Streaming components

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Introduction

Normally, when you want to store data on disk or to network streams, you must write code for loading and saving each property. This tutorial describes how to write classes, that can be loaded from and saved to streams without writing extra load/save code by using the RTTI.

There is an example in the lazarus sources, demonstrating how to save a TGroupBox with a TCheckBox child to a stream and read the stream back to create a copy of both components.

 See <lazaruspath>/examples/componentstreaming/

TComponent / TPersistent

The class TPersistent is defined in the unit Classes and is uses the {$M+} compiler switch. This switch tells the compiler to create Run Time Type Information (RTTI). This means it and all its descendants get a new class section published. 'Published' properties are visible as 'public', but additonally their structure is accessible at run time. That means all published properties can be read and written at run time. The IDE for instance uses this to work with components it never heard of.

TComponent extends TPersistent by the ability to have child components. This is important for streaming, where one component is the root component also called lookup root with a list of child components.

Writing a component to a stream =

The unit LResources has a function for that:

 procedure WriteComponentAsBinaryToStream(AStream: TStream; AComponent: TComponent);

It writes a component in binary format to the stream. For example:

procedure TForm1.Button1Click(Sender: TObject);
var
  AStream: TMemoryStream;
begin
  AStream:=TMemoryStream.Create;
  try
    WriteComponentAsBinaryToStream(AStream,AGroupBox);
    ... save stream somewhere ...
  finally
    AStream.Free;
  end;
end;

Reading a component from a stream =

The unit LResources has a function for that:

 procedure WriteComponentAsBinaryToStream(AStream: TStream; AComponent: TComponent);

It writes a component in binary format to the stream.


Streamable properties

There are some limitations, what types TReader/TWriter can stream:

  • Base types can be streamed: string, integer, char, single, double, extended, byte, word, cardinal, shortint, etc. .
  • TPersistent and descendants can be streamed

Streaming custom Data - DefineProperties