ConditionalRemover is a utility class that automatically removes listeners after the listeners are triggered and certain condition is satisfied.
ConditionalRemover is a pure functional class. After the member functions in ConditionalRemover are invoked, the ConditionalRemover object can be destroyed safely.
eventpp/utilities/conditionalremover.h
template <typename DispatcherType>
class ConditionalRemover;DispatcherType can be CallbackList, EventDispatcher, EventQueue, HeterCallbackList, HeterEventDispatcher, or HeterEventQueue.
explicit ConditionalRemover(DispatcherType & dispatcher);Constructs an instance of ConditionalRemover.
Member functions for EventDispatcher and EventQueue
template <typename Condition>
typename DispatcherType::Handle appendListener(
const typename DispatcherType::Event & event,
const typename DispatcherType::Callback & listener,
const Condition & condition
);
template <typename Condition>
typename DispatcherType::Handle prependListener(
const typename DispatcherType::Event & event,
const typename DispatcherType::Callback & listener,
const Condition & condition
);
template <typename Condition>
typename DispatcherType::Handle insertListener(
const typename DispatcherType::Event & event,
const typename DispatcherType::Callback & listener,
const typename DispatcherType::Handle & before,
const Condition & condition
);Member functions for CallbackList
template <typename Condition>
typename CallbackListType::Handle append(
const typename CallbackListType::Callback & listener,
const Condition & condition
);
template <typename Condition>
typename CallbackListType::Handle prepend(
const typename CallbackListType::Callback & listener,
const Condition & condition
);
template <typename Condition>
typename CallbackListType::Handle insert(
const typename CallbackListType::Callback & listener,
const typename CallbackListType::Handle & before,
const Condition & condition
);The member functions have the same names with the corresponding underlying class (CallbackList, EventDispatcher, or EventQueue), and also have the same parameters except there is one more parameter, condition. condition is a predicate function that returns a bool value. It's invoked after each trigger, if it returns true, the listener will be removed.
condition can have two kinds of prototype,
bool condition()
This condition doesn't receive any arguments.
bool condition(Args ...args)
This condition receives the arguments that passed to the listener.
template <typename DispatcherType>
ConditionalRemover<DispatcherType> conditionalRemover(DispatcherType & dispatcher);Since ConditionalRemover takes one template parameter and it's verbose to instantiate its instance, the function conditionalRemover is used to construct an instance of ConditionalRemover via the deduced argument.
#include "eventpp/utilities/conditionalRemover.h"
#include "eventpp/eventdispatcher.h"
eventpp::EventDispatcher<int, void ()> dispatcher;
constexpr int event = 3;
dispatcher.appendListener(event, []() {
// listener A
});
// Note the ConditionalRemover instance returned by conditionalRemover is invoked
// prependListener and destroyed immediately.
std::string removeWho;
eventpp::conditionalRemover(dispatcher).prependListener(event, [&dataList]() {
// listener B
}, [&removeWho]() -> bool {
return removeWho == "removeB";
});
auto handle = eventpp::conditionalRemover(dispatcher).appendListener(event, [&dataList]() {
// listener C
}, [&removeWho]() -> bool {
return removeWho == "removeC";
});
eventpp::conditionalRemover(dispatcher).insertListener(event, [&dataList]() {
// listener D
}, handle, [&removeWho]() -> bool {
return removeWho == "removeD";
});
dispatcher.dispatch(event);
// No listeners were removed since no conditions were met.
removeWho = "removeB";
dispatcher.dispatch(event);
// All listeners were triggered.
// Listener B was removed.
removeWho = "removeC";
dispatcher.dispatch(event);
// Listeners A, C, D were triggered.
// Listener C was removed.
removeWho = "removeD";
dispatcher.dispatch(event);
// Listeners A, D were triggered.
// Listener D was removed.
dispatcher.dispatch(event);
// Listeners A was triggered.