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Implementing object persistence using streams (Views: 100)


Problem/Question/Abstract:

How do I save data entered in a list box at run time without resorting to a text file or having to deal with the overhead of a table?

Answer:

This question actually asks much more than just saving a list box at runtime. It brings to the surface some of the internal workings of Delphi itself. But I should clarify that what I present here does not deal with the workings of Delphi's Runtime Type Information (RTTI). That's probably best left for another article. But what I'm going to discuss is a practical way of implementing object persistence in your programs by using the saving the of a listbox at runtime as an example. Okay, here we go...

Any OOP class library worth its salt supports what is called streamable persistent objects. Simply put, this means that an instance of a class (or at least its data) can be saved to a disk file and restored later. When a program reloads the object, it is restored in its last state, just prior to being written. The cool thing about this is that the program doesn't have to have any advance knowledge of the state of the object; the object itself contains all the information it needs to recreate itself when it's restored.

For example, let's say you've created a program that has a list box in which people append various bits of information at run time. For many folks, saving the information to disk means iterating through all the items in the list and writing them to a text file or even a table. The program must reload the data from the external file and add the data, line by line. This is not so bad, but it can be a bit of a chore to write the code.

On the other hand, using object persistence, the same program mentioned above instructs the list box to write its data to a disk file of some sort. When it wants to reload the object, all it has to do is stream it back into memory and specify the base class to write to. Remember, since all the data of the object was saved with it when it was written to disk, the object comes back to life in its original form. That's the whole idea behind object persistence.

Delphi itself makes heavy use of object persistence. Every time you save a project, it streams out to disk the data contained in your objects' properties so that everything you set during your session is saved. When you reload a project, Delphi streams the object data back into your form(s) to restore everything you previously set. In fact, a form file itself is streamed to and from disk. I should note here that Delphi uses a couple of specialized stream classes, TWriter and TReader which are derived from a superclass called TFiler. I won't go into the details of these classes here, since I'm providing a much simpler demonstration of employing object persistence in your programs. I'll leave it up to you to research this topic further.

Moving on, you might ask, "Where does employing streamable persistent objects come in handy?" The most useful cases I've found for employing them are when I've written programs that provide parameter or input criteria for processes, where the range of possible values to search on remain fairly constant from one run of the program to the next.

For instance, in my line of work, almost all of my programs are typically front-ends to very complex query operations. However, the range of domains and their values don't change very often, and from client to client, the same questions are typically asked. So in these cases, I've found that simply streaming my criteria objects (these are all list objects) out to disk when I close the forms and streaming them back in when I open the forms provides a much cleaner solution to saving my criteria sets from session to session. Besides, this is very low overhead programming, since once the programs are finished with the streams, they're immediately destroyed. Not only that, I don't have to use DB.PAS or DBTables.PAS for data operations.

A simple example

The example I've provided here is by no means a full-fledged search program of the type I normally write. I've merely taken the parts pertinent to this article for your use. Feel free to include or modify this code to your heart's content. In any case, here's the code listing for the main form of the program. We'll discuss particulars below.

unit main;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics,
  Controls, Forms, Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    ListBox1: TListBox;
    Edit1: TEdit;
    Memo1: TMemo;
    procedure Edit1KeyPress(Sender: TObject; var Key: Char);
    procedure FormCreate(Sender: TObject);
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
    procedure ListBox1DblClick(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.Edit1KeyPress(Sender: TObject; var Key: Char);
begin
  if Key = #13 then
  begin
    Key := #0;
    ListBox1.Items.Add(Edit1.Text);
    Edit1.Text := '';
  end;
end;

procedure TForm1.FormCreate(Sender: TObject);
var
  strm: TFileStream;
begin
  if FileExists('MyList.DAT') then
  begin
    strm := TFileStream.Create('MyList.DAT', fmOpenRead);
    strm.ReadComponent(ListBox1);
    strm.Free;
  end;
end;

procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
var
  strm: TFileStream;
begin
  strm := TFileStream.Create('MyList.DAT', fmCreate);
  strm.WriteComponent(ListBox1);
  strm.Free;
end;

procedure TForm1.ListBox1DblClick(Sender: TObject);
begin
  ListBox1.Items.Delete(ListBox1.ItemIndex);
end;

end.

You were expecting some complex code, weren't you? In actuality, this stuff is incredibly simple. So why isn't it documented very well? I'd say it's because this is one of the more uncommon things done in Delphi. But for those of you who wish to really get into the innards of the environment, this stuff is a must to understand and master. Let's look a little deeper into the code.

The program consists of a form with a TEdit and a TListBox dropped onto it. It has just two meaningful methods: FormCreate and FormClose. In the FormCreate method,

procedure TForm1.FormCreate(Sender: TObject);
var
  strm: TFileStream;
begin
  if FileExists('MyList.DAT') then
  begin
    strm := TFileStream.Create('MyList.DAT', fmOpenRead);
    strm.ReadComponent(ListBox1);
    strm.Free;
  end;
end;

the program checks for the existence of MyList.DAT with a call to FileExists, which is the stream file that holds the list box information. If it exists, the file is streamed into ListBox1; otherwise, it does nothing. With the FormClose method,

procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
var
  strm: TFileStream;
begin
  strm := TFileStream.Create('MyList.DAT', fmCreate);
  strm.WriteComponent(ListBox1);
  strm.Free;
end;

the program writes ListBox1 out to MyList.DAT, overwriting any previous versions of the file.

That's all there is to this program. Surprisingly, this is one of the more simple things to do in Delphi, but paradoxically it's one of the most difficult things to find good information about in the manuals or help file. Granted, as I mentioned above, doing this type of stuff is fairly uncommon, but think of the implication: simple, low overhead, persistent storage without the need for tables. What was accomplished above was done in fewer than 10 lines of code &mdash that's absolutely incredible!

I urge you to play around with this technique and apply it to other things. I think you'll get a lot of mileage out of it.

A demonstration program is available.

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