Delphi XE2 TZipFile: replace a file in zip archive

Question

I'd like to replace a file (= delete old and add new) in a zip archive with the Delphi XE2/XE3 standard System.Zip unit. But there are no replace/delete methods. Does anybody have an idea how it could be achieved without needing to extract all files and add them to a new archive?

I have this code, but it adds the "document.txt" once more if it's already present:

var
  ZipFile: TZipFile;
  SS: TStringStream;
const
  ZipDocument = 'E:\document.zip';
begin
  ZipFile := TZipFile.Create; //Zipfile: TZipFile
  SS := TStringStream.Create('hello');
  try
    if FileExists(ZipDocument) then
      ZipFile.Open(ZipDocument, zmReadWrite)
    else
      ZipFile.Open(ZipDocument, zmWrite);

    ZipFile.Add(SS, 'document.txt');

    ZipFile.Close;
  finally
    SS.Free;
    ZipFile.Free;
  end;
end;

Note: I used TPAbbrevia before (that did the job), but I'd like to use Delphi's Zip unit now. So please do not answer something like "use another library". Thank you.

You have answered your own question. The built in ZIP library doesn't support that functionality. — David Heffernan
Why don't you use Abbrevia? I've been told that it is very good. — David Heffernan
I do use it and it's great. But I've written a library that needs zip functionality and I do want to reduce the dependencies. Furthermore TPAbbrevia is a little bit tricky to install for unexperienced users if you need OSX plattform support. — oxo

Zoë Peterson Zoë Peterson · Accepted Answer · 2012-10-31T19:07:51

I'd recommend Abbrevia because I'm biased :), you already know it, and it doesn't require any hacks. Barring that, here's your hack:

type
  TZipFileHelper = class helper for TZipFile
    procedure Delete(FileName: string);
  end;

{ TZipFileHelper }

procedure TZipFileHelper.Delete(FileName: string);
var
  i, j: Integer;
  StartOffset, EndOffset, Size: UInt32;
  Header: TZipHeader;
  Buf: TBytes;
begin
  i := IndexOf(FileName);
  if i <> -1 then begin
    // Find extents for existing file in the file stream
    StartOffset := Self.FFiles[i].LocalHeaderOffset;
    EndOffset := Self.FEndFileData;
    for j := 0 to Self.FFiles.Count - 1 do begin
      if (Self.FFiles[j].LocalHeaderOffset > StartOffset) and
         (Self.FFiles[j].LocalHeaderOffset <= EndOffset) then
        EndOffset := Self.FFiles[j].LocalHeaderOffset;
    end;
    Size := EndOffset - StartOffset;
    // Update central directory header data
    Self.FFiles.Delete(i);
    for j := 0 to Self.FFiles.Count - 1 do begin
      Header := Self.FFiles[j];
      if Header.LocalHeaderOffset > StartOffset then begin
        Header.LocalHeaderOffset := Header.LocalHeaderOffset - Size;
        Self.FFiles[j] := Header;
      end;
    end;
    // Remove existing file stream
    SetLength(Buf, Self.FEndFileData - EndOffset);
    Self.FStream.Position := EndOffset;
    if Length(Buf) > 0 then
      Self.FStream.Read(Buf[0], Length(Buf));
    Self.FStream.Size := StartOffset;
    if Length(Buf) > 0 then
      Self.FStream.Write(Buf[0], Length(Buf));
    Self.FEndFileData := Self.FStream.Position;
  end;
end;

Usage:

ZipFile.Delete('document.txt');
ZipFile.Add(SS, 'document.txt');

Delphi XE2 TZipFile: replace a file in zip archive

1 Answers