eventpp/doc/eventdispatcher.md

Class EventDispatcher reference

Table Of Contents

• Tutorials

  • Tutorial 1 -- Basic usage
  • Tutorial 2 -- Listener with parameters
  • Tutorial 3 -- Customized event struct
  • Tutorial 4 -- Event queue
  • Tutorial 5 -- Event filter

• API reference

• Event getter

• Event filter

• Argument passing mode

• Nested listener safety

• Thread safety

• Exception safety

• Time complexities

• Internal data structure

Tutorials

Tutorial 1 -- Basic usage

Code

// The namespace is eventpp
// The first template parameter int is the event type,
// the event type can be any type such as std::string, int, etc.
// The second is the prototype of the listener.
eventpp::EventDispatcher<int, void ()> dispatcher;

// Add a listener. As the type of dispatcher,
// here 3 and 5 is the event type,
// []() {} is the listener.
// Lambda is not required, any function or std::function
// or whatever function object with the required prototype is fine.
dispatcher.appendListener(3, []() {
	std::cout << "Got event 3." << std::endl;
});
dispatcher.appendListener(5, []() {
	std::cout << "Got event 5." << std::endl;
});
dispatcher.appendListener(5, []() {
	std::cout << "Got another event 5." << std::endl;
});

// Dispatch the events, the first argument is always the event type.
dispatcher.dispatch(3);
dispatcher.dispatch(5);

Output

Got event 3.
Got event 5.
Got another event 5.

Remarks
First let's define a dispatcher.

eventpp::EventDispatcher<int, void ()> dispatcher;

class EventDispatcher takes two template arguments. The first argument is the event type, here is int. The second is the prototype of the listener.
The event type must be able to use as the key of std::map, that's to say, it must support operator <.
The prototype is C++ function type, such as void (int), void (const std::string &, const MyClass &, int, bool).

Now let's add a listener.

dispatcher.appendListener(3, []() {
	std::cout << "Got event 3." << std::endl;
});

Function appendListener takes at least two arguments. The first argument is the event of type event type, here is int. The second is the callback.
The callback can be any callback target -- functions, pointers to functions, , pointers to member functions, lambda expressions, and function objects. It must be able to be called with the prototype declared in dispatcher.
In the tutorial, we also add two listeners for event 5.

Now let's dispatch some event.

dispatcher.dispatch(3);
dispatcher.dispatch(5);

Here we dispatched two events, one is event 3, the other is event 5.
During the dispatching, all listeners of that event will be invoked one by one in the order of they were added.

Tutorial 2 -- Listener with parameters

Code

// The listener has two parameters.
eventpp::EventDispatcher<int, void (const std::string &, const bool)> dispatcher;

dispatcher.appendListener(3, [](const std::string & s, const bool b) {
	std::cout << std::boolalpha << "Got event 3, s is " << s << " b is " << b << std::endl;
});
// The listener prototype doesn't need to be exactly same as the dispatcher.
// It would be find as long as the arguments is compatible with the dispatcher.
dispatcher.appendListener(5, [](std::string s, int b) {
	std::cout << std::boolalpha << "Got event 5, s is " << s << " b is " << b << std::endl;
});
dispatcher.appendListener(5, [](const std::string & s, const bool b) {
	std::cout << std::boolalpha << "Got another event 5, s is " << s << " b is " << b << std::endl;
});

// Dispatch the events, the first argument is always the event type.
dispatcher.dispatch(3, "Hello", true);
dispatcher.dispatch(5, "World", false);

Output

Got event 3, s is Hello b is true
Got event 5, s is World b is false
Got another event 5, s is World b is false

Remarks
Now the dispatcher callback prototype takes two parameters, const std::string & and const bool.
The listener's prototype is not required to be same as the dispatcher, it's fine as long as the prototype is compatible with the dispatcher. See the second listener, [](std::string s, int b), its prototype is not same as the dispatcher.

