How to save a TObjectList in a Tstream - delphi

Question

I need to save a TObjectList<TStrings> (or <TStringList>) in a TStream and then retrive it.
To be clear, how to apply SaveToStream and LoadFromStream to a TObjectList?

Answer 1

Try something like this:
procedure SaveListOfStringsToStream(List: TObjectList<TStrings>; Stream: TStream);
var
Count, I: Integer;
MStrm: TMemoryStream;
Size: Int64;
begin
Count := List.Count;
Stream.WriteBuffer(Count, SizeOf(Count));
if Count = 0 then Exit;
MStrm := TMemoryStream.Create;
try
for I := 0 to Count-1 do
begin
List[I].SaveToStream(MStrm);
Size := MStrm.Size;
Stream.WriteBuffer(Size, SizeOf(Size));
Stream.CopyFrom(MStrm, 0);
MStrm.Clear;
end;
finally
MStrm.Free;
end;
end;
procedure LoadListOfStringsFromStream(List: TObjectList<TStrings>; Stream: TStream);
var
Count, I: Integer;
MStrm: TMemoryStream;
Size: Int64;
SList: TStringList;
begin
Stream.ReadBuffer(Count, SizeOf(Count));
if Count <= 0 then Exit;
MStrm := TMemoryStream.Create;
try
for I := 0 to Count-1 do
begin
Stream.ReadBuffer(Size, SizeOf(Size));
SList := TStringList.Create;
try
if Size > 0 then
begin
MStrm.CopyFrom(Stream, Size);
MStrm.Position := 0;
SList.LoadFromStream(MStrm);
MStrm.Clear;
end;
List.Add(SList);
except
SList.Free;
raise;
end;
end;
finally
MStrm.Free;
end;
end;
Alternatively:
procedure SaveListOfStringsToStream(List: TObjectList<TStrings>; Stream: TStream);
var
LCount, SCount, Len, I, J: Integer;
SList: TStrings;
S: UTF8String;
begin
LCount := List.Count;
Stream.WriteBuffer(LCount, SizeOf(LCount));
if LCount = 0 then Exit;
for I := 0 to LCount-1 do
begin
SList := List[I];
SCount := SList.Count;
Stream.WriteBuffer(SCount, SizeOf(SCount));
for J := 0 to SCount-1 do
begin
S := UTF8String(SList[J]);
// or, if using Delphi 2007 or earlier:
// S := UTF8Encode(SList[J]);
Len := Length(S);
Stream.WriteBuffer(Len, SizeOf(Len));
Stream.WriteBuffer(PAnsiChar(S)^, Len * SizeOf(AnsiChar));
end;
end;
end;
procedure LoadListOfStringsFromStream(List: TObjectList<TStrings>; Stream: TStream);
var
LCount, SCount, Len, I, J: Integer;
SList: TStrings;
S: UTF8String;
begin
Stream.ReadBuffer(LCount, SizeOf(LCount));
for I := 0 to LCount-1 do
begin
Stream.ReadBuffer(SCount, SizeOf(SCount));
SList := TStringList.Create;
try
for J := 0 to SCount-1 do
begin
Stream.ReadBuffer(Len, SizeOf(Len));
SetLength(S, Len);
Stream.ReadBuffer(PAnsiChar(S)^, Len * SizeOf(AnsiChar));
SList.Add(String(S));
// or, if using Delphi 2007 or earlier:
// SList.Add(UTF8Decode(S));
end;
List.Add(SList);
except
SList.Free;
raise;
end;
end;
end;

Answer 2

What's in your list?
It depends on what type of objects you have in your objectlist.
You loop over the list and save each item in turn.
However the objects inside your list need to have a SaveToStream method.
For reasons unknown SaveToStream is not a method of TPersistent, instead it is implemented independently in different classes.
Test for stream support
If the VCL were built with interfaces in mind, in newer versions has been solved with the IStreamPersist interface.
If all your stuff in the list descents from a base class that has streaming built-in (e.g. TComponent) then there is no problem and you can just use TComponent.SaveToStream.
type
TStreamableClass = TStrings; //just to show that this does not depend on TStrings.
procedure SaveToStream(List: TObjectList; Stream: TStream);
var
i: integer;
begin
for i:= 0 to List.Count -1 do begin
if List[i] is TStreamableClass then begin
TStreamableClass(List[i]).SaveToStream(Stream);
end;
end; {for i}
end;
Add stream support
If you have items in your list that do not derive from a common streamable ancestor then you'll have to have multiple if list[i] is TX tests in your loop.
If the object does not have a SaveToStream method, but you have enough knowledge of the class to implement it yourself, then you have twothree options.
A: implement a class helper that adds SaveToStream to that class or B: add a descendent class that implements that option.
If these are your own objects, then see option C: below.
type
TObjectXStreamable = class(TObjectX)
public
procedure SaveToStream(Stream: TStream); virtual;
procedure LoadFromStream(Stream: TStream); virtual;
end;
procedure SaveToStream(List: TObjectList; Stream: TStream);
...
if List[i] is TObjectX then TObjectXStreamable(List[i]).SaveToStream(Stream);
...
Note that this approach fails if TObjectX has subclasses with additional data. The added streaming will not know about this extra data.
Option C: implement System.Classes.IStreamPersist
type
IStreamPersist = interface
['<GUID>']
procedure SaveToStream(Stream: TStream);
procedure LoadFromStream(Stream: TStream);
end;
//enhance your streamable objects like so:
TInterfaceBaseObject = TInterfacedObject //or TSingletonImplementation
TMyObject = class(TInterfaceBaseObject, IStreamPersist)
procedure SaveToStream(Stream: TStream); virtual;
procedure LoadFromStream(Stream: TStream); virtual;
See: Bypassing (disabling) Delphi's reference counting for interfaces
You test the IStreamPersist support using the supports call.
if Supports(List[i], IStreamPersist) then (List[i] as IStreamPersist).SaveToStream(Stream);
If you have a newer version of Delphi consider using a generic TObjectList, that way you can limit your list to: MyList: TObjectList<TComponent>;
Now you can just call MyList[i].SaveToStream, because Delphi knows that the list only contains (descendents of) TComponent.

Answer 3

You will need to create your own routine to do this: One for saving, the other for loading.
For saving, loop through the list, convert each pointer of the list into is hexadecimal (decimal, octal) then add a separator character like ','; When done write the string contain to the stream.
For loading, loop through the list, search for the first separator character, extract the value, convert it back as a pointer then add it to the list.
Procedure ObjListToStream(objList: TObjectList; aStream: TStream);
var
str: String;
iCnt: Integer;
Begin
if not assigned(aStream) then exit; {or raise exception}
for iCnt := 0 to objList.Count - 1 do
begin
str := str + IntToStr(Integer(objList.Items[iCnt])) + ',';
end;
aStream.Write(str[1], Length(str));
End;
Procedure StreamToObjList(objList: TObjectList; aList: String);
var
str: String;
iCnt: Integer;
iStart, iStop: Integer;
Begin
try
if not assigned(aStream) then exit; {or raise exception}
iStart := 0;
Repeat
iStop := Pos(',', aList, iStart);
if iStop > 0 then
begin
objList.Add(StrToInt(Copy(sList, iStart, iStop - iStart)));
iStart := iStop + 1;
end;
Until iStop = 0;
except
{something want wrong}
end;
End;
I haven't test it and wrote it from memory. But it should point you in the right direction.