# inset-foundation/src
Flutter home: packages/flutter/lib/src/foundation
Ported against: ed2132410ee94b5a590cb7f67cee7a6ea9101a60
## Identical
- observer_list.rs → observer_list.dart
## app.rs
- Change: `App` and `Handle<T>` live here; Flutter has no counterpart.
Reason: language — Rust has no isolate-global GC, so a named owner has to hold the arena.
Affect: Flutter objects are `Handle`s; callbacks take `&mut App` plus a handle to themselves.
- Change: a Flutter object is one struct in the arena and its methods take `self: Handle<Self>` plus the `App`.
Reason: language — `Handle` is foreign to every other crate, so `impl Handle<T>` there is an orphan; nightly `arbitrary_self_types` puts the methods on `T` instead.
Affect: a crate that calls an inherent handle-receiver method needs `#![feature(arbitrary_self_types)]`; trait methods resolve without it.
- Change: Dart's isolate microtask queue is on `App`, and a drain has a budget that panics on a cycle.
Reason: language — there is no isolate event loop.
Affect: the queue drains after platform events and between begin-frame and draw-frame, and a microtask cycle panics where the isolate would hang.
- Change: `dart:async`'s `Timer` runs on a logical clock that `AppCell::elapse` advances (microtasks first, then due order, microtasks after each); `Timer.periodic` is absent.
Reason: language — there is no isolate event loop; tests play FakeAsync through `elapse`.
Affect: no timer fires until the host or a test elapses the clock, and a repeating timer has to re-arm itself.
- Change: a Dart `async` method is a `Task`: the part before its first `await` runs inline, the rest at the next checkpoint.
Reason: language — a Rust future cannot hold the `App` across an `await`; it borrows the cell per step, as gpui does.
Affect: everything after an `await` lands at a later checkpoint, and dropping a `Task` does not cancel it.
- Change: `App::retain(handle)` returns a `RetainedHandle<T>` (and `retain_id` a `RetainedHandleId`) that keeps an object alive past its `destroy`.
Reason: language — Dart keeps an object alive while anything references it; the arena frees it, and a cached hit-test path or the mouse tracker may still name it.
Affect: a `Listener` rebuilt mid-press still receives its release, and a retained entry lingers until its holder lets go.
- Change: `App` holds the host `Platform`: `AppCell::new` uses an inert one and `AppCell::with_platform` installs a live one before user code.
Reason: platform — dart:ui is host-bound; there is no isolate global to hang it on.
Affect: host requests and view queries go through `app.platform()`, and one built without a live platform answers nothing.
- Change: the callbacks Flutter assigns onto `PlatformDispatcher` (`onPlatformBrightnessChanged`, `onLocaleChanged`) are `App::platform_callbacks`, which `WidgetsBinding::instance` fills and the shell invokes.
Reason: platform — there is no isolate-global dispatcher to assign a handler onto.
Affect: a host reports the change through its client and never calls a binding.
## key.rs → key.dart
- Change: `Key` equality and hashing go through `eq_key` / `hash_key`, so `dyn Key` can be `PartialEq` and `Hash`; a `UniqueKey`'s identity is a monotonic id, not object identity.
Reason: language — `PartialEq` and `Hash` are not object-safe, and a Rust value type has no implicit object identity.
Affect: two `UniqueKey::new` calls are never equal; copying a `UniqueKey` keeps the same id.
## constants.rs → constants.dart
- Change: `kProfileMode` is the constant `false`.
Reason: language — `cfg(debug_assertions)` is the only build distinction Rust exposes, and it splits two ways where Dart splits three.
Affect: Dart's `kProfileMode` branches never run.
- Change: `kIsWeb` and `kIsWasm` are always equal (`cfg!(target_family = "wasm")`).
Reason: language — Rust has one wasm target family and no JS-compilation path.
Affect: `kIsWeb && !kIsWasm` is never true.
## date_time.rs → dart:core `DateTime`
- Change: only UTC exists: `DateTime::utc(..)`, `now()` read as UTC, and epoch constructors with no `isUtc`.
Reason: platform — the standard library has no time zone database, and its duration is unsigned.
Affect: every `DateTime` reads as UTC whatever the host's zone; local time, `parse` and `difference` are absent, so a difference is a subtraction of `microseconds_since_epoch()`.
## change_notifier.rs → change_notifier.dart
- Change: `remove_listener` matches a `Listener` by identity (`Rc::ptr_eq`, or the `(Handle, function)` pair for `handle_method`).
Reason: language — Rust closures have no identity; Dart's `VoidCallback` compares by identity and tear-offs canonicalize.
Affect: an equivalent-looking closure built at the removal site removes nothing and the listener keeps firing; keep the `Listener` that was added, or rebuild the `handle_method` tear-off.
- Change: `remove_listener` on a `Handle` whose entry is gone is a no-op.
Reason: language — Dart allows `removeListener` on a disposed `ChangeNotifier`, whose object the collector still holds; here the entry may already be destroyed, and a stale `app.get_mut` panics.
Affect: a state that unmounts after its listenable (a route's `TickerMode`, during a hero flight's pop) removes its listener without panicking; a handle that was never valid is ignored too.
- Change: a panicking listener ends the notification, and `dispose` also clears the reentrant-removal count.
Reason: language — Rust has no catchable exception for ordinary control flow, so a panic skips the compaction Dart's `catch` always reaches.
Affect: a panicking listener skips every listener after it; Flutter still calls them.
## Deferred
- date_time.rs: local time, `parse` / `tryParse`, `difference`. Trigger: a host with a time zone database; a signed duration.
- The form a `Listenable` takes when held as a value (stored, re-pointed, compared). Trigger: `AnimatedBuilder` / `ValueListenableBuilder`.
- `ChangeNotifier.maybeDispatchObjectCreation` / `memory_allocations`. Trigger: leak tracker / devtools.
- Diagnostics / `FlutterError` structured trees; messages are `debug_assert!` strings meanwhile. Trigger: porting diagnostics.
- `GlobalKey` / `ObjectKey`. Trigger: `widgets/framework.dart`.
- `IterableFilter`. Trigger: the first iterable-filter call site.
- Waking a task from another thread; it would need a `Send` poke into the host's event loop. Trigger: the first background executor or platform callback off the main thread.