With C++Builder, how to run AniIndicator along whith WaitFor() in main thread?

Question

Following this Delphi sample and this other one , I built three scenarios in order to run properly TAniIndicator along with WaitFor() in the main thread.

In the first two tests, I use the Synchronize() and Queue() methods with FreeOnTerminate=false/FreeAndNil() . In the third test, I got rid of the WaitFor() without success. In all cases, the app freezes with message:

Project1 isn't responding. Do you want to close it? Wait/OK

Note that in the third scenario, the TAniIndicator spun correctly if the variable PHONENumber is assigned before the execution of the REST request. Curiously, after this line, the app freezes again. This would be my preferred solution.

Below is my code:

.h

// ...
//---------------------------------------------------------------------------
class TAlphaThread : public TThread
{private:
 protected:
    void __fastcall Execute();
 public:
    __fastcall TAlphaThread(bool CreateSuspended);
    //void __fastcall OnTerminate(TObject *Sender); Never triggered
};
//---------------------------------------------------------------------------
//===========================================================================
//---------------------------------------------------------------------------
class TBetaThread : public TThread
{private:
 protected:
    void __fastcall Execute();
 public:
    __fastcall TBetaThread(bool CreateSuspended);
    //void __fastcall OnTerminate(TObject *Sender); Never triggered
};
//---------------------------------------------------------------------------
//===========================================================================
//---------------------------------------------------------------------------
class TDeltaThread : public TThread
{private:
 protected:
    void __fastcall Execute();
 public:
    __fastcall TDeltaThread(bool CreateSuspended);
    //void __fastcall OnTerminate(TObject *Sender); Never triggered
};
//---------------------------------------------------------------------------
//===========================================================================
//---------------------------------------------------------------------------
class TForm1 : public TForm
{
__published:    // Composants gérés par l'EDI
//  ...
private:    // Déclarations utilisateur
public:     // Déclarations utilisateur
    __fastcall TForm1(TComponent* Owner);

    UnicodeString PHONENumber; // Mobile number

    TRESTClient *REST1Client;
    TRESTRequest *REST1Request;
    TRESTResponse *REST1Response;
    TAlphaThread *AlphaThread;
    void __fastcall AlphaThreadTerminated(TObject *Sender);

    TRESTClient *REST2Client;
    TRESTRequest *REST2Request;
    TRESTResponse *REST2Response;
    TBetaThread *BetaThread;
    void __fastcall BetaThreadTerminated(TObject *Sender);

    TRESTClient *REST3Client;
    TRESTRequest *REST3Request;
    TRESTResponse *REST3Response;
    TDeltaThread *DeltaThread;
    void __fastcall DeltaThreadTerminated(TObject *Sender);
};
//---------------------------------------------------------------------------
//...

.cpp

//...
TForm1 *Form1;
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
    : TForm(Owner)
{//
 REST1Client = new TRESTClient(this);
 REST1Request = new TRESTRequest(this);
 REST1Response = new TRESTResponse(this);

 REST2Client = new TRESTClient(this);
 REST2Request = new TRESTRequest(this);
 REST2Response = new TRESTResponse(this);

 REST3Client = new TRESTClient(this);
 REST3Request = new TRESTRequest(this);
 REST3Response = new TRESTResponse(this);
}
//---------------------------------------------------------------------------
//===========================================================================
// Scenario 1 => Project1 isn't responding. Do you want to close it ? Wait/OK
//---------------------------------------------------------------------------
void __fastcall TForm1::Button1Tap(TObject *Sender, const TPointF &Point)
{//
 PHONENumber = NULL;
 AlphaThread = new TAlphaThread(true);
 AlphaThread->OnTerminate = &AlphaThreadTerminated;
 AlphaThread->Start();

