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Use afterEach and afterAll to Run Callback Functions

You create a Future when you run functions in the background or on a parallel pool using parfeval, parfevalOnAll, afterEach, or afterAll. You can use afterEach and afterAll to automatically run a callback function after one or more Future objects finish.

  • If you use afterEach, MATLAB® runs the callback function after each Future object finishes. If the Future array has M elements, the MATLAB client runs the callback function M times.

  • If you use afterEach, MATLAB runs the callback function after all Future object finish. If the Future array has M elements, the MATLAB client runs the callback function only runs once.

Call afterEach on parfeval Computations

You can use afterEach to automatically invoke functions on each of the results of parfeval computations.

Use parfeval to compute random vectors in the workers. With default preferences, parfeval creates a parpool automatically if there is not one already created.

for idx = 1:10
    f(idx) = parfeval(@rand, 1, 1000, 1);
end

Display the maximum element in each of those vectors after they are created. afterEach executes the function handle on the output of each future when they become ready.

afterEach(f, @(r) disp(max(r)), 0);
    0.9975

    0.9990

    0.9982

    0.9991

    0.9982

    0.9998

    0.9999

    0.9986

    0.9996

    0.9990

Call afterAll on parfeval Computations

You can use afterAll to automatically invoke functions on all of the combined outputs of your parfeval computations.

Use parfeval to compute random vectors in the workers. With default preferences, parfeval creates a parpool automatically if there is not one already created.

for idx = 1:10
    f(idx) = parfeval(@rand, 1, 1000, 1);
end

Display the maximum element among all of those vectors after they are created. afterAll executes the function handle on the combined output of all the futures when they all become ready.

afterAll(f, @(r) disp(max(r)), 0);
    0.9998

Combine afterEach and afterAll

You can combine afterEach and afterAll to automatically invoke more functions on the results of futures. Both afterEach and afterAll generate future variables that can be used again in afterEach and afterAll.

Use parfeval to compute random vectors in the workers. With default preferences, parfeval creates a parpool automatically if there is not one already created.

for idx= 1:10
    f(idx) = parfeval(@rand, 1, 1000, 1);
end
Starting parallel pool (parpool) using the 'local' profile ...
connected to 8 workers.

Compute the largest element in each of those vectors when they become ready. afterEach executes the function handle on the output of each future when they become ready and creates another future to hold the results.

maxFuture = afterEach(f, @(r) max(r), 1);

To compute the minimum value among them, call afterAll on this new future. afterAll executes a function on the combined output arguments of all the futures after they all complete. In this case, afterAll executes the function min on the outputs of maxFuture after completing and creates another future to hold the result.

minFuture = afterAll(maxFuture, @(r) min(r), 1);

You can fetch the result using fetchOutputs. fetchOutput waits until the future completes to gather the results.

fetchOutputs(minFuture)
ans = 0.9973

You can check the result of afterEach by calling fetchOutputs on its future variable.

fetchOutputs(maxFuture)
ans = 10×1

    0.9996
    0.9989
    0.9994
    0.9973
    1.0000
    1.0000
    0.9989
    0.9994
    0.9998
    0.9999

Update User Interface Asynchronously Using afterEach and afterAll

This example shows how to update a user interface as computations complete. When you offload computations to workers using parfeval, all user interfaces are responsive while workers perform these computations. In this example, you use waitbar to create a simple user interface.

  • Use afterEach to update the user interface after each computation completes.

  • Use afterAll to update the user interface after all the computations complete.

Use waitbar to create a figure handle, h. When you use afterEach or afterAll, the waitbar function updates the figure handle. For more information about handle objects, see Handle Object Behavior.

h = waitbar(0,'Waiting...');

Use parfeval to calculate the real part of the eigenvalues of random matrices. With default preferences, parfeval creates a parallel pool automatically if one is not already created.

for idx = 1:100
    f(idx) = parfeval(@(n) real(eig(randn(n))),1,5e2); 
end

You can use afterEach to automatically invoke functions on each of the results of parfeval computations. Use afterEach to compute the largest value in each of the output arrays after each future completes.

maxFuture = afterEach(f,@max,1);

You can use the State property to obtain the status of futures. Create a logical array where the State property of the futures in f is "finished". Use mean to calculate the fraction of finished futures. Then, create an anonymous function updateWaitbar. The function changes the fractional wait bar length of h to the fraction of finished futures.

updateWaitbar = @(~) waitbar(mean({f.State} == "finished"),h);

Use afterEach and updateWaitbar to update the fractional wait bar length after each future in maxFuture completes. Use afterAll and delete to close the wait bar after all the computations are complete.

updateWaitbarFutures = afterEach(f,updateWaitbar,0);
afterAll(updateWaitbarFutures,@(~) delete(h),0);

Use afterAll and histogram to show a histogram of the results in maxFuture after all the futures complete.

showsHistogramFuture = afterAll(maxFuture,@histogram,0);

Handle Errors in Future Variables

When computations for future variables result in an error, by default, afterEach does not evaluate its function on the elements that failed. If you want to handle any errors, for example, you have a user interface that you want to update, you can use the name-value pair PassFuture. When set to true, the future variable is passed to the callback function. You can call fetchOutputs on it, process the outputs, and handle any possible errors.

Send computations to the workers using parfeval. With default preferences, parfeval creates a parpool automatically if there is not one already created. If your parfeval computations result in an error, the future variable errors, and its Error property reflects it.

errorFuture = parfeval(@(n) randn(n), 0, 0.5);
wait(errorFuture);
errorFuture.Error
ans = 
  ParallelException with properties:

     identifier: 'MATLAB:NonIntegerInput'
        message: 'Size inputs must be integers.'
          cause: {}
    remotecause: {[1×1 MException]}
          stack: [1×1 struct]

If you use afterEach on that future, the callback function is not evaluated on those elements in the future that errored. In the code below, the msgbox is not executed because the future errors.

afterEach(errorFuture, @() msgbox('Operation completed'), 0);

To handle futures that result in errors, use the name-value pair PassFuture when calling afterEach. The future variable is passed to the callback function instead of its outputs. Call fetchOutputs on the future variable, and process its outputs. If the future results in an error, fetchOutputs throws an error that you can catch and handle. The following code shows an error dialog box.

afterEach(errorFuture, @handleError, 0, 'PassFuture', true);

function handleError(f)
    try
        output = fetchOutputs(f);
        % Do something with the output
    catch
        errordlg('Operation failed');
    end
end