flutter_html_css_percent_size #

A flutter_html extension that renders a percentage width, height, padding and margin. flutter_html 3.0.0 parses all four but reads no percentage value, so width: 50% is a width of zero, padding: 5% is no padding, and an <img style="width: 50%"> disappears. There is no error and no warning. Resolving a percentage needs a LayoutBuilder, which cannot do dry layout.

Features #

Measured in a containing block 784 wide: 50% is 392, 5% is 39.2.

• Writes logical pixels into Style.width, Style.height, Style.padding and Style.margin at the building step, so the element renders through its normal renderer and a background colour still covers the padding.
• Resolves a percentage padding or margin against the containing block's width on all four sides, as CSS does. Reads the shorthands, the longhands and the logical -inline and -block forms.

Getting started #

dependencies:
  flutter_html: ^3.0.0
  flutter_html_css_percent_size: ^0.3.0

Usage #

import 'package:flutter_html/flutter_html.dart';
import 'package:flutter_html_css_percent_size/flutter_html_css_percent_size.dart';

Html(
  data: '<img src="asset:photo.png" style="width: 50%; padding: 5%">',
  extensions: const [CssPercentSizeHtmlExtension()],
);

An Html widget under an IntrinsicHeight or an IntrinsicWidth throws, because a LayoutBuilder cannot do dry layout. Register CssPercentSizeHtmlExtension(resolvePercentages: false) there: a percentage width is then auto, and a percentage padding or margin is zero.

Before you register it #

List this extension before any other building step extension that renders the same element. HtmlParser.buildFromExtension returns on the first match.

Html(
  data: '<img src="asset:photo.png" style="width: 50%; object-fit: cover">',
  extensions: const [
    CssSizeConstraintsHtmlExtension(), // first, so max-width still clamps
    CssPercentSizeHtmlExtension(),     // before object-fit
    CssObjectFitHtmlExtension(),       // builds the image, delegates to nobody
  ],
);

CssObjectFitHtmlExtension builds the image itself and never delegates. List it first and this extension never runs.

CssSizeConstraintsHtmlExtension delegates, so both run in either order, and neither order matches CSS. Measured on width: 50%; max-width: 100px: 50 with it first, 392 with it second, and CSS says 100. Listed second, the max-width is lost with no error and no warning, so list it first. That order gives padding: 5%; max-width: 100px a padding of 5, where CSS says 39.2.

A TagExtension never reads a Style, so a percentage never reaches it in either order, with no error and no warning.

Limitations #

• A percentage height almost never resolves. The basis is an unbounded maximum height in an ordinary document. Measured inside a SizedBox(height: 600): still dropped, where CSS computes 300.
• A percentage margin does not collapse, because flutter_html collapses margins before this extension writes the pixels. Measured on two adjacent <div style="margin: 5%">: the paragraphs sit 100 apart, against 61 for the same margin in pixels.
• calc() and the other CSS functions stay with flutter_html, which renders the percentage as zero. There is no error and no warning.
• Only the inline style="" attribute is read.

The numbers are in doc/measurements.md and the order matrix in doc/composition.md.

Additional information #

Issues and merge requests go to the GitLab repository. Clamping the same element is flutter_html_css_size_constraints. Fitting an image inside its box is flutter_html_css_object_fit. A font-size in rem is flutter_html_css_font_size.

CI, publishing and Renovate are described in CONTRIBUTING.md.