Modifying a Menu

In certain situations a change of a menu entry may be desirable. GLUT allows us to change and delete menu entries. To alter a menu entry use:

void glutChangeToMenuEntry(int entry, char *name, int value);

Parameters:

• entry – the index of the entry, this must be between 1 and the total number of entries
• name – the name of the new entry
• value – The value that will be return to the callback function when the entry is selected.

To swap a submenu use:

void glutChangeToSubMenu(int entry, char *name, int menu);

Parameters:

• entry – the index of the entry, this must be between 1 and the total number of entries
• name – the name of the new entry
• menu – The menu index to be used.

To following function deletes an item.

void glutRemoveMenuItem(int entry);

Parameters:

• entry – the index of the entry, this must be between 1 and the total number of entries

One last thing, you can query at any time the number of items of the current menu with glutGet.

The next example shows an example of changing a menu:

void processMenuEvents(int option) {

	red = 0.0;
	green = 0.0;
	blue = 0.0;

	switch (option) {
		case RED :
			red = 1.0; break;
		case GREEN :
			green = 1.0; break;
		case BLUE :
			blue = 1.0; break;
		case WHITE :
			red = 1.0;
			green = 1.0;
			blue = 1.0; break;
	}
}

void processKeys(unsigned char c, int x, int y) {

	int num = glutGet(GLUT_MENU_NUM_ITEMS);
	switch (c) {
		case 'a':
			glutChangeToMenuEntry(1,"Blue",BLUE);
			glutChangeToMenuEntry(3,"Red",RED);
			break;
		case 'b':
			glutChangeToMenuEntry(3,"Blue",BLUE);
			glutChangeToMenuEntry(1,"Red",RED);
			break;
		case 'c':
			if (num > 3)
				glutRemoveMenuItem(num);
			break;
		case 'd': if (num == 3)
				glutAddMenuEntry("White",WHITE);
			break;
	}
	glutSetMenu(menu);
}

void createGLUTMenus() {

	menu = glutCreateMenu(processMenuEvents);
	glutAddMenuEntry("Red",RED);
	glutAddMenuEntry("Green",GREEN);
	glutAddMenuEntry("Blue",BLUE);
	glutAddMenuEntry("White",WHITE);
	glutAttachMenu(GLUT_RIGHT_BUTTON);
}

Note that we changed the menu in the keyboard callback function as opposed to the menu callback function. This is because we shouldn’t do any changes to a menu while it is in use. A menu is in use until the callback is over, so we couldn’t change the menu’s structure inside the menu’s own callback.

As mentioned before, when a menu is in use it can’t, or at least it shouldn’t, be altered. In order to prevent messing up we must make sure if a menu is not in use before we change the menu entries. GLUT allows us to register a callback function that will ba called whenever a menu pops-up, and when it goes away. The function to register the callback is glutMenuStatusFunc.

void glutMenuStatusFunc(void (*func)(int status, int x, int y);

Parameters:

• func – the name of the callback function

This function can be called in our main function, so we’ll just add it there.

As seen by the signature of glutMenuStatusFunc the callback function must take three parameters. These are:

• status – one of GLUT_MENU_IN_USE or GLUT_MENU_NOT_IN_USE
• x – The left coordinate of the menu relative to the window client area.
• y – The top coordinate of the menu relative to the window client area.

Bellow an example function is presented where a flag is set when the menu is in use.

void processMenuStatus(int status, int x, int y) {

	if (status == GLUT_MENU_IN_USE)
		flag = 1;
	else
		flag = 0;
}

We can now use this flag when processing keyboard events as in the next example:

void processKeys(unsigned char c, int x, int y) {

	if (!flag) {
		int num = glutGet(GLUT_MENU_NUM_ITEMS);
		switch (c) {
			case 'a':
				glutChangeToMenuEntry(1,"Blue",BLUE);
				glutChangeToMenuEntry(3,"Red",RED);
				break;
			case 'b':
				glutChangeToMenuEntry(3,"Blue",BLUE);
				glutChangeToMenuEntry(1,"Red",RED);
				break;
			case 'c':
				if (num > 3)
					glutRemoveMenuItem(num);
				break;
			case 'd': if (num == 3)
					glutAddMenuEntry("White",WHITE);
				break;
		}
	}
}