When you send a message to a DataQueue object, the message waits in the queue until it is processed by a listener. Each message adds 1 to the queue length. In this example, you use the QueueLength property to find the length of a DataQueue object.

When a MATLAB process creates a DataQueue object, any messages that are sent to the queue are held in the memory of that process. Therefore, the QueueLength property on all other processes is 0. In this example, you create a DataQueue object on the client, and send data from a worker.

First, create a parallel pool with one worker.

parpool(1);

Starting parallel pool (parpool) using the 'local' profile ...
Connected to the parallel pool (number of workers: 1).

Then, create a DataQueue.

q = parallel.pool.DataQueue

q = 
  DataQueue with properties:

    QueueLength: 0

A newly created DataQueue has an empty queue. You can use parfor to find q.QueueLength on the worker. Find the queue length on the client, and the queue length on the worker.

fprintf('On the client: %i\n', q.QueueLength)

On the client: 0

parfor i = 1
    fprintf('On the worker: %i\n', q.QueueLength)
end

On the worker: 0

As the queue is empty, the QueueLength is 0 for both the client and the worker. Next, send a message to the queue from the worker. Then, use the QueueLength property to find the length of the queue.

% Send a message first
parfor i = 1
    send(q, 'A message');
end

% Find the length
fprintf('On the client: %i\n', q.QueueLength)

On the client: 1

parfor i = 1
    fprintf('On the worker: %i\n', q.QueueLength)
end

On the worker: 0

The QueueLength property is 1 on the client, and 0 on the worker. Create a listener to process the queue by immediately displaying the data.

el = afterEach(q, @disp);

Wait until the queue is empty, then delete the listener.

while q.QueueLength > 0
    pause(0.1);
end
delete(el);

Use the QueueLength property to find the length of the queue.

fprintf('On the client: %i\n', q.QueueLength)

On the client: 0

QueueLength is 0 because the queue processing is complete.

Construct a DataQueue, and call afterEach.

q = parallel.pool.DataQueue;
afterEach(q, @disp);

parfor i = 1:3
    send(q, i); 
end;

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Create a DataQueue, and use afterEach to specify the function to execute each time the queue receives data. This example calls a subfunction that updates the wait bar.

Create a parfor-loop to carry out a computationally demanding task in MATLAB^®. Use send to send some dummy data on each iteration of the parfor-loop. When the queue receives the data, afterEach calls nUpdateWaitbar in the client MATLAB, and you can observe the wait bar progress.

function a = parforWaitbar

D = parallel.pool.DataQueue;
h = waitbar(0, 'Please wait ...');
afterEach(D, @nUpdateWaitbar);

N = 200;
p = 1;

parfor i = 1:N
    a(i) = max(abs(eig(rand(400))));
    send(D, i);
end

    function nUpdateWaitbar(~)
        waitbar(p/N, h);
        p = p + 1;
    end
end

If you call afterEach and there are items on the queue waiting to be dispatched, these items are immediately dispatched to the function handle specified by afterEach. Call afterEach before sending data to the queue, to ensure that on send, the function handle @disp is called.

q = parallel.pool.DataQueue;
afterEach(q, @disp);

parfor i = 1:3
    send(q, i); 
end
send(q, 0);

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q = parallel.pool.DataQueue;
parfor i = 1:3
    send(q, i); 
end
afterEach(q, @disp);

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