Question

I am new with this stuff of Threading in Delphi. so, I am trying to make a simple query aplication that make a bit call up for the database and take a bit of time, so I want to alert the user that there is a background process and have to be patient.

I tried many samples, but none of them work for me, Please, could somebody show me a simple sample that could work?

I know that I have to Declare a Type of TThread, with Create and Override Execute… etc.. but since that I am lost…

Using Delphi 7, SQL Server 2005 and ADO, Windows XP sp3.-

Thanks.

Answer 1

You can find many examples on the web of threads. The only special feature, if you are using ADO connections inside the Thread, is that you can’t share the same connection.
Each thread must create its own connection, otherwise they are equal (should follow the same rules as any other thread.)

An sample that I have used is this:

  TADOSQLThread = class(TThread)
  private
    FADOQ: TADOQuery;  // Internal query
    FSQL: string;      // SQL To execute
    FID: integer;      // Internal ID

  public
    constructor Create(CreateSuspended:Boolean; AConnString:String;
                       ASQL:string; IDThread:integer);
    destructor Destroy; override;
    procedure Execute(); override;

    property ID:integer read FID write FID;
    property SQL:string read FSQL write FSQL;
    property ADOQ:TADOQuery read FADOQ write FADOQ;
  end;

The Create constructor is overrided, and look like this:

constructor TADOSQLThread.Create(CreateSuspended:Boolean; AConnString:String;
                                 ASQL:string; IDThread:integer);
begin

  inherited Create(CreateSuspended);

  // ini
  Self.FreeOnTerminate := False;

  // Create the Query
  FADOQ := TAdoquery.Create(nil);
  // assign connections
  FADOQ.ConnectionString := AConnString;
  FADOQ.SQL.Add(ASQL);
  Self.FID := IDThread;
  Self.FSQL:= ASQL;
end;

And the execute method is very simple:

procedure TADOSQLThread.Execute();
begin

  inherited;

  try
    // Ejecutar la consulta
    Self.FADOQ.Open;
  except
    // Error al ejecutar
    ...Error treattement
  end;
end;

To start and create a thread you can use code similar to this:

  //crear el Thread
  th := TADOSQLThread.Create(True, mmConnection.Lines.Text, ASQL, AId);
  // internal for me (for controled the number of active threads and limete it)
  inc(numThreads);
  // evento finalizacion
  th.OnTerminate := TerminateThread;
  // Ejecutarlo
  th.Resume;

I have create a TerminateThread method that receive the control of threads when they finish. The only different to other threads is the connection problem. You must create a new connection on every thread, It can’t share the same ADOConnections with others.
I hope this example will be useful for you.

Regards