Add to and Clear TObjectList from a Thread

onr of few ways to do it:

unit Unit1;

interface

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

type
  TForm1 = class(TForm)
    ListView1: TListView;
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
  private
    FObjects: TObjectList;
  public
    { Public declarations }
  end;

  TBreakData = class(TObject)
  private
    FBreakNo: Integer;
    FBreakText: string;
  public
    constructor Create(ABreakNo: Integer;const ABreakText: string);
    property BreakNo: Integer read FBreakNo;
    property BreakText: string read FBreakText;
  end;


  TMyThread = class(TThread)
  private
    FListItemsRef: TListItems;
    FObjectsRef: TObjectList;
    procedure CreateObjects;
    procedure FillListView;
  protected
    constructor Create(AListItems: TListItems;AObjects: TObjectList);reintroduce;
    procedure Execute;override;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

{ TBreakData }

constructor TBreakData.Create(ABreakNo: Integer; const ABreakText: string);
begin
  inherited Create;
  FBreakNo := ABreakNo;
  FBreakText := ABreakText;
end;

{ TMyThread }

constructor TMyThread.Create(AListItems: TListItems;
  AObjects: TObjectList);
begin
  inherited Create(False);
  FreeOnTerminate := True;
  FListItemsRef := AListItems;
  FObjectsRef := AObjects;
end;

procedure TMyThread.CreateObjects;
var cnt: Integer;
begin
  if Assigned(FObjectsRef) then
    for cnt := 0 to 150 do
      FObjectsRef.Add(TBreakData.Create(cnt, IntToStr(cnt)));
end;

procedure TMyThread.Execute;
begin
  CreateObjects;
  Synchronize(FillListView);
end;

procedure TMyThread.FillListView;
var cnt: Integer;
begin
  if Assigned(FListItemsRef) and Assigned(FObjectsRef) then begin
    FListItemsRef.BeginUpdate;
    try
      for cnt := 0 to FObjectsRef.Count - 1 do
        with FListItemsRef.Add do begin
          Caption := IntToStr(TBreakData(FObjectsRef[cnt]).BreakNo);
          SubItems.Add(TBreakData(FObjectsRef[cnt]).BreakText);
        end;
    finally
      FListItemsRef.EndUpdate;
    end;
  end;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  FObjects := TObjectList.Create;
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  FreeAndNil(FObjects);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  TMyThread.Create(ListView1.Items, FObjects);
end;

end.

ziolko.

>>when the thread is executed a second time it simply adds an additional list of objects without clearing the listview and objectlist first.

little correction:

procedure TMyThread.CreateObjects;
var cnt: Integer;
begin
  if Assigned(FObjectsRef) then begin
    FObjectsRef.Clear;
    for cnt := 0 to 150 do
      FObjectsRef.Add(TBreakData.Create(cnt, IntToStr(cnt)));
  end;
end;

procedure TMyThread.FillListView;
var cnt: Integer;
begin
  if Assigned(FListItemsRef) and Assigned(FObjectsRef) then begin
    FListItemsRef.BeginUpdate;
    try
      FListItemsRef.Clear;
      for cnt := 0 to FObjectsRef.Count - 1 do
        with FListItemsRef.Add do begin
          Caption := IntToStr(TBreakData(FObjectsRef[cnt]).BreakNo);
          SubItems.Add(TBreakData(FObjectsRef[cnt]).BreakText);
        end;
    finally
      FListItemsRef.EndUpdate;
    end;
  end;
end;

>>This takes some time to process and you find that the screen freezes while the list view is being filled.

ok, I'll try to explain this one.
TListView was created and exists in main thread but you fill it from other thread so you have situation when two threads access same memory. To avoid AccessViolations you need to synchronize (see .Execute method of TMyThread) unfortunatelly TListView and TTreeView doesn't handle large number of items too quickly that's why app frozes. Of course you can get rid of synchronization part so your app will respond (but still it will take some time to load large number of items) however not using synchronization puts you in big risk of getting AccessViolation errors and it's not recommended.