Tutorial 3 -- Customized event struct

Code

// Define an Event to hold all parameters.
struct MyEvent {
	int type;
	std::string message;
	int param;
};

// Define an event type getter to let the dispatcher knows how to
// extract the event type.
// The getter must derive from eventpp::EventGetterBase
// The getter must have:
// 1, A type named Event indicating the event type.
// 2, A static member function named getEvent. It receives all parameters
// same as the dispatcher prototype, and returns Event.
struct MyEventTypeGetter : public eventpp::EventGetterBase
{
	using Event = int;

	static Event getEvent(const MyEvent & e, bool b) {
		return e.type;
	}
};

// Pass MyEventTypeGetter as the first template argument of EventDispatcher
eventpp::EventDispatcher<
	MyEventTypeGetter,
	void (const MyEvent &, bool)
> dispatcher;

// Add a listener.
// Note: the first argument, event type, is MyEventTypeGetter::Event,
// not Event
dispatcher.appendListener(3, [](const MyEvent & e, bool b) {
	std::cout
		<< std::boolalpha
		<< "Got event 3" << std::endl
		<< "Event::type is " << e.type << std::endl
		<< "Event::message is " << e.message << std::endl
		<< "Event::param is " << e.param << std::endl
		<< "b is " << b << std::endl
	;
});

// Dispatch the event.
// The first argument is Event.
dispatcher.dispatch(MyEvent { 3, "Hello world", 38 }, true);

Output

Got event 3
Event::type is 3
Event::message is Hello world
Event::param is 38
b is true

Remarks Previous tutorials pass the event type as the first argument in dispatch, and all other event parameters as other arguments of dispatch. Another common situation is an Event class is defined as the base, all other events derive from Event, and the actual event type is a data member of Event (think QEvent in Qt).

Tutorial 4 -- Event queue

Code

eventpp::EventDispatcher<int, void (const std::string &, const bool)> dispatcher;

dispatcher.appendListener(3, [](const std::string & s, const bool b) {
	std::cout << std::boolalpha << "Got event 3, s is " << s << " b is " << b << std::endl;
});
// The listener prototype doesn't need to be exactly same as the dispatcher.
// It would be find as long as the arguments is compatible with the dispatcher.
dispatcher.appendListener(5, [](std::string s, int b) {
	std::cout << std::boolalpha << "Got event 5, s is " << s << " b is " << b << std::endl;
});
dispatcher.appendListener(5, [](const std::string & s, const bool b) {
	std::cout << std::boolalpha << "Got another event 5, s is " << s << " b is " << b << std::endl;
});

// Enqueue the events, the first argument is always the event type.
// The listeners are not triggered during enqueue.
dispatcher.enqueue(3, "Hello", true);
dispatcher.enqueue(5, "World", false);

// Process the event queue, dispatch all queued events.
dispatcher.process();

Output

Got event 3, s is Hello b is true
Got event 5, s is World b is 0
Got another event 5, s is World b is false

Remarks
EventDispatcher<>::dispatch() invokes the listeners synchronously. Sometimes an asynchronous event queue is more useful (think about Windows message queue, or an event queue in a game). EventDispatcher supports such kind of event queue.
EventDispatcher<>::enqueue() puts an event to the queue. Its parameters are exactly same as dispatch.
EventDispatcher<>::process() must be called to dispatch the queued events.
A typical use case is in a GUI application, each components call EventDispatcher<>::enqueue() to post the events, then the main event loop calls EventDispatcher<>::process() to dispatch the events.

Tutorial 5 -- Event filter

Code

eventpp::EventDispatcher<int, void (int e, int i, std::string)> dispatcher;

dispatcher.appendListener(3, [](const int e, const int i, const std::string & s) {
	std::cout << "Got event 3, i was 1 but actural is " << i << " s was Hello but actural is " << s << std::endl;
});
dispatcher.appendListener(5, [](const int e, const int i, const std::string & s) {
	std::cout << "Shout not got event 5" << std::endl;
});

// Add three event filters.

