Android Modular Architecture

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Christopher Alexander “It is the ﬁrst steps in a design process which count for most. The ﬁrst few strokes, creates the form.”

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Challenges

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Challenges • Independence: be able to work in a feature without impacting other developers.

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Challenges • Independence: be able to work in a feature without impacting other developers. • Encapsulation: effective use of access modiﬁers to hide information from different modules.

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Challenges • Independence: be able to work in a feature without impacting other developers. • Encapsulation: effective use of access modiﬁers to hide information from different modules. • Growth: a module should not increase size without bounds.

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Module by Layer In Module by Layer, the highest level modules reﬂect the various application "layers", instead of features.

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:app

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:app :ui

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:app :domain :ui DI injects use case in the presentation.

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:app :domain :ui :data DI injects repository implementation in the domain. DI injects use case in the presentation.

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:app :domain :ui :data DI injects repository implementation in the domain. DI injects use case in the presentation. :app knows about all modules, and need to set up the DI.

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Challenges

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Challenges • Independence: features has its implementation spread out over multiple modules.

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Challenges • Independence: features has its implementation spread out over multiple modules. • Encapsulation: modules contains items that usually aren't closely related to each other.

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Challenges • Independence: features has its implementation spread out over multiple modules. • Encapsulation: modules contains items that usually aren't closely related to each other. • Growth: As an application grows, the number of modules remains the same, while the number of classes in each module will increase without bound.

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This results in modules with low cohesion with high coupling between each other.

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The fundamental ﬂaw with Module by Layer is that it puts implementation details ahead of high level abstractions.

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Module by Feature Module by Feature uses modules to reﬂect the feature set. It tries to place all items related to a single feature (and only that feature) into a single module.

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What is a feature?

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What is a feature? A feature is fully independent, ﬁne-grained and self contained service those messages are delivered over Intent by URLs.

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:app

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:app :feature1

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:app :feature1 :feature2

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:app :feature1 :feature2 :feature3

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:app :feature1 :feature2 :feature3 :feature4

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Challenges

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Challenges • Independence: all items needed for a given feature are in the same module.

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Challenges • Independence: all items needed for a given feature are in the same module. • Encapsulation: allows classes to decrease their scope from public to internal.

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Challenges • Independence: all items needed for a given feature are in the same module. • Encapsulation: allows classes to decrease their scope from public to internal. • Growth: the number of classes within each package remains limited to the items related to a speciﬁc feature.

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This results in modules with high cohesion and low coupling between each other.

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Micro-Module Architecture

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:app

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:app :feature1

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:app :feature2 :feature1

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:app :feature2 :feature1 :component1

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:app :feature2 :common :feature1 :component1 :design :testing

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:app :feature2 :common :feature3 :feature1 :component1 :design :testing

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:app :feature2 :component2 :common :feature3 :feature1 :component1 :design :testing

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Module Structure

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:module

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:module ui

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:module ui activity fragment viewmodel

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:module domain ui activity fragment viewmodel

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:module domain ui model usecase   repository activity fragment viewmodel

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:module domain data ui model usecase   repository activity fragment viewmodel

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:module domain data ui model usecase   repository activity fragment viewmodel repository

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:module domain data ui model usecase   repository activity fragment viewmodel store repository local remote

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Thank you.