Added ADR about usage of mappers in the Proton Mail Appilication.

docs/0003-use-mappers-that-extends-core-mapper.md

@@ -0,0 +1,32 @@
+# Use Mappers that extends core Mapper
+
+* Status: Discussed and agreed in principle
+* Deciders: Marino, Zorica, Stefanija, Davide, Tomasz, Maciej
+* Date: 31/08/2021
+
+
+## Context and Problem Statement
+
+Currently in the application we have different approaches for mapping between clean architecture
+layers and models, these approaches vary in terms of complexity and readability. Going forward,
+especially having in mind current conversion to core networking and database models we would like 
+to have common approach to this basic problem.
+
+* We want to have a consistent approach to mappers in the application
+* We want the ability to fully unit test mappers and code related
+
+## Considered Options
+
+* Mapper based on kotlin extension functions e.g. ConversationMapper with ConversationApiModel.toLocal(userId: String)
+* Old school mapper (java style) with functions like .mapResponseToEntity(response: Response)
+* Mapper that extends interface Mapper<in In, out Out>  from core, and implements methods as extension function e.g. fun BusinessModel.toUiModel(): UiModel
+* Mapper that extends interface Mapper<in In, out Out>  from core, with plain function per parameter e.g. fun toUiModel(type: BusinessModel): UiModel
+
+## Decision Outcome
+
+* We choose to rely on a mapper that extends interface Mapper<in In, out Out>  from core, with plain function per parameter
+* We would like to use one Mapper per 2 types, therefore multiple mappers may be required
+
+## Consequences
+1. All Workers mappers in the application should follow the approach suggested above
+