Less 语言特性

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Results in:

.caller {
  width:  100%;
  height: 200px;
}

Variables defined directly in callers scope cannot be overridden. However, variables defined in callers parent scope is not protected and will be overridden:

.mixin() {
  @size: in-mixin;
  @definedOnlyInMixin: in-mixin;
}

.class {
  margin: @size @definedOnlyInMixin;
  .mixin();
}

@size: globaly-defined-value; // callers parent scope - no protection

Results in:

.class {
  margin: in-mixin in-mixin;
}

Finally, mixin defined in mixin acts as return value too:

.unlock(@value) { // outer mixin
  .doSomething() { // nested mixin
    declaration: @value;
  }
}

#namespace {
  .unlock(5); // unlock doSomething mixin
  .doSomething(); //nested mixin was copied here and is usable
}

Results in:

#namespace {
  declaration: 5;
}

Recursive Mixins

Edit the markdown source for "mixin-loops" 

Creating loops

In Less a mixin can call itself. Such recursive mixins, when combined with Guard Expressions and Pattern Matching, can be used to create various iterative/loop structures.

Example:

.loop(@counter) when (@counter > 0) {
  .loop((@counter - 1));    // next iteration
  width: (10px * @counter); // code for each iteration
}

div {
  .loop(5); // launch the loop
}

Output:

div {
  width: 10px;
  width: 20px;
  width: 30px;
  width: 40px;
  width: 50px;
}

A generic example of using a recursive loop to generate CSS grid classes:

.generate-columns(4);

.generate-columns(@n, @i: 1) when (@i =< @n) {
  .column-@{i} {
    width: (@i * 100% / @n);
  }
  .generate-columns(@n, (@i + 1));
}

Output:

.column-1 {
  width: 25%;
}
.column-2 {
  width: 50%;
}
.column-3 {
  width: 75%;
}
.column-4 {
  width: 100%;
}

Mixin Guards

Edit the markdown source for "mixin-guards" Guards are useful when you want to match on expressions, as opposed to simple values or arity. If you are familiar with functional programming, you have probably encountered them already.

In trying to stay as close as possible to the declarative nature of CSS, Less has opted to implement conditional execution via guarded mixins instead of if/else statements, in the vein of @media query feature specifications.

Let's start with an example:

.mixin(@a) when (lightness(@a) >= 50%) {
  background-color: black;
}
.mixin(@a) when (lightness(@a) < 50%) {
  background-color: white;
}
.mixin(@a) {
  color: @a;
}

The key is the when keyword, which introduces a guard sequence (here with only one guard). Now if we run the following code:

.class1 { .mixin(#ddd) }
.class2 { .mixin(#555) }

Here's what we'll get:

.class1 {
  background-color: black;
  color: #ddd;
}
.class2 {
  background-color: white;
  color: #555;
}

Guard Comparison Operators

The full list of comparison operators usable in guards are: >>===<<. Additionally, the keyword true is the only truthy value, making these two mixins equivalent:

.truth(@a) when (@a) { ... }
.truth(@a) when (@a = true) { ... }

Any value other than the keyword true is falsy:

.class {
  .truth(40); // Will not match any of the above definitions.
}

Note that you can also compare arguments with each other, or with non-arguments:

@media: mobile;

.mixin(@a) when (@media = mobile) { ... }
.mixin(@a) when (@media = desktop) { ... }

.max(@a; @b) when (@a > @b) { width: @a }
.max(@a; @b) when (@a < @b) { width: @b }

Guard Logical Operators

You can use logical operators with guards. The syntax is based on CSS media queries.

