face="宋體" lang="ZH-CN">對象的意義就在于：一方面可以使</font><font
face="Arial">Delphi</font><font face="宋體" lang="ZH-CN">應用程序充分利用多媒體數據庫的數據管理能力；另一方面又能利用</font><font
face="Arial">Object Pascal</font><font face="宋體" lang="ZH-CN">的強大程序設計能力給多媒體數據庫提供全方向的功能擴展余地。</p>
<p>　　使用</font><font face="Arial">TBlobStream</font><font face="宋體"
lang="ZH-CN">對象可以在多媒體數據庫的</font><font face="Arial">BLOB</font><font
face="宋體" lang="ZH-CN">字段存儲任意格式的數據。一般說來，許多多媒體數據庫只能支持圖像、語音或者</font><font
face="Arial">OLE</font><font face="宋體" lang="ZH-CN">服務器支持的數據。利用</font><font
face="Arial">TBlobStream</font><font face="宋體" lang="ZH-CN">則不同，只要是程序能夠定義的數據格式，它都能在</font><font
face="Arial">BLOB</font><font face="宋體" lang="ZH-CN">字段中讀寫，而不需要其它輔助工具。</p>
<p>　　</font><font face="Arial">TBlobStream</font><font face="宋體" lang="ZH-CN">用構造方法</font><font
face="Arial">Create</font><font face="宋體" lang="ZH-CN">建立數據庫域和</font><font
face="Arial">BLOB</font><font face="宋體" lang="ZH-CN">流的聯接。用</font><font
face="Arial">Read</font><font face="宋體" lang="ZH-CN">或</font><font face="Arial">Write
</font><font face="宋體" lang="ZH-CN">方法訪問和改變域中的內容；用</font><font
face="Arial">Seek</font><font face="宋體" lang="ZH-CN">方法，在域中定位；用</font><font
face="Arial">Truncate</font><font face="宋體" lang="ZH-CN">方法刪除域中當前位置起所有的數據。</p>
<p>　</font><font face="Arial"></p>
<p>20.1.7.1 TBlobStream</font><font face="宋體" lang="ZH-CN">的屬性和方法</p>
<p>　</p>
<p>　　</font><font face="Arial">TBlobStream</font><font face="宋體" lang="ZH-CN">對象從</font><font
face="Arial">TStream</font><font face="宋體" lang="ZH-CN">直接繼承，沒有增添新的屬性。它覆蓋了</font><font
face="Arial">Read</font><font face="宋體" lang="ZH-CN">、</font><font face="Arial">Write
</font><font face="宋體" lang="ZH-CN">和</font><font face="Arial">Seek</font><font
face="宋體" lang="ZH-CN">方法，提供了對</font><font face="Arial">BLOB</font><font
face="宋體" lang="ZH-CN">字段的訪問操作；它增添了</font><font face="Arial">Truncate</font><font
face="宋體" lang="ZH-CN">方法以實現</font><font face="Arial">BLOB</font><font
face="宋體" lang="ZH-CN">字段中的刪除操作。</p>
<p>　　</font><font face="Arial">1. Read</font><font face="宋體" lang="ZH-CN">方法</p>
<p>　　聲明：</font><font face="Arial">function Read(var Buffer; Count: Longint):
Longint; </p>
<p>Read</font><font face="宋體" lang="ZH-CN">方法從數據庫域的當前位置起復制</font><font
face="Arial">Count</font><font face="宋體" lang="ZH-CN">個字節的內容到</font><font
face="Arial">Buffer</font><font face="宋體" lang="ZH-CN">中。</font><font face="Arial">Buffer</font><font
face="宋體" lang="ZH-CN">也必須至少分配</font><font face="Arial">Count</font><font
face="宋體" lang="ZH-CN">個字節。</font><font face="Arial">Read</font><font
face="宋體" lang="ZH-CN">方法返回實際傳輸的字節數，因為傳輸的字節數可能小于</font><font
face="Arial">Count</font><font face="宋體" lang="ZH-CN">，所以需要選擇符的邊界判斷。</p>
<p>　　</font><font face="Arial">2. Write</font><font face="宋體" lang="ZH-CN">方法</p>
<p>　　聲明：</font><font face="Arial">function Write(const Buffer; Count: Longint);
