TN043: RFX Routines
备注
The following technical note has not been updated since it was first included in the online documentation. As a result, some procedures and topics might be out of date or incorrect. For the latest information, it is recommended that you search for the topic of interest in the online documentation index.
This note describes the record field exchange (RFX) architecture. It also describes how you write an RFX_ procedure.
Overview of Record Field Exchange
All recordset field functions are done with C++ code. There are no special resources or magic macros. The heart of the mechanism is a virtual function that must be overridden in every derived recordset class. It is always found in this form:
void CMySet::DoFieldExchange(CFieldExchange* pFX)
{
//{{AFX_FIELD_MAP(CMySet)
<recordset exchange field type call>
<recordset exchange function call>
//}}AFX_FIELD_MAP
}
The special format AFX comments allow ClassWizard to locate and edit the code within this function. Code that is not compatible with ClassWizard should be placed outside of the special format comments.
In the above example, <recordset_exchange_field_type_call> is in the form:
pFX->SetFieldType(CFieldExchange::outputColumn);
and <recordset_exchange_function_call> is in the form:
RFX_Custom(pFX, "Col2", m_Col2);
Most RFX_ functions have three arguments as shown above, but some (e.g. RFX_Text and RFX_Binary) have additional optional arguments.
More than one RFX_ may be included in each DoDataExchange function.
See 'afxdb.h' for a list of all the recordset field exchange routines provided with MFC.
Recordset field calls are a way of registering memory locations (usually data members) to store field data for a CMySet class.
Notes
Recordset field functions are designed to work only with the CRecordset classes. They are not generally usable by any other MFC classes.
Initial values of data are set in the standard C++ constructor, usually in a block with //{{AFX_FIELD_INIT(CMylSet) and //}}AFX_FIELD_INIT comments.
Each RFX_ function must support various operations, ranging from returning the dirty status of the field to archiving the field in preparation for editing the field.
Each function that calls DoFieldExchange (for instance SetFieldNull, IsFieldDirty), does its own initialization around the call to DoFieldExchange.
How Does It Work?
You do not need to understand the following in order to use record field exchange. However, understanding how this works behind the scenes will help you write your own exchange procedure.
The DoFieldExchange member function is much like the Serialize member function — it is responsible for getting or setting data to/from an external form (in this case columns from the result of an ODBC query) from/to member data in the class. The pFX parameter is the context for doing data exchange and is similar to the CArchive parameter to CObject::Serialize. The pFX (a CFieldExchange object) has an operation indicator, which is similar to, but a generalization of the CArchive direction flag. An RFX function may have to support the following operations:
BindParam — Indicate where ODBC should retrieve parameter data
BindFieldToColumn — Indicate where ODBC must retrieve/deposit outputColumn data
Fixup — Set CString/CByteArray lengths, set NULL status bit
MarkForAddNew — Mark dirty if value has changed since AddNew call
MarkForUpdate — Mark dirty if value has changed since Edit call
Name — Append field names for fields marked dirty
NameValue — Append "<column name>=?" for fields marked dirty
Value — Append "?" followed by separator, like ',' or ' '
SetFieldDirty — Set status bit dirty (i.e. changed) field
SetFieldNull — Set status bit indicating null value for field
IsFieldDirty — Return value of dirty status bit
IsFieldNull — Return value of null status bit
IsFieldNullable — Return TRUE if field can hold NULL values
StoreField — Archive field value
LoadField — Reload archived field value
GetFieldInfoValue — Return general information on a field
GetFieldInfoOrdinal — Return general information on a field
User Extensions
There are several ways to extend the default RFX mechanism. You can
Add new data types. For example:
CBookmarkAdd new exchange procedures (RFX_???).
void AFXAPI RFX_Bigint(CFieldExchange* pFX, const char *szName, BIGINT& value);Have the DoFieldExchange member function conditionally include additional RFX calls or any other valid C++ statements.
while (posExtraFields != NULL) { RFX_Text(pFX, m_listName.GetNext(posExtraFields), m_listValue.GetNext(posExtraValues)); }
备注
Such code cannot be edited by ClassWizard and should be used only outside of the special format comments.
Writing a Custom RFX
To write your own Custom RFX function, it is suggested that you copy an existing RFX function and modify it to your own purposes. Selecting the right RFX to copy can make your job much easier. Some RFX functions have some unique properties that you should take into account when deciding which to copy.
RFX_Long and RFX_Int:
These are the simplest RFX functions. The data value does not need any special interpretation, and the data size is fixed.RFX_Single and RFX_Double:
Like RFX_Long and RFX_Int above, these functions are simple and can make use of the default implementation extensively. They are stored in dbflt.cpp instead of dbrfx.cpp, however, to enable loading the runtime floating point library only when they are explicitly reference.RFX_Text and RFX_Binary:
These two functions preallocate a static buffer to hold string/binary information, and must register these buffers with ODBC SQLBindCol instead of registering &value. Because of this, these two functions have lots of special-case code.RFX_Date:
ODBC returns date and time information in their own TIMESTAMP_STRUCT data structure. This function dynamically allocates a TIMESTAMP_STRUCT as a "proxy" for sending and receiving date time data. Various operations must transfer the date and time information between the C++ CTime object and the TIMESTAMP_STRUCT proxy. This complicates this function considerably, but it is a good example of how to use a proxy for data transfer.RFX_LongBinary:
This is the only class library RFX function that does not use column binding to receive and send data. This function ignores the BindFieldToColumn operation and instead, during the Fixup operation, allocates storage to hold the incoming SQL_LONGVARCHAR or SQL_LONGVARBINARY data, then performs an SQLGetData call to retrieve the value into the allocated storage. When preparing to send data values back to the data source (such as NameValue and Value operations), this function uses ODBC's DATA_AT_EXEC functionality. See Technical Note 45 for more information on working with SQL_LONGVARBINARY and SQL_LONGVARCHARs.
When writing your own RFX_ function, you will often be able to use CFieldExchange::Default to implement a given operation. Look at the implementation of Default for the operation in question. If it performs the operation you would be writing in your RFX_ function you can delegate to the CFieldExchange::Default. You can see examples of calling CFieldExchange::Default in dbrfx.cpp
It is important to call IsFieldType at the start of your RFX function, and return immediately if it returns FALSE. This mechanism keeps parameter operations from being performed on outputColumns, and vice versa (like calling BindParam on an outputColumn). In addition, IsFieldType automatically keeps track of the count of outputColumns (m_nFields) and params (m_nParams).