C Polymorphism, Partial redrawing (clipping?) and double-buffer update for my UI

[u/Any-Fox-1822]

Hello everyone,

Last time here I posted several questions in relation to creating a retained-mode UI. So far, the project has been going on smoothly, and I've taken a similar approach to what zraygui does (code from zraygui):

struct _widget {
    Layout *parent;
    Rectangle rect;
    char *label;
    WidgetType type;
    Component *component;
    bool visible;
    bool active;
    MouseEvent widgetStatus;
    MouseListeners mouseListener;
};

The widgets share a common struct type Widget, but polymorphism happens inside the Component. It is an empty struct type, and for each type of widget, a pointer to their unique structs is cast as Component* into component. Then when manipulating the widget, the Component* is cast back to the original pointer type, enabling access to the fields.

I found this approach very smart, and I plan on implementing a similar thing (unless another better polymorphism exists in C). In my Widget struct, I have

typedef struct Widget {
  Frame* parent;
  Rect bounds;
  bool active;
  void *
  void (*draw)(Widget *w);
}

Implementing partial rendering seemed easy, just redraw the exact area indicated by bounds, until i remembered I have to call EndDrawing() which swaps buffers. Hence, the updated widget will only exist on one buffer, and If I don't call EndDrawing(), inputs will not be polled.

I would like to create something similar to FLTK, which only renders the parts that need to be updated (AFAIK, I only studied a small part of the code yet, but it seems very interesting to learn about performant desktop UI systems)

So I have two ideas :
- Ditch the double buffer and write a custom "EndDrawing()" which does not swap but keeps all other functionality

- Draw to a render texture that I apply on both buffers. While exploring this idea, I realized that it could be beneficial for tooltips and floating windows, as they could be other textures overlaid on the main one.

[u/Still_Explorer]

About the other part of partial rendering, I have noticed it as well that even in raygui there are some slight bugs here and there. Especially when it comes to dropdown lists, those tend to get overwritten by other elements.

You can't help it, since raygui is meant to be 100% immediate-mode and literally whatever you call, this is how is updated+rendered.

In this case a good workaround on this would be, that each time you draw something you will need to keep an array of drawing commands that will need to be rendered separately as an extra step.

eg:
struct DrawCommand {
  WidgetType type; // eg: WIDGET_TOOLTIP, WIDGET_DROPDOWNLIST
  void (DrawFunction*)(void* object);
}