// The first filter modifies the input arguments to other values, then the subsequence filters
// and listeners will see the modified values.
dispatcher.appendFilter([](const int e, int & i, std::string & s) -> bool {
	std::cout << "Filter 1, e is " << e << " passed in i is " << i << " s is " << s << std::endl;
	i = 38;
	s = "Hi";
	std::cout << "Filter 1, changed i is " << i << " s is " << s << std::endl;
	return true;
});

// The second filter filters out all event of 5. So no listeners on event 5 can be triggered.
// The third filter is not invoked on event 5 also.
dispatcher.appendFilter([](const int e, int & i, std::string & s) -> bool {
	std::cout << "Filter 2, e is " << e << " passed in i is " << i << " s is " << s << std::endl;
	if(e == 5) {
		return false;
	}
	return true;
});

// The third filter just prints the input arguments.
dispatcher.appendFilter([](const int e, int & i, std::string & s) -> bool {
	std::cout << "Filter 3, e is " << e << " passed in i is " << i << " s is " << s << std::endl;
	return true;
});

// Dispatch the events, the first argument is always the event type.
dispatcher.dispatch(3, 1, "Hello");
dispatcher.dispatch(5, 2, "World");

Output

Filter 1, e is 3 passed in i is 1 s is Hello
Filter 1, changed i is 38 s is Hi
Filter 2, e is 3 passed in i is 38 s is Hi
Filter 3, e is 3 passed in i is 38 s is Hi
Got event 3, i was 1 but actural is 38 s was Hello but actural is Hi
Filter 1, e is 5 passed in i is 2 s is World
Filter 1, changed i is 38 s is Hi
Filter 2, e is 5 passed in i is 38 s is Hi

Remarks
EventDispatcher<>::appendFilter(filter) adds an event filter to the dispatcher. The filter receives the arguments which types are the callback prototype with lvalue reference, and must return a boolean value. Return true to allow the dispatcher continues the dispatching, false to prevent the dispatcher from invoking any subsequence listeners and filters.
The event filters are invoked before any listeners are invoked.

API reference

Template parameters

template <
	typename EventGetter,
	typename Prototype,
	typename Callback = void,
	typename ArgumentPassingMode = ArgumentPassingAutoDetect,
	typename Threading = MultipleThreading
>
class EventDispatcher;

EventGetter: the event getter. The simplest form is an event type. For details, see Event getter for details.
Prototype: the listener prototype. It's C++ function type such as void(int, std::string, const MyClass *).
Callback: the underlying type to hold the callback. Default is void, which will be expanded to std::function.
ArgumentPassingMode: the argument passing mode. Default is ArgumentPassingAutoDetect. See Argument passing mode for details.
Threading: threading model. Default is 'MultipleThreading'. Possible values:

• MultipleThreading: the core data is protected with mutex. It's the default value.
• SingleThreading: the core data is not protected and can't be accessed from multiple threads.

Public types

Handle: the handle type returned by appendListener, prependListener and insertListener. A handle can be used to insert a listener or remove a listener. To check if a Handle is empty, convert it to boolean, false is empty. Handle is copyable.
Callback: the callback storage type.
Event: the event type. The event type must be able to be used as the key in std::map, so it must be comparable with operator <.
FilterHandle: the handle type returned by appendFilter. A filter handle can be used to remove a filter. To check if a FilterHandle is empty, convert it to boolean, false is empty. FilterHandle is copyable.

Functions

EventDispatcher() = default;
EventDispatcher(EventDispatcher &&) = delete;
EventDispatcher(const EventDispatcher &) = delete;
EventDispatcher & operator = (const EventDispatcher &) = delete;

EventDispatcher can not be copied, moved, or assigned.

