This function tells you if the wallpaper is being shown in the preview of the wallpaper selector. After the wallpaper is finally selected, it will be restarted in non-preview mode to run as the background until a new wallpaper is selected.
void setup() {
fullScreen();
}
void draw() {
if (wallpaperPreview()) {
text("This is a preview", 20, 20);
}
}