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flat assembler > Examples and Tutorials > Center MessageBox in Parent Window, Win64

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Joined: 21 Jul 2003
Posts: 2792
Location: dank orb
Who (besides Microsoft) doesn't want to center MessageBox's on the Window generating them?

I'm watch a movie on my primary monitor, and working on the secondary. "Beep!" WTF? My application appears to be frozen. Searching all over the screens. Thinking, "What stopped, huh?" Then under my movie is a MessageBox. Or, worse - hitting enter and not knowing what caused the MessageBox.

Clearly, there are times when MessageBox's don't logically result from a user's action. Yet, if the user is focused on your application and has caused an error/alert by their action, then the MessageBox should be put at their point of focus. It's just good UI design and respectful to the user.

Here is the routine that lets you do that, a CBT Hook which is only momentarily active. Simply replace:

    invoke MessageBox,[hWnd],[lpText],[lpCaption],[uType]

    fastcall CBTMessageBox,[hWnd],[lpText],[lpCaption],[uType]

...and include this code:
; somewhere in your global bss/data? segment
hHk rq 1 ;CBT HHOOK
.  in the code segment 
proc CBTMessageBox,hWnd,lpText,lpCaption,uType
        mov hWnd,rcx
        mov lpText,rdx
        mov lpCaption,r8
        mov uType,r9
        invoke GetCurrentThreadId
        invoke SetWindowsHookEx,WH_CBT,CBTProc,0,rax
        mov hHk,rax ; doesn't matter if it fails, we need to continue
        invoke MessageBox,hWnd,lpText,lpCaption,uType

proc CBTProc nCode,hChild,lpCBTact
        rParent RECT
        rChild RECT

        cmp ecx,HCBT_ACTIVATE
        jz .HCBT_ACTIVATE
        invoke CallNextHookEx,hHk,rcx,rdx,r8
        xor eax,eax
        mov hChild,rdx
        invoke GetWindowRect,rdx,addr rChild
        test eax,eax
        jz @F
        invoke GetParent,hChild
        test eax,eax
        jz @F
        invoke GetWindowRect,rax,addr rParent
        test eax,eax
        jz @F
        mov r9d,rChild.right
        mov eax,rChild.bottom
        mov edx,rParent.right
        mov r8d,rParent.bottom
        sub r9d,rChild.left ; width
        sub eax,rChild.top ; height
        add edx,rParent.left
        add r8d,rParent.top
        sub edx,r9d
        sub r8d,eax
        sar edx,1 ; start.x
        sar r8d,1 ; start.x
        invoke MoveWindow,hChild,rdx,r8,r9,rax,FALSE
; any error results in unhooking and just presenting without centering
    @@ invoke UnhookWindowsHookEx,hHk
        xor eax,eax
...and if you are curious to see it in action, check out this example.

¯\(°_o)/¯ unlicense.org
Post 28 Mar 2013, 14:25
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