Feat/injectable backend host - #258
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packages/backend previously assumed a live Figma plugin sandbox: the figma.mixed sentinel and figma.getNodeByIdAsync()/exportAsync() were referenced directly throughout the conversion path. That makes the package unusable anywhere without a running plugin, including a server converting already-fetched REST API JSON. setBackendHost() lets a caller supply mixed/getNodeExport/ getVariableName; when unset, getBackendHost() falls back to wrapping the real figma global, so the existing plugin app is unaffected.
getBackendHost().mixed is plain symbol (BackendHost.mixed avoids requiring @figma/plugin-typings for third-party host authors), but callers rely on TypeScript narrowing T | typeof figma.mixed unions after comparing against it. A plain symbol return type broke that narrowing wherever the branch result was used arithmetically afterward.
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Included review availability: Your plan includes up to 8 reviews per rolling hour; 5 remain after this review. 📝 WalkthroughWalkthroughThe backend adds a configurable host abstraction for exports, variable-name lookup, and mixed-value detection. Conversion modules now use host-provided operations instead of direct global Figma APIs. ChangesBackend host abstraction
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to The PR adds an injectable backend host while preserving the existing plugin default behavior, and no actionable merge-blocking risk remains beyond normal checks and review. Sequence Diagram(s)sequenceDiagram
participant exportAsyncProxy
participant getBackendHost
participant BackendHost
participant FigmaAPI
exportAsyncProxy->>getBackendHost: resolve backend host
getBackendHost-->>exportAsyncProxy: return BackendHost
exportAsyncProxy->>BackendHost: getNodeExport(node.id, settings)
BackendHost->>FigmaAPI: getNodeByIdAsync and exportAsync
FigmaAPI-->>BackendHost: return exported bytes
BackendHost-->>exportAsyncProxy: return export result
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Inline comments:
In `@packages/backend/src/host.ts`:
- Around line 9-20: Replace the open-ended ExportRequest interface with a closed
discriminated-union export contract based on the maintained Figma export types,
covering valid formats, constraint types, required format-specific options, and
the SVG_STRING return distinction; update defaultHost() to pass the validated
request without casting to ExportSettings.
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📒 Files selected for processing (15)
packages/backend/src/common/commonRadius.tspackages/backend/src/common/commonStroke.tspackages/backend/src/common/exportAsyncProxy.tspackages/backend/src/compose/composeMain.tspackages/backend/src/compose/composeTextBuilder.tspackages/backend/src/flutter/flutterContainer.tspackages/backend/src/host.tspackages/backend/src/html/builderImpl/htmlColor.tspackages/backend/src/html/htmlDefaultBuilder.tspackages/backend/src/html/htmlTextBuilder.tspackages/backend/src/index.tspackages/backend/src/swiftui/builderImpl/swiftuiTextWeight.tspackages/backend/src/swiftui/swiftuiMain.tspackages/backend/src/tailwind/conversionTables.tspackages/backend/src/tailwind/tailwindTextBuilder.ts
Included review availability: Your plan includes up to 8 reviews per rolling hour; 7 remain after this review.
Satisfies the PR's docstring-coverage check: every exported symbol in host.ts now has JSDoc, exportAsyncProxy's existing comment is converted to JSDoc format, and the touched variableToColorName gets one too.
ExportRequest was a loose custom interface (format?: string,
constraint?: {type: string, value: number}) accepting any string,
including invalid formats or {}. defaultHost() then cast that value
to ExportSettings to call exportAsync(), bypassing the type checker
entirely.
Aliasing ExportRequest to ExportSettings | ExportSettingsSVGString
(the actual overloaded exportAsync() parameter type) removes the
cast: narrowing on settings.format now statically selects the right
overload, the same way the pre-refactor exportAsyncProxy.ts did.
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Why
packages/backenddoes the actual auto-layout → CSS/gradient/etc. conversion, but it currently assumes it's always running inside a live Figma plugin sandbox: several core builders (html,tailwind,flutter,swiftui,compose) read thefigmaglobal directly — not just the plugin-messaging bridge (code.ts/messaging.ts), which already degrades gracefully outside a plugin.That coupling means
packages/backendcan only be used from inside the plugin. It can't be run headlessly — e.g. server-side, converting node trees already fetched via the Figma REST API — without a live document connection.This PR adds a small seam so the package can run either way, with no change in behavior for the existing plugin app.
What changed
packages/backend/src/host.tsexportssetBackendHost()/getBackendHost(). Nobody has to callsetBackendHost()to keep using this as before: left unset,getBackendHost()transparently falls back to wrapping the realfigmaglobal, so the plugin app's code path is unchanged.setBackendHost()once with a small object implementing:mixed— a sentinel standing in forfigma.mixedgetNodeExport(id, settings)— replacesfigma.getNodeByIdAsync(id).exportAsync(settings), e.g. backed by the Figma REST image-export endpointgetVariableName(id)(optional) — replacesfigma.variables.getVariableByIdAsync(id)?.namefigma.mixedcomparison (15 call sites acrosscompose,html,tailwind,flutter,swiftui,common) now goes throughgetMixed().exportAsyncProxy(node image/SVG export) andtailwind/conversionTables.ts(variable-based color naming) now go through the host instead of the bare global.Intentionally out of scope
common/retrieveUI/retrieveColors.ts'sfigma.getSelectionColors()is left as a direct global reference. It reads the current user selection in the Figma UI — there's no REST equivalent, and its only caller (code.ts) is itself plugin-only, so it was never going to run headlessly regardless.How to verify no regression
Since the default host wraps the real
figmaglobal 1:1, the plugin app's behavior should be provably unchanged:setBackendHost()is never called anywhere inapps/plugin, sogetBackendHost()always falls through todefaultHost()there.Summary by CodeRabbit
New Features
Bug Fixes