override; Longint; </p>
<p>Write</font><font face="宋體" lang="ZH-CN">方法從</font><font face="Arial">Buffer</font><font
face="宋體" lang="ZH-CN">中向數據庫域的當前位置復制</font><font
face="Arial">Count</font><font face="宋體" lang="ZH-CN">個字節的內容。</font><font
face="Arial">Buffer</font><font face="宋體" lang="ZH-CN">必須分配有</font><font
face="Arial">Count</font><font face="宋體" lang="ZH-CN">個字節的內存空間，函數返回實際傳輸的字節數，傳輸過程也要進行選擇符邊界判斷。</p>
<p>　　</font><font face="Arial">3. Seek</font><font face="宋體" lang="ZH-CN">方法</p>
<p>　　聲明：</font><font face="Arial">function Seek(Offset: Longint; Origin: Word):
Longint; </font><font face="宋體" lang="ZH-CN"></p>
<p>　　</font><font face="Arial">Seek</font><font face="宋體" lang="ZH-CN">方法重新設置</font><font
face="Arial">BLOB</font><font face="宋體" lang="ZH-CN">流中的指針位置。如果</font><font
face="Arial">Origin</font><font face="宋體" lang="ZH-CN">的值是</font><font
face="Arial">soFromBeginning</font><font face="宋體" lang="ZH-CN">，則新的指針位置是</font><font
face="Arial">Offset; </font><font face="宋體" lang="ZH-CN">如</font><font face="Arial">Origin</font><font
face="宋體" lang="ZH-CN">的值是</font><font face="Arial">soFromCurrent</font><font
face="宋體" lang="ZH-CN">，則新的指針位置是</font><font face="Arial">Position+Offset</font><font
face="宋體" lang="ZH-CN">；如果</font><font face="Arial">Origin</font><font
face="宋體" lang="ZH-CN">的值是</font><font face="Arial">SoFromCurrent</font><font
face="宋體" lang="ZH-CN">，則新的指針位置是</font><font face="Arial">Size+Offset</font><font
face="宋體" lang="ZH-CN">。函數返回新的指針位置值。當</font><font
face="Arial">Origin</font><font face="宋體" lang="ZH-CN">為</font><font face="Arial">0(SoFromBegin)</font><font
face="宋體" lang="ZH-CN">時，</font><font face="Arial">Offset</font><font
face="宋體" lang="ZH-CN">的值必須大于等于零</font><font face="Arial">; </font><font
face="宋體" lang="ZH-CN">當</font><font face="Arial">Origin</font><font face="宋體"
lang="ZH-CN">的值為</font><font face="Arial">2(SoFromEnd)</font><font face="宋體"
lang="ZH-CN">，</font><font face="Arial">Offset</font><font face="宋體" lang="ZH-CN">的值必須小于等于零。</p>
<p>　　</font><font face="Arial">4. Truncate</font><font face="宋體" lang="ZH-CN">方法</p>
<p>　　聲明：</font><font face="Arial">procedure Truncate; </p>
<p>Truncate</font><font face="宋體" lang="ZH-CN">方法撤消</font><font face="Arial">TBlobField</font><font
face="宋體" lang="ZH-CN">、</font><font face="Arial">TBytesField</font><font
face="宋體" lang="ZH-CN">或</font><font face="Arial">TVarBytesField</font><font
face="宋體" lang="ZH-CN">中從當前位置起的數據。</p>
<p>　　</font><font face="Arial">5. Create</font><font face="宋體" lang="ZH-CN">方法</p>
<p>　　聲明：</font><font face="Arial">constructor Create(Field: TBlobField; Mode:
TBlobStreamMode); </font><font face="宋體" lang="ZH-CN"></p>
<p>　　</font><font face="Arial">Create</font><font face="宋體" lang="ZH-CN">方法使用</font><font
face="Arial">Field</font><font face="宋體" lang="ZH-CN">參數建立</font><font
