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Peter Davies - GUI Computing
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The Task:
Create a class to access the list of files contained in a RAR (similar to a ZIP) archive in Delphi 2.0. Keep it generic enough so that other compression types can be added in the future.

The Solution:
An approach, used back in the days of Borland Pascal Turbo Vision to tackle a similar task, was to use the class TCollection to store a list of file objects. Unfortunately, TCollection in Delphi has taken on a whole new look and does not really suit the requirements. Instead, the class TList in Delphi is better suited. It basically stores a list of objects of any type. The beauty of TList, over the use of the conventional array, is that it dynamically grows with the addition of new objects to the list - whereas an array must be set to a finite size before a maximum size might be actually known. TList also takes care of many of the tasks of maintaining a sequential list, such as removing items in the middle of the list, adding items in the middle of a list and so on.

Certain details need to be stored about each file in the RAR archive. To do this we will create a small class that stores the details and knows how to create and destroy itself. The class will be of the following structure:

  TFileDetails = class
    Filename             : PChar;
    CompressedSize       : Longint;
    OriginalSize         : Longint;
    FileDate             : Longint;
    constructor          Create(AFileName : String;ACompressedSize : Longint;
                         AOriginalSize : Longint;AFileDate : Longint);
    destructor           Destroy; override;
  end;

On creating an instance of TFileDetails, the information about the file (passed as parameters), is stored in the object. Many of these objects create all the information we need to know about the RAR archive. A class is required to handle the creating of each instance of TFileDetails Class. This is accomplished by the TArchive Class. This class consists of the following structure:

  TArchive = class
    private
      FileName           : PChar;
      FileList           : TList;
      ArchiveOK          : boolean;
      ArchiveType        : Longint;
      TotOrigSize,
      TotCompSize,
      FileDate           : Longint;
    public
      constructor   Create(AFileName : String);
      destructor    Destroy; override;
      function      GetArchiveOk : boolean;
      function      GetTotalFiles : Integer;
      function      GetAFileName(Index : Integer) : String;
  end;

This class requires the filename of the RAR archive as input. An example of using this class might be:

  var
    Archive : TArchive;
    Counter : Integer;

  begin
    Archive := TArchive.Create('C:\TEST.RAR');
    if Archive.GetArchiveOk then begin
      For Counter := 1 to Archive.GetTotalFiles do begin
        ... do something
        do something with the filename ... := Archive.GetAFileName(Counter)
        ... do something
      end;
    end;
    Archive.Free;
  end;

Some tips used in the creation of the source, which may help other Delphi developers, are:

  1. When adding an object to the list of objects (Tlist), it is not necessary to actually create the object before adding. It can be dynamically created while adding it. Simply use the class name, then the create constructor, and the parameters required.

    Eg: FileList.Add(TFileDetails.Create(FName,FBlock.OrigSize,FBlock.PackSize,FBlock.FTime));

  2. When obtaining information about a particular object, in the list of objects (Tlist), it is not necessary to actually assign the object being read to an object. You can simply tell Delphi the type of object it is (type cast) and then tell it the variable/etc you wish to get the information from.

    Eg: sResult := StrPas(TFileDetails(FileList.Items[Index-1] ).FileName);

Image of How-To Icon Here is the relevant code. In future AVDF articles, this class will be expanded upon, creating many ways of accessing the data in an instance of the class. This class provides the basis for a general purpose way of accessing RAR archives.



Written by: Peter Davies
November '96

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