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深入浅出MFC文档/视图架构之文档模板 |
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时间: 2006-03-15 来自：天极开发 |
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CDocTemplate类的AddDocument、RemoveDocument成员函数使得CDocument* pDoc参数所指向的文档归属于本文档模板（通过将this指针赋值给pDoc所指向CDocument对象的m_pDocTemplate成员变量）或脱离与本文档模板的关系：
void CDocTemplate::AddDocument(CDocument* pDoc)
{
ASSERT_VALID(pDoc);
ASSERT(pDoc->m_pDocTemplate == NULL); // no template attached yet
pDoc->m_pDocTemplate = this;
}
void CDocTemplate::RemoveDocument(CDocument* pDoc)
{
ASSERT_VALID(pDoc);
ASSERT(pDoc->m_pDocTemplate == this); // must be attached to us
pDoc->m_pDocTemplate = NULL;
} |
而CDocTemplate类的CreateNewDocument成员函数则首先调用CDocument运行时类的CreateObject函数创建一个CDocument对象，再调用AddDocument成员函数将其归属于本文档模板类：
CDocument* CDocTemplate::CreateNewDocument()
{
// default implementation constructs one from CRuntimeClass
if (m_pDocClass == NULL)
{
TRACE0("Error: you must override CDocTemplate::CreateNewDocument.\n");
ASSERT(FALSE);
return NULL;
}
CDocument* pDocument = (CDocument*)m_pDocClass->CreateObject();
if (pDocument == NULL)
{
TRACE1("Warning: Dynamic create of document type %hs failed.\n",m_pDocClass->m_lpszClassName);
return NULL;
}
ASSERT_KINDOF(CDocument, pDocument);
AddDocument(pDocument);
return pDocument;
} |
文档类对象由文档模板类构造生成，单文档模板类CSingleDocTemplate只能生成一个文档类对象，并用成员变量 m_pOnlyDoc 指向该对象；多文档模板类可以生成多个文档类对象，用成员变量 m_docList 指向文档对象组成的链表。
CSingleDocTemplate的构造函数、AddDocument及RemoveDocument成员函数都在CDocTemplate类相应函数的基础上增加了对m_pOnlyDoc指针的处理：
CSingleDocTemplate::CSingleDocTemplate(UINT nIDResource,
CRuntimeClass* pDocClass, CRuntimeClass* pFrameClass,
CRuntimeClass* pViewClass)
: CDocTemplate(nIDResource, pDocClass, pFrameClass, pViewClass)
{
m_pOnlyDoc = NULL;
}
void CSingleDocTemplate::AddDocument(CDocument* pDoc)
{
ASSERT(m_pOnlyDoc == NULL); // one at a time please
ASSERT_VALID(pDoc);
CDocTemplate::AddDocument(pDoc);
m_pOnlyDoc = pDoc;
}
void CSingleDocTemplate::RemoveDocument(CDocument* pDoc)
{
ASSERT(m_pOnlyDoc == pDoc); // must be this one
ASSERT_VALID(pDoc);
CDocTemplate::RemoveDocument(pDoc);
m_pOnlyDoc = NULL;
} |
同样，CMultiDocTemplate类的相关函数也需要对m_docList所指向的链表进行操作（实际上AddDocument和RemoveDocument成员函数是文档模板管理其所包含文档的函数）：
// CMultiDocTemplate document management (a list of currently open documents)
void CMultiDocTemplate::AddDocument(CDocument* pDoc)
{
ASSERT_VALID(pDoc);
CDocTemplate::AddDocument(pDoc);
ASSERT(m_docList.Find(pDoc, NULL) == NULL); // must not be in list
m_docList.AddTail(pDoc);
}
void CMultiDocTemplate::RemoveDocument(CDocument* pDoc)
{
ASSERT_VALID(pDoc);
CDocTemplate::RemoveDocument(pDoc);
m_docList.RemoveAt(m_docList.Find(pDoc));
} |
由于CMultiDocTemplate类可包含多个文档，依靠其成员函数GetFirstDocPosition和GetNextDoc完成对文档链表m_docList的遍历：
POSITION CMultiDocTemplate::GetFirstDocPosition() const
{
return m_docList.GetHeadPosition();
}
CDocument* CMultiDocTemplate::GetNextDoc(POSITION& rPos) const
{
return (CDocument*)m_docList.GetNext(rPos);
} |
而CSingleDocTemplate的这两个函数实际上并无太大的意义，仅仅是MFC要玩的某种"招数"，这个"招数"高明吗？相信看完MFC的相关源代码后你或许不会这么认为，实际上CSingleDocTemplate的GetFirstDocPosition、GetNextDoc函数仅仅只能判断m_pOnlyDoc的是否为NULL：
POSITION CSingleDocTemplate::GetFirstDocPosition() const
{
return (m_pOnlyDoc == NULL) ? NULL : BEFORE_START_POSITION;
}
CDocument* CSingleDocTemplate::GetNextDoc(POSITION& rPos) const
{
CDocument* pDoc = NULL;
if (rPos == BEFORE_START_POSITION)
{
// first time through, return a real document
ASSERT(m_pOnlyDoc != NULL);
pDoc = m_pOnlyDoc;
}
rPos = NULL; // no more
return pDoc;
} |
笔者认为，MFC的设计者们将GetFirstDocPosition、GetNextDoc作为基类CDocTemplate的成员函数是不合理的，一种更好的做法是将GetFirstDocPosition、GetNextDoc移至CMultiDocTemplate派生类。
CDocTemplate还需完成对其对应文档的关闭与保存操作：
BOOL CDocTemplate::SaveAllModified()
{
POSITION pos = GetFirstDocPosition();
while (pos != NULL)
{
CDocument* pDoc = GetNextDoc(pos);
if (!pDoc->SaveModified())
return FALSE;
}
return TRUE;
}
void CDocTemplate::CloseAllDocuments(BOOL)
{
POSITION pos = GetFirstDocPosition();
while (pos != NULL)
{
CDocument* pDoc = GetNextDoc(pos);
pDoc->OnCloseDocument();
}
} |
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