Wait for thread without freezing the application - multithreading

Question

I'm trying to put an indy TIdHttp in a thread,
I have tried this :
type
TSendThread = class(TThread)
private
{ Private declarations }
protected
procedure Execute; override;
public
http : TIdHTTP;
URL : String;
Method : String;
property ReturnValue;
end;
procedure TSendThread.Execute;
begin
form1.Memo1.lines.Add(http.Get(URL));
ReturnValue := 1;
end;
And in the main :
procedure TForm1.Button1Click(Sender: TObject);
var t : TSendThread;
begin
t := TSendThread.Create(true);
t.URL := 'http://www.url.com/';
t.http := http;
t.Start;
showmessage(IntToStr(t.ReturnValue));
end;
My problem here is that the next instruction gets executed(showmessage) without waiting the thread to be done, i tried to use the "WaitFor" but it freezes the application.
Is there any other workaround?
Thank you.

Answer 1

Use the TThread.OnTerminate event to know when the thread has finished:
type
TSendThread = class(TThread)
private
http : TIdHTTP;
Line: string;
procedure AddLine;
protected
procedure Execute; override;
public
constructor Create; reintroduce;
destructor Destroy; override;
URL : String;
Method : String;
property ReturnValue;
end;
constructor TSendThread.Create;
begin
inherited Create(True);
FreeOnTerminate := True;
http := TIdHTTP.Create;
end;
destructor TSendThread.Destroy;
begin
http.Free;
inherited;
end;
procedure TSendThread.Execute;
begin
Line := http.Get(URL);
Synchronize(AddLine);
ReturnValue := 1;
end;
procedure TSendThread.AddLine;
begin
Form1.Memo1.Lines.Add(Line);
end;
procedure TForm1.Button1Click(Sender: TObject);
var
t : TSendThread;
begin
t := TSendThread.Create;
t.URL := 'http://www.url.com/';
t.OnTerminate := ThreadTerminated;
t.Start;
end;
procedure TForm1.ThreadTerminated(Sender: TObject);
begin
ShowMessage(IntToStr(TSendThread(Sender).ReturnValue));
end;
If you want to use a loop to wait for the thread to finish, without blocking the UI, then you can do it like this:
constructor TSendThread.Create;
begin
inherited Create(True);
//FreeOnTerminate := True; // <-- remove this
http := TIdHTTP.Create;
end;
procedure TForm1.Button1Click(Sender: TObject);
var
t : TSendThread;
h : THandle;
begin
t := TSendThread.Create;
try
t.URL := 'http://www.url.com/';
t.Start;
h := t.Handle;
repeat
case MsgWaitForMultipleObjects(1, h, 0, INFINITE, QS_ALLINPUT) of
WAIT_OBJECT_0: Break;
WAIT_OBJECT_0+1: Application.ProcessMessages;
WAIT_FAILED: RaiseLastOSError;
else
Break;
end;
until False;
ShowMessage(IntToStr(t.ReturnValue));
finally
t.Free;
end;
end;