add active() and count() methods to po.queue

[straup]

Prettymaps uses these to display a handy dialog indicating the number of tiles left to load.

[shawnbot]

+1. I went one step further and added "process" events for queuing in an experimental version of polymaps a while back, which makes it trivial to add a progress spinner to any page with polymaps on it. Does this seem worth adding?

po.queue = (function() {
  var queued = [], active = 0, size = 6;
  var queue = {};
  queue.dispatch = po.dispatch(queue);

  function process() {
    if ((active >= size) || !queued.length) {
    } else {
      active++;
      queued.pop()();
    }
    queue.dispatch({type: "process", active: active, queued: queued.length});
  }

  function dequeue(send) {
    for (var i = 0; i < queued.length; i++) {
      if (queued[i] == send) {
        queued.splice(i, 1);
        queue.dispatch({type: "process", active: active, queued: queued.length});
        return true;
      }
    }
    return false;
  }
  ...

[mbostock]

Do you need both methods? I wonder if it'd be simpler to have a size() method that returns queued.length + active.

[straup]

Strictly speaking, both methods aren't necessary but I there may be cases where people don't care what the size of the queue is only that it's being processed. I needed the size of the queue do display some amount of feedback and "active" is/was already being passed around to maintain state so I just decided to check that first.