Handle appendListener(const Event & event, const Callback & callback)

Add the callback to the dispatcher to listen to event.
The listener is added to the end of the listener list.
Return a handle which represents the listener. The handle can be used to remove this listener or insert other listener before this listener.
If appendListener is called in another listener during a dispatching, the new listener is guaranteed not triggered during the same dispatching.
The time complexity is O(1).

Handle prependListener(const Event & event, const Callback & callback)

Add the callback to the dispatcher to listen to event.
The listener is added to the beginning of the listener list.
Return a handle which represents the listener. The handle can be used to remove this listener or insert other listener before this listener.
If prependListener is called in another listener during a dispatching, the new listener is guaranteed not triggered during the same dispatching.
The time complexity is O(1).

Handle insertListener(const Event & event, const Callback & callback, const Handle before)

Insert the callback to the dispatcher to listen to event before the listener handle before. If before is not found, callback is added at the end of the listener list.
Return a handle which represents the listener. The handle can be used to remove this listener or insert other listener before this listener.
If insertListener is called in another listener during a dispatching, the new listener is guaranteed not triggered during the same dispatching.
The time complexity is O(1).

bool removeListener(const Event & event, const Handle handle)

Remove the listener handle which listens to event from the dispatcher.
Return true if the listener is removed successfully, false if the listener is not found.
The time complexity is O(1).

void dispatch(Args ...args);  

template <typename T>  
void dispatch(T && first, Args ...args);

Dispatch an event. The event type is deducted from the arguments of dispatch.
In both overloads, the listeners are called with arguments args.
The listeners are called in the thread same as the caller of dispatch.

void enqueue(Args ...args);

template <typename T>  
void enqueue(T && first, Args ...args);

Put an event into the event queue. The event type is deducted from the arguments of enqueue.
All arguments are copied to internal data structure, so the arguments must be copyable.

void process()

Process the event queue. All events in the event queue are dispatched once and then removed from the queue.
The listeners are called in the thread same as the caller of process.
Note: if process() is called from multiple threads simultaneously, the events in the event queue are guaranteed dispatched only once.

template <typename Func>  
void forEach(const Event & event, Func && func)

Apply func to all listeners of event.
The func can be one of the three prototypes:

AnyReturnType func(const EventDispatcher::Handle &, const EventDispatcher::Callback &);
AnyReturnType func(const EventDispatcher::Handle &);
AnyReturnType func(const EventDispatcher::Callback &);

Note: the func can remove any listeners, or add other listeners, safely.

template <typename Func>  
bool forEachIf(const Event & event, Func && func)

Apply func to all listeners of event. func must return a boolean value, and if the return value is false, forEachIf stops the looping immediately.
Return true if all listeners are invoked, or event is not found, false if func returns false.

FilterHandle appendFilter(const Filter & filter)

Add the filter to the dispatcher.
Return a handle which can be used in removeFilter.

bool removeFilter(const FilterHandle & filterHandle)

Remove a filter from the dispatcher.
Return true if the filter is removed successfully.

Event getter

The first template parameter of EventDispatcher is the event getter.

If event getter is not a struct or class inherits from eventpp::EventGetterBase, it's the event type.
For example EventDispatcher<std::string, void()> is a dispatcher with event type std::string, prototype void().

If event getter inherits from tag eventpp::EventGetterBase, it's the event type getter. See Tutorial 3 for example.
Assume we have an EventDispatcher which callback prototype is void (const MyEvent &, bool), where MyEvent is a unified event structure.

struct MyEvent {
	int type;
	std::string message;
	int param;
	// blah blah
};

A typical event getter looks like:

struct MyEventTypeGetter : public eventpp::EventGetterBase
{
	using Event = int;

	static Event getEvent(const MyEvent & e, bool b) {
		return e.type;
	}
};

Then we can define the dispatcher as

eventpp::EventDispatcher<MyEventTypeGetter, void (const MyEvent &, bool)> dispatcher;

An event getter must have a type Event which is the event type and used in the internal 'std::map, and it must have a static function getEvent`, which receives the arguments of callback prototype and return the event type.

