How to wait for signal in long running method?

Question

At the moment I am developing a simple programming language on the basis of flex&bison.

For this I first build the abstract syntax tree (AST) and continue with its evaluation. During the evaluation it shall be possible to receive user input. The user input is done in a Qt-GUI. The ast-evaluation-procedure runs in a thread-worker-object, that is created by the GUI. My problem: Which one is the best way, to "block" the running flex&bison evaluation-procedure to then receive a user input from the GUI and afterwards continue with executing the procedure? Unfortunately I have no idea, how to split the evaluation procedure into two parts to catch the user input-signal from the GUI and then to continue with the "splitted procedure" in a slot. My current solution works (to my opinion poor) with an infinite loop within the evaluation procedure and this runs until a boolean member variable (_hasUserInput) in the thread worker-object is set to true. After the user input the GUI emits the signal. The corresponding slot is inside the thread worker-object. The slot receives the user input e.g. as a QString and sets a corresponding member variable. Additionally the boolean variable is set to true, so the infinite loop of the evaluation-procedure can be left, the user input can be processed and the evaluation can be continued.

Functional principle:
GUI --> starts Thread (Worker-Object) --> starts the longterm running procedure

Example in pseudo code:

class ThreadWorker : public QObject {

    ...

    // Slot, that reacts to the signal from the GUI
    void userInput(const QString &input) {
        _userInput = input;
        _hasUserInput = true;
    }

    // evaluation-procedure
    void doEvaluation() {
        // evaluation of the AST

        ...

        emit userInputRequired();
        while (!hasUserInput){          // Check if user input has been done
            qApp -> processEvents();    // to react to events
        }
        do something with user input;
        continue valuation;             // Processing of user input and continuation of the procedure
    }

    ...

};

I tried to give a description of my problem as good as possible and hope it worked. Please ask, in case you need more detailed information. Thanks very much in advance for your assistance!

Maybe you need a non-blocking worker? https://stackoverflow.com/questions/32952474/non-blocking-worker-interrupt-file-copy The idea is you remove the `while ()` loop and replace it with an event driven one — dtech, Jul 28 '17 at 19:59
While a bytecode interpreter might require more work than an AST-based one upfront, it makes it a lot easier to just pause and resume execution whenever you want, so you might want to consider going that route instead. — sepp2k, Jul 28 '17 at 20:40

score 0 · Answer 1 · answered Jul 28 '17 at 20:13

Since you've already factored out the worker into a QObject, you should never need to call processEvents(). Instead, break down the AST evaluation into small chunks that are performed as long as necessary. I've also made the public interface thread-safe - thus you can call it directly from any thread.

// see https://stackoverflow.com/a/40382821/1329652
bool isSafe(QObject * obj);
template <typename Class, typename... Args> void postCall(Class * obj, void (Class::*method)(Args...), Args&& ...args);

class ASTWorker : public QObject {
  Q_OBJECT
  QBasicTimer m_evaluateTimer; // a bit lower overhead than QTimer
  void timerEvent(QTimerEvent * ev) override {
    if (ev->timerId() == m_evaluateTimer.timerId())
      evaluateImpl();
  }
  void evaluateImpl() {
    bool evaluationDone = {};
    QThread::msleep(10); // emulate evaluating a chunk of AST
    if (!evaluationDone && !m_evaluateTimer.isActive())
      m_evaluateTimer.start(0, this);
    else if (evaluationDone)
      m_evaluateTimer.stop();
  }
public:
  using QObject::QObject;
  /// Thread-safe
  Q_SLOT void setUserInput(const Data & data) {
    if (!isSafe(this))
      return postCall(this, &ASTWorker::setUserInput, data);
    QThread::msleep(1); // emulate processing user data
  }
  /// Thread-safe
  Q_SLOT void evaluate() {
    if (!isSafe(this))
      return postCall(this, &ASTWorker::evaluate, data);
    evaluateImpl();
  }
};

How to wait for signal in long running method?

1 Answers1