Async object initialization using QtConcurrent

manpreet Best Answer 3 years ago

I have asynchronous objects fetcher.

1. Fetcher during initialization loads from base ids of objects for fetch, and ready to work when finished. This init should be asynchronous.
2. After init user can call fetchMore(int count), which starts asynchronous objects loading.
3. Fetcher has two signals initFinished and fetchFinished`

I want to start initialization in Fetcher constructor. Can I be sure that signal initFinished will NOT be emitted before I connect that signal to FetcherUser?

class Fetcher : public QObject {
private:
    QFutureWatcher<QList<ObjectId> > watcherInit;
    QFutureWatcher<QList<Object> > watcherFetch;

    QList<ObjectId> listIds;

    QList<ObjectId> asyncInit() {
        // Do some ids load
    }

    QList<Object> asyncFetchMore(QList<ObjectId> ids) {
        // Do some objects load
    }

signals:
    void initFinished();
    void fetchFinished(const QList<Object> &list);

public:
    explicit Fetcher() 
    {
        connect(&watcherInit, &QFutureWatcher<QList<ObjectId>>::finished, [this]{
            // take ids and change init flag
            emit initFinished();
        });

        auto future = QtConcurrent::run(&Fetcher::asyncInit, this);
        watcherInit.setFuture(future)
    }

    void fetchMore(int count) {
        // take some ids from list and QtConcurrnt run asyncFetchMore
    }
};

//================================================

class FetcherUser : public QObject
{
    Fetcher *fetcher = nullptr;

private:
    void initFinished() {
        //Start interaction with fetcher
    }

public:
    FetcherUser() {
        fetcher = new Fetcher(); // Here async init started

        connect(fetcher, &Fetcher::initFinished, this, &FetcherUser::initFinished); //and here connect signal
    }
};

So can I be sure that after constructor finished and async init started, and before the signal is connected to FetcherUser, signal by Fetcher will not be emitted? Moreover if I will put some operations between Fetcher constructor and signal connection, how can i ensure that I haven't lost init signal?