To add a listener

dispatcher.appendListener(5, [](const MyEvent & e, bool b) {});

Note the first argument is the Event in the event getter, here is int, not MyEvent.

To dispatch or enqueue an event

MyEvent myEvent { 5, "Hello", 38 };
dispatcher.dispatch(myEvent, true);

Note the first argument is MyEvent, not Event.
dispatch and enqueue use the function getEvent in the event getter to deduct the event type.
dispatch and enqueue don't assume the meaning of any arguments. How to get the event type completely depends on getEvent. getEvent can simple return a member for the first argument, or concatenate all arguments, or even hash the arguments and return the hash value as the event type.

Event filter

EventDispatcher<>::appendFilter(filter) adds an event filter to the dispatcher. The filter receives the arguments which types are the callback prototype with lvalue reference, and must return a boolean value. Return true to allow the dispatcher continues the dispatching, false to prevent the dispatcher from invoking any subsequence listeners and filters.

The event filters are invoked for all events, and invoked before any listeners are invoked.
The event filters can modify the arguments since the arguments are passed as lvalue reference, no matter whether they are reference in the callback prototype (of course we can't modify a reference to const).

Below table shows the cases of how event filters receive the arguments.

Argument type in callback prototype	Argument type received by filter	Can filter modify the argument?	Comment
int, const int	int &, int &	Yes	The constness of the value is discarded
int &, std::string &	int &, std::string &	Yes
const int &, const int *	const int &, const int * &	No	The constness of the reference/pointer must be respected

Event filter is a powerful and useful technology, below is some sample use cases, though the real world use cases are unlimited.

1, Capture and block all interested events. For example, in a GUI window system, all windows can receive mouse events. However, when a window is under mouse dragging, only the window under dragging should receive the mouse events even when the mouse is moving on other window. So when the dragging starts, the window can add a filter. The filter redirects all mouse events to the window and prevent other listeners from the mouse events, and bypass all other events.

2, Setup catch-all event listener. For example, in a phone book system, the system sends events based on the actions, such as adding a phone number, remove a phone number, look up a phone number, etc. A module may be only interested in special area code of a phone number, not the actions. One approach is the module can listen to all possible events (add, remove, look up), but this is very fragile -- how about a new action event is added and the module forgets to listen on it? The better approach is the module add a filter and check the area code in the filter.

Argument passing mode

We have the dispatcher

eventpp::EventDispatcher<int, void(int, const std::string &)> dispatcher;

The event type is int.
The listener's first parameter is also int. Depending how the event is dispatched, the listener's first argument can be either the event type, or an extra argument.

dispatcher.dispatch(3, "hello");

The event 3 is dispatched with an argument "hello", the listener will be invoked with the arguments (3, "hello"), the first argument is the event type.

dispatcher.dispatch(3, 8, "hello");

The event 3 is dispatched with two arguments 8 and "hello", the listener will be invoked with the arguments (8, "hello"), the first argument is the extra argument, and the event type is omitted.

So by default, EventDispatcher automatically detects the argument count of dispatch and listeners prototype, and calls the listeners either with or without the event type.

The default rule is convenient, permissive, and, may be error prone. The second parameter typename ArgumentPassingMode in the policies can control the behavior.

struct ArgumentPassingAutoDetect;
struct ArgumentPassingIncludeEvent;
struct ArgumentPassingExcludeEvent;

ArgumentPassingAutoDetect: the default policy. Auto detects whether to pass the event type.
ArgumentPassingIncludeEvent: always passes the event type. If the argument count doesn't match, compiling fails.
ArgumentPassingExcludeEvent: always omits and doesn't pass the event type. If the argument count doesn't match, compiling fails.

