Observers may match terms on multiple different sources. However, when an observer matches components both on the`$this` source (default) and on a fixed source, the fixed source terms will not match events. The reason for this is that otherwise emitting an event for a fixed source term would mean iterating all matching entities for the`$this` term. If an observer only has fixed source terms, events will be matched for each of the terms.

Singletons are a special case of fixed source term, where the component is matched on itself. An example:

<divclass="flecs-snippet-tabs">

<ul>

<li><bclass="tab-title">C</b>

ecs_singleton_set(world, TimeOfDay, {0});

// Observer with singleton source

ecs_observer(world, {

.query.terms = {

{ ecs_id(TimeOfDay), .src.id = ecs_id(TimeOfDay) }

},

.events = { EcsOnSet },

.callback = MyObserver

});

// Triggers observer

ecs_singleton_set(world, TimeOfDay, {1});

// Does not trigger observer

ecs_entity_t e = ecs_insert(world, ecs_value(TimeOfDay, {0}));

</li>

<li><bclass="tab-title">C#</b>

world.Set(newTimeOfDay(0));

world.Observer<TimeOfDay>()

.TermAt(0).Singleton()

.Event(Ecs.OnSet)

.Each((Iterit,inti,refTimeOfDayt)=>

{

});

world.Set(newTimeOfDay(1));

Entitye=world.Entity().Set(newTimeOfDay(0));

</li>

<li><bclass="tab-title">Rust</b>

world.set(TimeOfDay {value:0.0 });

.observer::<flecs::OnSet,&TimeOfDay>()

.term_at(0)

.singleton()

.each_iter(|it,i,time| {

});

world.set(TimeOfDay {value:1.0 });

lete=world.entity().set(TimeOfDay {value:0.0 });

</li>

</ul>

</div>

When an observer has a query that uses (up) relationship traversal, events are propagated along the relationship edge. For example, when an observer requests component`Position` from a parent entity, setting`Position` on the parent will propagate an`OnSet` event along the`ChildOf` edge, notifying all child entities of the parent.

}

ecs_add_id(world, Game, EcsSingleton);

// System declaration using the ECS_SYSTEM macro

ECS_SYSTEM(world, PrintTime, EcsOnUpdate, Game($));

ECS_SYSTEM(world, PrintTime, EcsOnUpdate, Game);

// System declaration using the descriptor API

ecs_system(world, {

.add = ecs_ids( ecs_dependson(EcsOnUpdate) )

}),

.query.terms = {

{ .id = ecs_id(Game), .src.id = ecs_id(Game) } // Singleton source

{ .id = ecs_id(Game) }

},

.callback = PrintTime

});

<li><bclass="tab-title">C#</b>

world.Component<Game>().Entity.Add(flecs::Singleton);

world.System<Game>("PrintTime")

.TermAt(0).Singleton()

.Kind(Ecs.OnUpdate)

.Each((refGameg)=>

{

<li><bclass="tab-title">Rust</b>

world.component::<Game>().add_trait::<flecs::Singleton>();

.system_named::<&Game>("PrintTime")

.term_at(0)

.singleton()

.kind::<flecs::pipeline::OnUpdate>()

.run_iter(|it,game| {

println!("Time: {}",game[0].time);