Use the and keyword to combine guards:

.mixin(@a) when (isnumber(@a)) and (@a > 0) { ... }

You can emulate the or operator by separating guards with a comma ,. If any of the guards evaluate to true, it's considered a match:

.mixin(@a) when (@a > 10), (@a < -10) { ... }

Use the not keyword to negate conditions:

.mixin(@b) when not (@b > 0) { ... }

Type Checking Functions

Lastly, if you want to match mixins based on value type, you can use the is functions:

.mixin(@a; @b: 0) when (isnumber(@b)) { ... }
.mixin(@a; @b: black) when (iscolor(@b)) { ... }

Here are the basic type checking functions:

  • iscolor
  • isnumber
  • isstring
  • iskeyword
  • isurl

If you want to check if a value is in a specific unit in addition to being a number, you may use one of:

  • ispixel
  • ispercentage
  • isem
  • isunit

Aliasing Mixins

Edit the markdown source for "mixins-aliasing" Released v3.5.0

Assigning mixin calls to a variable

Mixins can be assigned to a variable to be called as a variable call, or can be used for map lookup.

#theme.dark.navbar {
  .colors(light) {
    primary: purple;
  }
  .colors(dark) {
    primary: black;
    secondary: grey;
  }
}

.navbar {
  @colors: #theme.dark.navbar.colors(dark);
  background: @colors[primary];
  border: 1px solid @colors[secondary];
}

This would output:

.navbar {
  background: black;
  border: 1px solid grey;
}

Variable calls

Entire mixin calls can be aliased and called as variable calls. As in:

#library() {
  .rules() {
    background: green;
  }
}
.box {
  @alias: #library.rules();
  @alias();
}

Outputs:

.box {
  background: green;
}

Note, unlike mixins used in root, mixin calls assigned to variables and called with no arguments always require parentheses. The following is not valid.

#library() {
  .rules() {
    background: green;
  }
}
.box {
  @alias: #library.colors;
  @alias();   // ERROR: Could not evaluate variable call @alias
}

This is because it's ambiguous if variable is assigned a list of selectors or a mixin call. For example, in Less 3.5+, this variable could be used this way.

.box {
  @alias: #library.colors;
  @{alias} {
    a: b;
  }
}

The above would output:

.box #library.colors {
  a: b;
}

CSS Guards

Edit the markdown source for "css-guards" 

"if"'s around selectors

Released v1.5.0

Like Mixin Guards, guards can also be applied to css selectors, which is syntactic sugar for declaring the mixin and then calling it immediately.

For instance, before 1.5.0 you would have had to do this:

.my-optional-style() when (@my-option = true) {
  button {
    color: white;
  }
}
.my-optional-style();

Now, you can apply the guard directly to a style.

button when (@my-option = true) {
  color: white;
}

You can also achieve an if type statement by combining this with the & feature, allowing you to group multiple guards.

& when (@my-option = true) {
  button {
    color: white;
  }
  a {
    color: blue;
  }
}

Note that you can also achieve a similar pattern by using the actual if() function and a variable call. As in:

@dr: if(@my-option = true, {
  button {
    color: white;
  }
  a {
    color: blue;
  }
});
@dr();

Detached Rulesets

Edit the markdown source for "detached-rulesets" 

Assign a ruleset to a variable

Released v1.7.0

A detached ruleset is a group of css properties, nested rulesets, media declarations or anything else stored in a variable. You can include it into a ruleset or another structure and all its properties are going to be copied there. You can also use it as a mixin argument and pass it around as any other variable.

Simple example:

// declare detached ruleset
@detached-ruleset: { background: red; }; // semi-colon is optional in 3.5.0+

// use detached ruleset
.top {
    @detached-ruleset(); 
}

compiles into:

.top {
  background: red;
}

Parentheses after a detached ruleset call are mandatory (except when followed by a lookup value). The call @detached-ruleset; would not work.

It is useful when you want to define a mixin that abstracts out either wrapping a piece of code in a media query or a non-supported browser class name. The rulesets can be passed to mixin so that the mixin can wrap the content, e.g.

