Channels and Notifications - musescore/MuseScore GitHub Wiki

Channels and Notifications

In Qt there is a powerful system for asynchronous interaction - signals and slots. But there are several problems with it:

  • Dependence on Qt (and we try to reduce the dependence on Qt as much as possible)
  • It is not convenient to use with interfaces; in interfaces, you need to add the virtual QObject* getQObject() = 0 method; and return a QObject*.
  • It's harder to do mocks for unit tests.
  • It is unsafe to use lambdas because there is no automatic disconnect for lambdas and there will be a crash if the receiver is deleted

Therefore, the use of Qt systems of signals and slots is not suitable for us.

At the same time, we plan that in MuseScore 4 there will be many asynchronous interactions. Accordingly, the system should be easy to use.

For these purposes, a number of primitives have been developed for simple asynchronous interaction. The channels in the GoLang language served as inspiration.

Channels

Сhannels is a primitive for bidirectional transmission of typed data.

Can subscribe to the channel and send data

Channel<int> ch;
ch.onReceive(nullptr, [](int val) {
    std::cout << "onReceive val: " << val << "\n";
});

ch.send(1); // will be printed: 'onReceive val: 1'

Channels can be returned from methods

struct Counter {
    Channel<int> m_ch;
    int m_val = 0;

    void increment()
    {
        ++m_val;
        m_ch.send(m_val);
    }

    Channel<int> channel() const
    {
        return m_ch;
    }
};


int main(int argc, char* argv[])
{
    Counter counter;

    counter.channel().onReceive(nullptr, [](int val) {
        std::cout << "counter: " << val << "\n";
    });

    for (int i = 0; i < 5; ++i) {
        counter.increment(); // will be printed: 'counter: n'
    }
}

Channels can be passed to methods

If the subscription occurs inside the class method, then it is required to inherit from the Asyncable interface and you need to pass this to the subscription method so that there is auto disconnect when deleting an object

class Receiver : public async::Asyncable
{
public:
    Receiver(const Channel<int>& ch)
        : m_ch(ch)
    {
        m_ch.onReceive(this, [](int val) {
            std::cout << "Receiver channel: " << val << "\n";
        });
    }

private:
    Channel<int> m_ch;
};

int main(int argc, char* argv[])
{
    Counter counter;
    Channel<int> ch = counter.channel();

    {
        Receiver r(ch);
        for (int i = 0; i < 5; ++i) {
            counter.increment(); // will be printed: 'Receiver channel: n'
        }
    } // When deleting an object, all its callbacks will be unsubscribed.
}

Can unsubscribe from the channel

class Receiver : public async::Asyncable
{
...
    void unsubscribe()
    {
        m_ch.resetOnReceive(this);
    }
}

int main(int argc, char* argv[])
{
    Counter counter;
    Channel<int> ch = counter.channel();

    Receiver r(ch);
    for (int i = 0; i < 5; ++i) {
        counter.increment(); // will be printed: 'Receiver channel: n'
    }

    r.unsubscribe();

    for (int i = 0; i < 5; ++i) {
        counter.increment(); // nothing will be printed
    }

}

Channel can be closed

Closing the channel notifies subscribers that there will be no data coming from the channel.

struct Processor
{
    Channel<int> m_ch;

    Channel<int> channel() const
    {
        return m_ch;
    }

    void process()
    {
        for (int task = 0; task < 5; ++task) {
            int val = doCalculate(task);
            m_ch.send(val);
        }

        m_ch.close();
    }
};

struct Saver : public async::Asyncable
{
    Stream m_stream;
    void saveFromChannel(const Channel<int>& ch)
    {
        m_stream.open();
        ch.onReceive(this, [this](int val) {
            m_stream.write(val);
        });

        ch.onClose(this, [this]() {
            m_stream.close();
        });
    }
};

int main(int argc, char* argv[])
{
    Processor p;
    Saver s;

    s.saveFromChannel(p.channel());

    p.process();
}

Notifications

Notification is designed to send notifications. They have all the qualities of channels, they are only used without arguments and have slightly different semantics.

// Notification sender
class Selector
{
public:
    void select(int val)
    {
        m_selection = val;
        m_selecttionChanged.notify();
    }

    int selection() const
    {
        return m_selection;
    }

    Notification selecttionChanged() const
    {
        return m_selecttionChanged;
    }

private:
    int m_selection = 0;
    Notification m_selecttionChanged;
};

// Notification receiver
class SelectionPrinter : public async::Asyncable
{
    void setSelector(Selector* s)
    {
        if (m_selector) {
            m_selector->selecttionChanged().resetOnNotify(this); // unsubscribe from previous
        }

        m_selector = s;
        m_selector->selecttionChanged().onNotify(this, [this]() {
            std::cout << "Selection changed, new selection: " << m_selector->selection() << "\n";
        });
    }

private:
    Selector* m_selector = nullptr;
};


int main(int argc, char* argv[])
{
    Selector* sel = new Selector();
    SelectionPrinter* p = new SelectionPrinter();

    p->setSelector(sel);

    sel->select(5); // will be printed: 'Selection changed, new selection: 5'
}
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