GDI实现透明窗口

bool SetTransparent(int alpha)    
{
    typedef DWORD (WINAPI *PSETLAYEREDWINDOWATTR)(HWND, DWORD, BYTE, DWORD);
    PSETLAYEREDWINDOWATTR pSetLayeredWindowAttributes = NULL;

    if ( pSetLayeredWindowAttributes == NULL )
    {

        HINSTANCE   hInst   =   LoadLibrary("User32.DLL");     
        if(hInst)     
        {     
             //取得SetLayeredWindowAttributes函数指针     
             pSetLayeredWindowAttributes =(PSETLAYEREDWINDOWATTR )GetProcAddress(hInst,   "SetLayeredWindowAttributes"); 
    }
    if ( pSetLayeredWindowAttributes == NULL )
        return false;

    LONG exstyle = GetWindowLong(this->GetSafeHwnd(), GWL_EXSTYLE);

    // if setting alpha to fully opaque then turn off the layered style
    if (alpha == 255)
    {
        SetWindowLong(GetHwnd(), GWL_EXSTYLE, exstyle & ~WS_EX_LAYERED);
        Refresh();
        return true;
    }

    // Otherwise, set the layered style if needed and set the alpha value
    if ((exstyle & WS_EX_LAYERED) == 0 )
        SetWindowLong(this->GetSafeHwnd(), GWL_EXSTYLE, exstyle | WS_EX_LAYERED);

    return pSetLayeredWindowAttributes(this->GetSafeHwnd(), 0, alpha, LWA_ALPHA) != 0;
}