For those interested in storing components onto a stream, take a look
at TWriter.WriteRootComponent and TWriter.WriteComponent. Then check
out the TReader.Read... counterparts. Unfortunately, there appears
to be no documentation showing HOW to use these methods properly.
The docs keep mentioning the "root" component, but never clearly
explain what it is or how you are suppose to use the root property
to store your components.
If you are interested in writing objects to the stream that are
not components, I recommend doing something like the follow:
type
TMyObject = class(TObject)
...
protected
procedure SaveToStream(writer : TWriter); virtual;
procedure LoadFromStream(writer : TWriter); virtual;
...
end;
procedure TMyObject.SaveToStream(writer : TWriter);
begin
with writer do begin
WriteListBegin;
{- write object state -}
WriteListEnd;
end;
end;
procedure TMyObject.LoadFromStream(reader : TReader);
begin
with reader do begin
ReadListBegin;
while not EndOfList do begin
{- read object state -}
end;
ReadListEnd;
end;
end;
Somewhere in the initialization section of the unit in which this
object is declared, call RegisterObject('TMyObject'). (See
RegisterObject() below.)
In the main program, where you specify the file to read/write,
you can do something like this:
var
RegisteredObjects : TStringList;
procedure RegisterObject(cname : string; ctype : TClass);
begin
RegisteredObjects.AddObject(cname, ctype);
end;
procedure GetObject(cname : string) : TClass;
var
i : integer;
begin
i := RegisteredObjects.IndexOf(cname);
if i > -1 then
Result := TClass(RegisteredObjects.Objects[i])
else
Result := nil;
end;
procedure SaveFile(const filename : string; objlist : TList);
var
stream : TFileStream;
writer : TWriter;
i : integer;
begin
stream := TFileStream.Create(filename, fmCreate or fmOpenWrite);
try
writer := TWriter.Create(stream, $ff);
try
with writer do begin
WriteSignature; {marker to indicate a Delphi filer object file.}
WriteListBegin; {outer list marker}
for i := 0 to objlist.Count - 1 do begin
WriteListBegin; {object marker}
WriteString(TMyObject(objlist[i]).ClassName);
TMyObject(objlist[i]).SaveToStream(writer);
WriteListEnd; {object marker}
end;
WriteListEnd; {outer list marker}
end;
finally
writer.Free;
end;
finally
stream.Free;
end;
end;
procedure OpenFile(const filename : string; objlist : TList);
var
stream : TFileStream;
writer : TWriter;
cname : string; {class name}
ctype : TClass; {class type}
obj : TObject;
begin
stream := TFileStream.Create(filename, fmOpenRead);
try
reader := TReader.Create(stream, $ff);
try
with reader do begin
ReadSignature; {check Delphi filer object signature.}
ReadListBegin; {outer list marker}
while not EndOfList do begin
ReadListBegin; {object marker}
while not EndOfList do begin
cname := ReadString;
ctype := GetObjectClass(cname);
obj := TObject(TObjectClass(ctype).Create;
try
obj.LoadFromStream(reader);
except
obj.Free;
raise;
end;
objlist.Add(obj);
end;
ReadListEnd; {object marker}
end;
ReadListEnd; {outer list marker}
end;
finally
reader.Free;
end;
finally
stream.Free;
end;
end;
Well, I don't know how far this will get you. I haven't tested ANY
of this code, so who knows if it could ever possibly work. The most
doubtful part is the whole dynamic instantiation taking place in
LoadFromStream(). Delphi provides a bunch of great functions for
registering TPersistent descendants and getting their class types
from their class names, etc.: RegisterClass(), FindFieldClass(),
FindClass(), GetClass(), etc. (They use it for loading components
off of streams....no surprise there.) However, if your objects
are not TPersistent descendants (and there's no reason they should
be), then you're basically out of luck (read: "you get to write
your own RegisteredClass()").
So, give this a try if you're feeling daring. Just don't come running
after me with a shotgun complaining about little voices in your heads
if you do. I suspect the above code will need a lot of polish before
it does what is expected of it... Nonetheless, I hope you find it
interesting, if nothing else.