 if(AlphaThread->WaitFor())
  {if(PHONENumber == NULL) ShowMessage(L"Null value!"); else Label1->Text =  PHONENumber;
  }
}
//---------------------------------------------------------------------------
void __fastcall TForm1::AlphaThreadTerminated(TObject * Sender)
{//
 FreeAndNil(Form1->AlphaThread);
}
//---------------------------------------------------------------------------
__fastcall TAlphaThread::TAlphaThread(bool CreateSuspended) : TThread(CreateSuspended)
{//
 FreeOnTerminate = false;
}
//---------------------------------------------------------------------------
/* Never triggered
void __fastcall TAlphaThread::OnTerminate(TObject * Sender)
{ShowMessage(L"TAlphaThread::OnTerminate!");
 FreeAndNil(Form1->AlphaThread);
}*/
//---------------------------------------------------------------------------
void __fastcall TAlphaThread::Execute()
{//
 Synchronize([this](){
   Form1->AniIndicator1->Align = TAlignLayout::Contents;
   Form1->AniIndicator1->Enabled = true;
   Form1->AniIndicator1->Visible = true;
  }
 );

 // Chargement des données
 Form1->REST1Client->BaseURL = "my.URL.php";
 Form1->REST1Request->AddParameter("ID", "123");
 Form1->REST1Request->Client = Form1->REST1Client;
 Form1->REST1Request->Response = Form1->REST1Response;
 Form1->REST1Response->ContentType = "application/json";
 Form1->REST1Request->Execute();

 // Assignation des variables
 TJSONValue *JV0TOPLevel; JV0TOPLevel = Form1->REST1Response->JSONValue;
 TJSONArray *JSONArray = dynamic_cast<TJSONArray*>(JV0TOPLevel);
 TJSONObject *JSONObject = dynamic_cast<TJSONObject*>(JSONArray->Items[0]);
 TJSONValue *JV0ITEMLevel;

 JV0ITEMLevel = JSONObject->GetValue("FIELDName"); Form1->PHONENumber = JV0ITEMLevel->AsType<String>();

 Synchronize([this](){
   Form1->AniIndicator1->Enabled = false;
   Form1->AniIndicator1->Visible = false;
  }
 );

 ReturnValue = 1;
 Terminate();
}
//---------------------------------------------------------------------------
//===========================================================================
// Scenario 2 => Project1 isn't responding. Do you want to close it ? Wait/OK
//---------------------------------------------------------------------------
void __fastcall TForm1::Button2Tap(TObject *Sender, const TPointF &Point)
{//
 PHONENumber = NULL;
 BetaThread = new TBetaThread(true);
 BetaThread->OnTerminate = &BetaThreadTerminated;
 BetaThread->Start();

 if(BetaThread->WaitFor())
  {if(PHONENumber == NULL) ShowMessage(L"Null value!"); else Label2->Text =  PHONENumber;
  }
}
//---------------------------------------------------------------------------
void __fastcall TForm1::BetaThreadTerminated(TObject * Sender)
{//
 FreeAndNil(Form1->BetaThread);
}
//---------------------------------------------------------------------------
__fastcall TBetaThread::TBetaThread(bool CreateSuspended)
    : TThread(CreateSuspended)
{//
 FreeOnTerminate = false;
}
//---------------------------------------------------------------------------
/*
void __fastcall TBetaThread::OnTerminate(TObject * Sender)
{// Never triggered
 ShowMessage(L"TBetaThread::OnTerminate!");
 FreeAndNil(Form1->BetaThread);
}*/
//---------------------------------------------------------------------------
void __fastcall TBetaThread::Execute()
{//
 Queue(NULL, [this](){
   Form1->AniIndicator1->Align = TAlignLayout::Contents;
   Form1->AniIndicator1->Enabled = true;
   Form1->AniIndicator1->Visible = true;
  }
 );

 // Chargement des données
 Form1->REST2Client->BaseURL = "my.URL.php";
 Form1->REST2Request->AddParameter("ID", "123");
 Form1->REST2Request->Client = Form1->REST2Client;
 Form1->REST2Request->Response = Form1->REST2Response;
 Form1->REST2Response->ContentType = "application/json";
 Form1->REST2Request->Execute();

