Struct MessageReader 

pub struct MessageReader<'w, 's, M>
where
    M: Message,
{ /* private fields */ }

Reads Messages of type T in order and tracks which messages have already been read.

Use PopulatedMessageReader<T> to skip the system if there are no messages.

Usage

#[derive(Message)]
pub struct MyMessage(String); // Custom message type.
fn my_system(mut reader: MessageReader<MyMessage>) {
    for msg in reader.read() {
        println!("{}", msg.0)
    }
}

Concurrency

Unlike MessageWriter<T>, systems with MessageReader<T> param can be executed concurrently (but not concurrently with MessageWriter<T> or MessageMutator<T> systems for the same message type).

Implementations

impl<'w, 's, M> MessageReader<'w, 's, M>

where M: Message,

pub fn read(&mut self) -> MessageIterator<'_, M>

Iterates over the messages this MessageReader has not seen yet. This updates the MessageReader’s message counter, which means subsequent message reads will not include messages that happened before now.

pub fn read_with_id(&mut self) -> MessageIteratorWithId<'_, M>

Like read, except also returning the MessageId of the messages.

pub fn par_read(&mut self) -> MessageParIter<'_, M>

Returns a parallel iterator over the messages this MessageReader has not seen yet. See also for_each.

Example

#[derive(Message)]
struct MyMessage {
    value: usize,
}

#[derive(Resource, Default)]
struct Counter(AtomicUsize);

// setup
let mut world = World::new();
world.init_resource::<Messages<MyMessage>>();
world.insert_resource(Counter::default());

let mut schedule = Schedule::default();
schedule.add_systems(|mut messages: MessageReader<MyMessage>, counter: Res<Counter>| {
    messages.par_read().for_each(|MyMessage { value }| {
        counter.0.fetch_add(*value, Ordering::Relaxed);
    });
});
for value in 0..100 {
    world.write_message(MyMessage { value });
}
schedule.run(&mut world);
let Counter(counter) = world.remove_resource::<Counter>().unwrap();
// all messages were processed
assert_eq!(counter.into_inner(), 4950);

pub fn len(&self) -> usize

Determines the number of messages available to be read from this MessageReader without consuming any.

pub fn is_empty(&self) -> bool

Returns true if there are no messages available to read.

Example

The following example shows a useful pattern where some behavior is triggered if new messages are available. MessageReader::clear() is used so the same messages don’t re-trigger the behavior the next time the system runs.

#[derive(Message)]
struct Collision;

fn play_collision_sound(mut messages: MessageReader<Collision>) {
    if !messages.is_empty() {
        messages.clear();
        // Play a sound
    }
}

pub fn clear(&mut self)

Consumes all available messages.

This means these messages will not appear in calls to MessageReader::read() or MessageReader::read_with_id() and MessageReader::is_empty() will return true.

For usage, see MessageReader::is_empty().

Trait Implementations

impl<'w, 's, M> Debug for MessageReader<'w, 's, M>

where M: Debug + Message,

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl<M> SystemParam for MessageReader<'_, '_, M>

where M: Message,

type State = FetchState<M>

Used to store data which persists across invocations of a system.

type Item<'w, 's> = MessageReader<'w, 's, M>

The item type returned when constructing this system param. The value of this associated type should be Self, instantiated with new lifetimes.

fn init_state( world: &mut World, ) -> <MessageReader<'_, '_, M> as SystemParam>::State

Creates a new instance of this param’s State.

fn init_access( state: &<MessageReader<'_, '_, M> as SystemParam>::State, system_meta: &mut SystemMeta, component_access_set: &mut FilteredAccessSet, world: &mut World, )

Registers any World access used by this [SystemParam].

fn apply( state: &mut <MessageReader<'_, '_, M> as SystemParam>::State, system_meta: &SystemMeta, world: &mut World, )

Applies any deferred mutations stored in this [SystemParam]’s state. This is used to apply Commands during ApplyDeferred.

fn queue( state: &mut <MessageReader<'_, '_, M> as SystemParam>::State, system_meta: &SystemMeta, world: DeferredWorld<'_>, )

Queues any deferred mutations to be applied at the next ApplyDeferred.

unsafe fn get_param<'w, 's>( state: &'s mut <MessageReader<'_, '_, M> as SystemParam>::State, system_meta: &SystemMeta, world: UnsafeWorldCell<'w>, change_tick: Tick, ) -> Result<<MessageReader<'_, '_, M> as SystemParam>::Item<'w, 's>, SystemParamValidationError>

Creates a parameter to be passed into a SystemParamFunction.

impl<'w, 's, M> ReadOnlySystemParam for MessageReader<'w, 's, M>

where M: Message, Local<'s, MessageCursor<M>>: ReadOnlySystemParam, Res<'w, Messages<M>>: ReadOnlySystemParam,