Agent Skills: Flutter Knowledge Patch

Flutter Knowledge Patch

Use this skill before changing Flutter or Dart applications, packages, plug-ins, embedders, build automation, or development tooling. Read the reference that matches the task before choosing an API or migration path; several old flags, generated files, properties, and platform assumptions are no longer valid.

Reference index

| Reference | Topics | | --- | --- | | accessibility-input-and-testing.md | Semantics, accessibility preferences, selection, text input, gestures, and tests | | android.md | Android SDK and Gradle requirements, rendering, platform views, system UI, and embedding | | apple-platforms.md | UIScene, Swift Package Manager, Xcode, Cupertino integration, signing, and Apple rendering | | dart-language.md | Dot shorthands, null-aware collection elements, flow analysis, constructors, and roadmap | | dart-tooling-and-packages.md | Formatter, analyzer plug-ins, Pub, workspaces, build hooks, native assets, and publishing | | desktop-and-embedding.md | Thread merging, add-to-app sizing, multi-window APIs, displays, input, and architectures | | migrations-and-breaking-changes.md | Cross-cutting removals, deprecations, callback changes, and required source migrations | | navigation-layout-and-animation.md | Routes, transitions, overlays, sheets, slivers, scrolling, layout, and animation | | tooling-build-assets-and-plugins.md | CLI contracts, Widget Preview, builds, assets, plug-in metadata, MCP, and release artifacts | | web-rendering-and-graphics.md | Web hot reload, Wasm, renderers, Impeller, shaders, images, filters, and web configuration | | widgets-and-theming.md | Material and Cupertino controls, menus, forms, themes, tooltips, and component styling |

Working method

1. Check pubspec.yaml, the platform project, and CI scripts before editing.
2. Read the topic reference for every platform or API involved.
3. Apply required migrations before adding new behavior.
4. Prefer current properties and command forms; do not preserve a removed flag or generated artifact merely because an older project still contains it.
5. Test on every affected renderer, operating system, input mode, and accessibility configuration when behavior differs by target.

Breaking changes and deprecations

Android projects

• Use the declarative Gradle plug-in setup. The imperative Flutter Gradle script application mechanism is gone.
• Remove Android v1 embedding APIs and migrate applications and plug-ins to the v2 embedding.
• Treat edge-to-edge content and resizable, orientation-flexible layouts as the platform baseline. Handle system-bar and large-screen insets explicitly.
• Keep the Android toolchain within Flutter's supported combination. Check the Android reference before adopting AGP 9 because its built-in Kotlin support changes both application and plug-in configuration.
• Do not rely on opting out of Impeller or restoring separate mobile UI and platform threads.

Apple projects

• Migrate iOS lifecycle work and lifecycle-aware plug-ins to UIScene; do not assign the now-read-only sceneLifeCycleDelegate.
• Expect Swift Package Manager to manage ordinary iOS and macOS application plug-ins. Audit customized Xcode targets and schemes after automatic migration. Add-to-app hosts still require a different dependency path.
• Use supported Xcode tooling and current platform deployment minimums. Regenerate platform configuration after raising a plug-in's minimum OS.
• Do not depend on the iOS Skia opt-out or on SkSL warm-up build artifacts.

Dart and package configuration

• After raising a package language constraint, run dart pub get before formatting. Use dart fix; dart format --fix no longer exists.
• Declare imported packages directly. The explicit-dependency check is unconditional.
• Replace browser work based on legacy web libraries with dart:js_interop and package:web, especially before enabling Wasm.
• Import generated localization code from its real output. The synthetic package:flutter_gen package is gone.
• Account for Pub enforcing the root Flutter SDK upper bound once the package opts into the relevant language behavior.

Framework source migrations

• Replace deprecated ...Theme value types with their ...ThemeData forms and use the replacement component properties documented in the widget reference.
• Put shared radio value and change handling in RadioGroup.
• Give DropdownMenu a non-nullable generic type, use initialValue on DropdownButtonFormField, and express DropdownButton.enabled independently of its callback.
• Replace removed or renamed callbacks and members before changing behavior: findItemIndexCallback, onReorderItem, maintainHintSize, Tooltip.constraints, and the current Cupertino sheet builder contract.
• Do not subclass IconData or TextDecoration; use instances or composition.

