Kotlin & RxJava in Android project. One year later. MO 2016

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Kotlin & RxJava in Android project One year later Anton Rutkevich, Juno Igor Korotenko, Juno

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Contents 1. Stack benefits 2. Interop 3. Challenges

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Stack benefits

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What’s in the stack?

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What’s is the stack?

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Core ideas

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Core ideas 1. Safety a. Immutability b. Easy concurrency 2. Concise code 3. Modularity (SOLID-inspired, Clean architecture) 4. Test-ready

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Safety. Immutability

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Why immutable? 1. No state - simpler program a. “Simple made easy” talk by Rich Hickey 2. Mutable state is the core of concurrency issues 3. Immutability allows to perform some optimizations

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Immutability is transitive Reference Data Collection

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Reference immutability

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Objects immutability. Data classes

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Collections immutability

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Rx 1. Implicitly relies on immutable data in streams 2. Hides the state in operators & subjects

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Safety. Easy concurrency

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Concurrency is hard

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Rx approach to concurrency

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Safety. Other

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Safe by default 1. Immutability by default 2. Classes & methods are final by default 3. ...

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Null safety

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Advanced type safety. Why? More checks are guaranteed at compile time

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Type safety. Sum types 1. TimeDiff

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Why sum types? Example Long Long Long if (timeDiff != -1) if (timeDiff != -1) TimeDiff if (timeDiff is TimeDiff.Known) TimeDiff.Known

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Type safety. Sum types 2. Network response

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Concise code

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Smart casts

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Stream-like API

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Easy to understand complex data-flow

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Modularity. Architecture

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High-level view on architecture ViewModel View starts here Retrofit BL home SOL ID Dagger 2 helps here Service A Service B Service D Networking ends here

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Testability

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Main ideas 1. Separation of concerns (clean architecture principles) 2. Dependencies of SUT are interfaces 3. Business logic is isolated from platform code 4. UI layer can be replaced for tests

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Kotlin

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Spek

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Interop

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Koltin interop’s rule of thumb 1. No black magic -> works fine 2. Black magic -> most probably works fine, but need to re-check

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Koltin + Dagger 2 Initially DI code was in Java Now, with kapt, DI can also be in Kotlin

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Koltin + Retrofit Just works :)

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Koltin + Rx Works even better than Java + RxJava 1. Lambdas (non-capturing) 2. Extension functions

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Koltin + Gson Gson uses reflection to set field values -> can set null into non-nullable field. Beware!

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Challenges

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Testing. Koltin + Mockito 1. Everything is final by default in Kotlin a. Solution 1: PowerMock. Dirty b. Solution 2: Clean architecture. Clean 2. Mockito returns nulls where Koltin expects non-nullable a. Use Mockito-Kotlin library or write your own matchers

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Java -> Kotlin interop Beware of nulls that come from the Java’s dark side String!

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Compile time 1. Was kind of a problem: a. Clean ~3 min b. Incremental ~55 sec c. Of which compile Kotlin ~50 sec 2. Much better with Koltin 1.0.2 & dex in process a. Clean ~2m b. Incremental ~30 sec c. Of which compile Kotlin clean ~50 sec, incremental ~20 sec

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Method count 1. Kotlin ~6K 2. RxJava ~5K 3. Multidex works

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Proguard Works, causes ~ same amount of issues as in Java

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Summay

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Core benefits 1. Runtime checks go to compile time 2. Kindly pushes you to write better code 3. Expressive code

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Challenges 1. Testing -> solved 2. Compile time -> not significant 3. Java interop -> be careful