Widgets are the foundation of Flutter's layout system.Almost everything in Flutter is a widget, including layout models.Widgets are images, icons, and text that you see in your Flutter app.Things you can't see, such as arranging, constraining, and aligning rows, columns, and grids of visible widgets, are also widgets.

RenderObject class is the core of the rendering library, it handles the visual element tree and specifies the layout, painting and composting process.RenderObject does not have any specific layout protocol, nor does it define submodels.Implementers of the RenderObject class such as RenderBox, RenderSliver, RenderAbstractViewport, and RenderView introduce a layout system and submodels.

Most RenderObjects are RenderBoxes.

RenderBox has its Cartesian coordinate system and box constraints that determine the minimum and maximum height and width of its children.Then use these values ​​to find out the size of the child.

Common examples of RenderBox are Container, SizedBox, etc.They follow the RenderBox protocol to describe and draw layout models based on Cartesian space and box constraints.

What is Sliver?

While RenderBoxes are great for rendering objects in Cartesian coordinates, they are not great for scrolling widgets.For rolling, there is the Sliver protocol.Render objects that implement scrolling effects in the viewport use RenderSliver as their base class.

The Sliver is the part of the viewport content that overlays the viewport.Sliver participates in the Sliver protocol.