height: event.nativeEvent.contentSize.height,
                usedLines: contentHeightTracker.usedLines + 1
            });
        } else {
            // the height decreased, we subtract a line from the line counter
            if (contentHeightTracker.height > event.nativeEvent.contentSize.height) {
                setContentHeightTracker({
                    height: event.nativeEvent.contentSize.height,
                    usedLines: contentHeightTracker.usedLines - 1
                });
            }
        }
    };

render() {
    console.log('usedLines', this.state.usedLines);
    return (
      <View style={styles.container}>
        <TextInput 
           multiline 
           style={{backgroundColor: 'green'}} 
           onContentSizeChange={_onContentHeightChange}
        />
      </View>
    );
  }

Answer 2

As far as I know, there is no offical way to get the currently used number of lines, but I have a workaround for you:

We make use of the onLayout method of our TextInput and we keep track of the currently used height. We need two state variables:

this.state = {
      height: 0, // here we track the currently used height
      usedLines: 0,// here we calculate the currently used lines 
    }

onLayout:

  _onLayout(e) {
     console.log('e', e.nativeEvent.layout.height);
     // the height increased therefore we also increase the usedLine counter
     if (this.state.height < e.nativeEvent.layout.height) {
         this.setState({usedLines: this.state.usedLines+1}); 
     } 
     // the height decreased, we subtract a line from the line counter 
     if (this.state.height > e.nativeEvent.layout.height){
         this.setState({usedLines: this.state.usedLines-1}); 
     }
     // update height if necessary
     if (this.state.height != e.nativeEvent.layout.height){
         this.setState({height: e.nativeEvent.layout.height});
     }

  }

Render Method

  render() {
    console.log('usedLines', this.state.usedLines);
    return (
      <View style={styles.container}>
        <TextInput multiline style={{backgroundColor: 'green'}} onLayout={(e)=> this._onLayout(e)} />
      </View>
    );
  }

Working Example:

https://snack.expo.io/B1vC8dvJH