Mimcss Guide: Defining rules
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Defining rules

Style definitions

In regular CSS, a unit of style definition is a rule. There are regular style rules that define a selector followed by a styleset, and at-rules: @import, @font-face, @keyframes, @media, @supports and others. Rules such as @media and @support are conditional grouping rules; that is, they define a condition and a set of nested rules, which, in turn, might be style rules or at-rules. Multiple rules are combined into a CSS file, which is sometimes called a stylesheet.

In Mimcss, a stylesheet is represented by a class - called a Style Definition Class. Individual rules are defined as properties of a style definition class. For rules producing names (such as classes, IDs, custom properties, animations, etc.), the property names, to which these rules are assigned, become the names by which these rules are referred to from the HTML rendering code. Rules can also be defined using arrays or plain objects of arbitrary structure.

Let’s create a simple style definition class:

import * as css from "mimcss";

class MyStyles extends css.StyleDefinition
    vbox = this.$class({ display: "flex", flexDirection: "column" })

    standout = this.$id({ boxShadow: {color: "red", blur: 4} })

    move = this.$keyframes([
        [ "from", { top: 0} ],
        [ 50, { top: css.percent(50) } ],
        [ "to", { top: "100%" } ]

    init = [
        this.$tag( "*", { boxSizing: "border-box" }),
        this.$tag( ["html", "body"], { height: "100%", margin: 0 }),
        this.$style( "a:hover", { color: "navy" }),

    specialColors = {
        background: this.$var({ "color", "cyan" }),
        foreground: this.$var({ "color", "magenta" }),

    special = this.$class({
        backgroundColor: this.specialColors.background,
        color: this.specialColors.foreground,

Hopefully, the rules defined above are more or less self-explanatory. The $tag,$class and $id methods define style rules where the selector is a tag name, a class name and an element ID respectively. The $style method defines a style rule that has a more complicated selector than just a tag or class name or an element ID. The $tag, $class, $id and $style methods accept a Styleset object, which is defined by Mimcss as an object with property names corresponding to the camel-cased names of CSS properties. The $var method defines a custom CSS property. The $keyframes method defines a @keyframes (animation) rule.

The rules that require names are assigned to the class’s properties. The names of these properties will be later used as names of the corresponding CSS entities (classes, IDs, etc.) when writing JSX code. Rules that don’t require names - such as simple tag rules or a universal rule (*) - are gathered into an array. The array does get assigned to a property, but this is only because the language’s syntax requires it; this property name is usually not used in any way.

Rules can be grouped using plain objects as we did in the specialColors property. This is done mostly for convenience to group rules of similar meaning or belonging to the same category. When applying these rules to elements in JSX or when referencing them internally their dotted path is used - as we did in the special property.

Note that we didn’t specify the name of the classes (nor of the ID, animation or custom property). This is because we usually don’t define the names that will be used in HTML; instead, we will use the properties to refer to the class and other entities. This is a fundamental aspect of Mimcss: names are hidden from the developers, so that the developers never have a chance to misspell the names. Mimcss mechanism generates the names that will be used in HTML and makes sure that the properties, to which the rules are assigned, refer to these names. If there is a need for a rule to have a pre-defined name, it can be specified as an optional parameter to $class, $id or $var() methods - this will override the name-generation mechanism.

Rule types

Mimcss supports all CSS rules except @charset - the latter is not needed because developers don’t actually write text-based CSS files. This section gives a brief description of StyleDefinition methods that create the rules.

