In general, a webhook is a way for an app developer to specify a URI to receive notifications based on some trigger. In the case of Dropbox, notifications get sent to your webhook URI every time a user of your app makes a file change. The payload of the webhook is a list of user IDs who have changes. Your app can then use the standard delta method to see what changed and respond accordingly.
This got me thinking about how I thought that Apple had at one point talked about Web APIs for iCloud. I couldn’t find a reference for that, but I did find the WWDC 2011 keynote where Steve Jobs says (at 96:33):
Documents and Key Value data works across all iOS devices and Macs and PCs, too.
I don’t see any evidence that this shipped, though.
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