halkyon/packages/halkyon_app/README.md

# halkyon_app

What every Halkyon app (Learn, Todo, Train) is built around, on an instance's
`halkyon_base` module: the instances a device keeps an account on, signing in,
each person's data opened and closed, and syncing it. Taken from Halkyon Learn,
which it was written for, so it behaves as Learn's own did; the texts are in
British English, American English and German, and where they name the app,
they name yours.

Written as the apps are: Dart 3.13's primary constructors, Material from
`package:material_ui`, Lucide icons, blocs (`flutter_bloc`), LeanCode's lints.

## What it holds

- **Instances** (`lib/src/instance/`): `Instance` (an address as people type
  it; `isSecure`: over HTTPS, or over plain HTTP on this machine alone),
  `InstanceChoice` (those the device keeps an account on, one in use;
  `FileInstanceStore`; `allows` an instance reached securely, or in a
  development build the build's own over HTTP, and `leave`s one it no longer
  allows, which `InstanceGate` never opens: it says why and offers another or
  removing it), `InstanceStorage` (each instance's people's folders and
  session key), `InstanceChecker` (does an address answer, where does it keep
  people's data; `ServerpodInstanceChecker` asks its `instance.basics()`),
  `InstanceScope`. The screens: `FirstStartScreen` (the build's instance and
  another, as tiles, `InstanceTile`), `InstanceStep` (an address, checked),
  `showAccountSwitcher` and `AddAccountScreen`, and where an instance keeps
  people's data, as a flag and words (`InstanceLocationBuilder`,
  `InstanceFlag`, `instanceLocationWords`): `EU`, a country or a subdivision
  (ISO 3166-2, `US-CA`) as its operator says, "Germany (EU)" for a member of
  the European Union (`europeanUnionMembers`) and "California, United States"
  for a subdivision, with the most specific flag there is (a few subdivisions
  have their own, such as Scotland; else the country's). Countries and
  subdivisions are named by Unicode CLDR's names in English and German, which
  come with the package (`AssetCountryNames`, `AssetSubdivisionNames`; the
  package's `tool/country_names.py` and `tool/subdivision_names.py` write
  them, from one release); tests give names of their own (`CountryNames`,
  `SubdivisionNames`).
- **Gates** (`lib/src/app/`): `InstanceGate` opens the app's dependencies on
  the instance in use (`AppDependencies`, handed down by `AppScope`), anew on
  a switch, the previous ones closed first; `SessionGate` keeps a person
  signed in, signing in anonymously at a first start and asking how to start
  once signed out (`StartScreen`); `PersonScope` opens each person's own
  dependencies and closes them when the person goes. `resolveAppLocale`
  picks the app's language as every Halkyon app does: the first preferred
  language it speaks, never by country alone, British English otherwise.
- **About** (`showAppAbout`): the app's name (its frame's), its version,
  icon and legalese where it gives them, where the source of what the
  instance runs is (`InstanceSourceCode`, from `AppDependencies.sourceCodeUrl`;
  both licences require offering it), the app's own parts (its credits), and
  the licence list.
- **The account** (`AccountTiles`): whose account on which instance,
  switching to the account on another, signing out (which of an anonymous
  account deletes it, so asks first) and deleting it (after asking), for an
  app's settings, in the app's words where only it can say them. While one
  runs, the person's app stays and its tile shows a spinner; one that fails
  keeps them signed in and says why once, in a snack bar (in a sheet, give
  it `hasSnackBars`). An instance out of reach signs out on the device all
  the same (Serverpod's `signOutDevice`), so only the device's own storage
  fails a sign-out.
- **Signing in** (`lib/src/auth/`): `AuthRepository` on an `AuthGateway`;
  `ServerpodAuthGateway` signs in anonymously and with GitHub (the system
  browser and a loopback address on Linux and Windows, halkyon_oidc_flutter's
  `LoopbackWebAuth`),
  through the instance's `halkyon_base`, and puts a session granted elsewhere
  in place (`useSession`); `SessionBloc`, `GitHubSignInBloc` and
  `SignInWaysCubit`; `SignedOutMark`, `SignInAddresses`, `PersonFolders`.
  Where the instance offers passwords, GitHub is added to nobody (ADR 0017):
  `gitHubOffer` says so from its basics, `GitHubSignInStateReady.linking`
  hands it to the screens, which then offer no Add, and the start screen
  offers an account to create in place of the app's sentence on adding
  GitHub; `gitHubProblemText` says why a GitHub sign-in was refused.
- **Content sealed with data keys** (`lib/src/privacy/`, ADR 0019):
  `ContentCipher` (`SodiumContentCipher`, on the isolate it is used on, as an
  app seals and opens rows one by one; `FakeContentCipher` in tests) makes a
  store's data keys, sealed to the account and signed by it, opens them, and
  seals and opens a row's content for the row it is stored in (its table, id
  and column), the payload holding the signature before the ciphertext. An
  anonymous person's device makes key pairs of its own
  (`AccountCrypto.newDeviceKeys`), which `makePassword` keeps as the account's
  once they set a password.
- **A password and TOTP** (`lib/src/password/`, ADR 0017): `PasswordRepository`
  on a `PasswordGateway` (`ServerpodPasswordGateway`), an `AccountCrypto`
  (`SodiumAccountCrypto`: halkyon_keys and SRP's client half, each step on an
