Reactive programming on Android - why and how Sasa Sekulic co-author of the upcoming Manning book “Grokking Rx” (with Fabrizio Chignoli and Ivan Morgillo) developer of the UN-WFP ShareTheMeal app cofounder of Alter Ego Solutions @sasa_sekulic | www.alterego.solutions | Grokking Rx: http://bit.ly/grokking-rx 

UN WFP ShareTheMeal We provide food to schoolchildren in need - over 12 million meals! It costs only 35p to feed one child for a day. www.sharethemeal.org Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

Team #codemobileuk Join the team by donating: http://bit.ly/2puITes Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

Abridged evolution of OOP language evolution 1995 - Java, JavaScript, Delphi 2004 - Java 1.5: generics, futures 2011 - C++ 11: lambdas, futures & promises 2014 - C++ 14: generic lambdas 2014 - Java 1.8: lambdas, streams, CompletableFuture 2015 - EcmaScript 6: futures & promises 2017? - Java 1.9: Flow (with limited operators) MAINSTREAM OOP LANGUAGES: OLD, IMPROVING SLOWLY! Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

Declarative vs. imperative Imperative/procedural/OOP: - we use statements to change program state (data) -> we need to interpret the current state to get to new state Declarative/functional: - we evaluate functions (no previous state, no mutable data) Declarative/dataflow/reactive: - changes are propagated -> marries well with event-based systems (event-driven prog.) Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

What's missing in Java/Android? pt.1 Collections are strange: - why static Collections.sort() and not list.sort()? - why static Arrays.toArray()? Arrays.sort()? OOP much?! - why Iterator.hasNext() and not list.hasNext? - Gang of four Iterator is forward biased! Finally, in Java 8 we have list.forEach()! Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

What's missing in Java/Android? pt.2 Observing changes is non-standard: - Observable/Observer: exist since Java 1.0, but were never used anywhere; rudimentary GoF implementation - no Collection listeners/observers - Android uses listeners everywhere, in a non-standardized manner (add() vs set() vs method parameter) Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

What's missing in Java/Android? pt.3 Multithreading management is very basic: - AsyncTask: must be created/invoked from UI thread; cannot be repeated; must use custom executors for multithreading; no thread-switching (beyond UI/non-UI); no repeating, syncing or daisy-chaining helpers - Executors (pure Java): default Android executor is single- threaded; have to create your own executors, and manage them manually Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

What's missing in Java/Android? pt.4 Multithreading mgmt, cont'd - no easy way to synchronize nested asynchronous calls: - callbacks are not universal - composing is very manual (you have to write everything) - reuse of methods with callbacks is (almost) impossible https://gist.github.com/benjchristensen/4677544 Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

What's missing in Java/Android? pt.5 Multithreading mgmt, cont'd - Future/Promise don't solve the problem: - introduced in Java 1.5, but no support for syncing, grouping, composing; Future.get() is blocking https://gist.github.com/benjchristensen/4671081 - CompletableFuture with grouping and composing only in Java 8, but with very limited features; ignored in JDK; represents only one future value Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

What's missing in Java/Android? - conclusion PREMISE: Mobile devices are asynchronous by default (event-based UI, async communication, event-based sensors) PREMISE: Mobile devices now all have multi-core CPUs PROBLEM: Asynchronous, multithreaded event and data handling on Android is PITA! Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

Why Rx? - Pros - standardized multiplatform library for functional and reactive programming (not FRP! https://github.com/conal/talk-2015-essence-and- origins-of-frp) - universal event interface instead of custom listeners - asynchronous by default; but synchronization is easy - easier to do event-driven programming (event sourcing?) - simplified multithreading management - composable (made for data flows) with lots of operators! Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

Why Rx? - Cons - it's an external library: v1.2.0 rx adds around 5k methods, 2.5k fields - uses more memory - but the data is immutable - complex flows and multithreading are still complex (but easier to control and implement) - lifecycle management - careful around Context references in Android (but that's always valid) - testing and debugging are harder (async multithreading is not easy!) Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

Why Rx? - Benchmarking calls Benchmarking receiving n events with 100ms delay https://github.com/ akarnokd/AgeraTest2 (Samsung S5 Neo, normal usage) Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

