Return String from Thread Delphi

Question

I'm using Delphi XE6.

I have a thread where I pass a ID and would like to get back a string created by the thread. I looked at all examples, but they all getting values back when thread is running I just need it OnTerminate.

Calling the thread from a form:

StringReturnedFromThread := PrintThread.Create(MacId);

PrintThread = class(TThread)
  private        
    MyReturnStr, PrinterMacId : String;
  public
        constructor Create(MacId: String); reintroduce;
        procedure OnThreadTerminate(Sender: TObject);
  protected
    procedure Execute; override;
  end;

constructor PrintThread.Create(MacId: String);
begin
    inherited Create(False);
    OnTerminate := OnThreadTerminate;
    FreeOnTerminate := True;
    PrinterMacId := MacId;
end;

procedure PrintThread.Execute;
begin
    PrepareConnection;
    MyReturnStr:= RequestPrintJobs(PrinterMacId);
end;

procedure PrintThread.OnThreadTerminate(Sender: TObject);
begin


end;

Thanks for any help.

Answer 1

You need to intercept thread termination. One way is to use TThread.OnTerminate event/callback.

Below a sample code.

Thread unit:

unit Processes;

interface

uses
  System.Classes;

type
  TProcess = class(TThread)
  private
    FReturnStr: string;
    FMacId: string;
  protected
    procedure Execute; override;
  public
    property MacId: string read FMacId write FMacId;
    property ReturnStr: string read FReturnStr write FReturnStr;
    constructor Create;
  end;

implementation

constructor TProcess.Create;
begin
  inherited Create(True);
  FreeOnTerminate := True;
end;

procedure TProcess.Execute;
begin
  // Some hard calculation here
  FReturnStr := FMacId + 'BLA';
end;

end.

Thread usage:

uses Processes;

procedure TForm1.Button1Click(Sender: TObject);
var P: TProcess;
begin
  // Create the thread
  P := TProcess.Create;
  // Initialize it
  P.MacId := 'MID123';
  // Callback handler
  P.OnTerminate := OnProcessTerminate;
  // Let's go
  P.Start;
end;

procedure TForm1.OnProcessTerminate(Sender: TObject);
var P: TProcess;
begin
  // The thread has been terminated
  P := TProcess(Sender);
  ShowMessage(P.ReturnStr);
end;

The thread will return MID123BLA on it's termination.