Struct wayland_client::EventQueue

pub struct EventQueue { /* fields omitted */ }

An event queue for protocol messages

Event dispatching in wayland is made on a queue basis, allowing you to organize your objects into different queues that can be dispatched independently, for example from different threads.

And EventQueue is not Send, and thus must stay on the thread on which they were created. However the Display object is Send + Sync, allowing you to create the queues directly in the threads that host them.

When a queue is dispatched (via the dispatch() or dispatch_pending() methods) all the incoming messages from the server designated to objects associated with the queue are processed sequentially, and the appropriate implementation for each is invoked. When all messages have been processed these methods return.

There are two main ways to driving an event queue forward. The first way is the simplest and generally sufficient for single-threaded apps that only process events from wayland. It consists of using the EventQueue::dispatch() method, which will take care of sending pending requests to the server, block until some events are available, read them, and call the associated handlers:

loop {
    // The dispatch() method returns once it has received some events to dispatch
    // and have emptied the wayland socket from its pending messages, so it needs
    // to be called in a loop. If this method returns an error, your connection to
    // the wayland server is very likely dead. See its documentation for more details.
    event_queue.dispatch().expect("An error occurred during event dispatching!");
}

The second way is more appropriate for apps that are either multithreaded (and need to process wayland events from different threads conccurently) or need to react to events from different sources and can't affort to just block on the wayland socket. It centers around three methods: Display::flush(), EventQueue::read_events() and EventQueue::dispatch_pending():

loop {
    // The first method, called on the Display, is flush(). It writes all pending
    // requests to the socket. Calling it ensures that the server will indeed
    // receive your requests (so it can react to them).
    if let Err(e) = display.flush() {
        if e.kind() != ::std::io::ErrorKind::WouldBlock {
            // if you are sending a realy large number of request, it might fill
            // the internal buffers of the socket, in which case you should just
            // retry flushing later. Other errors are a problem though.
            eprintln!("Error while trying to flush the wayland socket: {:?}", e);
        }
    }

    // The second method will try to read events from the socket. It is done in two
    // steps, first the read is prepared, and then it is actually executed. This allows
    // lower contention when different threads are trying to trigger a read of events
    // concurently
    if let Some(guard) = event_queue.prepare_read() {
        // prepare_read() returns None if there are already events pending in this
        // event queue, in which case there is no need to try to read from the socket
        if let Err(e) = guard.read_events() {
            if e.kind() != ::std::io::ErrorKind::WouldBlock {
                // if read_events() returns Err(WouldBlock), this just means that no new
                // messages are available to be read
                eprintln!("Error while trying to read from the wayland socket: {:?}", e);
            }
        }
    }

    // Then, once events have been read from the socket and stored in the internal
    // queues, they need to be dispatched to their handler. Note that while flush()
    // and read_events() are global and will affect the whole connection, this last
    // method will only affect the event queue it is being called on. This method
    // cannot error unless there is a bug in the server or a previous read of events
    // already errored.
    event_queue.dispatch_pending().expect("Failed to dispatch all messages.");

    // Note that none of these methods are blocking, as such they should not be used
    // as a loop as-is if there are no other sources of events your program is waiting on.

    // The wayland socket can also be integrated in a poll-like mechanism, using
    // either the integration with calloop provided by the "eventloop" cargo feature,
    // or the get_connection_fd() method.
}

Methods

impl EventQueue

pub fn dispatch(&mut self) -> Result<u32>

Dispatches events from the internal buffer.

Dispatches all events to their appropriators. If no events were in the internal buffer, will block until some events are read and dispatch them. This process can insert events in the internal buffers of other event queues.

If an error is returned, your connection with the wayland compositor is probably lost. You may want to check Display::protocol_error() to see if it was caused by a protocol error.

pub fn dispatch_pending(&mut self) -> Result<u32>

Dispatches pending events from the internal buffer.

Dispatches all events to their appropriators. Never blocks, if no events were pending, simply returns Ok(0).

If an error is returned, your connection with the wayland compositor is probably lost. You may want to check Display::protocol_error() to see if it was caused by a protocol error.

pub fn sync_roundtrip(&mut self) -> Result<u32>

Synchronous roundtrip

This call will cause a synchronous roundtrip with the wayland server. It will block until all pending requests of this queue are sent to the server and it has processed all of them and send the appropriate events.

Handlers are called as a consequence.

On success returns the number of dispatched events. If an error is returned, your connection with the wayland compositor is probably lost. You may want to check Display::protocol_error() to see if it was caused by a protocol error.

pub fn get_token(&self) -> QueueToken

Create a new token associated with this event queue

See QueueToken documentation for its use.

pub fn prepare_read(&self) -> Option<ReadEventsGuard>

Prepare an concurrent read

Will declare your intention to read events from the server socket.

Will return None if there are still some events awaiting dispatch on this EventIterator. In this case, you need to call dispatch_pending() before calling this method again.

As long as the returned guard is in scope, no events can be dispatched to any event iterator.

The guard can then be destroyed by two means:

• Calling its cancel() method (or letting it go out of scope): the read intention will be cancelled
• Calling its read_events() method: will block until all existing guards are destroyed by one of these methods, then events will be read and all blocked read_events() calls will return.

This call will otherwise not block on the server socket if it is empty, and return an io error WouldBlock in such cases.

pub fn get_connection_fd(&self) -> RawFd

Retrieve the file descriptor associated with the wayland socket

This FD should only be used to integrate into a polling mechanism, and should never be directly read from or written to.