Delphi - Multithreading: Why I can't start thread again after thread.terminate()? - multithreading

Question

I've coding a multithread application that send and receive TCP packages. I'm with the problem that when I call twice event confirmBoxRecognized(peerIP: string) of the code bellow. I'm getting the following exception:
Cannot call Start on a running or suspended thread
If I check in the thread object I've that terminated == true and suspended == false. Why am I coding wrong?
Following the code:
TThreadReadTCP = class(TThread)
private
context: TfrmBoxTest;
looping: Boolean;
procedure readTCP;
protected
procedure DoTerminate; override;
procedure Execute; override;
public
peerIP: String;
responseObject: TProtocolObject;
constructor Create(CreateSuspended: Boolean; ctx: TFrmBoxTest); overload;
end;
{ TThreadReadTCP }
constructor TThreadReadTCP.Create(CreateSuspended: Boolean; ctx: TFrmBoxTest);
begin
inherited Create(CreateSuspended);
Self.context := ctx;
FreeOnTerminate := True;
end;
procedure TThreadReadTCP.DoTerminate;
begin
looping := false;
inherited DoTerminate();
end;
procedure TThreadReadTCP.Execute;
begin
inherited;
looping := true;
readTCP;
end;
procedure TThreadReadTCP.readTCP;
var
buffer: TBytes;
begin
while looping do
begin
if context.tcpClientBox.Connected then
begin
try
buffer := TEncoding.ASCII.GetBytes(context.tcpClientBox.Socket.ReadLn());
//do something else
except on E:Exception do
ShowMessage('Error receiving TCP buffer with message: ' + e.Message);
end;
end;
end;
end;
procedure TfrmBoxTest.confirmBoxRecognized(peerIP: string);
begin
if (connectBoxTCP(peerIP)) then
begin
if Assigned(threadReadTCP) then
begin
threadReadTCP.Terminate();
threadReadTCP.Start(); // I get the exception here when I run this code twice...
end;
showBoxRecognized();
end;
sendBoxRecognized();
end;
Are there running thread status can I get? Or anyone can explain how can I improve this code to solve this problem?
Thanks a lot!

Answer 1

You get the exception because you can only call Start() on a TThread object one time. Once the thread has been started, you cannot restart it. Once it has been signaled to terminate, all you can do is wait for it to finish terminating, and then destroy the object.
If you want another thread to start running, you have to create a new TThread object, eg:
type
TThreadReadTCP = class(TThread)
private
context: TfrmBoxTest;
protected
procedure Execute; override;
procedure TerminatedSet; override;
public
peerIP: String;
responseObject: TProtocolObject;
constructor Create(ctx: TFrmBoxTest); reintroduce;
end;
constructor TThreadReadTCP.Create(ctx: TFrmBoxTest);
begin
inherited Create(False);
Self.context := ctx;
// NEVER use FreeOnTerminate=True with a thread object that you keep a reference to!
// FreeOnTerminate := True;
end;
procedure TThreadReadTCP.Execute;
var
buffer: TBytes;
begin
while not Terminated do
begin
try
buffer := TEncoding.ASCII.GetBytes(context.tcpClientBox.Socket.ReadLn());
// do something else
except
on E: Exception do
begin
// do something
raise;
end;
end;
end;
end;
procedure TThreadReadTCP.TerminatedSet;
begin
try
context.tcpClientBox.Disconnect(False);
except
end;
end;
...
procedure TfrmBoxTest.confirmBoxRecognized(peerIP: string);
begin
if Assigned(threadReadTCP) then
begin
threadReadTCP.Terminate();
threadReadTCP.WaitFor();
FreeAndNil(threadReadTCP);
end;
if connectBoxTCP(peerIP) then
begin
threadReadTCP := TThreadReadTCP.Create(Self);
showBoxRecognized();
end;
sendBoxRecognized();
end;