使用Sass扩展媒体查询中的选择器

Question

I have an item class and a compact "modifier" class:

.item { ... }
.item.compact { /* styles to make .item smaller */ }

This is fine. However, I'd like to add a @media query that forces the .item class to be compact when the screen is small enough.

On first thought, this is what I tried to do:

.item { ... }
.item.compact { ... }
@media (max-width: 600px) {
  .item { @extend .item.compact; }
}

But this generates the following error:

You may not @extend an outer selector from within @media. You may only @extend selectors within the same directive.

How would I accomplish this using SASS without having to resort to copy/pasting styles?

Answer 1

The simple answer is: you can't because Sass can't (or won't) compose the selector for it. You can't be inside of a media query and extend something that's outside of a media query. It certainly would be nice if it would simply take a copy of it instead of trying to compose the selectors. But it doesn't so you can't.

Use a mixin

If you have a case where you're going to be reusing a block of code inside and outside of media queries and still want it to be able to extend it, then write both a mixin and an extend class:

@mixin foo {
    // do stuff
}

%foo {
    @include foo;
}

// usage
.foo {
    @extend %foo;
}

@media (min-width: 30em) {
    .bar {
        @include foo;
    }
}

Extend the selector within a media query from the outside

This won't really help your use case, but it is another option:

%foo {
  @media (min-width: 20em) {
    color: red;
  }
}

@media (min-width: 30em) {
  %bar {
    background: yellow;
  }
}

// usage
.foo {
  @extend %foo;
}

.bar {
  @extend %bar;
}

Wait until Sass lifts this restriction (or patch it yourself)

There are a number of ongoing discussions regarding this issue (please don't contribute to these threads unless you have something meaningful to add: the maintainers are already aware that users desire this functionality, it's just a question of how to implement it and what the syntax should be).

• https://github.com/sass/sass/issues/1050
• https://github.com/sass/sass/issues/456

Answer 2

For the record, here is how I ended up solving the problem with only duplicating generated styles once:

// This is where the actual compact styles live
@mixin compact-mixin { /* ... */ }

// Include the compact mixin for items that are always compact
.item.compact { @include compact-mixin; }

// Here's the tricky part, due to how SASS handles extending
.item { ... }
// The following needs to be declared AFTER .item, else it'll
// be overridden by .item's NORMAL styles.
@media (max-width: 600px) {
  %compact { @include compact-mixin; }

  // Afterwards we can extend and
  // customize different item compact styles
  .item {
    @extend %compact;
    /* Other styles that override %compact */
  }
  // As shown below, we can extend the compact styles as many
  // times as we want without needing to re-extend
  // the compact mixin, thus avoiding generating duplicate css
  .item-alt {
    @extend %compact;
  }
}

Answer 3

I believe SASS/SCSS does not support the @extend directive inside of a media query. http://designshack.net/articles/css/sass-and-media-queries-what-you-can-and-cant-do/

You might need to use a mixin instead, though the code bloat needs to be weighed against your objective.

Answer 4

This is the cleanest, partial solution I've found. It takes advantage of @extend where possible and falls back to mixins when inside media queries.

Cross-Media Query @extend Directives in Sass

See the article for full details but the gist is that you call a mixin 'placeholder' that then decides whether to output @extend or an @include.

@include placeholder('clear') {
   clear: both;
   overflow: hidden;
}

.a {
    @include _(clear);
}
.b {
    @include _(clear);
}
.c {
    @include breakpoint(medium) {
      @include _(clear);
   }
}

Ultimately it may not be better than just using mixins, which is currently the accepted answer.

Answer 5

I use breakpoints, but it's the same idea:

@mixin bp-small {
    @media only screen and (max-width: 30em) {
        @content;
    }

How to use it:

.sidebar {
    width: 60%;
    float: left;
    @include bp-small {
        width: 100%;
        float: none;
    }
}

There is a text about mixins where you can find out more about this option.

Answer 6

Could you restructure?

.compact { //compact-styles }
.item {}
.item.compact { @extend .compact } 

@media (max-width: 600px) {
    .item { @extend .compact; }
}

If I understand the documentation correctly, that should work. I think the reason the way you're trying won't work is that it doesn't see .item.compact when it's parsing the @extend, but that's an uninformed guess, so take that with a truck load of salt! :)