Using Automatic Placement

[Context: HL Kit … Basic Concepts and Terminology … Manipulation of Visual Elements]

In an effort to make things as easy as possible, the Kit provides a mechanism called "automatic placement" that makes positioning certain visual elements significantly easier. Although primarily intended for use within sheets, automatic placement can also be used in various places with on-screen visual elements.

Automatic placement can only be used on visual elements within layouts, panels, and sheets. Every one of these element types manages internal logic to support automatic placement, so you can use the mechanism whenever it suits your needs. Automatic placement assumes that you are placing a progression of visual elements in a vertical arrangement, with each successive element appearing beneath the previous element.

All placement is performed within a rectangular region. Before anything is placed, the bounds of this region are initialized to be the full height and width of the visual container (i.e. the layout, panel, or sheet). When placement begins, each new placement consumes vertical space within the region. This automatically shrinks the region, moving the top of the region downward to the bottom of each new visual element that is placed.

Automatic placement is performed via the "autoplace" target reference. Each placement can specify a gap that should appear between the new element and the one previously placed. Additional target references provide the author with complete control over the bounds of the region within which automatic placement is performed. This makes it possible to place specific elements at the top and/or bottom of the visual container, then adjust the automatic placement region accordingly, and finally perform automatic placement of the remaining visual elements. You can also place elements automatically and then retrieve the bounds of the remaining unused space, in which you can manually place visual elements.

Until at least one visual element with actual contents is successfully placed within the container, the gap is always considered to be zero. This ensures that the first item actually placed in the container always starts at the top, regardless of how many elements render no contents.

   -When an element is automatically placed, the width of that element is set
       to the "autowidth" of the visual container
   -When a template is automatically placed, it is rendered upon completion
   -When a layout is automatically placed, it is estimated upon completion
   -When automatically placed, most visual elements have their height set to the
       full remaining height of the automatic placement region
       -Lone exception is a non-table portal being placed within a layout, for
           which the height is assumed to be whatever is already set
       -Since heights are set the full region during automatic placement, it is
           assumed that visual elements will properly truncate their height as
           part of their rendering
       -For example, a template placed within a layout or a layout placed within
           a scene must properly set their height at the end of the Position
           script, based on the extent of the bottommost item within them
       -Table portals automatically determine their extent, so automatic
           placement of tables will work smoothly
   -When automatic placement attempt to place a visual element that will not
       fully fit in the remaining space, the region is considered to be fully
       utilized and no further elements can be placed
   -If a table is placed and it does not fully fit in the remaining space, as
       many items as will fit are output, with all remaining items being treated
       as "spillover" for rendering in subsequent tables
   -Any visual element that is not displayed at all is designated as non-visible
       -Tables that contain zero items are non-visible
       -Templates and layout that do not fully fit are non-visible