Convert a function that uses callbacks to an iterator.
Let *func* be a function that takes a `callback` keyword argument.
For example:
>>> def func(callback=None):
... for i, c in [(1, 'a'), (2, 'b'), (3, 'c')]:
... if callback:
... callback(i, c)
... return 4
Use ``with callback_iter(func)`` to get an iterator over the parameters
that are delivered to the callback.
>>> with callback_iter(func) as it:
... for args, kwargs in it:
... print(args)
(1, 'a')
(2, 'b')
(3, 'c')
The function will be called in a background thread. The ``done`` property
indicates whether it has completed execution.
>>> it.done
True
If it completes successfully, its return value will be available
in the ``result`` property.
>>> it.result
4
Notes:
* If the function uses some keyword argument besides ``callback``, supply
*callback_kwd*.
* If it finished executing, but raised an exception, accessing the
``result`` property will raise the same exception.
* If it hasn't finished executing, accessing the ``result``
property from within the ``with`` block will raise ``RuntimeError``.
* If it hasn't finished executing, accessing the ``result`` property from
outside the ``with`` block will raise a
``more_itertools.AbortThread`` exception.
* Provide *wait_seconds* to adjust how frequently the it is polled for
output.