WEL TUTORIAL 

Step 4: Drawing lines in a window

In the next few steps, you will build a simple painting program that lets the user draw on the main window. You will do the following steps:

1. Respond to left button clicks and drag by connecting the dots, resulting drawn lines.

2. Respond to the right click by bringing up an input dialog, allowing the user to change the thickness of the line.

3. Automatically redraw the window's contents by storing the points and redrawing them in response to a paint message.

Typically, a window will receive one left button down message, followed by a series of mouse move messages (one for each point dragged over), then followed by a single left button up message. The drawing model will be implemented as follows:

	on_left_button_down (keys, x_pos, y_pos: INTEGER) is
			-- Initiate the drawing process.
		do
			if  not  button_down then
				button_down := true
				dc.get
				dc.move_to (x_pos, y_pos)
			end
		end
	
	on_mouse_move (keys, x_pos, y_pos: INTEGER) is
			-- Connect the points to make lines.
		do
			if  button_down then
				dc.line_to (x_pos, y_pos)
			end
		end
	
	on_left_button_up (keys, x_pos, y_pos: INTEGER) is
			-- Terminate the drawing process.
		do
			if  button_down then
				button_down := false
				dc.release
			end
		end

	button_down: BOOLEAN
			-- Is the left mouse button down?

Figure 5

Here is the full text of MAIN_WINDOW:

class
	MAIN_WINDOW

inherit
	WEL_FRAME_WINDOW
		redefine
			on_left_button_down, on_left_button_up, 
			on_right_button_down, on_mouse_move,
			closeable
		end

creation
	make

feature  {NONE} -- Initialization

	make is
			-- Make the main window.
		do
			make_top ("My application")
			!! dc.make (Current)
		end
	
feature   -- Access

	dc: WEL_CLIENT_DC
			-- Device context associated to the current
			-- client window
			
	button_down: BOOLEAN
			-- Is the left mouse button down?
			
feature  {NONE} -- Implementation

	on_left_button_down (keys, x_pos, y_pos: INTEGER) is
			-- Initiate the drawing process.
		do
			if  not  button_down then
				button_down := true
				dc.get
				dc.move_to (x_pos, y_pos)
			end
		end
	
	on_mouse_move (keys, x_pos, y_pos: INTEGER) is
			-- Connect the points to make lines.
		do
			if  button_down then
				dc.line_to (x_pos, y_pos)
			end
		end
	
	on_left_button_up (keys, x_pos, y_pos: INTEGER) is
			-- Terminate the drawing process.
		do
			if  button_down then
				button_down := false
				dc.release
			end
		end
	
	on_right_button_down (keys, x_pos, y_pos: INTEGER) is
			-- Invalidate window.
		do
			invalidate
		end
	
	closeable: BOOLEAN is
			-- Does the user want to quit?
		local
			msgBox: WEL_MSG_BOX
		do
			!! msgBox.make
			msgBox.question_message_box (Current, "Do you want to quit?", "Quit")
			Result := msgBox.message_box_result = Mb_ok
		end
	
end  -- class MAIN_WINDOW