Avalonia includes a group of elements that derive from Panel. These Panel elements enable many complex layouts. For example, stacking elements can easily be achieved by using the StackPanel element, while more complex and free flowing layouts are possible by using a Canvas.
The following table summarizes the available Panel controls:
Lays out all children to fill the bounds of the Panel
Defines an area within which you can explicitly position child elements by coordinates relative to the Canvas area.
Defines an area within which you can arrange child elements either horizontally or vertically, relative to each other.
Defines a flexible grid area that consists of columns and rows.
Arranges child elements relative to other elements or the panel itself.
Arranges child elements into a single line that can be oriented horizontally or vertically.
Positions child elements in sequential position from left to right, breaking content to the next line at the edge of the containing box. Subsequent ordering occurs sequentially from top to bottom or right to left, depending on the value of the Orientation property.
In WPF, Panel is an abstract class and laying out multiple controls to fill the available space is usually done with a Grid with no rows/columns. In Avalonia Panel is a usable control that has the same layout behavior as a Grid with no rows/columns, but with a lighter runtime footprint.

Element Bounding Boxes

When thinking about layout in Avalonia, it is important to understand the bounding box that surrounds all elements. Each Control consumed by the layout system can be thought of as a rectangle that is slotted into the layout. The Bounds property returns the boundaries of an element's layout allocation. The size of the rectangle is determined by calculating the available screen space, the size of any constraints, layout-specific properties (such as margin and padding), and the individual behavior of the parent Panel element. Processing this data, the layout system is able to calculate the position of all the children of a particular Panel. It is important to remember that sizing characteristics defined on the parent element, such as a Border, affect its children.

The Layout System

At its simplest, layout is a recursive system that leads to an element being sized, positioned, and drawn. More specifically, layout describes the process of measuring and arranging the members of a Panel element's Children collection. Layout is an intensive process. The larger the Children collection, the greater the number of calculations that must be made. Complexity can also be introduced based on the layout behavior defined by the Panel element that owns the collection. A relatively simple Panel, such as Canvas, can have significantly better performance than a more complex Panel, such as Grid.
Each time that a child control changes its position, it has the potential to trigger a new pass by the layout system. Therefore, it is important to understand the events that can invoke the layout system, as unnecessary invocation can lead to poor application performance. The following describes the process that occurs when the layout system is invoked.
  1. 1.
    A child UIElement begins the layout process by first having its core properties measured.
  2. 2.
    Sizing properties defined on Control are evaluated, such as Width, Height, and Margin.
  3. 3.
    Panel-specific logic is applied, such as Dock direction or stacking Orientation.
  4. 4.
    Content is arranged after all children have been measured.
  5. 5.
    The Children collection is drawn on the screen.
  6. 6.
    The process is invoked again if additional Children are added to the collection
This process and how it is invoked are defined in more detail in the following sections.

Measuring and Arranging Children

The layout system completes two passes for each member of the Children collection, a measure pass and an arrange pass. Each child Panel provides its own MeasureOverride and ArrangeOverride methods to achieve its own specific layout behavior.
During the measure pass, each member of the Children collection is evaluated. The process begins with a call to the Measure method. This method is called within the implementation of the parent Panel element, and does not have to be called explicitly for layout to occur.
First, native size properties of the Visual such as Clip and IsVisible are evaluated. This generates a constraint that is passed to MeasureCore.
First, framework properties which affects the value of the constraint are processed. These properties generally describe the sizing characteristics of the underlying Control, such as its Height, Width and Margin. Each of these properties can change the space that is necessary to display the element. MeasureOverride is then called with the constraint as a parameter.
Because Bounds is a calculated value, you should be aware that there could be multiple or incremental reported changes to it as a result of various operations by the layout system. The layout system may be calculating required measure space for child elements, constraints by the parent element, and so on.
The ultimate goal of the measure pass is for the child to determine its DesiredSize, which occurs during the MeasureCore call. The DesiredSize value is stored by Measure for use during the content arrange pass.
The arrange pass begins with a call to the Arrange method. During the arrange pass, the parent Panel element generates a rectangle that represents the bounds of the child. This value is passed to the ArrangeCore method for processing.
The ArrangeCore method evaluates the DesiredSize of the child and evaluates any additional margins that may affect the rendered size of the element. ArrangeCore generates an arrange size, which is passed to the ArrangeOverride method of the Panel as a parameter. ArrangeOverride generates the finalSize of the child. Finally, the ArrangeCore method does a final evaluation of offset properties, such as margin and alignment, and puts the child within its layout slot. The child does not have to (and frequently does not) fill the entire allocated space. Control is then returned to the parent Panel and the layout process is complete.

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