Trickle: Spotify's library for composing asynchronous code in Java

AA is for simple UI related asynchronous calls.

While this lib is for complicated nested asynchronous calls. On Android a good example would be in app purchases. To consume an IAP there's an async call to connect to Google Play, followed by an async call to query inventory, followed by an async call to consume the item. It gets hairy fast!

I'm not very familiar with Android Annotations, but they seem to be primarily a way to shuffle some processing to a background thread. All those methods return `void`, so if that method wants to return some results (or failures) to the caller, it needs to find some other mechanism to do so. It's not impossible, but it's bound to be more inconvenient than a plain old return value.

A common solution is to return what Trickle calls a `ListenableFuture`. The method can do any amount of async processing (on a background thread, or using async I/O, or whatever) and eventually will 'populate' that future with a value. You can test if that value has been set at any time, or add a callback to be called when the future completes / fails.

The problem with that solution comes when you're dealing with multiple APIs, and need the results of some as inputs to others. (This is commonly known as 'callback hell'.) Trickle's solution to this is to make you define the information-flow graph explicitly, and it does all the shuttling-of-intermediate-results around for you. It seems like an improvement, but I much prefer the RxJava / Akka approach to this, since the types are checked and error handling is more explicit and convenient.