.desktop-and-old-ie(@rules) {
  @media screen and (min-width: 1200px) { @rules(); }
  html.lt-ie9 &                         { @rules(); }
}

header {
  background-color: blue;

  .desktop-and-old-ie({
    background-color: red;
  });
}

Here the desktop-and-old-ie mixin defines the media query and root class so that you can use a mixin to wrap a piece of code. This will output

header {
  background-color: blue;
}
@media screen and (min-width: 1200px) {
  header {
    background-color: red;
  }
}
html.lt-ie9 header {
  background-color: red;
}

A ruleset can be now assigned to a variable or passed in to a mixin and can contain the full set of Less features, e.g.

@my-ruleset: {
    .my-selector {
      background-color: black;
    }
  };

You can even take advantage of media query bubbling, for instance

@my-ruleset: {
    .my-selector {
      @media tv {
        background-color: black;
      }
    }
  };
@media (orientation:portrait) {
    @my-ruleset();
}

which will output

@media (orientation: portrait) and tv {
  .my-selector {
    background-color: black;
  }
}

A detached ruleset call unlocks (returns) all its mixins into caller the same way as mixin calls do. However, it does not return variables.

Returned mixin:

// detached ruleset with a mixin
@detached-ruleset: { 
    .mixin() {
        color: blue;
    }
};
// call detached ruleset
.caller {
    @detached-ruleset(); 
    .mixin();
}

Results in:

.caller {
  color: blue;
}

Private variables:

@detached-ruleset: { 
    @color:blue; // this variable is private
};
.caller {
    color: @color; // syntax error
}

Scoping

A detached ruleset can use all variables and mixins accessible where it is defined and where it is called. Otherwise said, both definition and caller scopes are available to it. If both scopes contains the same variable or mixin, declaration scope value takes precedence.

Declaration scope is the one where detached ruleset body is defined. Copying a detached ruleset from one variable into another cannot modify its scope. The ruleset does not gain access to new scopes just by being referenced there.

Lastly, a detached ruleset can gain access to scope by being unlocked (imported) into it.

Note: unlocking variables into scope via a called mixin is deprecated. Use property / variable accessors.

Definition and Caller Scope Visibility

A detached ruleset sees the caller's variables and mixins:

@detached-ruleset: {
  caller-variable: @caller-variable; // variable is undefined here
  .caller-mixin(); // mixin is undefined here
};

selector {
  // use detached ruleset
  @detached-ruleset(); 

  // define variable and mixin needed inside the detached ruleset
  @caller-variable: value;
  .caller-mixin() {
    variable: declaration;
  }
}

compiles into:

selector {
  caller-variable: value;
  variable: declaration;
}

Variable and mixins accessible from definition win over those available in the caller:

@variable: global;
@detached-ruleset: {
  // will use global variable, because it is accessible
  // from detached-ruleset definition
  variable: @variable; 
};

selector {
  @detached-ruleset();
  @variable: value; // variable defined in caller - will be ignored
}

compiles into:

selector {
  variable: global;
}

Referencing Won't Modify Detached Ruleset Scope

A ruleset does not gain access to new scopes just by being referenced there:

@detached-1: { scope-detached: @one @two; };
.one {
  @one: visible;
  .two {
    @detached-2: @detached-1; // copying/renaming ruleset 
    @two: visible; // ruleset can not see this variable
  }
}

.use-place {
  .one > .two(); 
  @detached-2();
}

throws an error:

ERROR 1:32 The variable "@one" was not declared.

Unlocking Will Modify Detached Ruleset Scope

A detached ruleset gains access by being unlocked (imported) inside a scope:

#space {
  .importer-1() {
    @detached: { scope-detached: @variable; }; // define detached ruleset
  }
}

.importer-2() {
  @variable: value; // unlocked detached ruleset CAN see this variable
  #space > .importer-1(); // unlock/import detached ruleset
}

.use-place {
  .importer-2(); // unlock/import detached ruleset second time
   @detached();
}

compiles into:

.use-place {
  scope-detached: value;
}

Property / variable accessors

(Lookup values)

Released v3.5.0

Starting in Less 3.5, you can use property/variable accessors (also called "lookups") to select a value from variable (detached) rulesets.

