Unified Architectures in SwiftUI: A Comprehensive Design Strategy for iOS, iPadOS, macOS, and visionOS

1. Introduction: The Convergence of Spatial and Flat Computing

The contemporary landscape of Apple software development is defined by a radical convergence of interaction paradigms. Historically, the demarcation lines between platforms were rigid: iOS was governed by the touch target and the direct manipulation of glass; macOS was the domain of the precise cursor and the indirect abstraction of the mouse; and iPadOS occupied a liminal space, attempting to bridge the two. The introduction of visionOS, however, has fundamentally disrupted this tiered architecture. It has necessitated a new design language that is not merely responsive in terms of screen size, but adaptive in terms of physical dimension, input modality, and user intent.

This report provides an exhaustive analysis of the architectural strategies required to build truly multiplatform applications using SwiftUI. It moves beyond the superficial concept of "cross-platform" code sharing---often synonymous with the "lowest common denominator"