Creating a Child Window

To create an MDI child window, an application can either call the CreateMDIWindow function or send an WM_MDICREATE message to the MDI client window. (The application can use the CreateWindowEx function with the WS_EX_MDICHILD style to create MDI child windows.) A single-threaded MDI application can use either method to create a child window. A thread in a multithreaded MDI application must use the CreateMDIWindow or CreateWindowEx function to create a child window in a different thread.

The lParam parameter of a WM_MDICREATE message is a far pointer to an MDICREATESTRUCT structure. The structure includes four dimension members: x and y, which indicate the horizontal and vertical positions of the window, and cx and cy, which indicate the horizontal and vertical extents of the window. Any of these members may be assigned explicitly by the application, or they may be set to CW_USEDEFAULT, in which case Windows selects a position, size, or both, according to a cascading algorithm. In any case, all four members must be initialized. Multipad uses CW_USEDEFAULT for all dimensions.

The last member of the MDICREATESTRUCT structure is the style member, which may contain style bits for the window. To create an MDI child window that can have any combination of window styles, specify the MDIS_ALLCHILDSTYLES window style. When this style is not specified, an MDI child window has the WS_MINIMIZE, WS_MAXIMIZE, WS_HSCROLL, and WS_VSCROLL styles as default settings.

Multipad creates its MDI child windows by using its locally defined AddFile function (located in the source file MPFILE.C). The AddFile function sets the title of the child window by assigning the szTitle member of the window's MDICREATESTRUCT structure to either the name of the file being edited or to "Untitled." The szClass member is set to the name of the MDI child window class registered in Multipad's InitializeApplication function. The hOwner member is set to the application's instance handle.

The following example shows the AddFile function in Multipad.

HWND APIENTRY AddFile(pName)

CHAR * pName;

{

HWND hwnd;

CHAR sz[160];

MDICREATESTRUCT mcs;

if (!pName) {

// If the pName parameter is NULL, load the "Untitled"

// string from the STRINGTABLE resource and set the szTitle

// member of MDICREATESTRUCT.

LoadString(hInst, IDS_UNTITLED, sz, sizeof(sz));

mcs.szTitle = (LPCTSTR) sz;

}

else

// Title the window with the full path and filename,

// obtained by calling the OpenFile function with the

// OF_PARSE flag, which is called before AddFile().

mcs.szTitle = of.szPathName;

mcs.szClass = szChild;

mcs.hOwner = hInst;

// Use the default size for the child window.

mcs.x = mcs.cx = CW_USEDEFAULT;

mcs.y = mcs.cy = CW_USEDEFAULT;

// Give the child window the default style. The styleDefault

// variable is defined in MULTIPAD.C.

mcs.style = styleDefault;

// Tell the MDI client window to create the child window.

hwnd = (HWND) SendMessage (hwndMDIClient, WM_MDICREATE, 0,

(LONG) (LPMDICREATESTRUCT) &mcs);

// If the file is found, read its contents into the child

// window's client area.

if (pName) {

if (!LoadFile(hwnd, pName)) {

// Cannot load the file; close the window.

SendMessage(hwndMDIClient, WM_MDIDESTROY,

(DWORD) hwnd, 0L);

}

}

return hwnd;

}

The pointer passed in the lParam parameter of the WM_MDICREATE message is passed to the CreateWindow function and appears as the first member in the CREATESTRUCT structure, passed in the WM_CREATE message. In Multipad, the child window initializes itself during WM_CREATE message processing by initializing document variables in its extra data and by creating the edit control's child window.

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