 // Assignation des variables
 TJSONValue *JV0TOPLevel; JV0TOPLevel = Form1->REST2Response->JSONValue;
 TJSONArray *JSONArray = dynamic_cast<TJSONArray*>(JV0TOPLevel);
 TJSONObject *JSONObject = dynamic_cast<TJSONObject*>(JSONArray->Items[0]);
 TJSONValue *JV0ITEMLevel;

 JV0ITEMLevel = JSONObject->GetValue("FIELDName"); Form1->PHONENumber = JV0ITEMLevel->AsType<String>();

 Queue(NULL, [this](){
   Form1->AniIndicator1->Enabled = false;
   Form1->AniIndicator1->Visible = false;
  }
 );

 ReturnValue = 1;
 Terminate();
}
//---------------------------------------------------------------------------
//===========================================================================
// Scenario 3 => Project1 isn't responding. Do you want to close it ? Wait/OK
//---------------------------------------------------------------------------
void __fastcall TForm1::Button3Tap(TObject *Sender, const TPointF &Point)
{//
 PHONENumber = NULL;
 DeltaThread = new TDeltaThread(true);
 /*DeltaThread->OnTerminate = &DeltaThreadTerminated;*/
 DeltaThread->Start();

 while(PHONENumber == NULL) {
  [this]()->String{return PHONENumber;};
 }
 if(PHONENumber == NULL) ShowMessage(L"Null value!"); else Label3->Text =  PHONENumber;
}
//---------------------------------------------------------------------------
void __fastcall TForm1::DeltaThreadTerminated(TObject * Sender)
{//
 FreeAndNil(Form1->DeltaThread);
}
//---------------------------------------------------------------------------
__fastcall TDeltaThread::TDeltaThread(bool CreateSuspended)
    : TThread(CreateSuspended)
{//
 FreeOnTerminate = true;
}
//---------------------------------------------------------------------------
/* //Never triggered
void __fastcall TDeltaThread::OnTerminate(TObject * Sender)
{//ShowMessage(L"OnTerminate!");
 //FreeAndNil(Form1->DeltaThread);
} */
//---------------------------------------------------------------------------
void __fastcall TDeltaThread::Execute()
{//
 //Form1->PHONENumber = 123; Working!!

 Queue(NULL, [this](){
   Form1->AniIndicator1->Align = TAlignLayout::Contents;
   Form1->AniIndicator1->Enabled = true;
   Form1->AniIndicator1->Visible = true;
  }
 );

 //Form1->PHONENumber = 123; Working!!

 // Chargement des données
 Form1->REST3Client->BaseURL = "my.URL.php";
 Form1->REST3Request->AddParameter("ID", "123");
 Form1->REST3Request->Client = Form1->REST3Client;
 Form1->REST3Request->Response = Form1->REST3Response;
 Form1->REST3Response->ContentType = "application/json";
 Form1->REST3Request->Execute();

 // Form1->PHONENumber = 123; Freezing

 // Assignation des variables
 TJSONValue *JV0TOPLevel; JV0TOPLevel = Form1->REST3Response->JSONValue;
 TJSONArray *JSONArray = dynamic_cast<TJSONArray*>(JV0TOPLevel);
 TJSONObject *JSONObject = dynamic_cast<TJSONObject*>(JSONArray->Items[0]);
 TJSONValue *JV0ITEMLevel;

 JV0ITEMLevel = JSONObject->GetValue("FIELDName"); Form1->PHONENumber = JV0ITEMLevel->AsType<String>(); // Freezing

 Queue(NULL, [this](){
   Form1->AniIndicator1->Enabled = false;
   Form1->AniIndicator1->Visible = false;
  }
 );

 ReturnValue = 1;
 Terminate();
}
//---------------------------------------------------------------------------

Any idea of what I missed?

Answer 1

Scenario 1:

• TThread::WaitFor() is blocking the main UI thread until the thread finishes running. WaitFor() does not process new GUI messages while waiting, but it does process pending Synchronize() and Queue() requests (and does dispatch cross-thread messages sent with SendMessage...() to avoid deadlocks).

