Follow these best practices when you write USS to style visual elementsA node of a visual tree that instantiates or derives from the C# VisualElement
class. You can style the look, define the behaviour, and display it on screen as part of the UI. More info
See in Glossary.
Use USS files instead of inline styles when you can for more efficient memory usage.
Inline styles are per element and can cause memory overhead. When you use inline styles in a C# script or a UXML file on many elements, the memory usage becomes high quickly.
All USS selectors are applied at runtime so the architecture affects initialization performance. USS selectors are applied when an element first appears or when its classes change:
:hover
selector is the main culprit for selectors to cause interactivity issues and a re-styling.Usually, it’s not a problem if you have a lot of selectors because each USS file is turned into a lookup table. However, the performance decreases linearly as you add classes to an element. Each class in the list is used to query the lookup table. The complexity is N1 x N2
, where:
N1
is the number of class on the elementN2
is the current number of applicable USS filesThe number of elements in the hierarchy is the main fact that affects performance. Update Styling might be negligible for simple UIs(User Interface) Allows a user to interact with your application. Unity currently supports three UI systems. More info
See in Glossary but is significant for large UIs that have several thousands of elements. If an element matches a lot of selectors, it causes overhead to merge the styles coming from each rule.
In general, complex selectors have more impact on performance than simple selectors. Complex selectors depend on the ancestors of an element to match it. When possible, consider the following:
selector1 > selector2 > selector3
) instead of the descendant selector (selector1 selector2 selector3
).selector1 > selector2 > *
) or the combination of descendant selector with the universal selector (selector1 * selector2
). The universal selector tests every potential element against the selector which can impact performance.:hover
pseudo-class in selectors on elements with many descendants, such as .yellow:hover > * > Button
. Mouse movements invalidate the entire hierarchy of elements targeted by an :hover
selector.You can use the Block Element Modifier(BEM) convention to reduce hierarchical selectors. With BEM, each element receives a class that combines its specific existence inside another element.
BEM stands for Block Element Modifier. BEM is a simple system that helps you write structured, non-ambiguous, easy to maintain selectors. With BEM, you assign classes to elements and then use these classes as the selectors in style sheets.
BEM classes have up to three components:
menu
, button
, list-view
menu__item
, button__icon
, or list-view__item
menu--disabled
, menu__item--disabled
, button--small
, or list-view__item--selected
.Each name part may consist of Latin letters, digits, and dashes. Each name part is joined together with either a double underscore __
or a double dash --
.
The following example shows UXML code for a menu:
<VisualElement class="menu">
<Label class="menu__item" text="Banana" />
<Label class="menu__item" text="Apple" />
<Label class="menu__item menu__item--disabled" text="Orange" />
</VisualElement>
Each element is equipped with classes that describe its role and appearance, you can write most of your selectors with only one class name:
.menu {
}
.menu__item {
}
.menu__item--disabled {
}
You can style elements with a single class name. Sometimes, you might need to use complex selectors. For example, you can use a complex selector when the style of an element depends on the modifier of its block:
.button {
}
.button__icon {
}
.button--small {
}
.button--small .button__icon {
}
Note:
Button
, Label
) or element names (#my-button
) in your BEM selectors.UI Toolkit adheres to BEM. Each visual element has the necessary class names attached. For example, all TextElement
have the unity-text-element
class. Each instance of Button
, which derives from TextElement
, has its class list populated with the unity-button
and unity-text-element
classes.
If you derive a new element from VisualElement
or one of its descendants, following these guidelines to ensure that your element is easy to style using the BEM methodology:
AddToClassList()
in the constructor to add the relevant USS classes to your element instances.my-block__first-child
, my-block__other-child
.