  isolate of its own, whose results are copied before they cross back:
  sodium's are views of memory freed with the isolate that made them) and an
  `AccountKeyStore` (`SecureAccountKeyStore`: the account's unlocked keys in
  the system's keyring, per instance and person, gone once another person
  signs in or nobody; on iOS and macOS `unlocked_this_device`, so readable
  only while the device is unlocked, never in a backup restored elsewhere nor
  in iCloud Keychain, each write deleting the entry before it, and forgetting
  finding too the entries earlier builds kept with another accessibility, a
  keyring that can't be read now stopping nothing). Signing in (SRP's M2 checked and the keys opened before
  anything is trusted), signing up (the recovery phrase shown once and three of
  its words typed back before anything reaches the instance), a setup never
  finished, the authenticators (QR codes drawn with pub.dev's `qr`), recovery
  codes, changing the password, a new phrase, and getting back in. The
  screens: the start screen's Sign in and Create an account,
  `showPasswordSignInSheet`, `showAccountSetupSheet`,
  `showAccountRecoverySheet`, and `AccountSecuritySection` for an app's
  account part. Every change of how the account is secured asks for the
  password with the code, one answer per sheet (`OwnerAnswer`, the owner's
  proof: a session alone gets no code checked), each answer bound to the
  instance's base URL and to the change it approves (its purpose and the
  digest of what it sets, `proof_digests.dart`), so nobody between the device
  and the instance swaps anything in; deleting by a code by mail
  follows `secondFactorWorks`, as the instance tells it, and asks for the
  address only where the device doesn't know it. Secrets shown stay the
  person's: nothing secret is copied (no app can mark a copy as sensitive
  through Flutter, so clipboard histories and cloud clipboards would keep
  it): the recovery phrase and codes are saved or written down, an
  authenticator's key scanned, opened or typed; all of them show in a
  `SecretScreen` (the phrase typed back too, its fields offering Paste and
  Select all alone, their copy keys doing nothing), kept out of screenshots,
  screen recordings and recents where the app's window answers
  `app.halkyon/screen_guard`: Android's `FLAG_SECURE` (as Learn's and Todo's
  `ScreenGuard.kt` in their `MainActivity`), macOS's `sharingType = .none`
  (`MainFlutterWindow.swift`; screenshots, but not ScreenCaptureKit's
  captures on macOS 15 and later) and Windows' `WDA_EXCLUDEFROMCAPTURE`
  from Windows 10 2004 on, `WDA_MONITOR` (black in captures) before, by the
  version `RtlGetVersion` tells (`flutter_window.cpp`); neither run yet;
  covered on iOS in the app switcher's picture while the app isn't in front,
  screenshots there not held back; nothing on Linux; keyboards learn nothing typed into a password, a code or
  the phrase; no state's text (which a debug build's bloc observer logs) holds
  a secret, a code or the address; only an `otpauth:` URI is shown as a QR
  code or opened; and anything a step didn't expect (bytes of the wrong
  length from an instance, libsodium's or the keyring's own errors) fails it
  rather than leaving its sheet busy. `register_interop_test.dart` holds the
  real cryptography to the register's own checks. Anonymous people get no
  keys until something is encrypted (step 4). Every build of an app needs a C
  compiler and make: sodium's build hook compiles libsodium.
- **Files** (`FileSaver`, `PlatformFileSaver`): keeping a file the app made
  where the person chooses (a recovery phrase, recovery codes, an app's
  exports): on Linux and macOS the own user's alone (`writePrivately`). On
  Linux a new file beside it, made 0600 with `O_EXCL`, written through its
  descriptor, renamed over the one chosen, never written into, and the
  folder flushed. On macOS, whose sandbox lets an app reach the file chosen
  and nothing beside it, the file itself, opened without following a link
  or waiting on a pipe, refused unless it is a file of the own user's, made
  0600 and emptied before anything is written; whoever held it open reads
  what is saved. Each app's macOS entitlements grant reading and writing
  the files the person chooses (`com.apple.security.files.user-selected.read-write`),
  without which nothing could be saved there, Learn's exports neither. On
  both, a symbolic link or another user's file is refused with nothing
  written. On Windows written in place with its folder's rights, on iOS
  without the copy file_picker leaves in the temporary folder
  (`forgetPickerCopy`). None of it has run on a Mac yet.
- **Sync** (`lib/src/sync/`): `SyncService` (once at start, shortly after
  local changes, on resume and every few minutes, one sync at a time, counting
  the changes not on the server yet, `PendingChangesStore`), `AppCompatibility`
  and `compatibleSync` (the instance's minimum app build, the engine's table
  check), `SyncStatusBloc`, `syncAndWait`, and the widgets: `SyncStatusCard`,
  `AppMismatchBanner`, `SyncTroubleBadge`.
- **A client of `halkyon_base` alone** (`BaseClient`): to ask an instance
  about itself, and to sign in as another person than the app's client holds.
- **Testing** (`package:halkyon_app/testing.dart`): fakes of every boundary
  (`FakeAuthGateway`, `FakePasswordGateway`, `FakeAccountCrypto`,
  `FakeInstanceChecker`, the stores in memory), a frame
  (`testFrame`), `testApp`, and `TestAppDependencies`.

