Give generated Android apps synthetic input (MOB-160)

[GenericJam]

Generated Android apps shipped no way to synthesise a tap. An agent driving one over dist could read state but not touch it, so Mob.Test.tap_xy/3 and friends returned {:error, :not_loaded} on every Android app ever generated.

Adds tapXy, longPressXy, swipeXy, typeText and deleteBackward to the bridge template, dispatching MotionEvents at the activity's decor view in-process. The obvious route, Instrumentation.sendPointerSync, needs INJECT_EVENTS — a signature permission no ordinary app can hold.

Verified on a physical device

moto g power (2021), each assertion against observed app state rather than a return code:

primitive	evidence
tap_xy	navigated HomeScreen → TextScreen by coordinate
long_press_xy	on_long_press counter 0 → 1
swipe_xy	scroll offset 0 → 1033px (full extent)
type_text	focused field "" → "abc"; {:error, :no_first_responder} unfocused
delete_backward	removed exactly one character
clear_text	absent by design, returns {:error, :not_loaded}

That distinction earned its keep — every method here returned :ok while doing nothing at some point during development.

Two findings worth reading

Gestures must be dispatched over real elapsed time. The first implementation built each gesture inside one main-thread block with fabricated timestamps. Every event reached the view and every gesture was ignored: Android's long-press detector waits on a posted callback, and Compose resolves drags in a pointer-input coroutine that only resumes when the looper is free. A block that runs a gesture to completion without yielding starves exactly the machinery meant to interpret it.

clearText ships absent rather than broken. Two implementations reported success while clearing nothing — dispatched backspaces coalesce within a frame (~4 of 200 registered), and Ctrl+A does not select in a Compose text field. The JNI lookup is a cacheOptional, so an absent method leaves the handle null, capabilities/1 reports clear_text: false, and calls return :not_loaded. An agent can plan around a capability it knows it lacks; it cannot plan around a lie.

Hardened after adversarial review

The review caught the same sin shipped elsewhere in this change: results were reporting success unconditionally.

• Results report what was consumed, not what was sent. Discarding the dispatch return made :dispatch_failed and :no_first_responder unreachable, so typing into an unfocused field returned :ok.
• ACTION_CANCEL on any failure between DOWN and UP. A dangling DOWN leaves the view tree believing a finger is down, and every later touch in the session reads as a second pointer, silently.
• A mutex across each gesture — these arrive as concurrent :rpc calls, and a tap's DOWN landing inside a swipe's MOVEs corrupted both while both reported success.
• The latch timeout is honoured rather than discarded, ok is an AtomicBoolean, the job is cancelled on timeout, and InterruptedException is caught — letting it propagate would unwind into JNI with a pending exception, and those call sites do not ExceptionCheck.
• Refuses main-thread reentry, which would deadlock into an ANR.

Known limits, all documented in Mob.Test

Gestures block for their real duration; only the activity's own window is reachable (not dialogs or modal sheets); typeText is ASCII-only and rejects a whole string containing one unmappable character.

MobBridge.kt is generated once and never re-rendered, so existing apps must be regenerated to pick this up.

Tests

Assert the methods exist, are @JvmStatic, and that each Kotlin signature matches the JNI descriptor mob_nif.zig caches it with — a mismatch is not a compile error, it is a null JMethodID and a silent :not_loaded on device. Also asserts clearText stays absent so the deliberate omission is not "fixed" in good faith later. Mutation-checked: flipping durationMs to Int fails the signature test.

Suite: 411/413, the 2 failures pre-existing MOB_DIR env ones. ktlint clean.

Requires

GenericJam/mob#135 — these NIFs block a scheduler for the gesture's duration and must run dirty.

🤖 Generated with Claude Code