Assumes the number of arguments in the listener prototype is P, the number of arguments (include the event type) in dispatch is D, then the relationship of P and D is,
For ArgumentPassingAutoDetect: P == D or P + 1 == D
For ArgumentPassingIncludeEvent: P == D
For ArgumentPassingExcludeEvent: P + 1 == D

Note: the same rules also applies to EventDispatcher<>::enqueue, since enqueue has same parameters as dispatch.

Examples to demonstrate argument passing mode

eventpp::EventDispatcher<
	int,
	void(int, const std::string &),
	ArgumentPassingAutoDetect
> dispatcher;
// or just
//eventpp::EventDispatcher<int, void(int, const std::string &)> dispatcher;
dispatcher.dispatch(3, "hello"); // Compile OK
dispatcher.dispatch(3, 8, "hello"); // Compile OK
dispatcher.enqueue(3, "hello"); // Compile OK
dispatcher.enqueue(3, 8, "hello"); // Compile OK

eventpp::EventDispatcher<
	int,
	void(int, const std::string &),
	ArgumentPassingIncludeEvent
> dispatcher;
dispatcher.dispatch(3, "hello"); // Compile OK
//dispatcher.dispatch(3, 8, "hello"); // Compile failure
dispatcher.enqueue(3, "hello"); // Compile OK
//dispatcher.enqueue(3, 8, "hello"); // Compile failure

eventpp::EventDispatcher<
	int,
	void(int, const std::string &),
	ArgumentPassingExcludeEvent
> dispatcher;
//dispatcher.dispatch(3, "hello"); // Compile failure
dispatcher.dispatch(3, 8, "hello"); // Compile OK
//dispatcher.enqueue(3, "hello"); // Compile failure
dispatcher.enqueue(3, 8, "hello"); // Compile OK

Nested listener safety

1. If a listener adds another listener of the same event to the dispatcher during a dispatching, the new listener is guaranteed not to be triggered within the same dispatching. This is guaranteed by an unsigned 64 bits integer counter. This rule will be broken is the counter is overflowed to zero in a dispatching, but this rule will continue working on the subsequence dispatching.
2. Any listeners that are removed during a dispatching are guaranteed not triggered.
3. All above points are not true in multiple threading. That's to say, if one thread is invoking a callback list, the other thread add or remove a callback, the added or removed callback may be triggered during the invoking.

Thread safety

EventDispatcher is thread safe. All public functions can be invoked from multiple threads simultaneously. If it failed, please report a bug.
EventDispatcher guarantees the integration of each append/prepend/insert/remove/dispatching operations, but it doesn't guarantee the order of the operations in multiple threads. For example, if a thread is dispatching an event, the other thread removes a listener in the mean time, the removed listener may be still triggered after it's removed.
But the operations on the listeners, such as copying, moving, comparing, or invoking, may be not thread safe. It depends on listeners.

Exception safety

EventDispatcher doesn't throw any exceptions.
Exceptions may be thrown by underlying code when,

1. Out of memory, new memory can't be allocated.
2. The listeners throw exceptions during copying, moving, comparing, or invoking.

Time complexities

The time complexities being discussed here is about when operating on the listener in the underlying list, and n is the number of listeners. It doesn't include the event searching in the underlying std::map which is always O(log n).

• appendListener: O(1)
• prependListener: O(1)
• insertListener: O(1)
• removeListener: O(1)
• enqueue: O(1)

Internal data structure

Beside using CallbackList to manage the listener callbacks, EventDispatcher uses three std::list to manage the event queue.
The first busy list holds all nodes with queued events.
The second idle list holds all idle nodes. After an event is dispatched and removed from the queue, instead of freeing the memory, EventDispatcher moves the unused node to the idle list. This can improve performance and avoid memory fragment.
The third list is a local temporary list used in function process(). During processing, the busy list is swapped to the temporary list, all events are dispatched from the temporary list, then the temporary list is returned and appended to the idle list.