69 lines
2.6 KiB
C++
69 lines
2.6 KiB
C++
#include <basics.h>
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#include <iostream>
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// Observers provide a mechanism for responding to builtin and user defined
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// events. They are similar to systems, in that they have the same callback
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// signature and use the same query interface to match with entities, but
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// instead of a phase they have an event kind.
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//
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// The most commonly used builtin events are:
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// - flecs::OnAdd: a component was added
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// - flecs::OnRemove: a component was removed
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// - flecs::OnSet: a component's value was changed
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//
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// The OnAdd and OnRemove events are only thrown when a component is
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// actually added or removed. If an application invokes add and the entity
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// already has the component, no event is emitted. Similarly, if an application
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// invokes remove for a component the entity doesn't have, no event is
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// emitted. That is in contrast to OnSet, which is invoked each time set
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// or modified is invoked.
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//
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// Observers are different from component hooks in a number of ways:
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// - A component can only have one hook, whereas it can match many observers
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// - A hook matches one component, whereas observers can match complex queries
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// - Hooks are for add/set/remove events, observers can match custom events.
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struct Position {
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double x, y;
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};
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int main(int, char *[]) {
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flecs::world ecs;
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// Create an observer for three events
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ecs.observer<Position>()
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.event(flecs::OnAdd)
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.event(flecs::OnRemove)
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.event(flecs::OnSet)
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.each([](flecs::iter& it, size_t i, Position& p) {
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if (it.event() == flecs::OnAdd) {
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// No assumptions about the component value should be made here. If
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// a ctor for the component was registered it will be called before
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// the EcsOnAdd event, but a value assigned by set won't be visible.
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std::cout << " - OnAdd: " << it.event_id().str() << ": "
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<< it.entity(i).name() << "\n";
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} else {
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// EcsOnSet or EcsOnRemove event
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std::cout << " - " << it.event().name() << ": "
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<< it.event_id().str() << ": "
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<< it.entity(i).name()
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<< ": {" << p.x << ", " << p.y << "}\n";
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}
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});
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// Create entity, set Position (emits EcsOnAdd and EcsOnSet)
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flecs::entity e = ecs.entity("e")
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.set<Position>({10, 20});
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// Remove component (emits EcsOnRemove)
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e.remove<Position>();
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// Remove component again (no event is emitted)
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e.remove<Position>();
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// Output
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// - OnAdd: Position: e
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// - OnSet: Position: e: {10, 20}
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// - OnRemove: Position: e: {10, 20}
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}
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