face="Arial">BLOB</font><font face="宋體" lang="ZH-CN">流與</font><font face="Arial">BLOB</font><font
face="宋體" lang="ZH-CN">字段的聯接。</font><font face="Arial">Mode </font><font
face="宋體" lang="ZH-CN">的值可為</font><font face="Arial">bmRead</font><font
face="宋體" lang="ZH-CN">、</font><font face="Arial">bmWrite</font><font face="宋體"
lang="ZH-CN">和</font><font face="Arial">bmReadWrite</font><font face="宋體"
lang="ZH-CN">。</p>
<p>　</font><font face="Arial"></p>
<p>20.1.7.2 TBlobStream</font><font face="宋體" lang="ZH-CN">的實現原理</p>
<p>　</p>
<p>　　說明</font><font face="Arial">TBlobStream</font><font face="宋體"
lang="ZH-CN">對象的實現原理，不可避免地要涉及它的私有域，下面是私有域的定義：</p>
<p>　</p>
<p></font><font face="Arial">TBlobStream = class(TStream)</p>
<p>private</p>
<p>FField: TBlobField;</font><font face="宋體" lang="ZH-CN"></p>
<p></font><font face="Arial">FDataSet: TDataSet;</font><font face="宋體" lang="ZH-CN"></p>
<p></font><font face="Arial">FRecord: PChar;</p>
<p>FBuffer: PChar;</p>
<p>FFieldNo: Integer;</p>
<p>FOpened: Boolean;</p>
<p>FModified: Boolean;</p>
<p>FPosition: Longint;</p>
<p></font><font face="宋體" lang="ZH-CN">…</p>
<p></font><font face="Arial">public</p>
<p></font><font face="宋體" lang="ZH-CN">…</p>
<p></font><font face="Arial">end;</p>
<p>　</font><font face="宋體" lang="ZH-CN"></p>
<p>　　</font><font face="Arial">FField</font><font face="宋體" lang="ZH-CN">是與</font><font
face="Arial">BLOB</font><font face="宋體" lang="ZH-CN">流相聯的數據庫</font><font
face="Arial">BLOB</font><font face="宋體" lang="ZH-CN">域，該域用于</font><font
face="Arial">BLOB</font><font face="宋體" lang="ZH-CN">流的內部訪問。</font><font
face="Arial">FDataSet</font><font face="宋體" lang="ZH-CN">是代表</font><font
face="Arial">FField</font><font face="宋體" lang="ZH-CN">所在的數據庫，它可以是</font><font
face="Arial">TTable</font><font face="宋體" lang="ZH-CN">部件，也可以是</font><font
face="Arial">TQuery </font><font face="宋體" lang="ZH-CN">部件。</font><font
face="Arial">FRecord</font><font face="宋體" lang="ZH-CN">和</font><font face="Arial">FBuffer</font><font
face="宋體" lang="ZH-CN">都是</font><font face="Arial">BLOB</font><font face="宋體"
lang="ZH-CN">流內部使用的緩沖區，用于存儲</font><font face="Arial">FField</font><font
face="宋體" lang="ZH-CN">所在記錄的數據，該數據記錄中不包含</font><font
face="Arial">BLOB</font><font face="宋體" lang="ZH-CN">數據，</font><font
face="Arial">TBlobStream</font><font face="宋體" lang="ZH-CN">使用</font><font
face="Arial">FRecord</font><font face="宋體" lang="ZH-CN">作為調用</font><font
face="Arial">BDE API</font><font face="宋體" lang="ZH-CN">函數的參數值。</font><font
face="Arial">FFieldNo</font><font face="宋體" lang="ZH-CN">代表</font><font
face="Arial">BLOB</font><font face="宋體" lang="ZH-CN">字段的字段號，也用于</font><font
face="Arial">BDE API</font><font face="宋體" lang="ZH-CN">的參數傳遞，</font><font
face="Arial">FOpened</font><font face="宋體" lang="ZH-CN">和</font><font face="Arial">FMocified</font><font
