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148 lines
5.1 KiB
Plaintext
148 lines
5.1 KiB
Plaintext
## Units
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Sass has powerful support for manipulating units based on how
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[real-world unit calculations][] work. When two numbers are multiplied, their
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units are multiplied as well. When one number is divided by another, the result
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takes its numerator units from the first number and its denominator units from
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the second. A number can have any number of units in the numerator and/or
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denominator.
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[real-world unit calculations]: https://en.wikipedia.org/wiki/Unit_of_measurement#Calculations_with_units_of_measurement
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<% example(autogen_css: false) do %>
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@debug 4px * 6px; // 24px*px (read "square pixels")
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@debug 5px / 2s; // 2.5px/s (read "pixels per second")
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@debug 5px * 30deg / 2s / 24em; // 3.125px*deg/s*em
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// (read "pixel-degrees per second-em")
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$degrees-per-second: 20deg/1s;
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@debug $degrees-per-second; // 20deg/s
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@debug 1 / $degrees-per-second; // 0.05s/deg
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===
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@debug 4px * 6px // 24px*px (read "square pixels")
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@debug 5px / 2s // 2.5px/s (read "pixels per second")
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@debug 5px * 30deg / 2s / 24em // 3.125px*deg/s*em
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// (read "pixel-degrees per second-em")
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$degrees-per-second: 20deg/1s
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@debug $degrees-per-second // 20deg/s
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@debug 1 / $degrees-per-second // 0.05s/deg
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<% end %>
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<% heads_up do %>
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Because CSS doesn't support complex units like square pixesl, using a number
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with complex units as a [property value][] will produce an error. This is a
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feature in disguise, though; if you aren't ending up with the right unit, it
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usually means that something's wrong with your calculations! And remember, you
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can always use the [`@debug` rule][] to check out the units of any variable or
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[expression][].
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[property value]: /documentation/style-rules/declarations
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[`@debug` rule]: /documentation/at-rules/debug
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[expression]: /documentation/syntax/structure#expressions
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<% end %>
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Sass will automatically convert between compatible units, although which unit it
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will choose for the result depends on which implementation of Sass you're
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using.If you try to combine incompatible units, like `1in + 1em`, Sass will
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throw an error.
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<% example(autogen_css: false) do %>
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// CSS defines one inch as 96 pixels.
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@debug 1in + 6px; // 102px or 1.0625in
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@debug 1in + 1s;
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// ^^^^^^^^
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// Error: Incompatible units s and in.
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===
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// CSS defines one inch as 96 pixels.
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@debug 1in + 6px // 102px or 1.0625in
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@debug 1in + 1s
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// ^^^^^^^^
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// Error: Incompatible units s and in.
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<% end %>
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As in real-world unit calculations, if the numerator contains units that are
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compatible with units in the denominator (like `96px / 1in`), they'll cancel
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out. This makes it easy to define a ratio that you can use for converting
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between units. In the example below, we set the desired speed to one second per
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50 pixels, and then multiply that by the number of pixels the transition covers
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to get the time it should take.
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<%# TODO(nweiz): auto-generate this CSS once we're compiling with Dart Sass %>
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<% example do %>
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$transition-speed: 1s/50px;
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@mixin move($left-start, $left-stop) {
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position: absolute;
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left: $left-start;
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transition: left ($left-stop - $left-start) * $transition-speed;
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&:hover {
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left: $left-stop;
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}
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}
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.slider {
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@include move(10px, 120px);
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}
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===
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$transition-speed: 1s/50px
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@mixin move($left-start, $left-stop)
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position: absolute
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left: $left-start
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transition: left ($left-stop - $left-start) * $transition-speed
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&:hover
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left: $left-stop
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.slider
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@include move(10px, 120px)
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===
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.slider {
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position: absolute;
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left: 10px;
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transition: left 2.2s;
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}
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.slider:hover {
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left: 120px;
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}
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<% end %>
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<% heads_up do %>
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If your arithmetic gives you the wrong unit, you probably need to check your
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math. You may be leaving off units for a quantity that should have them!
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Staying unit-clean allows Sass to give you helpful errors when something isn't
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right.
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You should especially avoid using interpolation like `#{$number}px`. This
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doesn't actually create a number! It creates an [unquoted string][] that
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*looks* like a number, but won't work with any [number operations][] or
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[functions][]. Try to make your math unit-clean so that `$number` already has
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the unit `px`, or write `$number * 1px`.
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[unquoted string]: /documentation/values/strings#unquoted
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[number operations]: /documentation/operators/numeric
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[functions]: /documentation/functions/math
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<% end %>
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<% heads_up do %>
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Percentages in Sass work just like every other unit. They are *not*
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interchangeable with decimals, because in CSS decimals and percentages mean
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different things. For example, `50%` is a number with `%` as its unit, and
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Sass considers it different than the number `0.5`.
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You can convert between decimals and percentages using unit arithmetic.
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`$percentage / 100%` will return the corresponding decimal, and `$decimal *
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100%` will return the corresponding percentage. You can also use the
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[`percentage()` function][] as a more explicit way of writing `$decimal *
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100%`.
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[`percentage()` function]: /documentation/functions/math#percentage
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<% end %>
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