多次回调后触发延迟列表

Question

一个^{}是这样做的：

I combine a group of deferreds into one callback.

I track a list of Deferreds for their callbacks, and make a single callback when they have all completed, a list of (success, result) tuples, 'success' being a boolean.

Note that you can still use a Deferred after putting it in a DeferredList. For example, you can suppress 'Unhandled error in Deferred' messages by adding errbacks to the Deferreds after putting them in the DeferredList, as a DeferredList won't swallow the errors. (Although a more convenient way to do this is simply to set the consumeErrors flag)

def __init__(self, deferredList, fireOnOneCallback=0, fireOnOneErrback=0, consumeErrors=0): (source)
    overrides twisted.internet.defer.Deferred.__init__
    Initialize a DeferredList.
    Parameters  deferredList    The list of deferreds to track. (type: list of Deferreds )
    fireOnOneCallback   (keyword param) a flag indicating that only one callback needs to be fired for me to call my callback
    fireOnOneErrback    (keyword param) a flag indicating that only one errback needs to be fired for me to call my errback
    consumeErrors   (keyword param) a flag indicating that any errors raised in the original deferreds should be consumed by this DeferredList. This is useful to prevent spurious warnings being logged.

具体来说：

fireOnOneCallback (keyword param) a flag indicating that only one callback needs to be fired for me to call my callback

我正在寻找像fireOnOneCallback=True这样的行为，但我希望触发n回调。我试过这么做，但已经变得一团糟了。我相信有更好的办法。你知道吗

def _get_fired_index(deferred_list):
    for index, (success, value) in enumerate(deferred_list):
        if success:
            return index
    raise ValueError('No deferreds were fired.')


def _fire_on_other_callback(already_fired_index, deferred_list, callback, ):
    dlist_except_first_fired = (
        deferred_list[:already_fired_index]
        + deferred_list[already_fired_index + 1:]
    )
    dlist2 = DeferredList(dlist_except_first_fired, fireOnOneCallback=True)
    dlist2.addCallback(callback, deferred_list)


def _fire_on_two_callbacks(deferreds, callback, errback):
    dlist1 = DeferredList(deferreds, fireOnOneCallback=True)
    dlist1.addCallback(_get_fired_index)
    dlist1.addCallback(_fire_on_other_callback, deferreds, callback, errback)

Answer 1

这里有一种可能的方法。你知道吗

from __future__ import print_function

import attr
from twisted.internet.defer import Deferred

def fireOnN(n, ds):
    acc = _Accumulator(n)
    for index, d in enumerate(ds):
        d.addCallback(acc.one_result, index)
    return acc.n_results

@attr.s
class _Accumulator(object):
    n = attr.ib()
    so_far = attr.ib(default=attr.Factory(dict))
    done = attr.ib(default=False)
    n_results = attr.ib(default=attr.Factory(Deferred))

    def one_result(self, result, index):
        if self.done:
            return result
        self.so_far[index] = result
        if len(self.so_far) == self.n:
            self.done = True
            so_far = self.so_far
            self.so_far = None
            self.n_results.callback(so_far)

dx = list(Deferred().addCallback(print, i) for i in range(3))
done = fireOnN(2, dx)
done.addCallback(print, "done")

for i, d in enumerate(dx):
    d.callback("result {}".format(i))

注意，这个实现不处理错误，可能还有其他缺点（比如保留n_results引用）。然而，基本思想是正确的：从回调中积累状态，直到达到所需的条件，然后触发另一个延迟状态。你知道吗

DeferredList只会给这个问题带来不必要的复杂性，因为它的无关特性和接口不是为解决这个问题而设计的。你知道吗

Answer 2

下面是另一种使用DeferredSemaphore来处理潜在竞争条件的方法。这将在n延迟者被触发后立即触发，并取消其余的延迟者。你知道吗

from twisted.internet import defer


def fireAfterNthCallback(deferreds, n):
    if not n or n > len(deferreds):
        raise ValueError

    results = {}
    finished_deferred = defer.Deferred()
    sem = defer.DeferredSemaphore(1)

    def wrap_sem(result, index):
        return sem.run(callback_result, result, index)

    def cancel_remaining():
        finished = [deferreds[index] for index in results.keys()]
        for d in finished:
            deferreds.remove(d)
        for d in deferreds:
            d.addErrback(lambda err: err.trap(defer.CancelledError))
            d.cancel()

    def callback_result(result, index):
        results[index] = result
        if len(results) >= n:
            cancel_remaining()
            finished_deferred.callback(results.values())
        return result

    for deferred_index, deferred in enumerate(deferreds):
        deferred.addCallback(wrap_sem, deferred_index)

    return finished_deferred