face="宋體" lang="ZH-CN">都是狀態信息，</font><font face="Arial">FPosition</font><font
face="宋體" lang="ZH-CN">表示</font><font face="Arial">BLOB</font><font face="宋體"
lang="ZH-CN">流的當前位置，下面介紹</font><font face="Arial">TBlobStream</font><font
face="宋體" lang="ZH-CN">方法實現。</p>
<p>　　</font><font face="Arial">1. Create</font><font face="宋體" lang="ZH-CN">方法和</font><font
face="Arial">Destroy</font><font face="宋體" lang="ZH-CN">方法的實現</p>
<p>　　</font><font face="Arial">Create</font><font face="宋體" lang="ZH-CN">方法的功能主要是建立</font><font
face="Arial">BlobStream</font><font face="宋體" lang="ZH-CN">流與</font><font
face="Arial">BLOB</font><font face="宋體" lang="ZH-CN">字段的聯系并初始化某些私有變量。其實現如下：</p>
<p>　　</p>
<p></font><font face="Arial">constructor TBlobStream.Create(Field: TBlobField; Mode:
TBlobStreamMode);</p>
<p>var</p>
<p>OpenMode: DbiOpenMode;</p>
<p>begin</p>
<p>FField := Field;</p>
<p>FDataSet := Field.DataSet;</p>
<p>FRecord := FDataSet.ActiveBuffer;</p>
<p>FFieldNo := Field.FieldNo;</p>
<p>if FDataSet.State = dsFilter then</p>
<p>DBErrorFmt(SNoFieldAccess, [FField.DisplayName]);</p>
<p>if not FField.FModified then</p>
<p>begin</p>
<p>if Mode = bmRead then</p>
<p>begin</p>
<p>FBuffer := AllocMem(FDataSet.RecordSize);</p>
<p>FRecord := FBuffer;</p>
<p>if not FDataSet.GetCurrentRecord(FBuffer) then Exit;</p>
<p>OpenMode := dbiReadOnly;</p>
<p>end else</p>
<p>begin</p>
<p>if not (FDataSet.State in [dsEdit, dsInsert]) then DBError(SNotEditing);</p>
<p>OpenMode := dbiReadWrite;</p>
<p>end;</p>
<p>Check(DbiOpenBlob(FDataSet.Handle, FRecord, FFieldNo, OpenMode));</p>
<p>end;</p>
<p>FOpened := True;</p>
<p>if Mode = bmWrite then Truncate;</p>
<p>end;</p>
<p>　</p>
<p></font><font face="宋體" lang="ZH-CN">　該方法首先是用傳入的</font><font
face="Arial">Field</font><font face="宋體" lang="ZH-CN">參數給</font><font
face="Arial">FField</font><font face="宋體" lang="ZH-CN">，</font><font face="Arial">FDataSet</font><font
face="宋體" lang="ZH-CN">，</font><font face="Arial">FRecord</font><font face="宋體"
lang="ZH-CN">和</font><font face="Arial">FFieldNo</font><font face="宋體" lang="ZH-CN">賦值。方法中用</font><font
face="Arial">AllocMem</font><font face="宋體" lang="ZH-CN">按當前記錄大小分配內存，并將指針賦給</font><font
face="Arial">FBuffer</font><font face="宋體" lang="ZH-CN">，用</font><font
face="Arial">DataSet</font><font face="宋體" lang="ZH-CN">部件的</font><font
face="Arial">GetCurrentRecord</font><font face="宋體" lang="ZH-CN">方法，將記錄的值賦給</font><font
face="Arial">FBuffer</font><font face="宋體" lang="ZH-CN">，但不包括</font><font
face="Arial">BLOB</font><font face="宋體" lang="ZH-CN">數據。</p>
<p>　　方法中用到的</font><font face="Arial">DbiOpenBlob</font><font face="宋體"
lang="ZH-CN">函數是</font><font face="Arial">BDE</font><font face="宋體" lang="ZH-CN">的</font><font
face="Arial">API</font><font face="宋體" lang="ZH-CN">函數，該函數用于打開數據庫中的</font><font