----------------------------------------------------------------------
Well, shoot. After a bit of research and review, I came to realize just
how unnecessary all of this work with trying to store plain objects
on a stream is. When I wrote up the TStream2 message back in May
(only shortly after Delphi came out), I did not have a good understanding
of the VCL class heirarchy.
Here's a quote from the Component Writer's Guide (TPersistent):
The TPersistent object is the abstract base object for all objects
stored and loaded on Delphi stream objects. In addition to the methods
it inherits from its ancestor, TObject, TPersistent defines three new
methods: AssignTo and DefineProperties, which are protected, and Assign,
which is public.
The GetClass() and RegisterClass() functions work for TPersistent objects.
So, after being in the dark for so long, I sat down and just took a
good long look at TPersistent and other related matters. Then, after
looking back at what you wanted to do, I wrote up a little program
and tested it out to make sure it actually worked. Below is the result.
Below I have included two units: Unit1, which is a form definition, and
Unit2, which contains the TPlayer and TObjectList classes. The form
(Unit1) has for buttons labelled "Create," "Save," "Load," and "Exit."
Create -- create 5 TPlayers and add them to the object list
Save -- save the object list to a file
Load -- load the object list from a file
Exit -- free the object list and exit
In Unit2, the TPlayer object is declared as a TPersistent descendant
and is given two methods: ReadData() and WriteData().
ReadData() -- read property data with given TReader object
WriteData() -- write property data with given TWriter object
The TPlayer class is registered with a call to RegisterClass in the
initialization section of Unit2 when the program first begins.
Also in Unit2, the TObjectList class is declared as a TList and
given the methods Clear(), SaveToStream(), LoadFromStream(),
SaveToFile(), and LoadFromFile(). The SaveToFile() and LoadFromFile()
just create a TFileStream object and then pass it to the corresponding
SaveToStream()/LoadFromStream() method, which do the actually accessing
via the TFiler objects (TWriter & TReader). A destructor was also
added to TObjectList to ensure that it frees the items in the list
when it is destroyed.
I think it would be a good idea to go back into the TObjectList and
add a new kind of Items property that is specifically typed as
TPersistent or TObject instead of just Pointer since many of the
operations we perform on its Items property could cause problems
if a non-object were accidentally stored in the list. It would also
reduce the need for all of the extra type casting.
Anyhow, here are the two units that worked for me. Let me know what you
think.
--Mark Johnson
--------------------------------- UNIT1.PAS ---------------------------------
unit Unit1;
interface
uses
SysUtils, WinTypes, WinProcs, Messages, Classes, Graphics, Controls,
Forms, Dialogs, StdCtrls;
type
TForm1 = class(TForm)
btnCreate: TButton;
btnSave: TButton;
btnLoad: TButton;
btnExit: TButton;
procedure btnCreateClick(Sender: TObject);
procedure btnSaveClick(Sender: TObject);
procedure btnLoadClick(Sender: TObject);
procedure btnExitClick(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form1: TForm1;
implementation
{$R *.DFM}
uses
Unit2;
const
ObjFilename = 'C:\players.dat';
var
objList : TObjectList;
procedure TForm1.btnCreateClick(Sender: TObject);
var
player : TPlayer;
i : integer;
begin
{Creates five players and adds them to ObjList}
objList.Clear;
for i := 1 to 5 do begin
player := TPlayer.Create;
try
with player do begin
Name := 'Name' + IntToStr(i);
EmpireName := 'EmpireName' + IntToStr(i);
Wins := i;
Losses := 5 - i;
Ranking := 6 - i;
end;
objList.Add(player);
except
player.Free;
raise;
end;
end;
end;
procedure TForm1.btnSaveClick(Sender: TObject);
begin
{Stores objList to file}
objList.SaveToFile(ObjFilename);
end;
procedure TForm1.btnLoadClick(Sender: TObject);
begin
{Loads objList from file}
objList.LoadFromFile(ObjFilename);
end;
procedure TForm1.btnExitClick(Sender: TObject);
begin
{Frees objList (and everything in list) and exits}
objList.Free;
Close;
end;
initialization
objList := TObjectList.Create;
end.
