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TN025: Document, View, and Frame Creation
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This note describes the creation and ownership issues for WinApps, DocTemplates, Documents, Frames and Views.
There is one CWinApp object in the system.
It is statically constructed and initialized by the framework's internal implementation of WinMain. You must derive from CWinApp to do anything useful (exception: extension DLLs should not have a CWinApp instance — initialization is done in DllMain instead).
The one CWinApp object owns a list of document templates (a CPtrList). There is one or more document template per application. DocTemplates are usually loaded from the resource file (that is, a string array) in CWinApp::InitInstance.
pTemplate = new CDocTemplate(IDR_MYDOCUMENT, ...); AddDocTemplate(pTemplate);
The one CWinApp object owns all frame windows in the application. The main frame window for the application should be stored in CWinApp::m_pMainWnd; usually you set m_pMainWnd in the InitInstance implementation if you have not let AppWizard do it for you. For single document interface (SDI) this is one CFrameWnd that serves as the main application frame window as well as the only document frame window. For multiple document interface (MDI) this is an MDI-Frame (class CMDIFrameWnd) that serves as the main application frame window that contains all the child CFrameWnds. Each child window is of class CMDIChildWnd (derived from CFrameWnd) and serves as one of potentially many document frame windows.
The CDocTemplate is the creator and manager of documents. It owns the documents that it creates. If your application uses the resource-based approach described below, it will not need to derive from CDocTemplate.
For an SDI application, the class CSingleDocTemplate keeps track of one open document. For an MDI application, the class CMultiDocTemplate keeps a list (a CPtrList) of all the currently open documents created from that template. CDocTemplate::AddDocument and CDocTemplate::RemoveDocument provide the virtual member functions for adding or removing a document from the template. CDocTemplate is a friend of CDocument so we can set the protected CDocument::m_pDocTemplate back pointer to point back to the doc template that created the document.
CWinApp handles the default OnFileOpen implementation, which will in turn query all the doc templates. The implementation includes looking for already open documents and deciding what format to open new documents in.
CDocTemplate manages the UI binding for documents and frames.
CDocTemplate keeps a count of the number of unnamed documents.
A CDocument is owned by a CDocTemplate.
Documents have a list of currently open views (derived from CView) that are viewing the document (a CPtrList).
Documents do not create/destroy the views, but they are attached to each other after they are created. When a document is closed (that is, through File/Close), all attached views will be closed. When the last view on a document is closed (that is, Window/Close) the document will be closed.
The CDocument::AddView, RemoveView interface is used to maintain the view list. CDocument is a friend of CView so we can set the CView::m_pDocument back pointer.
A CFrameWnd (also known as a frame) plays the same role as in MFC 1.0, but now the CFrameWnd class is designed to be used in many cases without deriving a new class. The derived classes CMDIFrameWnd and CMDIChildWnd are also enhanced so many standard commands are already implemented.
The CFrameWnd is responsible for creating windows in the client area of the frame. Normally there is one main window filling the client area of the frame.
For an MDI-Frame window, the client area is filled with the MDICLIENT control which is in turn the parent of all the MDI-Child frame windows. For an SDI-Frame window or an MDI-Child frame window, the client area is usually filled with a CView-derived window object. In the case of CSplitterWnd, the client area of the view is filled with the CSplitterWnd window object, and the CView-derived window objects (one per split pane) are created as child windows of the CSplitterWnd.