Function on_message 

pub fn on_message<M>(reader: MessageReader<'_, '_, M>) -> bool
where
    M: Message,

A SystemCondition-satisfying system that returns true if there are any new messages of the given type since it was last called.

Example

app.add_systems(
    my_system.run_if(on_message::<MyMessage>),
);

#[derive(Message)]
struct MyMessage;

fn my_system(mut counter: ResMut<Counter>) {
    counter.0 += 1;
}

// No new `MyMessage` messages have been push so `my_system` won't run
app.run(&mut world);
assert_eq!(world.resource::<Counter>().0, 0);

world.resource_mut::<Messages<MyMessage>>().write(MyMessage);

// A `MyMessage` message has been pushed so `my_system` will run
app.run(&mut world);
assert_eq!(world.resource::<Counter>().0, 1);