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WM_SYSCOMMAND
WM_SYSCOMMAND消息。该消息参数为：
    uCmdType = wParam; // type of system command requested
    xPos = LOWORD(lParam); // horizontal postion, in screen coordinates
    yPos = HIWORD(lParam); // vertical postion, in screen coordinates
    其中uCmdType表示用户的选择：
    SC_CLOSE Closes the window.
    SC_CONTEXTHELP Changes the cursor to a question mark with a pointer. If the user then clicks a control in the dialog box, the control receives a WM_HELP message.
    SC_DEFAULT Selects the default item; the user double-clicked the System menu.
    SC_HOTKEY Activates the window associated with the application-specified hot key. The low-order word of lParam identifies the window to activate.
    SC_HSCROLL Scrolls horizontally.
    SC_KEYMENU Retrieves the System menu as a result of a keystroke.
    SC_MAXIMIZE (or SC_ZOOM) Maximizes the window.
    SC_MINIMIZE (or SC_ICON) Minimizes the window.
    SC_MONITORPOWER Windows 95 only: Sets the state of the display. This command supports devices that have power-saving features, such as a battery-powered personal computer.
    SC_MOUSEMENU Retrieves the System menu as a result of a mouse click.
    SC_MOVE Moves the window.
    SC_NEXTWINDOW Moves to the next window.
    SC_PREVWINDOW Moves to the previous window.
    SC_RESTORE Restores the window to its normal position and size.
    SC_SCREENSAVE Executes the screen saver application specified in the [boot] section of the SYSTEM.INI file.
    SC_SIZE Sizes the window.
    SC_TASKLIST Executes or activates Windows Task Manager.
    SC_VSCROLL Scrolls vertically.