#Custom properties (--*): CSS variables

#Introduction

CSS variables, are defined with a specific syntax (e.g., --main-color: black;) and their values are accessed using thevar()function (e.g., color: var(--main-color);). Custom properties store a value in one place and allow it to be referenced in multiple other places throughout the stylesheet, promoting reusability and maintainability.

#Examples

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csssrc/css_variable_theme/index.tsx

Code

Preview

#Syntax

/* Declare css variables */
--somekeyword: left;
--somecolor: #0000ff;

/* Reference css variables */
background: var(--somecolor);

#Basic Usage

#Declaration

CSS Variable is a property whose name starts with two hyphens (-), such as --foo, which can be referenced with var() after it is defined.

:root {
  --main-bg-color: yellow;
}

.two {
  --main-height: 200px;
  color: white;
  background-color: black;
  width: 100%;
  height: 100%;
}

You can delcare CSS variables in style attribute as well.

<view
  style={{
    '--main-bg-color': 'yellow',
    '--main-height': '200px',
    color: 'white',
    width: '100%',
  }}
/>

#How To Reference CSS variables

You use CSS variable by referencing the property in a var() function in place of a standard property value

.three {
  /* --main-bg-color: blue; */
  color: white;
  background-color: var(--main-bg-color);
  width: 50%;
  border: 1px blue solid;
}

You can use them in style attributes as well:

<view
  style={{
    color: 'var(--main-bg-color)',
    width: '100%',
  }}
/>

And a CSS variable is able to be a value of another:

.nested-variables {
  --first-var: blue;

  /* Nested reference */
  --nested-reference: var(--first-var); 
}

#Default Values

When a CSS variable cannot find a defined CSS variable value, a preset default value can be used:

.two {
  width: var(--view-width, 100px);
}

#CSS Variable With Calc expressions

In the CSS Variable declaration, the property value cannot be directly mathematically operated, and needs to use the calc() function, similar to:

.four {
  background-color: var(--main-bg-color);
  width: 25%;
  height: calc(var(--main-height) - 100px);
}

#CSS Variable Scope

Like ordinary properties, the inheritance and priority of CSS variables also follow the CSS cascading rules. If no CSS variable is defined on an element, the value of that variable is inherited from its parent element. Look up according to the cascading relationship. Finally, the root element is found, which is the variable defined by the :root selector used in the previous example. Therefore, the scope of a variable is the valid scope of the selector it is in.

<view className="one" bindtap="tap1">
  <view className="two">
    <view className="three"></view>
    <view className="four"></view>
    <comp></comp>
  </view>
</view>

:root {
  --main-bg-color: yellow;
}

.one {
  color: white;
  width: 100%;
  height: 100%;
}

.two {
  --main-bg-color: red;
  --main-height: 200px;
  color: white;
  background-color: black;
  width: 100%;
  height: 100%;
}

.three {
  --main-bg-color: blue;
  color: white;
  background-color: var(--main-bg-color);
  width: 50%;
  border: 1px blue solid;
}

.four {
  background-color: var(--main-bg-color);
  width: 25%;
  height: calc(var(--main-height) - 100px);
}

For the above example, the values of the var(--main-bg-color) variable are:

• For elements with class="three": blue;
• For elements with class="four": red;

#Modifying the Value of CSS Variables

#Modifying CSS Variables via JavaScript API

You can modify the value of css-variable by using the API on the JS side: Get an element node through the lynx.getElementById().setProperty method, and modify the value of css-var on the node:

// Modify a css variable at a time
lynx.getElementById('test').setProperty('--main-height', '300px');

// Batch modify css variables
lynx.getElementById('test').setProperty({
  '--main-height1': '300px',
  '--main-height2': '400px',
});

#Modifying the Selector that Declares CSS Variables

In a selector, you can declare the value of CSS variables. By switching the selector applied to the corresponding ancestor node, you can modify the corresponding value of the CSS variables.

.a {
  --main-bg-color: red;
}

.b {
  --main-bg-color: blue;
}

.child {
  background-color: var(--main-bg-color);
  width: 25%;
}

<view className={flag ? 'a' : 'b'}>
  <view>
    <view className="child"></view>
  </view>
</view>

In the example above, by toggling the class 'a' or 'b', you can achieve the effect of modifying the corresponding variable --main-bg-color.

#Compatibility