sCiSSors Build Status Pub Package

Smarter resources for Angular apps: CSS pruning, bidirectional layouts, SVG & PNG optimization, Sass compilation, locale permutations, automatic reload.

Disclaimer: This is not an official Google product.


All of the following features are lazy (only triggered when needed) and most of them are disabled or optimized for speed with pub serve in debug mode. (note: may need pub serve --force-poll on MacOS X)

  • CSS pruning for Angular (see example/angular1, example/angular2):

    • Finds which .css rules are not used by Angular templates and removes them.
    • Supports ng-class and class with programmatic interpolated fragments (e.g. class="some-{{fragmented}}-class and-some-normal-class", ng-class="{'some-class': isSome}").
    • Disabled by default in debug mode.
  • CSS mirroring / bidirectionalization that works with Angular2's transformer:

    • Uses CSSJanus to produce a single CSS file that supports both RTL & LTR layouts!

    • Given foo { color: blue; float: left }, it generates:

      foo { color: blue; }
      :host-context([dir="ltr"]) foo { float: left }
      :host-context([dir="rtl"]) foo { float: right }

      So you just need the supporting code in your main.dart to support bidirectional layouts (see example/mirroring):

      document.body.dir = Bidi.isRtlLanguage(Intl.getCurrentLocale()) ? 'rtl' : 'ltr';
  • Sass compilation:

    • Compiles *.sass and *.scss files with sassc, the lightning-fast C++ port of Ruby Sass.
    • Rebuilds .css files whenever their .sass sources are modified.
  • Image inlining:

    • Expands inline-image calls in CSS files into data URI links, like Compass does.
    • By default in debug mode, links to images instead of inlining them.
  • PNG optimization:

    • Calls pngcrush to remove all metadata that is useless for rendering.
    • Disabled by default in debug mode.
  • SVG optimization:

    • Removes comments, doctypes, unused namespaces.
    • Disabled by default in debug mode.
  • Locale-specific permutations generation (Optional, see example/permutations):

    • Generates one .js per locale (e.g. main_en.js, main_fr.js...) with the deferred parts needed for that locale, which speeds up load time.
    • Supports deferred messages and deferred LTR/RTL template caches.
    • Optionally optimizes the resulting .js files with the Closure Compiler.
  • Automatic reload support (Optional): zero-turnaround for Dart!

  • Experimental static checker that detects unawaited futures (a common cause of sneaky bugs in async code):


Defaults vs. debug vs. release

sCiSSors is fine-tuned for fast build in debug mode (default for pub serve) and small code size in release mode (default for pub build).

Its behaviour can be fully customized through transformer settings in pubspec.yaml. For instance, to enable PNG optimizations in all modes, and enable SVG optimizations in debug only:

- scissors:
    optimizePng: true
        optimizeSvg: false
        optimizeSvg: true


If you checked out scissors's sources, you can run . ./script/ to get all the required dependencies, and skip the rest of this section :-)

Installing CSSJanus

You'll need a local install of CSSJanus for CSS mirroring.

Note that this transformer uses Google's original, not its .js port (, which might work if packaged as a binary that consumes css from stdin and outputs mirrored css on stdout).

To install CSSJanus, run the following command in a console (ensure you have something like export PATH=~/bin:$PATH in your ~/.profile or ~/.bashrc):

  mkdir ~/bin
  curl > ~/bin/
  chmod +x ~/bin/

Other deps: SassC, pngcrush...

These packages are quite standard, you can get them with brew install on MacOS X and with sudo apt-get install on Ubuntu:

  • sassc
  • pngcrush

Using the scissors transformer

The default transformer will build Sass files in a blink of an eye and will optimize CSS, PNG and SVG assets in release mode (pub build).

Please only setup sCiSSors's transformer on projects you know respect sCiSSors' conventions and limitations (see below).

Examples: see example/angular1, example/angular2).


    scissors: ^0.6.0
  - scissors

Valid settings:

  • pruneCss (boolean): false by default
  • imageInlining: default is inlineInlinedImages
    • inlineAllUrls: treats url as inline-image
    • inlineInlinedImages: simply honours inline-image
    • linkInlinedImages: replaces inline-image by url
    • disablePass: leaves inline-image untouched
  • optimizePng (boolean): by default, true in release only
  • optimizeSvg (boolean): by default, true in release only
  • sasscPath: default is sassc
  • compiledCssExtension: default is append
    • append: append the .css extension to the SASS / SCSS file name: foo.scss will be compiled to foo.scss.css.
    • replace: replace the SASS / SCSS file extension by .css: foo.scss will be compiled to foo.css.
  • pngCrushPath: default is pngcrush
  • bidiCss (boolean): default is false (note that this is true by default in the scissors/css_mirroring_transformer, see below)
  • cssJanusPath: by default, see prerequisites


