Context

A majority of element’s member functions require a drawing “context”. The context class provides information such as the element’s bounds, a rectangle that indicates where the element needs to be drawn. The context class inherits from the basic_context, a struct which contains a reference to the view —the main content view inside a window where elements are placed, and a canvas, a drawing surface where the elements are drawn.

struct basic_context
{
   /*...*/

   elements::view&      view;
   elements::canvas&    canvas;
};

The context class inherits from basic_context and additionally includes a pointer to the “current” element being drawn, a pointer to its parent element in the elements hierarchy (that which contains it), and the element’s bounds, a rectangle that indicates where the element needs to be drawn.

Only one element member function takes in a basic_context argument, the limits member function (more on that later). Other than that, all the other element member functions take in a context argument. The reason for this is that the at the point where the limits is called, the element’s position and parent-child relationships in the elements hierarchy have not been established yet.

class context : public basic_context
{
public:

   /*...*/

   element*          element;
   context const*    parent;
   rect              bounds;
};