Most modern mobile applications need to be available on iOS and Android. Frequent code sharing is desirable. While dedicated cross- and multiplatform development tools achieve this goal, there are also situations where more flexibility is necessary. If code for one platform already exists or if, for example, the UI needs to be highly platform-optimized, a high volume of shared business logic could be one of the main implementation goals. The Hexagonal Architecture is a global architecture, providing an extendable approach suitable for small and large projects and helping to achieve more independence from frameworks and external agencies and better testability. This paper discusses the use of the Hexagonal Architecture to simplify business logic sharing.

The size and complexity of mobile apps has been increasing in recent years resulting in a rising need for formal specification similar to traditional software projects with languages like UML. However, mobile apps are driven more strongly by user interface needs and user interface design plays a more prominent role in the software engineering process in comparison to traditional software projects. User Interface design on the other hand, often lacks techniques for formal specification. In this paper, UML statecharts are proposed for the preceding process of user interface specification as part of the app interaction design. In addition, a technique for specifying single screens of a mobile application is presented.

Complex mobile applications require an appropriate global architecture. If used correctly, the high-level design patterns promoted by Apple and Google such as MVC, MVVM, and MVI/MVU may make an important contribution to the architecture, but they often require supplementary architectural concepts. General architectures such as Clean Architecture may come to the rescue but leave room for interpretation as to how they work best on iOS and Android. This paper gives an overview of the current situation and choices and a vision of using a mix-and-match approach with the following goals: provide an extendable approach suitable for small and large projects, use minimal integration into the platform, and sidestep traps and erroneous implementations.