Summary:
What it says on the tin. This allows Nodes to effectively participate in the responder chain.
Caveat: see note in implementation below. Would have been nice to use `-targetForAction:withSender:` the way the docs imply you can, but alas.
Test Plan: Observe a node in the responder chain get invoked to handle an action it implements.
Reviewers: bcunning, zsh, sma, jpasqualini, suv, nyn531, b3ll, aaronpang, kimon, grp, jonathan, rnystrom, nadi
Reviewed By: nadi
Subscribers: trunkagent, rnystrom
Differential Revision: https://phabricator.fb.com/D1894023
Signature: t1:1894023:1426026643:203945b6a7c318f6d2c9b94d876da61da31327bd
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