Callback function types for compiler events.

Pass an object implementing any subset of these callbacks to FilterCompilerOptions.events or WorkerCompilerOptions.events. All handlers are optional; you only need to implement the events you care about.

Lifecycle order

onCompilationStart
  for each source:
    onSourceStart
    onSourceComplete  (or onSourceError)
  for each transformation (via bridge hook):
    onTransformationStart
    onTransformationComplete
  onProgress  (emitted throughout)
onCompilationComplete

Relationship to TransformationHookManager

onTransformationStart and onTransformationComplete are now routed through the hook system via a bridge hook rather than direct emitter calls. This is an implementation detail that is transparent to callers — the events still arrive at the same callbacks as before.

For async hooks or error hooks on individual transformations, use FilterCompilerOptions.hookManager with a TransformationHookManager.

Error safety

All handlers are wrapped in try-catch by CompilerEventEmitter.safeEmit(). Throwing inside a handler logs the error but does not abort compilation.