# Phase 6: 页面几何与交互命中层 - Patterns

## Reusable Patterns

### Offset continuity is already the compatibility contract

- `Sources/RDReaderView/EPUBTextRendering/RDEPUBTextBookBuilder.swift` and `Sources/RDReaderView/LegacyRDReaderController/RDEPUBTextPaging.swift` both preserve `pageStartOffset`, `pageEndOffset`, and `fragmentOffsets`.
- `Sources/RDReaderView/EPUBCore/RDEPUBReadingModels.swift` keeps `RDEPUBTextOffsetRangeInfo` as the absolute-offset payload used by selection and highlight consumers.
- Phase 6 should keep that contract intact even after a geometry overlay is introduced.

### The layouter already knows more than the view layer

- `Sources/RDReaderView/EPUBTextRendering/RDEPUBTextLayouter.swift` already determines `blockRange`, `attachmentRanges`, `attachmentKinds`, `breakReason`, and `trailingFragmentID`.
- That makes the layouter the right place to enrich `RDEPUBTextLayoutFrame` with queryable page geometry instead of re-deriving page rules inside `RDEPUBTextContentView`.

### Reader coordination already belongs to the controller

- `Sources/RDReaderView/EPUBUI/RDEPUBReaderController.swift` already owns selection persistence, current-location updates, highlight creation, search state, and repagination.
- Any new geometry-backed selection flow should plug into the controller rather than creating a second reader state owner.

### Content view should become a thin host

- `Sources/RDReaderView/EPUBUI/RDEPUBTextContentView.swift` currently mixes display, selection, highlight painting, and offset conversion.
- For this phase, the safest direction is to let the view host text plus an overlay and keep the selection model conversion in one place.

## Closest Existing Analogs

- `RDEPUBTextLayoutFrame` is the natural home for page geometry metadata.
- `RDEPUBTextBookBuilder` is the natural place to lift geometry results into page-level book state.
- `RDEPUBReaderController` is the natural adapter layer for user interactions and persistence.
- `RDEPUBTextContentView` is the natural host for an overlay layer because it already owns the page view composition.

## Design Guardrails

- Do not move fixed/interactive EPUB off `WKWebView`.
- Do not change `RDReaderView`.
- Do not break the existing offset-based consumers while introducing geometry queries.
- Do not let search/highlight/tap-locate become separate geometry implementations; they should reuse the same page geometry API later.