  • $class() - creates CSS style rule with a class selector.
  • $id() - creates CSS style rule with an element ID selector.
  • $tag() - creates CSS style rule with a selector containing one or more tags. If more than one tag names are specified, they are separated by commas.
  • $style() - creates CSS style rule with an arbitrary complex selector. For more information see Defining Selectors
  • $abstract() - creates a styleset, which can be used (extended) by other style rules. Abstract rules by themselves don’t generate any CSS rules. For more information see Reusing Styles.
  • $var() - creates CSS custom property definition. All custom properties defined in a style definition class will go under a single :root CSS rule in the <style> element. For more information see Custom Properties.
  • $property() - creates CSS @property rule that defines a custom CSS property with a given syntax, initial value and inheritance flag. For more information see Custom Properties.
  • $keyframes() - creates CSS @keyframes rule defining animation parameters.
  • $use() - references another style definition class.
  • $fontface() - creates CSS @font-face rule defining a font.
  • $media() - creates CSS @media rule.
  • $supports() - creates CSS @supports rule.
  • $import() - creates CSS @import rule referencing a CSS file.
  • $namespace() - creates CSS @namespace rule.
  • $page() - creates CSS @page rule that specifies styles used for printing.
  • $layer() - creates CSS @layer rule.
  • $counterStyle() - creates CSS @counter-style rule.
  • $counter() - creates counter name that can be used with style properties such as counter-increment and counter-reset as well as with CSS functions such as counter() and counters(). This method doesn’t create any CSS rule but is used to generate a unique counter name.
  • $gridarea() - creates grid area name that can be used with style properties such as grid-area. This method doesn’t create any CSS rule but is used to generate a unique grid area name.
  • $gridline() - creates grid line name that can be used with style properties such as grid-template-columns and grid-temaplate-rows. This method doesn’t create any CSS rule but is used to generate a unique grid line name.
  • $classname() - creates class name by combining several class names. This allows using a single property whenever an element should have more than one CSS class applied to it. This method doesn’t create any CSS rule but is used for convenience only.
  • $const() - creates a constant corresponding to a certain type, which can be used wherever values of this type can be used. This rule doesn’t create any CSS rule and is used to substitute custom CSS properties if the value is not going to change during the application life time and if the specific characteristics of CSS custom properties such as cascading are not required. For more information see Custom Properties vs. Constants.

Under the CSS specification, @import and @namespace rules should precede all style rules in the style sheet. Mimcss doesn’t impose such a restriction: when Mimcss inserts the CSS rules into the DOM, it creates the @import statements first and the @namespace rules second, followed by other rules - regardless of their position in the style definition class. Note that the order of all other rules is preserved. Mimcss will ignore any @import and @namespace rules specified under the nested grouping rules, such as @media and @supports - also in accordance with the CSS specification.

Rules activation

By now we have defined our rules with a TypeScript class, but how do we insert the rules into the DOM so that they start applying to the HTML? This process is called “activation” and is accomplished using the activate function.

let myStyles = css.activate( MyStyles);

Notice that we passed the class object to the activate function - we didn’t create an instance of the class by ourselves. There are special situations in which you will want to create instances of the style definition class (see Styled Components unit later in this guide); however, normally you pass the class object to the activate function and Mimcss creates an instance of it.

The activate function can be invoked multiple times for the same class - Mimcss makes sure that only a single instance is created and the rules are inserted into the DOM only once.

The result of the activate call is two-fold: first, the rules defined in the class are inserted into the DOM, and second, the myStyles variable can now be used to refer to the rule names. Here is how we do it in a hypothetical HTML rendering code:

    return <div class={myStyles.vbox.name}>
        <div id={myStyles.standout.name}>Hello!</div>

The return value of the activate method is the instance of the style definition class. Properties created using one of the rule definition functions ($class, $id, etc.) implement different interfaces for different types of rules. The rules that produce names (such as class name or animation name) have the name property. In addition, each rule interface contains a reference to the CSSOM rule objects such as CSSStyleRule. These can be used to manipulate styles programmatically.

While the activate function inserts the rules into the DOM, the deactivate function removes the rules from the DOM:

css.deactivate( myStyles);

The activate and deactivate functions use a reference counting mechanism. If you call the activate function several times on the same style definition class, the styles will only be inserted once into the DOM. However, in order to remove them from the DOM, the deactivate function has to be called the same number of times.