>>Unfortunatly - this will not work for what I am trying to do. Do you have other suggestions?

threads can be used on many different ways and it really depends on what you trying to do, there's no one general step-by-step recipe how to use threads. for instance if you have two listboxes it might be good idea to fill them with two threads so they're filled simultaniously. other scenario is when items must "produceed" by reading bunch of files or reading data from network, device over COM port, etc. this is also good indication to use thread.

ziolko.

here's other approach to problem, maybe this will be better for you:
unit Unit1;

interface

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

const
  WM_ADDITEM = WM_USER + $0001;

type
  TForm1 = class(TForm)
    ListView1: TListView;
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
  private
    FObjects: TObjectList;
    procedure AddItemMessage(var Msg: TMessage); message WM_ADDITEM;
  public
    { Public declarations }
  end;

  TBreakData = class(TObject)
  private
    FBreakNo: Integer;
    FBreakText: string;
  public
    constructor Create(ABreakNo: Integer;const ABreakText: string);
    property BreakNo: Integer read FBreakNo;
    property BreakText: string read FBreakText;
  end;


  TMyThread = class(TThread)
  private
    FObjectsRef: TObjectList;
    FNotify: THandle;
    procedure CreateObjects;
    procedure FillListView;
  protected
    constructor Create(ANotify:THandle;AObjects: TObjectList);reintroduce;
    procedure Execute;override;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

{ TBreakData }

constructor TBreakData.Create(ABreakNo: Integer; const ABreakText: string);
begin
  inherited Create;
  FBreakNo := ABreakNo;
  FBreakText := ABreakText;
end;

{ TMyThread }

constructor TMyThread.Create(ANotify:THandle;AObjects: TObjectList);
begin
  inherited Create(False);
  FreeOnTerminate := True;
  FNotify := ANotify;
  FObjectsRef := AObjects;
end;

procedure TMyThread.CreateObjects;
var cnt: Integer;
begin
  if Assigned(FObjectsRef) then begin
    FObjectsRef.Clear;
    for cnt := 0 to 15000 do
      FObjectsRef.Add(TBreakData.Create(cnt, IntToStr(cnt)));
  end;
end;

procedure TMyThread.Execute;
begin
  CreateObjects;
  FillListView;
end;

procedure TMyThread.FillListView;
var cnt: Integer;
begin
  if Assigned(FObjectsRef) then begin
    for cnt := 0 to FObjectsRef.Count - 1 do begin
      PostMessage(FNotify, WM_ADDITEM, cnt, 0);
      Sleep(1);
    end;
  end;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  FObjects := TObjectList.Create;
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  FreeAndNil(FObjects);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  TMyThread.Create(Handle, FObjects);
end;

procedure TForm1.AddItemMessage(var Msg: TMessage);
var it: TListItem;
    obj: TBreakData;
begin
  it := ListView1.Items.Add;
  obj := TBreakData(FObjects[Msg.WParam]);
  it.Caption := IntToStr(obj.BreakNo);
  it.SubItems.Add(obj.BreakText);
end;

end.

ziolko.

.... forgot clearing stuff again.. sorry.

procedure TForm1.Button1Click(Sender: TObject);
begin
  ListView1.Items.BeginUpdate;
  try
    ListView1.Items.Clear;
  finally
    ListView1.Items.EndUpdate;
  end;
  TMyThread.Create(Handle, FObjects);
end;


ziolko.

and also some extra precoutions:

procedure TForm1.AddItemMessage(var Msg: TMessage);
var it: TListItem;
    obj: TBreakData;
begin
  if (Msg.WParam >= 0) and (Msg.WParam < FObjects.Count) then begin
    it := ListView1.Items.Add;
    obj := TBreakData(FObjects[Msg.WParam]);
    it.Caption := IntToStr(obj.BreakNo);
    it.SubItems.Add(obj.BreakText);
  end;
end;


sorry for posting in pieces but it's 1.30am  and my mind doesn't work fast enough (just like TListView:-) )

ziolko.

any progress ?

ziolko.