@config: {
  option1: true;
  option2: false;
}

.mixin() when (@config[option1] = true) {
  selected: value;
}

.box {
  .mixin();
}

Outputs:

.box {
  selected: value;
}

If what is returned from a lookup is another detached ruleset, you can use a second lookup to get that value.

@config: {
  @colors: {
    primary: blue;
  }
}

.box {
  color: @config[@colors][primary];
}

Variable variables in lookups

The lookup value that is returned can itself be variable. As in, you can write:

@config: {
  @dark: {
    primary: darkblue;
  }
  @light: {
    primary: lightblue;
  }
}

.box {
  @lookup: dark;
  color: @config[@@lookup][primary];
}

This will output:

.box {
  color: darkblue;
}

@import At-Rules

Edit the markdown source for "imports" 

Import styles from other style sheets

In standard CSS, @import at-rules must precede all other types of rules. But Less doesn't care where you put @import statements.

Example:

.foo {
  background: #900;
}
@import "this-is-valid.less";

File Extensions

@import statements may be treated differently by Less depending on the file extension:

  • If the file has a .css extension it will be treated as CSS and the @import statement left as-is (see the inline option below).
  • If it has any other extension it will be treated as Less and imported.
  • If it does not have an extension, .less will be appended and it will be included as a imported Less file.

Examples:

@import "foo";      // foo.less is imported
@import "foo.less"; // foo.less is imported
@import "foo.php";  // foo.php imported as a Less file
@import "foo.css";  // statement left in place, as-is

The following options can be used to override this behavior.

Import Options

Less offers several extensions to the CSS @import CSS at-rule to provide more flexibility over what you can do with external files.

Syntax: @import (keyword) "filename";

The following import options have been implemented:

  • reference: use a Less file but do not output it
  • inline: include the source file in the output but do not process it
  • less: treat the file as a Less file, no matter what the file extension
  • css: treat the file as a CSS file, no matter what the file extension
  • once: only include the file once (this is default behavior)
  • multiple: include the file multiple times
  • optional: continue compiling when file is not found

More than one keyword per @import is allowed, you will have to use commas to separate the keywords:

Example: @import (optional, reference) "foo.less";

reference

Use @import (reference) to import external files, but without adding the imported styles to the compiled output unless referenced.

Released v1.5.0

Example: @import (reference) "foo.less";

Imagine that reference marks every at-rule and selector with a reference flag in the imported file, imports as normal, but when the CSS is generated, "reference" selectors (as well as any media queries containing only reference selectors) are not output. reference styles will not show up in your generated CSS unless the reference styles are used as mixins or extended.

Additionally, reference produces different results depending on which method was used (mixin or extend):

  • extend: When a selector is extended, only the new selector is marked as not referenced, and it is pulled in at the position of the reference @import statement.
  • mixins: When a reference style is used as an implicit mixin, its rules are mixed-in, marked "not reference", and appear in the referenced place as normal.

reference example

This allows you to pull in only specific, targeted styles from a library such as Bootstrap by doing something like this:

.navbar:extend(.navbar all) {}

And you will pull in only .navbar related styles from Bootstrap.

inline

Use @import (inline) to include external files, but not process them.

Released v1.5.0

Example: @import (inline) "not-less-compatible.css";

You will use this when a CSS file may not be Less compatible; this is because although Less supports most known standards CSS, it does not support comments in some places and does not support all known CSS hacks without modifying the CSS.

So you can use this to include the file in the output so that all CSS will be in one file.

less

Use @import (less) to treat imported files as Less, regardless of file extension.

Released v1.4.0

Example:

@import (less) "foo.css";

css

Use @import (css) to treat imported files as regular CSS, regardless of file extension. This means the import statement will be left as it is.

Released v1.4.0

Example:

@import (css) "foo.less";

outputs

@import "foo.less";

once

The default behavior of @import statements. It means the file is imported only once and subsequent import statements for that file will be ignored.