In many cases, the rules don’t need to be removed from the DOM and should stay active for the lifetime of the application. There are, however, situations when a set of CSS rules is only used by a specific component. In this case, it is desirable that the styles will be inserted into DOM only when the component is mounted. Moreover, when the component is unmounted, it is desirable to remove the rules from the DOM. In Mimcss, this can be accomplished by placing the calls to the activate and deactivate functions into the mounting and unmounting code respectively, for example:

class MyComponent
    private styles;

        this.styles = css.activate( MyStyles);

         css.deactivate( this.styles);

        return <div class={this.styles.vbox.name}>
            <div id={this.styles.standout.name}>Hello!</div>

What if multiple instances of the component are used at the same time? No problem! The activation infrastructure keeps the reference count of how many times each style definition class has been activated and deactivated. The rules are inserted only upon first activation and removed only upon last deactivation. And, obviously, only a single instance of the style definition class is created.

There are more sophisticated activation strategies possible and they are discussed in Activation Strategies unit.

Activating and deactivating style definitions is a DOM writing activity. Without the proper care, writing to the DOM can have adverse effects such as layout thrashing. Mimcss provides several methods of activation scheduling. The activate and deactivate functions have an optional parameter schedulerType that can be used to specify what scheduling/activation method to use. Alternatively (and preferably) a default scheduling method can be set using the setDefaultSchedulerType function.

Mimcss supports several built-in scheduler types and allows the library users to create their own schedulers. For more information see the Activation Scheduling unit.

Referencing external style definitions

So far we used a single style definition class in our examples. In practice, it is usually desirable to divide application styles into several areas and use a separate style definition class for each of them. The styles defined by these classes are not usually completely isolated from one another though; that is, rules from one definition class may need to use the rules from another one. For example, a rule in class A may need to extend the rule from class B or a selector may need to combine CSS classes from two or more style definition classes.

Mimcss allows one style definition class to reference another one via the $use method as in the following example:

// CommonStyles.ts
class CommonStyles extends css.StyleDefinition
    shadow = this.$var( "boxShadow", { x: 10, y: 5, blur: 5, color: "gray" })

// MyStyles.ts
import {CommonStyles} from "./CommonStyles"

class MyStyles extends css.StyleDefinition
    common = this.$use( CommonStyles)

    // use the boxShadow custom CSS property from the CommonStyles stylesheet
    sidebar = this.$class({
        boxShadow: this.common.shadow,
        position: "absolute",
        width: css.em(15),
        height: css.em(50)

The $use method returns the instance of the referenced style definition class. From that moment on, we can use any rules in the referenced style definition - as this.common.shadow does in our example.

When our style definition class is activated and deactivated, all the referenced style definition classes are activated and deactivated along with it. The activation reference-counting mentioned above makes sure that only a single instance of the style definition exists no matter how many times it was referenced from other style definitions or activated directly. This provides a nice encapsulation of the referenced classes and makes the style definition classes self-contained units.

Embedding style definitions

When creating a set of components, developers often define small stylesheets for each component, which contain only styles for that single component. These small stylesheets are later combined into a bigger CSS files via the compile-time tools in order to reduce the number of CSS files that are loaded into a Web page.

In Mimcss, the small stylesheets would be created as separate style definition classes. Normally, each style definition class creates its own <style> element when activated. With small style definition files defined for each component, the number of <style> elements would be the same as the number of components - which can be quite big. In order to reduce the number of <style> elements, Mimcss allows marking style definition classes as embedded by applying the @embedded TypeScript decorator. The decorator takes a string parameter called category. All style definition classes marked as belonging to the same category string will be embedded in a single <style> element.

For example, developers of a widget library can create a separate style definition class for each widget component - maybe co-locating the styles and the component code in the same files. Marking all these small style definition classes as embedded with the same category will make all rules defined in all of these classes to be inserted into a single <style> element.

