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This example builds upon the 02_basic_window guide. It demonstrates how to constantly update the screen to simulate a bouncing square moving freely inside the window bounds.

Concepts Introduced

• Maintaining application state across frames (Velocity/Position).
• Drawing primitives (ui_fill_rect, ui_draw_string).
• The importance of clearing the screen on a new frame.
• Explicitly forcing standard visual updates via ui_mark_dirty().
• Declaring app metadata via source annotations.

---

The Code (src/userland/gui/bounce.c)

// BOREDOS_APP_DESC: Bouncing ball animation demo.
// BOREDOS_APP_ICONS: /Library/images/icons/colloid/applications-games.png
#include <stdlib.h>
#include <libui.h>
#include <syscall.h>

// Window Dimensions
#define W_WIDTH  400
#define W_HEIGHT 300
// Square Dimensions
#define SQ_SIZE  30

int main(void) {
    ui_window_t wid = ui_window_create("Bouncing Box Animation", 50, 50, W_WIDTH, W_HEIGHT);
    if (wid < 0) return 1;

    // Define object state variables
    int pos_x = 50;
    int pos_y = 50;
    int vel_x = 2; // Move 2 pixels per frame horizontally
    int vel_y = 2; // Move 2 pixels per frame vertically

    gui_event_t event;
    while (1) {
        // 1. Process Events
        while (ui_get_event(wid, &event)) {
            if (event.type == GUI_EVENT_CLOSE) {
                return 0; // Exit cleanly
            }
        }

        // 2. Physics & Logic Update
        pos_x += vel_x;
        pos_y += vel_y;

        // Collision logic (Bounce off edges)
        // The window has a 20px title bar, so the usable client height is W_HEIGHT - 20.
        if (pos_x <= 0 || pos_x + SQ_SIZE >= W_WIDTH) {
            vel_x = -vel_x; // Reverse horizontal direction
        }
        if (pos_y <= 0 || pos_y + SQ_SIZE >= W_HEIGHT - 20) {
            vel_y = -vel_y; // Reverse vertical direction
        }

        // 3. Rendering Update
        // Step A: Clear the entire background to Black (0xFF000000)
        ui_draw_rect(wid, 0, 0, W_WIDTH, W_HEIGHT, 0xFF000000);

        // Step B: Draw our shape in Red (0xFFFF0000) at the new position
        ui_draw_rect(wid, pos_x, pos_y, SQ_SIZE, SQ_SIZE, 0xFFFF0000);
        
        // Step C: Draw some UI text over the animation in White
        ui_draw_string(wid, 10, 10, "BoredOS Animation Demo!", 0xFFFFFFFF);

        // Step D: Instruct the compositor to flush our drawing buffer to the physical screen
        ui_mark_dirty(wid, 0, 0, W_WIDTH, W_HEIGHT);

        // 4. Yield and throttle (targeting ~60 FPS)
        sys_system(SYSTEM_CMD_SLEEP, 16, 0, 0, 0);
    }

    return 0;
}

How it Works

1. State Management: We store pos_x, pos_y, vel_x, and vel_y. These variables represent the "physics" of our system. Notice that they update outside the event-checking logic so that the animation runs even if the user isn't clicking the mouse.
2. Screen Clearing: We must fill the screen with black (ui_draw_rect(wid, 0, 0, W_WIDTH, W_HEIGHT, ...)). If we don't clear the screen, the red square will leave a permanent trailing smear everywhere it goes!
3. The Double Buffer: ui_draw_rect and ui_draw_string do not immediately appear on your monitor. They just color a hidden buffer within the kernel.
4. ui_mark_dirty: This is the crucial command that tells the kernel Window Manager, "I'm done drawing my frame. Can you quickly copy my hidden buffer over to the real screen now?"
5. BOREDOS_APP_DESC / BOREDOS_APP_ICONS: Embedded into the compiled .elf as a BoredOS NOTE section. The Desktop and File Explorer read this to show the game's icon instead of the generic binary icon. See elf_metadata.md for full details.