unit ServerMain;

interface

uses

  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,

  IdBaseComponent, IdComponent, IdTCPServer;

type

  TformMain = class(TForm)

    IdTCPServer1: TIdTCPServer;

    procedure FormCreate(Sender: TObject);

    procedure FormDestroy(Sender: TObject);

    procedure IdTCPServer1Connect(AThread: TIdPeerThread);

    procedure IdTCPServer1Execute(AThread: TIdPeerThread);

  private

    ZipCodeList: TStrings;

  public

  end;

var

  formMain: TformMain;

implementation

{R *.DFM}

procedure TformMain.IdTCPServer1Connect(AThread: TIdPeerThread);

begin

  AThread.Connection.WriteLn(''''Indy Zip Code Server Ready.'''');

end;

procedure TformMain.IdTCPServer1Execute(AThread: TIdPeerThread);

var

  sCommand: string;

begin

  with AThread.Connection do begin

    sCommand := ReadLn;

    if SameText(sCommand, ''''QUIT'''') then begin

      Disconnect;

    end else if SameText(Copy(sCommand, 1, 8), ''''ZipCode '''') then begin

      WriteLn(ZipCodeList.Values[Copy(sCommand, 9, MaxInt)]);

    end;

  end;

end;

procedure TformMain.FormCreate(Sender: TObject);

begin

  ZipCodeList := TStringList.Create;

  ZipCodeList.LoadFromFile(ExtractFilePath(Application.EXEName) + ''''ZipCodes.dat'''');

end;

procedure TformMain.FormDestroy(Sender: TObject);

begin

  ZipCodeList.Free;

end;

end.

The only parts that are Indy specific are the IdTCPServer1 component, IdTCPServer1Connect method, and the IdTCPServer1Execute method.

IdTCPServer1 is a TIdTCPServer and is a component on the form. The following properties were altered from the default:

• Active = True - Set the server to listen when the application is run.
• DefaultPort = 6000 - An arbitrary number for this demo. This is the port the listener will listen on for incoming client requests.

The IdTCPServer1Execute method is hooked to the OnExecute event of the server. The OnExecute event is fired by the server after a client connection has been accepted. The OnExecute event is uniquely different from other events you may be familiar with. OnExecute is executed in the context of a thread. The thread the event is called from is passed in the AThread argument of the method. This is important as many OnExecute events may be executing at the same time. This was done with an event so that a server could be built without the requirement of building a new component. There are also methods that can be overridden when descendant components are created.

The OnConnect is called after a connection has been accepted, and a thread created for it. In this server it is used to send the welcome message to the client. This could also be done in the OnExecute event if desired.

The OnExecute event will be called repeatedly until the connection is disconnected or broken. This eliminates the need to check the connection and loop inside the event.

IdTCPServer1Execute uses two basic Indy functions, ReadLn and WriteLn. ReadLn reads a line from the connection and WriteLn writes a line to the connection.

sCommand := ReadLn;

The above line reads the command from the client and puts the input into the local string variable sCommand.

if SameText(sCommand, ''''QUIT'''') then begin

  Disconnect;

end else if SameText(Copy(sCommand, 1, 8), ''''ZipCode '''') then begin

  WriteLn(ZipCodeList.Values[Copy(sCommand, 9, MaxInt)]);

end;

Next the input in sCommand is parsed to see which command the client issued.

If the command is "Quit" the connection is disconnected. No more reading or writing of the connection is permitted after a disconnect call. When the event is exited after this, the listener will not call it again. The listener will clean up the thread and the connection.

If the command is "ZipCode" the parameter after the command is extracted and used to look up the city and state. The city and state is then written to the connection, or an empty string if one a match for the parameter is not found.

Finally the method is exited. The server will recall the event again as long as the connection is connected, allowing the client to issue multiple commands.

Client Source Code

unit ClientMain;

interface

uses

  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,

  StdCtrls, ExtCtrls, IdAntiFreezeBase,

  IdAntiFreeze, IdBaseComponent, IdComponent, IdTCPConnection, IdTCPClient;

type

  TformMain = class(TForm)

    Client: TIdTCPClient;

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