LESS CSS set variables in media query?

问题:

I'm working on a iPad-specific website. To make my website work on both the retina display iPad and older versions of iPads, I want to set a variable in LESS CSS in media query like:

@media all and (max-width: 768px) {
    @ratio: 1;
}

@media all and (min-width: 769px) {
    @ratio: 2;  
}

So when you are setting anything in pixels, you can do

width: 100px * @ratio;

But I got a compile error saying @ratio is not defined. Is it possible to have a workaround to make it work? Thanks.

回答1:

It would be nice, but this is impossible to do like this.

LESS compiles your media queries to actual media queries, so at compile time there is no indication of which media query will be relevant to the browser.

After compile time you just have CSS not less so you can't have variables anymore.

You can do this instead but it isn't as elegant:

@base_width: 100px;

@media all and (max-width: 768px) {
    .something {
        width: @base_width;
    }
}

@media all and (min-width: 769px) {
    .something {
        width: @base_width * 2;
    }
}

回答2:

I know I'm late with my answer but someone may find this useful.

You can move your styles to a separate file

// styles.less
.foo {
  width: 100px * @ratio;
}

And then import the file multiple times after changing variables' values

// main.less
@ratio: 1; // initial value

@media all and (max-width: 768px) {
  @ratio: 1;
  @import (multiple) "styles";
}

@media all and (min-width: 769px) {
  @ratio: 2;
  @import (multiple) "styles";
}

Note that (multiple) is important here

Compiled code will look like this

// main.css
@media all and (max-width: 768px) {
  .foo {
    width: 100px;
  }
}
@media all and (min-width: 769px) {
  .foo {
    width: 200px;
  }
}

回答3:

That's a little hackish, but a possible solution is to set the font-size for a wrapper element and set all units to em:

HTML:

<div id="wrap">
    <div class="child1"></div>
    <div class="child2"></div>
</div>

LESS:

#wrap 
    {
    font-size: 100px;
    width: 10em; // = 1000px;
    @media all and (max-width: 768px)
        {
        font-size: 60px;
        }
    .child1
        {
        width: 5em; // 500px normally, 300px on small screens
        }
    .child1
        {
        width: 2.5em; // 250px normally, 150px on small screens
        }
    }

That of course does not work if you have elements that contain text AND children.

回答4:

LESS currently cannot do this, although it would be technically possible for it to do it. This feature has been requested in the GitHub issue entitled Less variables in media queries.

See also this question: CSS pre-processor with a possibility to define variables in a @media query

回答5:

I found the accepted solution not to work, as the compiler would complain the mixin was not defined. An alternative solution:

@base_width: 100px;

.mixin {
    width: @base_width;

    @media all and (min-width: 769px) {
        width: @base_width * 2;
    }
}

回答6:

For those who might allow supporting relatively modern browsers only (Chrome 49+, FF 31+, no IE), you can use css variables instead.

Here is browser support table from "Can I Use".

html {
    --width-var: 300px;

    @media only screen and (max-width: 750px) {
        --width-var: 200px;
   }
}

.your-class {
    max-width: calc( var(--width-var) * 2 );
    .... // tons of other props
}

With the code above, whenever screen is getting smaller than 750px, max-width property from .your-class is recalculated (since --width-var is changed), and of course when screen is resized to be bigger - css variable gets back to its original value.

回答7:

With less we can define variables for media query.
first of all detect iPad resolution:

@iPad: ~"only screen and (min-width: 40.063em) and (max-width: 64em);

These are em equivalent of 641px and 1024px.


Now detect high resolution:

@highResolution: ~"only screen and (-webkit-min-device-pixel-ratio: 1.5)",
          ~"only screen and (min--moz-device-pixel-ratio: 1.5)",
          ~"only screen and (-o-min-device-pixel-ratio: 3/2)",
          ~"only screen and (min-device-pixel-ratio: 1.5)";

Now we can combine these 2 variables in a media query like this:

@media @iPad, @highResolution{ .yourCssClass{} }

this will be valid just on iPad (or similar resolutions) with retina display (or similar pixel density).