## Using it in an app

1. **The server** includes `halkyon_base` with the nickname `base` (its
   README), so the app's generated client has `client.modules.base`.
2. **Depend on it** by path, within the repository's workspace, as on
   `halkyon_ui`:

   ```yaml
   dependencies:
     halkyon_app:
       path: ../../../packages/halkyon_app
   ```

3. **Texts**: add `HalkyonAppLocalizations.delegate` to every `MaterialApp`'s
   delegates, beside `HalkyonUiLocalizations.delegate` and material_ui's
   `GlobalMaterialLocalizations.delegates`.
4. **The frame** (`AppFrame`): the app's name and wordmark word, its icon, an
   example address, its theme, delegates, languages (`resolveAppLocale` as
   its resolution) and launch screen, and the three sentences only it can
   say: what the first start's instance keeps of the person's, what a switch
   to another account leaves behind, and where GitHub is added later (shown
   only where the instance offers no passwords).
5. **Android**: the app's `MainActivity` answers `app.halkyon/screen_guard`
   (copy Learn's or Todo's `ScreenGuard.kt` and call `ScreenGuard.attach` in
   `configureFlutterEngine`), or secrets show in screenshots and recents.
6. **The dependencies**: the app's composition root implements
   `AppDependencies<P>` (`serverName`, `auth`, `signedOut`, `passwords`,
   `fileSaver`, `sourceCodeUrl`, `openPerson`, `dispose`), its person's
   dependencies `Disposable`. Its `passwords` is a `PasswordRepository` on
   `ServerpodPasswordGateway(client, base: client.modules.base)`,
   `SodiumAccountCrypto()`, `SecureAccountKeyStore(storage.keysOf(instance))`,
   the auth gateway and the app's `SignInAddresses`. Its `auth` is an
   `AuthRepository` on `ServerpodAuthGateway(client, base:
   client.modules.base, callbackScheme: …, appName: …)`; the scheme is the
   app's own (`app.halkyon.learn`), registered with each platform and in the
   instance's GitHub OAuth app, whose loopback address on Linux and Windows is
   `http://127.0.0.1/github-callback` (port 43823 unless given).
7. **The tree**:

   ```dart
   InstanceGate(
     frame: frame,
     choice: choice,
     checker: ServerpodInstanceChecker(app: 'todo'),
     readLook: …,           // the device's theme mode and language
     open: Dependencies.create,
     app: SessionGate(
       look: (context) => …,
       app: (person) => PersonApp(person: person), // with PersonScope inside
     ),
   )
   ```

   A person's widgets find their dependencies with
   `PersonScope.of<PersonDependencies>(context)` and the app's with
   `AppScope.of<Dependencies>(context)`.
8. **Licences**: call `registerHalkyonAppLicenses()` once at startup (the
   flags' and Unicode CLDR's notices), beside halkyon_ui's
   `registerFontLicenses()`.

## Licence

GNU Affero General Public License 3.0 or later (`LICENSE`). The flags are
lipis' flag-icons, drawn by the `flag` package, under the MIT licence
(`assets/flags/LICENSE-flag-icons.txt`). The subdivisions' names are Unicode
CLDR's (release 48.2), under the Unicode License v3
(`assets/subdivisions/LICENSE-CLDR.txt`, `assets/subdivisions/ATTRIBUTION.md`).