How to highlight a word in string in flutter programmatically

Question

Is there any way to change color on particular word in a string ?

Text("some long string")

now i want to give color to only long word.

can someone tell me how can i do this programatically ?

eg:-

I am long a really long and long string in some variable, a long one

now here i want to seperate long word. I can seperate simple string to highlight one word but not sure how to find and highlight each of these words.

Answer 1

Wrap the word in a TextSpan and assign style properties to change the text appearance and use RichText instead of Text

RichText(
  text: TextSpan(
    text: 'Hello ',
    style: DefaultTextStyle.of(context).style,
    children: <TextSpan>[
      TextSpan(text: 'bold', style: TextStyle(fontWeight: FontWeight.bold)),
      TextSpan(text: ' world!'),
    ],
  ),
)

or use the Text.rich constructor https://docs.flutter.io/flutter/widgets/Text-class.html

const Text.rich(
  TextSpan(
    text: 'Hello', // default text style
    children: <TextSpan>[
      TextSpan(text: ' beautiful ', style: TextStyle(fontStyle: FontStyle.italic)),
      TextSpan(text: 'world', style: TextStyle(fontWeight: FontWeight.bold)),
    ],
  ),
)

Answer 2

Here is my code.

import 'package:flutter/material.dart';

class HighlightText extends StatelessWidget {
  final String text;
  final String highlight;
  final TextStyle style;
  final TextStyle highlightStyle;
  final Color highlightColor;
  final bool ignoreCase;

  HighlightText({
    Key key,
    this.text,
    this.highlight,
    this.style,
    this.highlightColor,
    TextStyle highlightStyle,
    this.ignoreCase: false,
  })  : assert(
          highlightColor == null || highlightStyle == null,
          'highlightColor and highlightStyle cannot be provided at same time.',
        ),
        highlightStyle = highlightStyle ?? style?.copyWith(color: highlightColor) ?? TextStyle(color: highlightColor),
        super(key: key);

  @override
  Widget build(BuildContext context) {
    final text = this.text ?? '';
    if ((highlight?.isEmpty ?? true) || text.isEmpty) {
      return Text(text, style: style);
    }

    var sourceText = ignoreCase ? text.toLowerCase() : text;
    var targetHighlight = ignoreCase ? highlight.toLowerCase() : highlight;

    List<TextSpan> spans = [];
    int start = 0;
    int indexOfHighlight;
    do {
      indexOfHighlight = sourceText.indexOf(targetHighlight, start);
      if (indexOfHighlight < 0) {
        // no highlight
        spans.add(_normalSpan(text.substring(start)));
        break;
      }
      if (indexOfHighlight > start) {
        // normal text before highlight
        spans.add(_normalSpan(text.substring(start, indexOfHighlight)));
      }
      start = indexOfHighlight + highlight.length;
      spans.add(_highlightSpan(text.substring(indexOfHighlight, start)));
    } while (true);

    return Text.rich(TextSpan(children: spans));
  }

  TextSpan _highlightSpan(String content) {
    return TextSpan(text: content, style: highlightStyle);
  }

  TextSpan _normalSpan(String content) {
    return TextSpan(text: content, style: style);
  }
}

Answer 3

Extending on zhpoo's awesome answer, here's a widget snippet that would allow you to style/highlight anything in a string programmatically using regular expressions (dart's RegExp).

Link to dartpad demo: https://dartpad.dev/d7a0826ed1201f7247fafd9e65979953

class RegexTextHighlight extends StatelessWidget {
  final String text;
  final RegExp highlightRegex;
  final TextStyle highlightStyle;
  final TextStyle nonHighlightStyle;

  const RegexTextHighlight({
    @required this.text,
    @required this.highlightRegex,
    @required this.highlightStyle,
    this.nonHighlightStyle,
  });

  @override
  Widget build(BuildContext context) {
    if (text == null || text.isEmpty) {
      return Text("",
          style: nonHighlightStyle ?? DefaultTextStyle.of(context).style);
    }

    List<TextSpan> spans = [];
    int start = 0;
    while (true) {
      final String highlight =
          highlightRegex.stringMatch(text.substring(start));
      if (highlight == null) {
        // no highlight
        spans.add(_normalSpan(text.substring(start)));
        break;
      }

      final int indexOfHighlight = text.indexOf(highlight, start);

      if (indexOfHighlight == start) {
        // starts with highlight
        spans.add(_highlightSpan(highlight));
        start += highlight.length;
      } else {
        // normal + highlight
        spans.add(_normalSpan(text.substring(start, indexOfHighlight)));
        spans.add(_highlightSpan(highlight));
        start = indexOfHighlight + highlight.length;
      }
    }

    return RichText(
      text: TextSpan(
        style: nonHighlightStyle ?? DefaultTextStyle.of(context).style,
        children: spans,
      ),
    );
  }

  TextSpan _highlightSpan(String content) {
    return TextSpan(text: content, style: highlightStyle);
  }

  TextSpan _normalSpan(String content) {
    return TextSpan(text: content);
  }
}