Mastering Unidirectional Data Flow with State Holders in Android

Implementing Unidirectional Data Flow (UDF) with State Holders in Android

• State holders are responsible for producing UI state and contain the necessary logic for that task.
• State holders come in different sizes depending on the UI elements they manage, from a single widget to a whole screen or navigation destination.
• The typical implementation of a state holder for a screen or navigation destination is an instance of a ViewModel, although a simple class can be used if requirements are minimal.
• UDF is a pattern where state flows down and events flow up, and it is used to model the codependency between the UI and its state producer.
• The ViewModel holds and exposes the state to be consumed by the UI, while the UI notifies the ViewModel of user events.
• The ViewModel handles user actions and updates the state, which is then fed back to the UI to render.
• The ViewModel works with repositories or use case classes to get data and transform it into the UI state while incorporating the effects of events that may cause mutations of the state.
• Business logic, such as bookmarking an article in the case study app, is usually placed in the domain or data layers, but never in the UI layer.
• UI behavior logic or UI logic, such as obtaining the right text to show on the screen or navigating to a particular screen, should live in the UI and not in the ViewModel.
• UDF allows for data consistency, testability, and maintainability by separating the place where state changes originate, where they are transformed, and where they are consumed.
• The produced state should be exposed in an observable data holder like LiveData or StateFlow so that the UI can react to any changes made in the state without having to manually pull data directly from the ViewModel.
• LiveData and StateFlow also have the benefit of always having the latest version of the UI state cached, which is useful for quick state restoration after configuration changes.