Interactive
Software Engineering		 Windows Eiffel Library (WEL) Tutorial

[ISE Home] [ Home ] [ Release Notes ] [ Technology Papers ] [ Installation Notes ] [ About Eiffel ]

Step 2: Defining a main window class

In this step, you will learn how to define your window type for the main window, including application-specific behavior and appearance. You will create a more specialized main window class which is descending from WEL_FRAME_WINDOW.

The quickest way to make a window useful is to explain it how to respond to Windows messages. For example, when the user clicks the left mouse button in the main window of My application, the corresponding window object receives a Wm_lbuttondown message from Windows. This tells the window object that the user clicked the mouse in it. It also passes the coordinates of the point where the user clicked. To intercept and respond to Windows messages, WEL defines several procedures corresponding to the most common Windows messages. For instance, WEL_WINDOW has a procedure named on_left_button_down corresponding to the Wm_lbuttondown message. To respond to the message, you just need to redefine this procedure as follows: 

	on_left_button_down (keys, x_pos, y_pos: INTEGER) is
			-- Display a message box when the user presses the
			-- the left mouse button.
		local
			msgBox: WEL_MSG_BOX
		do
			create msgBox.make
			msgBox.information_message_box (Current, "You have pressed the left mouse button.",
				"Message received")
		end
Since you have defined a customized main window class, you have to change the main_window function in APPLICATION in order to return an instance of MAIN_WINDOW. Class APPLICATION must be defined as follows: 
class
	APPLICATION
inherit
	WEL_APPLICATION
creation
	make
feature   
	main_window: MAIN_WINDOW is 
			-- Create the application's main window.
		once 
			create Result.make
		end
	
end  -- class APPLICATION
Figure 3 shows the message box displayed when the user clicks in the main window.

Figure 3

empty2

The program created here closes when the user clicks on the system close box. But in serious applications, you may want to change this default behavior, for instance, to ask the user if he wants to save his work. WEL provides an easy way to do that, you just need to redefine the boolean function closeable to perform tests and actions needed. If you return True (which is the default value) the user will be able to close the window, otherwise the user will be unable to close it. A possible implementation could be the following: 

	closeable: BOOLEAN is 
			-- Does the user want to quit?
		local
			msgBox: WEL_MSG_BOX
		do
			create msgBox.make
			msgBox.question_message_box (Current, "Do you want to quit?", "Quit")
			Result := msgBox.message_box_result = Mb_ok
		end

Here is the full source code of MAIN_WINDOW: 

class 
	MAIN_WINDOW
inherit 
	WEL_FRAME_WINDOW
		redefine 
			on_left_button_down, closeable
		end 
creation  
	make

feature  {NONE} -- Initialization

	make is 
			-- Make the main window.
		do 
			make_top ("My application")
		end
	
feature  {NONE} -- Implementation

	on_left_button_down (keys, x_pos, y_pos: INTEGER) is 
			-- Display a message box when the user presses the
			-- the left mouse button.
		local
			msgBox: WEL_MSG_BOX
		do 
			create msgBox.make
			msgBox.information_message_box (Current, "You have pressed the left mouse button.",
				"Message received")
		end
	
	closeable: BOOLEAN is 
			-- Does the user want to quit?
		local
			msgBox: WEL_MSG_BOX
		do
			create 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

Previous Table of Contents Next