You draw an overlay mask over an image using a single call to **Draw**. The parameters include the index of the image to draw and the index of an overlay mask. You must use the [INDEXTOOVERLAYMASK](http://msdn.microsoft.com/library/windows/desktop/bb761408) macro to specify the index of the overlay mask. You can also specify an overlay image when calling the [DrawIndirect](../mfc/reference/cimagelist-class.md#drawindirect) member function.

Documents are created in two different ways, so your document class must support both ways. First, the user can create a new, empty document with the File New command. In that case, initialize the document in your override of the [OnNewDocument](../mfc/reference/cdocument-class.md#onnewdocument) member function of class [CDocument](../mfc/reference/cdocument-class.md). Second, the user can use the Open command on the File menu to create a new document whose contents are read from a file. In that case, initialize the document in your override of the [OnOpenDocument](../mfc/reference/cdocument-class.md#onopendocument) member function of class **CDocument**. If both initializations are the same, you can call a common member function from both overrides, or `OnOpenDocument` can call `OnNewDocument` to initialize a clean document and then finish the open operation.

After the dialog box and all of its controls are created but just before the dialog box (of either type) appears on the screen, the dialog object's [OnInitDialog](../mfc/reference/cdialog-class.md#oninitdialog) member function is called. For a modal dialog box, this occurs during the `DoModal` call. For a modeless dialog box, `OnInitDialog` is called when **Create** is called. You typically override `OnInitDialog` to initialize the dialog box's controls, such as setting the initial text of an edit box. You must call the `OnInitDialog` member function of the base class, `CDialog`, from your `OnInitDialog` override.

For example, your application might need to implement direct mouse drawing in the view. The Scribble sample shows how to handle the `WM_LBUTTONDOWN`, `WM_MOUSEMOVE`, and `WM_LBUTTONUP` messages respectively to begin, continue, and end the drawing of a line segment. On the other hand, you might sometimes need to interpret a mouse click in your view as a selection. Your view's `OnLButtonDown` handler function would determine whether the user was drawing or selecting. If selecting, the handler would determine whether the click was within the bounds of some object in the view and, if so, alter the display to show the object as selected.

When you add a new view to the mouse manager, you give it a unique ID. If you want to support mouse customization for a window, you must process the `WM_LBUTTONDBLCLICK` message and call the [CWinAppEx::OnViewDoubleClick](../mfc/reference/cwinappex-class.md#onviewdoubleclick) function. When you call this function, one of the parameters is the ID for that window. It is the responsibility of the programmer to keep track of the ID numbers and the objects associated with them.

To create a list control, you need to supply image lists to be used when you insert new items into the list. The following example demonstrates this procedure, where `m_pImagelist` is a pointer of type `CImageList` and `m_listctrl` is a `CListCtrl` data member.

Your views access the document object by using the view's pointer to the document, installed in the view at creation time. You can retrieve this pointer in a view's member functions by calling the `CView` member function **GetDocument**. Be sure to cast this pointer to your own document type. Then you can access public document members through the pointer.

MDI main frame windows (one per application) contain a special child window called the **MDICLIENT** window. The **MDICLIENT** window manages the client area of the main frame window, and itself has child windows: the document windows, derived from `CMDIChildWnd`. Because the document windows are frame windows themselves (MDI child windows), they can also have their own children. In all of these cases, the parent window manages its child windows and forwards some commands to them.

In an MDI frame window, the frame window manages the **MDICLIENT** window, repositioning it in conjunction with control bars. The **MDICLIENT** window, in turn, manages all MDI child frame windows. The following figure shows the relationship between an MDI frame window, its **MDICLIENT** window, and its child document frame windows.