TStream.ReadComponent Method

Initiates streaming of components and their properties.

Pascal

function ReadComponent(Instance: TComponent): TComponent;

C++

__fastcall TComponent * ReadComponent(TComponent * Instance);

Description

ReadComponent is called indirectly by the global routine ReadComponentRes, by the ReadComponentRes method, or it can be called directly to initiate component streaming.

ReadComponent reads data values from the stream and assigns them to Instance's properties. It then constructs a reader object and calls the reader's ReadRootComponent method to read Instance's property values and construct child objects defined in the stream as children of the Instance. ReadComponent returns the component.

If Instance is nil (Delphi) or NULL (C++), ReadComponent constructs a component based on the type information in the stream and returns the newly-constructed component.

C++ Examples:

Copy Code

/*
This example shows how to use the built-in component
streaming support to convert any component into a string and
convert that string back into a component.
*/

#include <memory>       //for STL auto_ptr class

class MyScrollBar : public TScrollBar
{
__published:    // IDE-managed Components
private:    // User declarations
public:     // User declarations
    __fastcall MyScrollBar(TComponent* Owner);
};

__fastcall MyScrollBar::MyScrollBar(TComponent* Owner)
    : TScrollBar(Owner)
{
}

void RegisterClassesWithStreamingSystem(void)
{
  // Make sure that as part of the startup
  // code our streaming classes are registered
  // with the streaming system.

  #pragma startup RegisterClassesWithStreamingSystem

  Classes::RegisterClass(__classid(MyScrollBar));
}

MyScrollBar *ScrollBar1;

AnsiString __fastcall ComponentToString(TComponent *c)
{
  AnsiString as;
  std::auto_ptr<TMemoryStream> pms(new TMemoryStream);
  std::auto_ptr<TStringStream> pss(new TStringStream(as));

  try
  {
    pms->WriteComponent(c);
    pms->Seek(0, soFromBeginning);
    ObjectBinaryToText(pms.get(), pss.get());
    pss->Seek(0, soFromBeginning);
    as = pss->DataString;
  }
  catch(...)
  {
    ShowMessage("Streaming error.");
  }

  return as;
}

void __fastcall TForm1::Button1Click(TObject *Sender)
{
  Memo1->Text = ComponentToString(ScrollBar1);
}

TComponent *__fastcall StringToComponent(AnsiString as)

{
  std::auto_ptr<TMemoryStream> pms(new TMemoryStream);
  std::auto_ptr<TStringStream> pss(new TStringStream(as));
  TComponent *pc;

  try
  {
    ObjectTextToBinary(pss.get(), pms.get());
    pms->Seek(0, soFromBeginning);
  }
  catch(...)
  {
    ShowMessage("Streaming error.");
  }

  pc = pms->ReadComponent(NULL);
  return pc;
}

void __fastcall TForm1::Button2Click(TObject *Sender)
{
  TComponent *temp = StringToComponent(Memo1->Text);
  ScrollBar1->Free();

  ScrollBar1 = dynamic_cast<MyScrollBar *>(temp);
  ScrollBar1->Parent = Form1;
  ScrollBar1->Visible = TRUE;
}

void __fastcall TForm1::FormCreate(TObject *Sender)
{
  ScrollBar1 = new MyScrollBar(Form1);  // Form1 will clean up the scroll bar.
  ScrollBar1->Parent = Form1;
  ScrollBar1->Visible = TRUE;
  ScrollBar1->Top = 48;
  ScrollBar1->Left = 250;
  ScrollBar1->Name = "Ricksbar";
}

Delphi Examples:

Copy Code

{
This example shows how to use the built-in component
streaming support to convert any component into a string and
convert that string back into a component.
}
type
  TForm1 = class(TForm)
    Memo1: TMemo;
    Button1: TButton;
    Button2: TButton;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;
  MyScrollBar = class(TScrollBar)
  end;

var
  Form1: TForm1;
  ScrollBar1: MyScrollBar;
  
implementation

{$R *.dfm}

function ComponentToStringProc(Component: TComponent): string;
var
  BinStream:TMemoryStream;
  StrStream: TStringStream;
  s: string;
begin
  BinStream := TMemoryStream.Create;
  try
    StrStream := TStringStream.Create(s);
    try
      BinStream.WriteComponent(Component);
      BinStream.Seek(0, soFromBeginning);
      ObjectBinaryToText(BinStream, StrStream);
      StrStream.Seek(0, soFromBeginning);
      Result:= StrStream.DataString;
    finally
      StrStream.Free;
    end;
  finally
    BinStream.Free
  end;
end;

function StringToComponentProc(Value: string): TComponent;
var
  StrStream:TStringStream;
  BinStream: TMemoryStream;
begin
  StrStream := TStringStream.Create(Value);
  try
    BinStream := TMemoryStream.Create;
    try
      ObjectTextToBinary(StrStream, BinStream);
      BinStream.Seek(0, soFromBeginning);
      Result:= BinStream.ReadComponent(nil);
    finally
      BinStream.Free;
    end;
  finally
    StrStream.Free;
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  Memo1.Text:= ComponentToStringProc(ScrollBar1);
end;

// Edit the scrollbar parameters in the memo, click this
// button and see the scrollbar change.
procedure TForm1.Button2Click(Sender: TObject);
begin
  ScrollBar1.Free;
  ScrollBar1:= (StringToComponentProc(Memo1.Text) as MyScrollBar);
  ScrollBar1.Parent:= Form1;
  ScrollBar1.Visible:= TRUE;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  ScrollBar1:= MyScrollBar.Create(Form1);
  ScrollBar1.Parent:= Form1;
  ScrollBar1.Visible:= TRUE;
  ScrollBar1.Top:= 48;
  ScrollBar1.Left:= 250;
  ScrollBar1.Name:= 'Ricksbar';
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  ScrollBar1.Free;
end;

initialization

RegisterClass(MyScrollBar);