// WidgetAAA.tsx
class WidgetAAAStyles extends css.StyleDefinition
    aaa = this.$class( { background: css.Colors.orange, color: -css.Colors.orange })

let aaaStyles = css.activate(WidgetAAAStyles);
// WidgetBBB.tsx
class WidgetBBBStyles extends css.StyleDefinition
    bbb = this.$class( { background: css.Colors.green, color: -css.Colors.green })

let bbbStyles = css.activate(WidgetBBBStyles);

After being decorated with @embedded, the style definition classes are activated as usual. In the example above, the classes are activated right after being defined; however, it is possible to activate the classes only when needed and deactivate them when they are not needed any longer. Activating one embedded class will insert rules from all classes belonging to the same category. Activating multiple embedded classes will insert the rules only once; however, in order to remove the rules (if desired at all), all activated classes must be deactivated.

Conditional grouping rules

CSS defines several conditional rules, such as @supports and @media. These rules contain other CSS rules. In Mimcss, these rules are modeled in the same way as the top-level style definition class. The only difference is that for the grouping rules it is beneficial (but optional) to pass the class name of the parent as a generic parameter. Here is an example of the @media rule:

class MyStyles extends css.StyleDefinition
    box = this.$class( { margin: 8 })

    ifSmallScreen = this.$media( { maxWidth: 600 },
        class extends css.StyleDefinition<MyStyles>
            box = this.$class({ margin: 4 })

The $media method accepts a class that extends the StyleDefinition class with the generic type parameter set to the enclosing style definition class.

For the named rules (classes, IDs, custom properties, etc.), Mimcss will create names that would be actually inserted into DOM. There is a significant caveat here though: if a nested rule is assigned to a property with the name that already exists in the enclosing class, the actual name for the nested rule will be the same as the actual name for the existing property. This is done because the group rules such as @supports and @media are conditional rules and the styles defined by them are supposed to override the styles defined outside of the conditions.

The box property in our example is used to define a CSS class in two places: as a property of the MyStyles class and as a property of the object passed to the $media method. Mimcss will generate a single actual class name for the box property and the margin value of 4 pixels will be used on smaller devices while the value of 8 pixels will be used on the larger ones.

Every style definition has the special property: $parent, which points to the style definition object that is the parent of the current style definition object in the chain of grouping rules. The type of the $parent property is the type of the generic parameter passed to the StyleDefinition class. For the top-level class itself the value of the $parent property is undefined.

For the grouping rules, the $parent property allows referencing rules defined anywhere in the chain of grouping rules. Here is an example of how we can use the $parent property from the @media rule:

class MyStyles extends css.StyleDefinition
    defaultColor = this.$var( "color", "blue")

    ifSmallScreen = this.$media( { maxWidth: 600 },
        class extends css.StyleDefinition<MyStyles>
            p = this.$style( "p", { color: this.$parent.defaultColor })

In the top-level class, we defined a custom CSS variable that defines font color and in the @media rule, we referred to it using the this.$parent.defaultColor notation. Since we defined MyStyles class as a generic parameter for the StyleDefinition class, the TypeScript compiler knows the type of the $parent property and will help us with the autocomplete feature. Note that since we only use the $parent property, we don’t need to define the second generic type.

Style definition properties

Style definition classes are regular TypeScript classes and thus can have any types of properties and methods. Since the main purpose of a style definition class is to define CSS style and at-rules, the majority of properties will be initialized using methods like $tag, $class, $id, $style, etc. Note that property initializations are actually part of object construction, which run even before the body of the constructor (if given) does.

Properties can refer to other properties defined in the same class using the this. notation. The only requirement is that if property A references property B, property A must be defined after property B.

Depending on the method used to initialize a property, it has a type that may allow some actions on the property value after the style definition has been activated (and thus there is an instance of the style definition class). For example, all style rules have a method setProp, which allows setting a value of a style property at run-time. You can also create methods in the style definition class that manipulate property values. This can be useful, for example, if you want to change values of multiple properties at once.

Style definition instances

In all of the examples above, we never instantiated the style definition classes directly; instead, the instances were created by Mimcss upon activation of style definition classes. In fact it is possible to created style definition instances directly using the new operator and then activate these instances using the activate() function. In this case, the names for CSS entities will be generated uniquely for each instance. This is the basis for creating styled components and this will be discussed in more details in the Styled Components unit.

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