iOS architecture starts with pressure

Architecture is a response to pressure. A pattern earns its cost when it protects behavior from a change, lifetime, test, integration, or ownership problem the product actually has.

Name the pressure first

Field Notes has several different pressures:

Feature	Change	Criticality	External dependency	Lifetime	Useful boundary
title validation	low	high	none	one command	domain value or use case
note storage	medium	high	database	app lifetime	repository contract
weather context	high	low	network	cancellable request	client contract
editor focus	medium	low	UI framework	one presentation	keep in UI adapter
sync	high	high	server and disk	background and retry	explicit state machine and ports

One architecture label cannot answer all five rows.

Score a proposed boundary

Add a boundary when it buys at least one concrete outcome:

a business rule runs without UI, storage, or network setup
SwiftUI and UIKit reuse the same application behavior
a local adapter replaces a cloud or device dependency
cancellation, transaction, authorization, or retry has one owner
team ownership or build isolation reduces real coordination cost

Keep code direct when a wrapper only renames a framework call, has one trivial implementation, and protects no rule or volatile dependency.

Draw dependency direction

SwiftUI and UIKit adapters
           |
           v
   application use cases
           |
           v
      domain policies
           ^
           |
storage, network, clock, ID, and device adapters

The arrows represent source dependencies. Runtime calls can travel outward through interfaces owned by the application layer.

Revisit decisions with evidence

Record the pressure, chosen boundary, rejected alternatives, cost, and signal that would trigger review. Folder symmetry and protocol count are not architecture outcomes. Change cost, test distance, failure isolation, and ownership clarity are.

Validation boundary

The scorecard is a design tool, not runtime evidence. Its value must be checked against actual Field Notes changes, tests, build times, failures, and team work.

Previous: Part 58: UIKit Field Notes capstone
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Series index: Zero to iOS Hero

References

Swift Package Manager supplies module boundaries when they earn their cost.
SwiftUI model data shows framework-level data flow.
URLSession is one external adapter boundary used by Field Notes.