Layout and sizing
PCL.UI.Next Runtime computes PXML layout incrementally; it does not delegate its core Measure/Arrange contract to Avalonia.
Row and Column
<Column Padding="20" Gap="12">
<Text Text="Downloads" />
<Row Gap="8" Align="Center">
<Text Text="Current version" />
<Button Text="Refresh" Command="{cmd Versions.Refresh}" />
</Row>
</Column>Column arranges along the vertical axis; Row uses the horizontal axis. Gap appears only between adjacent children, Padding is inside the container, and Margin is outside a node.
Constraints
| Syntax | Meaning |
|---|---|
Width="120" | fixed logical pixels |
Width="50%" | half the parent available width |
Width="auto" | content measured under the parent constraint |
Width="1*" | weighted remaining track space |
MinWidth / MaxWidth | lower/upper measure bounds |
Percentage and star values resolve only after the parent provides an available constraint. Wrapped text therefore uses its actual allocated content width and remeasures when the viewport changes.
Choosing a container
- Row/Column: one-dimensional content.
- Grid: two-dimensional fixed/auto/percentage/star tracks. Constrained star tracks redistribute space iteratively.
- Overlay: children share one layout region; it does not create a modal interaction barrier by itself.
- Absolute: local canvases and anchored decoration, not ordinary responsive pages.
- Scroll: small scrollable content.
- VirtualList: large dynamic collections; never precreate thousands of children in Scroll.
Grid track declarations are validated by the target UI profile's component schema. Do not treat an unregistered ad-hoc track string as portable syntax.
Text wrapping
<Column Width="50%" Padding="12">
<Text Text="{bind Article.Summary}" />
</Column>Layout Measure passes the actual available content width to text shaping. Viewport shrink changes line count, desired height, and LayoutRect in the same layout convergence—never estimate height from character count.