Released v1.4.0

This is the default behavior of @import statements.

Example:

@import (once) "foo.less";
@import (once) "foo.less"; // this statement will be ignored

multiple

Use @import (multiple) to allow importing of multiple files with the same name. This is the opposite behavior to once.

Released v1.4.0

Example:

// file: foo.less
.a {
  color: green;
}
// file: main.less
@import (multiple) "foo.less";
@import (multiple) "foo.less";

Outputs

.a {
  color: green;
}
.a {
  color: green;
}

optional

Use @import (optional) to allow importing of a file only when it exists. Without the optional keyword Less throws a FileError and stops compiling when importing a file that can not be found.

Released v2.3.0

@plugin At-Rules

Edit the markdown source for "plugins" Released v2.5.0

Import JavaScript plugins to add Less.js functions and features

Writing your first plugin

Using a @plugin at-rule is similar to using an @import for your .less files.

@plugin "my-plugin";  // automatically appends .js if no extension

Since Less plugins are evaluated within the Less scope, the plugin definition can be quite simple.

registerPlugin({
    install: function(less, pluginManager, functions) {
        functions.add('pi', function() {
            return Math.PI;
        });
    }
})

or you can use module.exports (shimmed to work in browser as well as Node.js).

module.exports = {
    install: function(less, pluginManager, functions) {
        functions.add('pi', function() {
            return Math.PI;
        });
    }
};

Note that other Node.js CommonJS conventions, like require() are not available in the browser. Keep this in mind when writing cross-platform plugins.

What can you do with a plugin? A lot, but let's start with the basics. We'll focus first on what you might put inside the install function. Let's say you write this:

// my-plugin.js
install: function(less, pluginManager, functions) {
    functions.add('pi', function() {
        return Math.PI;
    });
}
// etc

Congratulations! You've written a Less plugin!

If you were to use this in your stylesheet:

@plugin "my-plugin";
.show-me-pi {
  value: pi();
}

You would get:

.show-me-pi {
  value: 3.141592653589793;
}

However, you would need to return a proper Less node if you wanted to, say, multiply that against other values or do other Less operations. Otherwise the output in your stylesheet is plain text (which may be fine for your purposes).

Meaning, this is more correct:

functions.add('pi', function() {
    return new tree.Dimension(Math.PI);
});

Note: A dimension is a number with or without a unit, like "10px", which would be less.Dimension(10, "px"). For a list of units, see the Less API.

Now you can use your function in operations.

@plugin "my-plugin";
.show-me-pi {
  value: pi() * 2;
}

You may have noticed that there are available globals for your plugin file, namely a function registry (functions object), and the less object. These are there for convenience.

Plugin Scope

Functions added by a @plugin at-rule adheres to Less scoping rules. This is great for Less library authors that want to add functionality without introducing naming conflicts.

For instance, say you have 2 plugins from two third-party libraries that both have a function named "foo".

// lib1.js
// ...
    functions.add('foo', function() {
        return "foo";
    });
// ...

// lib2.js
// ...
    functions.add('foo', function() {
        return "bar";
    });
// ...

That's ok! You can choose which library's function creates which output.

.el-1 {
    @plugin "lib1";
    value: foo();
}
.el-2 {
    @plugin "lib2";
    value: foo();
}

This will produce:

.el-1 {
    value: foo;
}
.el-2 {
    value: bar;
}

For plugin authors sharing their plugins, that means you can also effectively make private functions by placing them in a particular scope. As in, this will cause an error:

.el {
    @plugin "lib1";
}
@value: foo();

As of Less 3.0, functions can return any kind of Node type, and can be called at any level.

Meaning, this would throw an error in 2.x, as functions had to be part of the value of a property or variable assignment:

.block {
    color: blue;
    my-function-rules();
}

In 3.x, that's no longer the case, and functions can return At-Rules, Rulesets, any other Less node, strings, and numbers (the latter two are converted to Anonymous nodes).

