wrapTextAsLines function

List<String> wrapTextAsLines(String text, { int start = 0, int? length, })

Wraps a block of text into lines no longer than length, starting at the start column, and returns the result as a list of strings.

Tries to split at whitespace, but if that's not good enough to keep it under the limit, then splits in the middle of a word. Preserves embedded newlines, but not indentation (it trims whitespace from each line).

If length is not specified, then no wrapping occurs, and the original text is returned after splitting it on newlines. Whitespace is not trimmed in this case.

Implementation

List<String> wrapTextAsLines(String text, {int start = 0, int? length}) {
  assert(start >= 0);

  /// Returns true if the code unit at [index] in [text] is a whitespace
  /// character.
  ///
  /// Based on: https://en.wikipedia.org/wiki/Whitespace_character#Unicode
  bool isWhitespace(String text, int index) {
    var rune = text.codeUnitAt(index);
    return rune >= 0x0009 && rune <= 0x000D ||
        rune == 0x0020 ||
        rune == 0x0085 ||
        rune == 0x1680 ||
        rune == 0x180E ||
        rune >= 0x2000 && rune <= 0x200A ||
        rune == 0x2028 ||
        rune == 0x2029 ||
        rune == 0x202F ||
        rune == 0x205F ||
        rune == 0x3000 ||
        rune == 0xFEFF;
  }

  if (length == null) return text.split('\n');

  var result = <String>[];
  var effectiveLength = math.max(length - start, 10);
  for (var line in text.split('\n')) {
    line = line.trim();
    if (line.length <= effectiveLength) {
      result.add(line);
      continue;
    }

    var currentLineStart = 0;
    int? lastWhitespace;
    for (var i = 0; i < line.length; ++i) {
      if (isWhitespace(line, i)) lastWhitespace = i;

      if (i - currentLineStart >= effectiveLength) {
        // Back up to the last whitespace, unless there wasn't any, in which
        // case we just split where we are.
        if (lastWhitespace != null) i = lastWhitespace;

        result.add(line.substring(currentLineStart, i).trim());

        // Skip any intervening whitespace.
        while (isWhitespace(line, i) && i < line.length) {
          i++;
        }

        currentLineStart = i;
        lastWhitespace = null;
      }
    }
    result.add(line.substring(currentLineStart).trim());
  }
  return result;
}