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flat assembler > Examples and Tutorials > Alternate windows procedure

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Joined: 14 Feb 2013
Posts: 52
Location: Alberta
I devised this algorithm to facilitate runtime modification of event handlers in windows procedure. It also illuminates the busyness of excessive conditionals and jumps. ScanMap also deals with sub/super classed windows and post default processing.

MMap dd 0 ; Pointer to subclass procedure dd (MainWnd - MMap - 8) / 8 ; Number of handlers in map dd WM_COMMAND, CWndCmd ; Event # & Procedure pointer combos dd WM_DESTROY, MDest
Entry point to window procedure
MainWndProc: mov esi, MMap jmp ScanMap
Code to handle qualifying events
ScanMap: lodsd ; Load pointer to subclass procedure if applicable xchg ebx, eax ; Move it into EBX lodsd ; Load number of handlers in map xchg ecx, eax ; Move it into ECX @@: lodsd ; Load corresponding handlers message number cmp ax, [esp + 8] ; Is it same as kernel message lodsd ; Load pointer to procedure jz @Hd1 ; If it's the same, then handle message loopnz @B ; Keep going until end of map @@: or ebx, ebx ; Is there a subclass procedure jnz @Sub jmp [DefWindowProc] ; Do default processing @Sub: pop eax push ebx push eax jmp [CallWindowProc] @Hd1: lea esi, [esp + 4] ; ESI points to hwnd, msg, wParam, lParam mov edx, eax ; So EAX is NULL at the beginning of all handlers xor eax, eax call edx ; Execute handler jnc @B ; Branch if default handling required. xor eax, eax ; Handled messages return zero ret 16 ; Waste parameters
This may look a bit bizarre, but it does work, primarily because kernel code looks after cleaning up stack.

The handler simply looks like this and the only thing ScanMap needs is CF cleared if post default processing is required.
MDest: push 0 call [PostQuitMessage] stc ; Don't require default processing ret
Post 29 Sep 2015, 00:04
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