I really like the promise-based RPC capabilities provided by capnproto, and
am working to integrate them into an event-driven application that was
previously using libev.
Luckily, the application was already using libev-agnostic callback wrapper
classes for various event types, making it very easy to throw out libev
code and to replace it with kj. For example, here is the existing interface
of a periodic callback:
// Callback type called periodically.
class EventSystem::PeriodicCallback {
public:
// Construct a callback to function with the specified period.
template <typename T_Func>
PeriodicCallback(double seconds, T_Func &&function);
~PeriodicCallback();
// Change the period of the callback.
void setPeriod(double seconds);
// Enable or disable the callback.
void enable();
void disable();
private:
// Allocate and initialize the implementation structure.
void _initialize();
private:
std::function<void()> _function;
void *_impl;
};
I have however run into a problem regarding callbacks rescheduling
themselves, and would like to know what the "recommended" solution is. The
kj implementation struct for the periodic callback currently looks like so:
struct PeriodicCallbackImpl {
PeriodicCallbackImpl();
kj::Promise<void> promise;
kj::Duration period;
};
Since a promise can only execute once (please correct me if I'm wrong), the
callback has to reschedule the promise within itself. However, the new
promise has to be stored somewhere, and the only place to store it is
within the implementation struct. This however destroys the original
promise, which is still executing, resulting in an exception: expected
!firing; Promise callback destroyed itself.
I've come up with two methods that work, but don't quite feel right:
1) Add a second promise to the implementation struct, and move the
executing promise there (requires another promise that is only used
transiently).
2) Call detach on the executing promise (is this legal within the callback
itself? What is the error handler for in this case?).
What is the recommended method for rescheduling a promise during its own
callback execution?
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