Web builds

• Do not select the removed HTML renderer. Image.network HTML-element fallback is now an explicit webHtmlElementStrategy decision.
• Remove obsolete hot-reload and service-worker flags. Use the current web run/build controls and web_dev_config.yaml.
• Remember that a Wasm build includes a JavaScript fallback, is single-threaded without cross-origin isolation, and cannot compile dependencies that retain legacy browser interop.
• Do not assume Impeller exists on the web; both web renderer paths remain Skia-backed.

Frequently used additions

Widget Preview

Annotate top-level widget builders with @Preview and keep callable wrappers and themes publicly and statically accessible. Preview configuration supports size, brightness, localization, grouping, multiple variants, structured custom annotation values, IDE filtering, and Inspector integration. Treat platform-only calls as unavailable even when their imports compile in the preview environment.

@Preview(size: Size(320, 640), brightness: Brightness.dark)
Widget previewCard() => const CardExample();

Analyzer plug-ins

Configure analyzer plug-ins at the top level of analysis_options.yaml. Published and path-based plug-ins can provide diagnostics, fixes, and assists; enable individual plug-in lints under that plug-in's diagnostics mapping and restart analysis after configuration changes.

plugins:
  local_rules:
    path: tools/local_rules
    diagnostics:
      avoid_legacy_api: true

Build hooks and native assets

Place a package build hook at hooks/build.dart. Hooks run for run, build, and test in dependency order, receive a restricted environment, and place generated or downloaded intermediates in the shared output directory. Give emitted native code assets a package:<package>/<asset> identity and match it from @Native when the library URI does not supply the same identity.

Web development

Use stateful hot reload on Chrome and the web-server device. Put shared host, port, TLS, headers, and same-origin proxy settings in project-root web_dev_config.yaml; explicit command-line HTTPS and header settings win. Use --base-href when a development run must match a deployed subpath.

Accessibility and input

• Use semantic roles, identifiers, live regions, continuous sliver indexes, explicit hit-test behavior, and the semantics test matchers where appropriate.
• Honor reduced motion, forced colors, text-spacing overrides, progress semantics, and Apple autoplay or cursor preferences.
• Test keyboard, stylus, pointer-kind, selection, and context-menu behavior on the target platform; several defaults are platform-specific.

Navigation, overlays, and sheets

• Use the predictive-back-aware Material transition unless the application explicitly retains its previous transition.
• Use OverlayPortal.overlayChildLayoutBuilder for anchor-aware placement and OverlayChildLocation.rootOverlay for root overlays.
• Coordinate Cupertino sheet scrolling and drag dismissal through scrollableBuilder; use route settings when observers need sheet identity.
• Use Navigator.popUntilWithResult when one result must cross several popped routes.

Rendering and layout

• Group compatible backdrop filters with BackdropGroup and BackdropFilter.grouped.
• Use ImageFilter.shader, named shader bindings, sampler filter quality, synchronous texture creation, and float image formats only after checking renderer support.
• Prefer RoundedSuperellipseBorder and related primitives when the design calls for continuous corners; web support no longer falls back to ordinary rounded rectangles.
• Use shared scroll cache configuration, fixed grid main-axis extents, explicit sliver paint order, and custom maximum paint bounds instead of inferring geometry.

Desktop and embedding

• Assume UI/platform thread merging on supported desktop targets and mobile.
• Gate experimental multi-window, content-sized, undecorated, popup, tooltip, and dialog-window behavior; unsupported regular-window implementations can throw.
• Check platform architecture support and native input details before promising a Windows ARM, Linux RISC-V, stylus, or monitor-management workflow.

Verification checklist

• Run flutter analyze and relevant Dart analyzer plug-in diagnostics.
• Run unit, widget, semantics, and integration tests; use the documented timeout or installation-retention switches only when the harness needs them.
• Exercise route completion, predictive back, form reset/error clearing, keyboard traversal, and selection behavior affected by the change.
• Build each affected target and inspect warnings from Wasm dry runs, Gradle, Xcode, plug-in metadata, assets, and removed CLI options.
• Verify accessibility with the platform screen reader and user preferences enabled.
• Check profile or diagnostic tools when investigating microtasks, text layout, analysis-server latency, native symbols, or startup behavior.