  • Assumes if foo.html exists, foo.css is only used from there (conventions matter). This means sCiSSors should be disabled or used with caution when using Angular2 with ViewEncapsulation.None (see section below).
  • Very limited support of CSS rules (naive and hopefully pessimistic matching),
  • Bails out of pruning as soon as it doesn't recognize the (map literal) syntax of an ng-class (or if the map has non-string-literal keys),
  • Does not detect direct / handle DOM manipulations done in .dart companion files yet (html:Element.classes, etc).
  • No support for XML namespaces in CSS3 attribute selectors.
  • No CSS renaming yet (just pruning for now),
  • No Polymer.dart support yet.

Style Isolation in Angular

Angular(1,2) provide the following strategies:

  • Shadow DOM (default in AngularDart 1.x), implemented by ShadowDomComponentFactory in AngularDart 1.x and ViewEncapsulation.Native in Angular2
  • Shadow DOM emulation with "transclusion" (default in Angular2) implemented by TranscludingComponentFactory in AngularDart 1.x and ViewEncapsulation.Emulated in Angular2
  • Unscoped / no Shadow DOM, implemented by ViewEncapsulation.None in Angular2

The first two strategies (Shadow DOM & its transcluded emulation) provide strict encapsulation of style at the component level: styles defined in a component do not leak to any of its sub-components or parent components. This is the assumption by which sCiSSors lives, so you're safe with it.

The last "unscoped" strategy means there's no file- or component-local way of deciding if a style could be used elsewhere. You should not use sCiSSors on packages / projects with that strategy.

Using scissors/css_mirroring_transformer

See BidirectionalCss for more details.

Example: see example/mirroring.


  - scissors/css_mirroring_transformer

Valid settings:

  • bidiCss (boolean): true by default (Note: this is not the same default as in the scissors transformer)
  • originalCssDirection (ltr or rtl): ltr by default, defines the direction of input css.
  • cssJanusPath: by default.


  • The standalone scissors/css_mirroring_transformer transformer only consumes CSS files. If you need Sass support, please use the scissors transformer with bidiCss: true.
  • Does not handle directives like @keyframes and @page.

Using scissors/permutations_transformer

Example: see example/permutations.


  - scissors/permutations_transformer

Valid settings:

  • generatePermutations: true by default
  • ltrImport and rtlImport: unset by default. If you're deferred-loading LTR/RTL-specific template caches, these settings should take the alias you're importing them under. See example/permutations for a concrete example.
  • expectedPartCounts (map of .dart.js artifact to number of expected parts): unset by default. For instance: { web/main.dart.js: 3 }.
  • stripSourceMaps: false by default. Removes the sourceMappingURL links from all generated .js files, to avoid provoking 404s in production when sourcemaps aren't served (relevant only when the $dart2js transformer has setting sourceMaps: true).
  • reoptimizePermutations: false by default. Whether to optimize permutations with the Closure Compiler.
  • closureCompilerJarPath: compiler.jar by default
  • javaPath: java by default.

Using scissors/reloader/transformer

This provides an amazing development turnaround experience, whether you're using the other sCiSSors transformers or not.

With pub serve --force-poll, as soon as you save an asset (say, foo.scss) and it finished building the dependent assets (say, foo.scss.css), the app will reload. That's typically before you even have the time to tab-switch to the browser (+ no need to Ctrl+R).

The transformer ensures the automatic reload logic is removed in release builds (pub build), without interfering with source maps.

Example: see example/permutations.

Just edit pubspec.yaml (note: it's in dev_dependencies, not dependencies):

  - scissors/reloader/transformer

And edit main.dart:

  import 'package:scissors/reloader/reloader.dart';

  main() {

Valid settings:

  • serveTimestamps (boolean): by default, true in debug only
  • removeReloader (boolean): by default, true in release only

Using scissors/src/checker/transformer to detect unawaited futures

See UnawaitedFutures for more details.

Keep in mind that this transformer is very experimental, and slow. It aims to complement Dart's new strong-mode analyzer with more static checks, some of which could eventually graduate to the analyzer itself.



  - scissors/src/checker/transformer:
    unawaitedFutures: error

Valid settings:

  • unawaitedFutures (ignore, warning or error): warning by default


For things to do, please see issues.

To setup dependencies, please run:

. scripts/

This will download some executables used by Scissors and will export the following environment vars


Please run the following command to test your changes + reformat + analyze sources:


Please never force-push to master: use git revert to revert changes.