--------------------------------- UNIT2.PAS ---------------------------------
unit Unit2;
interface
uses
Classes;
type
TPlayer = class(TPersistent)
private
FName : string;
FEmpireName : string;
FWins : integer;
FLosses : integer;
FRanking : integer;
public
procedure ReadData(reader : TReader); dynamic;
procedure WriteData(writer : TWriter); dynamic;
published
property Name : string read FName write FName;
property EmpireName : string read FEmpireName write FEmpireName;
property Wins : integer read FWins write FWins;
property Losses : integer read FLosses write FLosses;
property Ranking : integer read FRanking write FRanking;
end;
TObjectlist=class(TList)
public
destructor Destroy; override;
procedure Clear;
procedure SaveToStream(stream : TStream);
procedure LoadFromStream(stream : TStream);
procedure SaveToFile(const filename : string);
procedure LoadFromFile(const filename : string);
end;
implementation
uses
SysUtils;
{TPlayer}
procedure TPlayer.ReadData(reader : TReader);
begin
with reader do begin
Name := ReadString;
EmpireName := ReadString;
Wins := ReadInteger;
Losses := ReadInteger;
Ranking := ReadInteger;
end;
end;
procedure TPlayer.WriteData(writer : Twriter);
begin
with writer do begin
WriteString(Name);
WriteString(EmpireName);
WriteInteger(Wins);
WriteInteger(Losses);
WriteInteger(Ranking);
end;
end;
{TObjectList}
destructor TObjectList.Destroy;
begin
{deallocate objects in list before termination}
Clear;
inherited Destroy;
end;
procedure TObjectList.Clear;
var
i : integer;
begin
{This routine deallocates all resources inside this list}
for i := 0 to Count - 1 do begin
TObject(Items[0]).Free;
Delete(0);
end;
end;
procedure TObjectList.SaveToStream(stream : TStream);
var
writer : TWriter;
i : integer;
begin
writer := TWriter.Create(stream, $ff);
try
with writer do begin
{mark beginning of file and beginning of object list}
WriteSignature;
WriteListBegin;
{loop through this list}
for i := 0 to Count - 1 do begin
{Store any TPersistent objects}
if TObject(Items[i]) is TPersistent then begin
WriteString(TPersistent(Items[i]).ClassName);
{Call WriteData() for TPlayer objects}
if (TPersistent(Items[i]) is TPlayer) then
TPlayer(Items[i]).WriteData(writer);
end;
end;
{mark end of object list}
WriteListEnd;
end;
finally
writer.Free;
end;
end;
procedure TObjectList.LoadFromStream(stream : TStream);
var
reader : TReader;
obj : TPersistent;
ctype : TPersistentClass;
cname : string;
i : integer;
begin
reader:=TReader.Create(stream,$ff);
try
with reader do begin
{read beginning of file and beginning of object list markers}
ReadSignature;
ReadListBegin;
{loop through file list of objects}
while not EndOfList do begin
{Load ClassName and use it to get ClassType}
cname := ReadString;
ctype := GetClass(cname);
if Assigned(ctype) then begin {"Assigned()" == " <> nil" but quicker}
{If a ClassType was found, create an instance}
obj := ctype.Create;
try
{if obj is a TPlayer, call its ReadData() method}
if obj is TPlayer then
TPlayer(obj).ReadData(reader);
except
obj.free;
raise;
end;
{add object to this list}
Add(obj);
end;
end;
ReadListEnd;
end;
finally
reader.Free;
end;
end;
procedure TObjectList.SaveToFile(const filename : string);
var
stream : TFileStream;
begin
stream := TFileStream.Create(filename, fmCreate or fmOpenWrite);
try
SaveToStream(stream);
finally
stream.Free;
end;
end;
procedure TObjectList.LoadFromFile(const filename : string);
var
stream : TFileStream;
begin
stream := TFileStream.Create(filename, fmOpenRead);
try
Clear;
LoadFromStream(stream);
finally
stream.Free;
end;
end;
initialization
{register TPlayer class here when program begins}
RegisterClass(TPlayer);
end.