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Jobs, threads and frame commits

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Use this page when changing event delivery, the native boundary or frame scheduling. A JavaScript job collects view changes into a typed batch; the UI thread applies committed batches in order.

02 / SCHEDULING & RENDERING One app, two execution threads The shared C++ queue batches property mutations before handing them to the native UI thread. HERMES THREADApp logic + async Signals · effects · eventsPromise jobs · timers · fetch One serialized VM owner C++ queue Deduplicateproperties Keep create /remove order NATIVE UI THREADPlatform renderer UIKit / Android ViewsApply ordered mutation batch Layout · animations · touch input Native events queue back to Hermes

Thread ownership

Work Owner
App JS, events, timers and network completions Serial Hermes thread/queue
Native views, layout and rendering Platform main thread
Motion and placed gesture callbacks Hermes UI runtime on the main thread
Background native operations The module's serial background thread

Android uses KuitoHermes; iOS uses the serial dev.kuito.hermes queue. A platform callback queues JS work onto its runtime owner. Do not call a Hermes runtime concurrently from arbitrary callback threads.

Commit a batch

src/runtime/wire.ts writes little-endian records into one ArrayBuffer, with a separate string table. __kuitoHostCommit hands it to C++ through JSI. native/shared/Wire.h decodes and validates the records before platform application.

Operation Purpose
Create / Remove Add or release a numbered view
Move Change parent or sibling position
Flatten Promote or demote a layout-only node
Prop Set or remove a property
Command Perform a view/app operation
UI and UI script/bundle records Install or update UI-runtime work

View numbers are not reused. Human-readable labels cross with creation for diagnostics. Built-in prop keys and value schemas come from src/protocol/manifest.ts and src/protocol/schemas.ts; generated Swift/Kotlin readers share that contract.

Validate properties once

Numbers, colors, enums and structured values decode into typed records. A bad field rejects the batch with the property path. Structured built-in props use generated schemas; custom module view props are decoded by that module's declared native prop types. Unsetting a prop restores its native default.

One transaction sends its records together so a rejected batch corresponds to the commit the kernel can undo. UI operations follow the views they reference. Event scopes and microtasks group ordinary signal writes; see reactivity.

Input and frame timing

An input handled on the JS thread can wait briefly for its resulting commit. For press/change events, the main thread waits up to 4 ms. If the job finishes, its batch can land in the input's frame; otherwise normal frame scheduling resumes. Other events do not wait. UI-thread handlers update placed properties directly.

This bounded wait trades a small amount of main-thread time for lower touch latency. Keep input handlers short and measure the tail latency when changing this path. The historical input experiment records the measured decision.

Check a scheduling change

Exercise a single signal write, several writes in one event, an async completion, a rejected prop batch and view removal during pending work. Compare native operation counts and device frame reports through performance tools. Headless tests check mutation behavior; device tests establish actual thread and frame timing.