Null Functions

There are times when you may want to call a function, but you don't want anything output (such as storing a value for later use). In that case, you just need to return false from the function.

var collection = [];

functions.add('store', function(val) {
    collection.push(val);  // imma store this for later
    return false;
});
@plugin "collections";
@var: 32;
store(@var);

Later you could do something like:

functions.add('retrieve', function(val) {
    return new tree.Value(collection);
});
.get-my-values {
    @plugin "collections";
    values: retrieve();   
}

The Less.js Plugin Object

A Less.js plugin should export an object that has one or more of these properties.

{
    /* Called immediately after the plugin is 
     * first imported, only once. */
    install: function(less, pluginManager, functions) { },

    /* Called for each instance of your @plugin. */
    use: function(context) { },

    /* Called for each instance of your @plugin, 
     * when rules are being evaluated.
     * It's just later in the evaluation lifecycle */
    eval: function(context) { },

    /* Passes an arbitrary string to your plugin 
     * e.g. @plugin (args) "file";
     * This string is not parsed for you, 
     * so it can contain (almost) anything */
    setOptions: function(argumentString) { },

    /* Set a minimum Less compatibility string
     * You can also use an array, as in [3, 0] */
    minVersion: ['3.0'],

    /* Used for lessc only, to explain 
     * options in a Terminal */
    printUsage: function() { },

}

The PluginManager instance for the install() function provides methods for adding visitors, file managers, and post-processors.

Here are some example repos showing the different plugin types.

Pre-Loaded Plugins

While a @plugin call works well for most scenarios, there are times when you might want to load a plugin before parsing starts.

See: Pre-Loaded Plugins in the "Using Less.js" section for how to do that.

Maps (NEW!)

Edit the markdown source for "maps" Released v3.5.0

Use rulesets and mixins as maps of values

By combining namespacing with the lookup [] syntax, you can turn your rulesets / mixins into maps.

@sizes: {
  mobile: 320px;
  tablet: 768px;
  desktop: 1024px;
}

.navbar {
  display: block;

  @media (min-width: @sizes[tablet]) {
    display: inline-block;
  }
}

Outputs:

.navbar {
  display: block;
}
@media (min-width: 768px) {
  .navbar {
    display: inline-block;
  }
}

Mixins are a little more versatile as maps because of namespacing and the ability to overload mixins.

#library() {
  .colors() {
    primary: green;
    secondary: blue;
  }
}

#library() {
  .colors() { primary: grey; }
}

.button {
  color: #library.colors[primary];
  border-color: #library.colors[secondary];
}

Outputs:

.button {
  color: grey;
  border-color: blue;
}

You can also make this easier by aliasing mixins. That is:

.button {
  @colors: #library.colors();
  color: @colors[primary];
  border-color: @colors[secondary];
}

Note, if a lookup value produces another ruleset, you can append a second [] lookup, as in:

@config: {
  @options: {
    library-on: true
  }
}

& when (@config[@options][library-on] = true) {
  .produce-ruleset {
    prop: val;
  }
}

In this way, rulesets and variable calls can emulate a type of "namespacing", similar to mixins.

As far as whether to use mixins or rulesets assigned to variables as maps, it's up to you. You may want to replace entire maps by re-declaring a variable assigned to a rulset. Or you may want to "merge" individual key/value pairs, in which case mixins as maps might be more appropriate.

Using variable variables in lookups

One important thing to notice is that the value in [@lookup] is the key (variable) name @lookup, and is not evaluated as a variable. If you want the key name itself to be variable, you can use the @@variable syntax.

E.g.

.foods() {
  @dessert: ice cream;
}

@key-to-lookup: dessert;

.lunch {
  treat: .foods[@@key-to-lookup];
}

This would output:

.lunch {
  treat: ice cream;
}
im, 无色
  • 本文由 发表于 2019年9月14日23:39:33
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