Hello,
One of the reasons developers have been avoiding pushPrefEnv compared to
the synchronous set*Pref (with a registerCleanupFunction) is because
pushPrefEnv required using a callback function to wait for the
preference change before moving on in the test file. This can make the
test flow more complicated (especially when using add_task) and
therefore harder to follow.
Bug 1197310[1] made pushPrefEnv/popPrefEnv/flushPrefEnv return a promise
which resolves when the callbacks would have been called so now you can
simply write test code like so:
add_task(function* setup() {
yield SpecialPowers.pushPrefEnv({"set": [["signon.debug", true]]});
…
})
As a reminder, the nice thing about pushPrefEnv is that the pref changes
are reverted at the end of the test file which avoids them leaking into
other tests unintentionally.
There are various places in the tree which wrote their own Promise
wrappers for pushPrefEnv so feel free to file follow-up bugs blocking
bug 1197310 to remove them.
Cheers,
Matthew N. (:MattN)
[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1197310
P.S. For those of you who didn't hear (since there was no announcement),
you can use add_task in mochitest-plain and mochitest-chrome thanks to
bug 1187701 if you load …/tests/SimpleTest/SpawnTask.js.
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