Why Rx? - Benchmarking calls Benchmarking receiving n events with 100ms delay https://github.com/ akarnokd/AgeraTest2 (Samsung S5 neo, normal usage) Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

Why Rx? - Benchmarking apps https://github.com/alter-ego/RxJavaAndroidBenchmark/ https://nimbledroid.com/my_apps/solutions.alterego.stack overflow.norx.test https://nimbledroid.com/my_apps/solutions.alterego.stack overflow.test https://nimbledroid.com/my_apps/solutions.alterego.stack overflow.rx2.test App without RxJava: 31.3k methods, 3.66MB, search page loading 1.8s App w/RxJava 1.x: +5.5k (18%), +340kB (9%), page loading -0.3s (-16.6%) App w/RxJava 2.0.0: +9k (29%), +560kB (15%), loading -0.4s (-22%) max RAM use, max CPU, startup time etc = same Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

Alternatives to Rx - non-reactive solutions Combining an event bus library (Otto) with data flow library (Guava) - Guava has Iterable and ListenableFuture, and Otto has universal event management - adds 9k methods, 1k fields and 450kB MORE than Rx1! Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

Alternatives to Rx - non-reactive solutions, cont'd Otto & Guava - bigger than Rx - have to write interfaces between libraries - data flows still synchronous - no threading management - missing advanced operators - Otto is deprecated in favour of RxJava! Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

Alternatives to Rx - other reactive libraries • Project Reactor (http://projectreactor.io/) - looks good, but slower than RxJava and needs Java 8; no Android support http://akarnokd.blogspot.hu/2015/10/comparison -of-reactive-streams.html • Java 9 Flow - just basic implementation of RS 1.0; also Java 9 • Google Agera - slow, buggy, untested, non-standard; supports Android http://akarnokd.blogspot.hu/2016/04/google- agera-vs-reactivex.html Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

Alternatives to Rx - DIY How about standard Java/Android with your own implementation? - if you're an expert reactive/functional developer with lots of time and lots of friends willing to write docs, debug, contribute and test (NIH fallacy) - Java 8 Streams are quick, but they're not reactive! (http://akarnokd.blogspot.hu/2015/10/comparison- of-reactive-streams.html) - also, Java 8! Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

Why Rx? - Costs Distinguish between absolute costs (compared to internal Android libraries and doing everything manually) and relative costs (compared to other libraries) - other libraries are slower and/or less compatible with Android - unless you have substantial time, expertise and resources at your disposal, DO NOT WRITE ANOTHER REACTIVE LIBRARY! - sync is always quicker than async, but it doesn't matter for most cases! Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

Why Rx? - sync vs async Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx http://akarnokd.blogspot.hu/2015/10/comparison-of-reactive-streams.html 

Rx and app/view architecture - indifferent! - marries well with MVP - marries even better with MVVM or MVI • https://developer.android.com/topic/libraries/data- binding/index.html • https://github.com/JakeWharton/RxBinding • http://hannesdorfmann.com/android/model-view- intent • https://github.com/alter-ego/androidbound Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

Why Rx? - SUMMARY - costs are as with any other library - supported by lots of other libraries (Retrofit...) - well tested, well documented, small, efficient - de facto standard for reactive programming in Java/Android RXJAVA HELPS YOU USE ANDROID HARDWARE BETTER - APPS ARE QUICKER (http://www.codemobile.co.uk/speaker/rick-schwartz/) ALSO, REACTIVE PROGRAMMING WILL MAKE YOU THINK DIFFERENTLY! Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx 

Future of Rx Intro to Rx 2.x: https://github.com/ReactiveX/RxJava/wiki/What's- different-in-2.0 https://www.youtube.com/watch?v=htIXKI5gOQU 1.x feature freeze on June 1, 2017; EOL March 31, 2018 2.x final release was on Oct 29, 2016; improving quickly! 3.x preview is here: https://github.com/akarnokd/RxJava3-preview Rx on Android | @sasa_sekulic | #GrokkingRx | http://bit.ly/grokking-rx