• you can't free the thread object from inside of its OnTerminated event handler. The RTL still needs access to the thread object after the handler exits. If you want the thread to free itself, set its FreeOnTerminate property to true instead. However, in this scenario, since you are waiting on the thread, you could just free the thread after WaitFor() exits instead.

A better way to handle this scenario is to update the TAniIndicator directly in Button1Tap() and get rid of the OnTerminate event handler altogether. The TAniIndicator logic doesn't really belong in the thread's Execute() method to begin with. However, you will have to manually pump the main thread's message queue for new messages while waiting for the thread to finish.

void __fastcall TForm1::Button1Tap(TObject *Sender, const TPointF &Point)
{
 PHONENumber = _D("");

 AlphaThread = new TAlphaThread(false);

 AniIndicator1->Align = TAlignLayout::Contents;
 AniIndicator1->Enabled = true;
 AniIndicator1->Visible = true;

 HANDLE h = (HANDLE) AlphaThread->Handle;
 while (MsgWaitForMultipleObjects(1, &h, FALSE, INFINITE, QS_ALLINPUT) == (WAIT_OBJECT_0 + 1))
 {
  Application->ProcessMessages();
 }

 int result = AlphaThread->WaitFor();
 delete AlphaThread;

 AniIndicator1->Enabled = false;
 AniIndicator1->Visible = false;

 if (result)
 {
  if(PHONENumber == _D(""))
   ShowMessage(_D("Null value!"));
  else
   Label1->Text = PHONENumber;
 }
}
//---------------------------------------------------------------------------
__fastcall TAlphaThread::TAlphaThread(bool CreateSuspended) : TThread(CreateSuspended)
{
 FreeOnTerminate = false;
}
//---------------------------------------------------------------------------
void __fastcall TAlphaThread::Execute()
{
 // Chargement des données
 Form1->REST1Client->BaseURL = _D("my.URL.php");
 Form1->REST1Request->AddParameter(_D("ID"), _D("123"));
 Form1->REST1Request->Client = Form1->REST1Client;
 Form1->REST1Request->Response = Form1->REST1Response;
 Form1->REST1Response->ContentType = _D("application/json");
 Form1->REST1Request->Execute();

 // Assignation des variables
 TJSONValue *JV0TOPLevel = Form1->REST1Response->JSONValue;
 TJSONArray *JSONArray = static_cast<TJSONArray*>(JV0TOPLevel);
 TJSONObject *JSONObject = static_cast<TJSONObject*>(JSONArray->Items[0]);
 TJSONValue *JV0ITEMLevel = JSONObject->GetValue(_D("FIELDName"));
 Form1->PHONENumber = JV0ITEMLevel->AsType<String>();

 ReturnValue = 1;
}

However, in this scenario, there is really no point whatsoever in having the main thread create a worker thread just to block itself waiting on that thread. You may as well just perform the REST logic directly in the main thread instead. You can use the REST components asynchronously to avoid blocking the main thread (ie, by using TRESTRequest::ExecuteAsync() instead of TRESTRequest::Execute() ).

---

Scenario 2:

• all of the same issues as Scenario 1.

---

Scenario 3:

• there is no call to TThread::WaitFor() in the main thread, so the Queue() requests go unprocessed, but at least they won't block the worker thread from running. That would not be the case had you used Synchronize() instead.

• the while loop in the main thread is creating a new lambda on each iteration, but is not actually calling the lambda. This is just a busy loop that eats up CPU cycles for no gain.

• same issue with freeing the thread as the other scenarios.

---

Now, with all of that said, the correct way to handle this situation using a worker thread is to not wait on the thread at all. Let it run normally, and let it notify the main thread when finished. DO NOT block the main thread at all in the meantime.

