Android Wear 2.0 preview 4 adds sign-in, in-app billing, more
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Android Wear 2.0 developer preview 4 adds a certain number of new APIs that will allow developers to create better standalone apps for smartwatches.
Thanks to the newly added Google Sign-In, authentication should be simpler than ever, as users will only need to select which account they want to authenticate with.
Another important new features added in this build is in-app billing support, which allows users to authorize purchases faster and much easier via a 4-digit Google Account PIN.
Also, there's new fragment and view support, as well as a hardware button that can be used as a “power” button instead of “back,” which means it can no longer be intercepted by apps.
It's also worth mentioning that in this preview build, Google implemented compatibility with Android Wear 1.0 apps.
source: Google
Things that are NOT allowed: