We will learn how to receive and respond to mouse input in our window procedure. The example program will wait for left mouse clicks and display a text string at the exact clicked spot in the client area.

Preliminary

As with keyboard input, Windows detects and sends notifications about relevant mouse activities to each window. These activities include left and right clicks, mouse cursor movements over window, double clicks. Unlike keyboard input which is directed to the window that has input focus, mouse messages are sent to any window that the mouse cursor is over, active or not. In addition, there are mouse messages about the non-client area too. But most of the time, we can blissfully ignore them. We can focus on those relating to the client area.

For each left and right mouse button, there are two associated messages: WM_LBUTTONDOWN,WM_RBUTTONDOWN and WM_LBUTTONUP, WM_RBUTTONUP messages. For a mouse with three buttons, there are also WM_MBUTTONDOWN and WM_MBUTTONUP. When the mouse cursor moves over the client area, Windows sends WM_MOUSEMOVE messages to the window under the cursor.

A window can receive double click messages, WM_LBUTTONDBCLK or WM_RBUTTONDBCLK, if and only if its window class has been defined to receive them by including CS_DBLCLKS flag in the class style,else the window will receive only a series of mouse button up and down messages. For all these messages, the value of lParam contains the position of the mouse. The low word is the x-coordinate, and the high word is the y-coordinate relative to upper left corner of the client area of the window. wParam indicates the state of the mouse buttons and Shift and Ctrl keys.

Content

include windows.inc
includelib user32.lib
includelib kernel32.lib
includelib gdi32.lib

.data
ClassName db "SimpleWinClass",0
AppName db "Our First Window",0
MouseClick db 0 ; 0=no click yet

.data?
hInstance HINSTANCE ?
CommandLine LPSTR ?
hitpoint POINT <>

.code
start:

invoke GetModuleHandle, NULL
mov hInstance,eax
invoke GetCommandLine
invoke WinMain, hInstance,NULL,CommandLine, SW_SHOWDEFAULT invoke ExitProcess,eax

WinMain proc
hInst:HINSTANCE,hPrevInst:HINSTANCE,CmdLine:LPSTR,CmdShow:SDWORD

LOCAL wc:WNDCLASSEX
LOCAL msg:MSG
LOCAL hwnd:HWND
mov wc.cbSize,SIZEOF WNDCLASSEX
mov wc.style, CS_HREDRAW or CS_VREDRAW mov wc.lpfnWndProc, OFFSET WndProc mov wc.cbClsExtra,NULL
mov wc.cbWndExtra,NULL
push hInstance
pop wc.hInstance
mov wc.hbrBackground,COLOR_WINDOW+1 mov wc.lpszMenuName,NULL
mov wc.lpszClassName,OFFSET ClassName invoke LoadIcon,NULL,IDI_APPLICATION mov wc.hIcon,eax
mov wc.hIconSm,0
invoke LoadCursor,NULL,IDC_ARROW
mov wc.hCursor,eax
invoke RegisterClassEx, addr wc
invoke CreateWindowEx,NULL,ADDR ClassName,ADDR AppName,\

           WS_OVERLAPPEDWINDOW,CW_USEDEFAULT,\
CW_USEDEFAULT,CW_USEDEFAULT,CW_USEDEFAULT,NULL,NULL,\
hInst,NULL

mov hwnd,eax
invoke ShowWindow, hwnd,SW_SHOWNORMAL invoke UpdateWindow, hwnd
.WHILE TRUE

                invoke GetMessage, ADDR msg,NULL,0,0
.BREAK .IF (!eax)
invoke DispatchMessage, ADDR msg
.ENDW
mov     eax,msg.wParam

ret
WinMain endp

WndProc proc hWnd:HWND, uMsg:UINT, wParam:WPARAM, lParam:LPARAM

LOCAL hdc:HDC
LOCAL ps:PAINTSTRUCT

mov eax,uMsg
.IF eax==WM_DESTROY

invoke PostQuitMessage,NULL
.ELSEIF eax==WM_LBUTTONDOWN

        mov eax,lParam
shl eax,16
shr eax,16
mov hitpoint.x,eax
mov eax,lParam
shr eax,16
mov hitpoint.y,eax
mov MouseClick,TRUE
invoke InvalidateRect,hWnd,NULL,TRUE
.ELSEIF eax==WM_PAINT
invoke BeginPaint,hWnd, ADDR ps
mov    hdc,eax
.IF MouseClick
invoke lstrlen,ADDR AppName
invoke TextOut,hdc,hitpoint.x,hitpoint.y,ADDR AppName,eax
.ENDIF
invoke EndPaint,hWnd, ADDR ps
.ELSE
invoke DefWindowProc,hWnd,uMsg,wParam,lParam
ret

.ENDIF
xor eax,eax
ret
WndProc endp
end start

Time to analyze the program.

.ELSEIF eax==WM_LBUTTONDOWN

        mov eax,lParam
and eax,0FFFFh
mov hitpoint.x,eax
mov eax,lParam
shr eax,16
mov hitpoint.y,eax
mov MouseClick,TRUE
invoke InvalidateRect,hWnd,NULL,TRUE

The window procedure waits for left mouse button click. When it receives WM_LBUTTONDOWN, lParam contains the coordinate of the mouse cursor in the client area. It saves the coordinate in a variable of type POINT which is defined as:

POINT STRUCT

x dd ?
y dd ?
POINT ENDS

and sets the flag, MouseClick, to TRUE, meaning that there's at least a left mouse button click in the client area.

        mov eax,lParam
and eax,0FFFFh
mov hitpoint.x,eax

Since x-coordinate is the low word of lParam and the members of POINT structure are 32-bit in size, we have to zero out the high word of eax prior to storing it in hitpoint.x.

        shr eax,16
mov hitpoint.y,eax

Because y-coordinate is the high word of lParam, we must put it in the low word of eax prior to storing it in hitpoint.y. We do this by shifting eax 16 bits to the right.

After storing the mouse position, we set the flag, MouseClick, to TRUE in order to let the painting code in WM_PAINT section know that there's at least a click in the client area so it can draw the string at the mouse position. Next we call InvalidateRect function to force the window to repaint its entire client area.

        .IF MouseClick
invoke lstrlen,ADDR AppName
invoke TextOut,hdc,hitpoint.x,hitpoint.y,ADDR AppName,eax
.ENDIF

The painting code in WM_PAINT section must check if MouseClick is true, since when the window was created, it received a WM_PAINT message which at that time, no mouse click had occurred so it should not draw the string in the client area. We initialize MouseClick to FALSE and change its value to TRUE when an actual mouse click occurs. If at least one mouse click has occurred, it draws the string in the client area at the mouse position. Note that it calls lstrlen to get the length of the string to display and sends the length as the last parameter of TextOut function.

      [Reprinted With permission from Iczelion's Win32 Assembly HomePage]
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