Controls

In most descendants of TWinControl, the job of drawing the control's surface belongs to the underlying Windows screen object. If a control has visible features that cannot be rendered by a Windows screen object, it requires access to a canvas object so it can do its own drawing. 

TCustomControl is one of two base classes for controls that draw their own surfaces. Because TCustomControl is descended from TWinControl, instances of its descendants can receive the focus and serve as containers. 

Classes of controls that perform their own screen rendering and do not need the features of TWinControl should be descendants of TGraphicControl instead of TCustomControl. TGraphicControl objects can render themselves with less overhead, because they do not have underlying Windows screen objects. 

TCustomControl objects have a Paint method, which is called whenever the control needs to render its image. Custom controls paint themselves using the Canvas property. 

TCanvas 

TGraphicControl 

TWinControl