Try this:

class TGetPhoneNumberThread : public TThread
{
 protected:
    void __fastcall Execute();
 public:
    __fastcall TGetPhoneNumberThread(bool CreateSuspended);
    String PHONENumber;
};

void __fastcall TForm1::Button1Tap(TObject *Sender, const TPointF &Point)
{
 TGetPhoneNumberThread *Thread = new TGetPhoneNumberThread(true);
 Thread->OnTerminate = &GetPhoneNumberThreadTerminated;
 Thread->Start();

 Button1->Enabled = false;
 AniIndicator1->Align = TAlignLayout::Contents;
 AniIndicator1->Enabled = true;
 AniIndicator1->Visible = true;
}
//---------------------------------------------------------------------------
void __fastcall TForm1::GetPhoneNumberThreadTerminated(TObject *Sender)
{
 AniIndicator1->Enabled = false;
 AniIndicator1->Visible = false;
 Button1->Enabled = true;

 TGetPhoneNumberThread *Thread = static_cast<TGetPhoneNumberThread*>(Sender);

 if (Thread->FatalException)
   ShowMessage(_D("Error! ") + static_cast<Exception*>(Thread->FatalException)->Message);

 else if (Thread->PHONENumber == _D(""))
   ShowMessage(_D("Null value!"));

 else
   Label1->Text = Thread->PHONENumber;
 }
}
//---------------------------------------------------------------------------
__fastcall TGetPhoneNumberThread::TGetPhoneNumberThread(bool CreateSuspended) : TThread(CreateSuspended)
{
 FreeOnTerminate = true;
}
//---------------------------------------------------------------------------
#include <memory>
void __fastcall TGetPhoneNumberThread::Execute()
{
 std::unique_ptr<TRESTClient> RESTClient(new TRESTClient(NULL));
 std::unique_ptr<TRESTRequest> RESTRequest(new TRESTRequest(NULL));
 std::unique_ptr<TRESTResponse> RESTResponse(new TRESTResponse(NULL));

 // Chargement des données
 RESTClient->BaseURL = _D("my.URL.php");
 RESTRequest->AddParameter(_D("ID"), _D("123"));
 RESTRequest->Client = RESTClient.get();
 RESTRequest->Response = RESTResponse.get();
 RESTResponse->ContentType = _D("application/json");
 RESTRequest->Execute();

 // Assignation des variables
 TJSONValue *JV0TOPLevel = RESTResponse->JSONValue;
 TJSONArray *JSONArray = static_cast<TJSONArray*>(JV0TOPLevel);
 TJSONObject *JSONObject = static_cast<TJSONObject*>(JSONArray->Items[0]);
 TJSONValue *JV0ITEMLevel = JSONObject->GetValue(_D("FIELDName"));

 PHONENumber = JV0ITEMLevel->AsType<String>();
}

Alternatively, you can get rid of the worker thread completely and use TRESTRequest asynchronously instead:

void __fastcall TForm1::Button1Tap(TObject *Sender, const TPointF &Point)
{
 // Chargement des données
 REST1Client->BaseURL = _D("my.URL.php");
 REST1Request->AddParameter(_D("ID"), _D("123"));
 REST1Request->Client = REST1Client;
 REST1Request->Response = REST1Response;
 REST1Response->ContentType = _D("application/json");

 REST1Request->ExecuteAsync(
  [this](){
    // Assignation des variables
    TJSONValue *JV0TOPLevel = REST1Response->JSONValue;
    TJSONArray *JSONArray = static_cast<TJSONArray*>(JV0TOPLevel);
    TJSONObject *JSONObject = static_cast<TJSONObject*>(JSONArray->Items[0]);
    TJSONValue *JV0ITEMLevel = JSONObject->GetValue(_D("FIELDName"));

    PHONENumber = JV0ITEMLevel->AsType<String>();

    TThread::Queue(nullptr,
     [this](){
      AniIndicator1->Enabled = false;
      AniIndicator1->Visible = false;
      Button1->Enabled = true;

      if (PHONENumber == _D(""))
        ShowMessage(_D("Null value!"));
      else
        Label1->Text = PHONENumber;
     }
    );
   }
  },
  false
 );

 Button1->Enabled = false;
 AniIndicator1->Align = TAlignLayout::Contents;
 AniIndicator1->Enabled = true;
 AniIndicator1->Visible = true;
}