How it works...

By setting the TDBGrid property Default Drawing to False, we told the grid that we wanted to manually draw all the data into every cell. OnDrawColumnCell allows us to actually draw using standard Delphi code. For each cell we are about to draw, the event handler was called with a list of useful parameters to know which cell we're about to draw and what data we have to read, considering the column we are currently drawing. In this case, we want to draw only the calculated columns and the Rating field in a custom way. This is not a rule, but this can be done to manipulate all cells. We can draw any cell in the way we like. For the cells where we don't want to do custom drawing, a simple call method, DefaultDrawColumnCell, passes the same parameters we got from the event and the VCL code will draw the current cell as usual.

Among the event parameters, there is a Rect object (of type TRect) that represents the specific area we're about to draw. There is a column object (of type TColumn), which is a reference to the current column of the grid, and a State (of type TGridDrawState), which is a set of the grid cell states (for example, Selected, Focused, HotTrack, and many more). If our drawing code ignores the State parameter, all the cells will be drawn in the same way, and users cannot see which cell or row is selected.

The event handler uses a Pascal Sets Intersect to know whether the current cell should be drawn as a Selected or Focused cell. Refer to the following code for better clarity:

  if [gdSelected, gdFocused] * State <> [] then 
    Grid.Canvas.Brush.Color := clHighlight;