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# facet_uikit

facet's **UIKit** backend — the iOS counterpart of `facet_appkit`.

> **Status: runs in the simulator; not on a device.** `cpc build --target
> ios-arm64` produces a real arm64 iOS static library (`platform 2`, minos 13.0)
> and `--target ios-arm64-simulator` produces the simulator one.
> `examples/facet_gallery_ios` links against it and runs: layout, measurement,
> view ownership, the run-loop tick and target/action delivery have all been
> observed working. What is **unvalidated** is FEEL — a drag, a fling, momentum,
> the exact moment a swipe opens — because those cannot be asserted from a
> screenshot and nobody has held the phone.

**Prop parity with `facet_appkit`: 305 of 318 (95%).** The thirteen-prop
difference is MANIFEST.md §1 in full — a date or time picker's font band (twelve
verbs UIDatePicker has no property for) and a window's chrome style. Nothing is
missing for want of writing.

That number is a measurement, not a claim — `tools/parity.py` walks facet's
contract modules and asks which bits each backend names:

```
python3 vendor/facet_uikit/tools/parity.py      # from the repo root
```

Anything it prints under UIKIT MISSING must appear in MANIFEST.md §1. It also
reports the other direction, which is how the four props `facet_uikit` honours
and `facet_appkit` does not were found.

## Test

```
vendor/facet_uikit/tools/run_ios_tests.sh      # builds, installs, runs, reports
```

`cpc test` cannot run this package — it builds a host binary and macOS has no
UIKit. The checks live in `src/selftest.cplus` and run as an iOS binary
(`tests/`) under `simctl`. The script exits non-zero when
anything fails.

## Build

```
cd vendor/facet_uikit
cpc build --target ios-arm64             # device: target/ios-arm64/debug/libfacet_uikit.a
cpc build --target ios-arm64-simulator   # simulator
cpc check --target ios-arm64             # sema + borrowck only, faster
```

Xcode owns the final link — an iOS target stops at object emission by design
(`plans/plan.backends.md`, rung 1). Add the `.a` and a bridging call to
`facet_uikit::install()` from the app's `main`.

## What it is

facet owns the description tree (`facet::Node`) and all layout (the shared
`flex_layout` engine). This package fills the same five-verb `Renderer` seam
`facet_appkit` fills, and no wider surface:

| File | Role |
|---|---|
| `facet_uikit.cplus` | `install()` — the whole registration surface |
| `views.cplus` | `create` / `apply` / `insert` / `remove` / `view_release`, the backing rule, measurement, and this package's tests |
| `controls.cplus` | one body per kind |
| `paint.cplus` | the shared band — colour, brush, shadow, clip, radius, transform, `animate_*` |
| `geometry.cplus` | the layout pass and the frame walk |
| `input.cplus` | target/action and tap recognisers → facet handlers, and the radio group |
| `scheduler.cplus` | the CFRunLoop tick, `run_on_main`, `after` |
| `text_input.cplus` | the field / editor / search band, style runs, the length limit |
| `recycler.cplus` | `list`, `table`, `tree` on UITableView; `collection`, `carousel` on UICollectionView |
| `drawing.cplus` | the canvas replay — facet's recorded display list into a `CGContext` |
| `swipe.cplus` | swipe-to-reveal, driven by a pan so `reveal_threshold` means something |
| `web.cplus` | `web` and `hybrid_web` over WKWebView |
| `dates.cplus` | facet's `Date` / `Time` ↔ `NSDate`, through `NSDateComponents` |
| `window.cplus` | the app delegate, the window, and the first tick |

## Where UIKit is SHORTER than AppKit

Worth reading before porting anything else from `facet_appkit`, because these
are the places where copying that code would add machinery for no reason:

- **No flip.** A UIView is top-left origin natively. `facet_appkit` creates a
  flipped subclass for every node and says so in three files.
- **Always layer-backed.** No `setWantsLayer:`, so every paint verb is one call
  shorter and every clear is unconditional.
- **No appearance dance.** A `UIColor` resolves against the current trait
  collection when asked; `NSColor` flattens against whatever appearance is
  current, which costs the AppKit backend a re-entrant saved-static wrapper
  around every configure.
- **No document view.** A `UIScrollView` scrolls its own subviews.
- **`insertSubview:atIndex:` exists**, so slot ordering is one call.
- **`sizeThatFits:` takes the bound**, so a wrapping label needs no
  `preferredMaxLayoutWidth` pre-step.
- **`isSecureTextEntry` is a property**, so there is no live reclass path at all
  — `views::reclass` exists in the AppKit backend for that single prop.
- **One synthesized class, not one per kind.** Target/action takes a separate
  target object, so no view has its class moved.

Three things are genuinely richer here: SF Symbols (`systemImageNamed:` is the
whole implementation), Dynamic Type, and the `keyboard` band — which facet
declares on every input and macOS can do nothing with.

## What is not here

`MANIFEST.md` is the authority, and it keeps two lists strictly apart:

- **decided absent** — iOS has no such thing (window buttons, the menu bar, a
  draggable split divider). Finished; silent at runtime.
- **not yet built** — iOS has an answer and this pass did not write it (the
  list/table/collection/tree recycling tier, web, canvas, swipe actions). Each
  renders its children through a plain backing view and **warns once on stderr**
  when mounted, so the debt is audible.

The largest single piece of work left is the recycling tier: `facet_appkit`'s
`recycler.cplus` is 2,900 lines and has no counterpart here.

## Wired into facet

`import "facet_runtime/runtime"` resolves to `facet_runtime/src/runtime_ios.cplus`
on an iOS target — the same filename override that picks `runtime_macos.cplus`
on a Mac, keyed off `target::active_platform()`. facet_runtime's manifest
carries `[ios.dependencies]`, so **facet_runtime's own build on iOS compiles
this package**,
exactly as it compiles `facet_appkit` on macOS. The consequence is the one
facet's manifest already states for the Mac: a backend that does not build turns
facet red.

`facet/src/agent_ios.cplus` shadows `agent.cplus` the same way, because that
module is written against `agent_appkit` and there is no `agent_uikit`. It keeps
the surface compiling and refuses loudly.

```
cd vendor/facet && cpc check --target ios-arm64     # facet + this backend
```

### Application lifetime and navigation

`App::run` registers the initial window with UIKit and enters the OS-owned
loop. It does not return on iOS. Window content and navigation are retained by
that window's session; there are no standalone component/screen host APIs.

Content commands use the window's navigator, changing its whole content or a
named slot. An explicit window-opening request never becomes a content push.
Additional iPad windows require an app that opts into multiple scenes. Shared
screens can be used by separate desktop and mobile app compositions.

Save persistent state on backgrounding rather than relying on `on_quit` or
process-exit cleanup. Closing an additional scene has its own window teardown;
process termination does not guarantee a final callback.

The current shared navigation engine supports screen-driven Back. The older
UIKit controller stack's swipe behavior does not validate interactive gestures
or scene restoration through the redesigned navigator. See the
[navigation guide](https://github.com/netdur/cplus/blob/v0.0.28/vendor/facet/docs/navigation.md#desktop-and-mobile) for the
current validation limits and API.