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Sets or gets the window's (not the client area) or a specific control's messaging procedure for use with low-level Windows API calls that require it. Generally used in sub-classing to track all Windows messages.

**Example:**

**  PROGRAM**

** MAP**

**Main           PROCEDURE**

**SubClassFunc1  PROCEDURE(UNSIGNED,UNSIGNED,UNSIGNED,LONG),LONG,PASCAL**

**SubClassFunc2  PROCEDURE(UNSIGNED,UNSIGNED,UNSIGNED,LONG),LONG,PASCAL**

**  MODULE('')                                 !SoftVelocity Win Library**

**CallWindowProc PROCEDURE(LONG,UNSIGNED,SIGNED,UNSIGNED,LONG),LONG,PASCAL,NAME('CallWindowProcA')**

**  END**

** END                        **

**SavedProc1   LONG**

**SavedProc2   LONG**

**WM_MOUSEMOVE EQUATE(0200H)**

**PT       GROUP**

**X         SIGNED**

**Y         SIGNED**

**         END**

** CODE**

** Main**

**Main          PROCEDURE**

**WinView WINDOW('View'),AT(0,0,320,200),HVSCROLL,MAX,TIMER(1),STATUS**

**       STRING('X Pos'),AT(1,1,,),USE(?String1)**

**       STRING(@n3),AT(24,1,,),USE(PT:X)**

**       STRING('Y Pos'),AT(44,1,,),USE(?String2)**

**       STRING(@n3),AT(68,1,,),USE(PT:Y)**

**       BUTTON('Close'),AT(240,180,60,20),USE(?Close)**

**      END**

** CODE**

** OPEN(WinView)**

** SavedProc1 = WinView{****PROP:WndProc****}                    !Save this procedure**

** WinView{****PROP:WndProc****} = ADDRESS(SubClassFunc1)        !Name subclass procedure**

** SavedProc2 = WinView{PROP:ClientWndProc}              !Save this procedure**

** WinView{PROP:ClientWndProc} = ADDRESS(SubClassFunc2)  !Name subclass procedure**

** ACCEPT**

**  CASE ACCEPTED()**

**  OF ?Close**

**   BREAK**

**  END**

** END**

**SubClassFunc1    PROCEDURE(hWnd,wMsg,wParam,lParam)     !Sub class procedure**

** CODE                                                   ! to track mouse movement in**

** IF  wMsg = WM_MOUSEMOVE                                ! window's status bar (only)**

**  PT.X = MOUSEX() ; PT.Y = MOUSEY()                     !Assign mouse position**

** END**

** RETURN(CallWindowProc(SavedProc1,hWnd,wMsg,wParam,lParam))!Pass control to SavedProc1**

**SubClassFunc2    PROCEDURE(hWnd,wMsg,wParam,lParam)     !Sub class procedure**

** CODE                                                   ! to track mouse movement in**

** IF  wMsg = WM_MOUSEMOVE                                ! window's client area**

**  PT.X = MOUSEX() ; PT.Y = MOUSEY()                     !Assign mouse position**

** END**

** RETURN(CallWindowProc(SavedProc2,hWnd,wMsg,wParam,lParam))!Pass control to SavedProc2**