parseLineHeight function

double parseLineHeight(
  1. String lineHeight, {
  2. double fontSize = 16,
  3. double rootFontSize = 16,
})

Parses a CSS line-height value to a pixel value based on the specified fontSize and optional rootFontSize.

Supports unitless values, percentages, pixels (px), ems (em), rems (rem), and the keyword normal. Adjusts the parsed value to fit within the supported range of line heights: 1.0, 1.15, 1.5, and 2.0.

Parameters:

  • lineHeight: The CSS line-height value to parse.
  • fontSize: The font size to use for unit conversions. Defaults to 16.
  • rootFontSize: The root font size, used for rem conversions. Defaults to fontSize.

Returns: The parsed line-height value as a double in pixels.

Examples:

print(parseLineHeight('0.8'));   // 1.0
print(parseLineHeight('1.1'));   // 1.0
print(parseLineHeight('1.2'));   // 1.15
print(parseLineHeight('1.3'));   // 1.5
print(parseLineHeight('1.7'));   // 1.5
print(parseLineHeight('2.2'));   // 2.0

// Example with different units
print(parseLineHeight('120%'));  // 19.2 (16 * 1.2)
print(parseLineHeight('1.5em')); // 24.0 (16 * 1.5)
print(parseLineHeight('1.2rem')); // 19.2 (16 * 1.2)

Implementation

double parseLineHeight(
  String lineHeight, {
  double fontSize = 16,
  double rootFontSize = 16,
}) {
  // Convert line-height values
  double parsedValue;
  if (lineHeight.endsWith('px')) {
    parsedValue = double.parse(lineHeight.replaceAll('px', ''));
  } else if (lineHeight.endsWith('%')) {
    parsedValue =
        fontSize * (double.parse(lineHeight.replaceAll('%', '')) / 100);
  } else if (lineHeight.endsWith('rem')) {
    parsedValue = rootFontSize * double.parse(lineHeight.replaceAll('rem', ''));
  } else if (lineHeight.endsWith('em')) {
    parsedValue = fontSize * double.parse(lineHeight.replaceAll('em', ''));
  } else if (lineHeight == 'normal') {
    parsedValue = fontSize * normalLineHeightMultiplier;
  } else if (lineHeight.startsWith(_cssVariables)) {
    parsedValue = 1.0;
  } else {
    parsedValue = fontSize * (double.tryParse(lineHeight) ?? 1.0);
  }

  // Apply additional constraints
  if (parsedValue < 1.0) {
    parsedValue = 1.0;
  } else if (parsedValue > 1.0 && parsedValue < 1.15) {
    parsedValue = 1.0;
  } else if (parsedValue > 1.15 && parsedValue < 1.25) {
    parsedValue = 1.15;
  } else if (parsedValue > 1.25 && parsedValue < 1.5) {
    parsedValue = 1.5;
  } else if (parsedValue > 1.5 && parsedValue < 2.0) {
    parsedValue = 1.5;
  } else if (parsedValue > 2.0) {
    parsedValue = 2.0;
  }

  return parsedValue;
}