Position element with the layout engine

UI Toolkit includes a layout engine that positions elements based on layout and styling properties. The layout engine uses the layout principles of Yoga, which implements a subset of Flexbox, a HTML/CSS layout system. UI Toolkit properties match Yoga layout behavior. UI Toolkit supports most properties in Flexbox.

By default, all elements are part of the layout. The layout has the following default behaviours:

• 容器垂直分配其子项。
• 容器矩形的位置包括其子矩形。其他布局属性可能限制此行为。
• A VisualElement with text uses the text size in its size calculations. This behavior can be limited by other layout properties.

UI Toolkit includes built-in controls for standard UI components, such as a button, toggle, or text field. These built-in controls have styles that affect their layout.

Main style properties

The following are the main style properties:

• Flex > Direction (flex-direction in USS): Defines the layout direction the main-axis. The default is column and that makes the main-axis run from top to bottom.
• Flex > Grow (flex-grow in USS): Defines how an element should grow in the main-axis. It’s a ratio that’s shared with all other siblings of the same parent. This property is useful when trying make an element stretch to take up the entire size of its parents (minus any siblings). To do this, set the Flex > Grow value to 1. If you have two siblings with flex-grow set to 1, they will each take 50% of the parent’s available size along the main-axis.
• Align > Align Items (align-items in USS): Defines the alignment of elements in the cross-axis, or the perpendicular axis to the main-axis. For example, if you have two Buttons in a VisualElement that has flex-direction: column and align-items: flex-end set, the two Buttons will squish against the container’s right edge. The options for align-items have names like flex-start and flex-end because they depend on the value of flex-direction.
• Align > Justify Content (justify-content in USS): Defines the alignment of elements in the main-axis. For example, if you have two Buttons in a VisualElement that has flex-direction: column and justify-content: flex-end set, the two Buttons squish against the container’s bottom edge. The options for justify-content are named like flex-start and flex-end because they depend on the value of flex-direction.

For more information, refer to Flex layout.

If the selected element has child elements, in UI Builder, you can use toggles in the header to toggle flex-related style properties in the Viewport. The image below shows the options available for the #menu element:

You can use the style properties to create complex, dynamic layouts. Here’s an example of a layout that anchors a Button on the bottom-right edge of the screen:

The UXML for this layout, showing the inline styles set on each container VisualElement, is below:

<ui:UXML xmlns:ui="UnityEngine.UIElements" xmlns:uie="UnityEditor.UIElements">
    <ui:VisualElement name="screen-is-blue" style="flex-grow: 1; justify-content: flex-end; background-color: blue;">
        <ui:VisualElement name="toolbar-is-orange" style="align-items: flex-end; background-color: orange;">
            <ui:Button text="Button" display-tooltip-when-elided="true" />
        </ui:VisualElement>
    </ui:VisualElement>
</ui:UXML>

The containers are colored to reveal their shape. You can use nested VisualElement containers to achieve any dynamic layout without resorting to explicitly setting the position and size of each element. This keeps the layout dynamic and automatically adjusts to the larger container changing size, like the screen changing size.

Absolute position

UI Builder also supports the Position style properties. The Position > Absolute mode makes an element invisible to the default Flexbox-based layout engine. It’s as if it no longer takes any space. Absolute-position elements appear on top of any siblings that are Relative-position.

While using Absolute position, you can use the Position > Left, Top, Right, Bottom style properties to offset and size the element from the respective edges of its parent. This doesn’t set absolute coordinates on the screen but sets offsets relative to the parent element. You can still use Relative mode to position the parent element. If you set both a Left offset and a Right offset, the Width style property of the element is ignored and the computed width will now come from the following formula:

element-computed-width = parent-width - left-offset - right-offset

Left, Top, Right, Bottom can also be interpreted as anchors. For example, you can anchor a Button on the bottom-left of the screen:

The UXML below displays the inline styles:

<ui:UXML xmlns:ui="UnityEngine.UIElements" xmlns:uie="UnityEditor.UIElements" editor-extension-mode="False">
    <ui:VisualElement style="flex-grow: 1;">
        <ui:VisualElement style="position: absolute; left: 0; bottom: 0;" />
    </ui:VisualElement>
</ui:UXML>

With positioning, there is a difference between having Left set to 0 and having Left unset:

• Left set to 0 means set an offset of 0.
• Left unset doesn’t set any offset and lets other style properties define the width or height of the element.

You can also modify these offset style properties directly in the Canvas via additional resize handles on each edge and corner of the element’s blue selection border. It’s important to differentiate between what’s set and what’s unset, so the Canvas also includes “anchor” toggles as orange squares off each side of the element. The Canvas handles will adjust which style properties are set when resizing the element visually, depending on which “anchors” are set. For example, say you are resizing the element in the Canvas via its right-border handle:

• If you set both Left and Right properties, the handle will update the Right property.
• If you set Left but not Right, the handle will update the Width property.

Use both Relative and Absolute

It’s typically discouraged to use Absolute position mode because it bypasses the built-in layout engine in UI Toolkit. It might also lead to high-maintenance UI where changing the overall layout would require manually updating individual elements. However, one of the legitimate use cases for Absolute position mode is for overlays. It’s useful to overlay some complex UI on top of other complex UI, such as a popup or a dropdown. You can use Absolute position only for the overlay container itself while continuing to use Relative mode inside the overlay. Here’s an example of a simple centered popup:

And here’s the UXML for your reference:

<ui:UXML xmlns:ui="UnityEngine.UIElements" xmlns:uie="UnityEditor.UIElements">
    <ui:VisualElement name="complex-ui-screen">
        <ui:Toggle label="Toggle" />
        <ui:MinMaxSlider picking-mode="Ignore" label="Min/Max Slider" min-value="10" max-value="12" low-limit="-10" high-limit="40" />
        <ui:Label text="Label" />
        <ui:Button text="Button" />
        <ui:Button text="Button" />
    </ui:VisualElement>
    <ui:VisualElement name="overlay" style="position: absolute; left: 0; top: 0; right: 0; bottom: 0; background-color: rgba(0, 0, 0, 0.71); align-items: center; justify-content: center;">
        <ui:VisualElement name="popup" style="background-color: rgba(70, 70, 70, 255);">
            <ui:Label text="Exit?" name="message" />
            <ui:Button text="Ok" name="ok-button" style="width: 108.3333px;" />
        </ui:VisualElement>
    </ui:VisualElement>
</ui:UXML>

The example above highlights the use of Absolute position. Set all Position > Left, Top, Right, Bottom to 0, so the position is 0 pixels away from the edges of the screen. This makes the #overlay element overlap the #complex-ui-screen container element. You can also set a semi-transparent background to the #overlay element to make the other UI appear darkened. Use #overlay to set Relative position to center our #popup container VisualElement.

最佳实践

下面列出了有关如何改进布局引擎性能的技巧：

• 设置 width 和 height 来定义元素的大小。

• Use the flexGrow property (USS: flex-grow: <value>;) to assign a flexible size to an element. The value of the flexGrow property assigns a base weight to the size of an element when it’s determined by its siblings.

• Set the flexDirection property to row (USS: flex-direction: row;) to switch to a horizontal layout.

• 使用相对定位根据元素的原始布局位置来偏移元素。

• 将 position 属性设置为 absolute 以便相对于父位置矩形来放置某个元素。这不会影响其同级和父项的布局。

其他资源

• USS properties reference
• USS common properties
• Style UI with USS
• Relative and absolute positioning C# example