Drggable widget in GTK 3 with C

Drag.c

#include <gtk/gtk.h>

// Source:
// http://www.linuxforums.org/forum/programming-scripting/117713-gtk-moving-widget-mouse-post906490.html#post906490

// higher values make movement more performant
// lower values make movement smoother
const gint Sensitivity = 1;

const gint EvMask = GDK_BUTTON_PRESS_MASK | GDK_BUTTON1_MOTION_MASK;

GtkWidget *fixed;
int offsetx, offsety, px, py, maxx, maxy;

inline static int Min(const int a, const int b) { return b < a ? b : a; }
inline static int Max(const int a, const int b) { return b > a ? b : a; }

inline static int RoundDownToMultiple(const int i, const int m)
{
	return i/m*m;
}
inline static int RoundToNearestMultiple(const int i, const int m)
{
	if (i % m > (double)m / 2.0d)
		return (i/m+1)*m;
	return i/m*m;
}

static void destroy( GtkWidget *widget, gpointer   data ) { gtk_main_quit (); }

static gboolean button_press_event( GtkWidget *w, GdkEventButton *event )
{
	if (event->button == 1) {
		GtkWidget* p = gtk_widget_get_parent(w);
		// offset == distance of parent widget from edge of screen ...
		gdk_window_get_position(gtk_widget_get_window(p), &offsetx, &offsety);
		// plus distance from pointer to edge of widget
		offsetx += (int)event->x;
		offsety += (int)event->y;
		// maxx, maxy both relative to the parent
		// note that we're rounding down now so that these max values don't get
		// rounded upward later and push the widget off the edge of its parent.
		maxx = RoundDownToMultiple(gtk_widget_get_allocated_width(p) - gtk_widget_get_allocated_width(w), Sensitivity);
		maxy = RoundDownToMultiple(gtk_widget_get_allocated_height(p) - gtk_widget_get_allocated_height(w), Sensitivity);
	}
	return TRUE;
}

static gboolean motion_notify_event( GtkWidget *widget, GdkEventMotion *event )
{
	// x_root,x_root relative to screen
	// x,y relative to parent (fixed widget)
	// px,py stores previous values of x,y

	// get starting values for x,y
	int x = (int)event->x_root - offsetx;
	int y = (int)event->y_root - offsety;
	// make sure the potential coordinates x,y:
	//   1) will not push any part of the widget outside of its parent container
	//   2) is a multiple of Sensitivity
	x = RoundToNearestMultiple(Max(Min(x, maxx), 0), Sensitivity);
	y = RoundToNearestMultiple(Max(Min(y, maxy), 0), Sensitivity);
	if (x != px || y != py) {
		px = x;
		py = y;
		gtk_fixed_move(GTK_FIXED(fixed), widget, x, y);
	}
	return TRUE;
}

GtkWidget* make_button(const gchar* const text)
{
	GtkWidget* b = gtk_button_new_with_label(text);
	gtk_widget_add_events(b, EvMask);
	g_signal_connect(b, "button_press_event", G_CALLBACK(button_press_event), NULL);
	g_signal_connect(b, "motion_notify_event",G_CALLBACK(motion_notify_event), NULL);
	gtk_widget_show(b);
	return b;
}

int main(int argc, char *argv[] )
{
	gtk_init (&argc, &argv);

	// top level window
	GtkWidget* window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
	gtk_container_set_border_width (GTK_CONTAINER (window), 0);
	g_signal_connect (G_OBJECT (window), "destroy",G_CALLBACK (destroy), NULL);

	// fixed container
	fixed=gtk_fixed_new();
	gtk_container_add((GtkContainer*)window,(GtkWidget*)fixed);
	gtk_widget_show (fixed);

	// buttons
	gtk_fixed_put(GTK_FIXED(fixed), make_button("A button"), 50, 50);
	gtk_fixed_put(GTK_FIXED(fixed), make_button("Another button"), 250, 100);

	gtk_widget_show (window);
	gtk_main ();
	return 0;
}