Multithread interactions

Question

Main Form:

unit Unit3;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;

type
  TForm3 = class(TForm)
    Memo1: TMemo;
    Button1: TButton;
    Button2: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
    class var counter : Integer;
  end;

var
  Form3: TForm3;

implementation

{$R *.dfm}

uses Unit4, Unit5;

procedure TForm3.Button1Click(Sender: TObject);
var
  th1 : thCounter;
  th2 : thPrinter;
begin
  th1:= thCounter.Create;
  th2:= thPrinter.Create;
end;

end.

Thread Counter :

unit Unit4;

interface

uses
  System.Classes, Unit3;

type
  thCounter = class(TThread)
  private
    { Private declarations }
  protected
    procedure Execute; override;
  end;

implementation

{ thCounter }

procedure thCounter.Execute;
var
  i: Integer;
  printVal : Integer;
begin
  { Place thread code here }
  printVal:= 50;
  for i := 0 to 1000000000 do
  begin
    Form3.counter:= i;
    if Form3.counter = printVal then
    begin
      // RUN print thread    ????
      printVal:= printVal + 50;
    end;
  end;
end;

end.

Thread Print :

unit Unit5;

interface

uses
  System.Classes, Unit3;

type
  thPrinter = class(TThread)
  private
    { Private declarations }
    procedure printIt;
  protected
    procedure Execute; override;
  end;

implementation

uses
  System.SysUtils;

{ thPrinter }

procedure thPrinter.Execute;
begin
  { Place thread code here }
  Synchronize(printIt);
end;

procedure thPrinter.printIt;
begin
  Form3.Memo1.Lines.Add(IntToStr(Form3.counter));
end;

end.

I am workin on simple project. But i stuck.

I have 2 thread which are thCounter and thPrint. My thCounter, increase the Counter to 1 billion. And i want to call another thread ( thPrint ) when the counter 50 and multiples like 100, 150, 200 to print the screen in TMemo....

How can i send a message to thPrint from thCounter?

have you looked at - http://stackoverflow.com/questions/438945/cross-thread-communication-in-delphi — RBA, May 15 '15 at 12:52
@Ken accessing a data member of a class is just fine, no VCL/threading issues there. The design is dubious, but the issue you point out does not apply here. — David Heffernan, May 15 '15 at 14:17
@Heart the issue is not read vs write. The issue is threead affinity of windows, thread safety of all access (read and write), and thread safety of MakeObjectInstance. — David Heffernan, May 15 '15 at 15:17

score 3 · Accepted Answer · answered May 15 '15 at 14:28

To signal the other thread, use a synchronization primitive, like a TSimpleEvent.

Let it be owned by the thCounter thread, and pass a reference to it when thPrinter is created.

In the thPrinter.Execute:

while not Terminated do
begin
  if waitEvent.WaitFor(100) = wrSignaled then // Keep it listening to the Terminated flag
  begin
    Synchronize(PrintIt);
  end;
end;

And in the thCounter:

waitEvent.SetEvent;  // Triggers the printIt

Just create the waitEvent so that it automatically is reset after the event has been triggered.

waitEvent := TSimpleEvent.Create(nil, false,false,'');

Multithread interactions

1 Answers1