Question

I am working on a Javascript simulator that runs in a web browser. It has a main loop:

do {
    updateVisualization(simulator);
    simulator.doStep();
} while (!reachedGoal(simulator));

And for every iteration, I need to run several workers that must be concurrently executed:

doStep = function() {
    ...
    for (every agent in the simulation) {
        var worker = new Worker('worker.js');
        worker.onmessage = function(event) {
            ...
        }
        worker.postMessage(...);
    }

    // Here is the problem
}

My question is: how could I wait for every worker to finish?

Answer 1

A suggestion:

doStep = function () {
    var i, worker;

    updateVisualization( simulator );    

    simulator.workers = []; // array of workers for this step

    for ( i = 0; i < agents.length; i++ ) {
        // set up new worker
        worker = new Worker( 'worker.js' );
        worker.onmessage = function ( e ) {
            var i;

            if ( e.data === 'finished' ) {
                this.finished = true;

                for ( i = 0; i < simulator.workers.length; i++ ) {
                    if ( !simulator.workers[i].finished ) {
                        return;
                    }
                }

                // survived for-loop = all workers finished
                if ( !reachedGoal( simulator ) ) { // another iteration?
                    simulator.doStep();    
                }
            }
        };
        worker.postMessage( 'doStep' );

        simulator.workers.push( worker ); // push worker into workers array
    }
};

So all the action happens in the onmessage callback of the workers. Every time a worker responds with a message, you inspect the simulator.workers array by checking if all workers have a finished property set to true. If that is the case, this means that all workers finished and you can move on (the “survived for-loop” part).

So basically, you instantiate all workers and then just wait for their responses… no loop required.