A functional way to capitalize each word in a sentence (a.k.a. Title Case). This is not efficient -- use something like string_scanner if you need to run this in a tight loop.

main.dart

main() {
  var city = "new york";
  print(titleCase(city));
}

/// Inefficient way of capitalizing each word in a string.
String titleCase(String text) {
  if (text.length <= 1) return text.toUpperCase();
  var words = text.split(' ');
  var capitalized = words.map((word) {
    var first = word.substring(0, 1).toUpperCase();
    var rest = word.substring(1);
    return '$first$rest';
  });
  return capitalized.join(' ');
}

OK, what is an efficient way?

main() {
  String spanish = "esto es una prueba con varias palabras";

  /// First option.
  print(titleCase(spanish));

  /// Second option.
  String titleCaseVar = spanish
      .split(' ')
      .map((word) => word[0].toUpperCase() + word.substring(1))
      .join(' ');
  print(titleCaseVar);
}

/// My way of capitalizing each word in a string.
String titleCase(String text) {
  if (text == null) throw ArgumentError("string: $text");

  if (text.isEmpty) return text;

  /// If you are careful you could use only this part of the code as shown in the second option.
  return text
      .split(' ')
      .map((word) => word[0].toUpperCase() + word.substring(1))
      .join(' ');
}

I've converted to String extension as below:

extension CapExtension on String {
  //first letter only
  String get inCaps => '${this[0].toUpperCase()}${this.substring(1)}';
  //all letter in string
  String get allInCaps => this.toUpperCase();
  //first letter for each word in a string
  String get titleCase => this
      .split(' ')
      .map((word) => word[0].toUpperCase() + word.substring(1))
      .join(' ');
}

Hope it helps. :)

Here's my implementation, using a StringBuffer and UTF-16 values (this capitalizes letters after periods as well):

  /// Fixes name cases; Capitalizes Each Word.
  String normaliseName(String name) {
    final stringBuffer = StringBuffer();

    var capitalizeNext = true;
    for (final letter in name.toLowerCase().codeUnits) {
      // UTF-16: A-Z => 65-90, a-z => 97-122.
      if (capitalizeNext && letter >= 97 && letter <= 122) {
        stringBuffer.writeCharCode(letter - 32);
        capitalizeNext = false;
      } else {
        // UTF-16: 32 == space, 46 == period
        if (letter == 32 || letter == 46) capitalizeNext = true;
        stringBuffer.writeCharCode(letter);
      }
    }

    return stringBuffer.toString();
  }

Here's my implementation, using a StringBuffer and UTF-16 values (this capitalizes letters after periods as well):

  /// Fixes name cases; Capitalizes Each Word.
  String normaliseName(String name) {
    final stringBuffer = StringBuffer();

    var capitalizeNext = true;
    for (final letter in name.toLowerCase().codeUnits) {
      // UTF-16: A-Z => 65-90, a-z => 97-122.
      if (capitalizeNext && letter >= 97 && letter <= 122) {
        stringBuffer.writeCharCode(letter - 32);
        capitalizeNext = false;
      } else {
        // UTF-16: 32 == space, 46 == period
        if (letter == 32 || letter == 46) capitalizeNext = true;
        stringBuffer.writeCharCode(letter);
      }
    }

    return stringBuffer.toString();
  }

Thank you so much! @hacker1024