二.Windows文件句柄(handle)
面向對象的Pascal的文件句柄封裝了Windows文件句柄類型,文件操作函數庫則封裝了Windows API函數,比如"Fileread"就是調用了Windows API 函數"ReadFile",Delphi提供了一個Windows API操作接口如果熟悉Windows API，可以用Windows文件句進行文件操作.
三.文件流(File Streams)
文件流是TFileStream類的對象實例,文件流是高層的文件操作類型,TFileStream提供了一個句柄屬性.用此屬性可操作Windows文件句柄類型.
如何選擇文件類型
Windows文件句柄是較底層的文件操作類型,提供了靈活的同步及異步文件讀寫控制,以下提供用Windows文件句柄類型對文件同步及異步操作的偽代碼描述:
同步操作:
bResult = ReadFile(hFile, &inBuffer, nBytesToRead, &nBytesRead, NULL) ;
// check for eof
if (bResult && nBytesRead == 0, ) {
// we're at the end of the file
}
異步操作:
// set up overlapped structure fIElds
gOverLapped.Offset = 0;
gOverLapped.OffsetHigh = 0;
gOverLapped.hEvent = NULL;
// attempt an asynchronous read Operation
bResult = ReadFile(hFile, &inBuffer, nBytesToRead, &nBytesRead,
&gOverlapped) ;
// if there was a problem, or the async. Operation's still pending ...
if (!bResult)
{
// deal with the error code
switch (dwError = GetLastError())
{
case ERROR_HANDLE_EOF:
{
// we're reached the end of the file
// during the call to ReadFile
// code to handle that
}
case ERROR_IO_PENDING:
{
// asynchronous i/o is still in progress
// do something else for a while
GoDOSomethingElse() ;
// check on the results of the asynchronous read
bResult = GetOverlappedResult(hFile, &gOverlapped,
&nBytesRead, FALSE) ;
// if there was a problem ...
if (!bResult)
{
// deal with the error code
switch (dwError = GetLastError())
{
case ERROR_HANDLE_EOF:
{
// we're reached the end of the file
file during asynchronous Operation
}
// deal with other error cases
}
}
} // end case
// deal with other error cases
} // end switch
} // end if
雖然Windows文件句柄提供靈活的文件控制,但須編寫更多的出錯處理代碼,如果對
WindowsAPI不熟悉,使用Delphi推薦的舊文件變量類型.
Delphi的舊文件類型使用AssignFile,使文件變量和物理文件關聯,通過Delphi定義的
對文件變量的各種操作,完成文件的存取和操作.使用方便.以下提供對文件變量類
型的操作代碼描述:
var
F: TextFile;
S: string;
begin
if OpenDialog1.Execute then { Display Open dialog box }
begin
AssignFile(F, OpenDialog1.FileName); { File selected in dialog box }
Reset(F);
Readln(F, S); { Read the first line out of the file }
Edit1.Text := S; { Put string in a TEdit control }
CloseFile(F);
end;
end;
文件流是流(stream classes)的子類,所以使用他的一個優點就是能自動繼承其父類的屬性他能很容易的和其他的流類互操作,比如你如果想把一塊動態內存塊寫入磁盤,可以使用一個TFileStream和一個TMemoryStream來完成.
