8.0 inter app communication documentation

The thing is, using `UIActivityViewController` is *not* the way to build a direct “Share to Tumblr” button since both the extension and all sorts of other things will show up there too, unless you go out of your way to disable them, thus removing the benefit of using `UIActivityViewController` to begin with.

You'd be better off just creating a `UIButton` whose action uses one of our URL schemes.