Builder API and output watching

Builder API

Use the MetaSound Builder Subsystem for runtime/editor creation and mutation of transient Sources or Patches. Use the MetaSound Editor Subsystem for editor-only asset creation/serialization workflows. Prefer authored assets unless dynamic graph topology is a real requirement.

Builder handles identify graph nodes/pins but may change when documents rebuild/version. Never serialize them as durable identity. Define ownership of transient builders, audition/live-update state, dependency loading, and teardown. Validate generated graphs before playback/export.

Output watching

Watch Output can observe supported Source Outputs—Float, Int32, Bool, Time, String, and Trigger—while the Source is playing. Patch Outputs cannot be watched directly. The delegate can fire for every output update, including multiple updates within a frame.

Use watched outputs for presentation/analysis callbacks, not audio-thread authority over gameplay. Throttle or aggregate high-rate outputs before expensive game-thread work. For envelope data, watch the Audio Output with Epic's Envelope Follower analyzer identifiers.

Failure gates:

• no callback -> Source not playing, wrong exact output/analyzer name, unsupported type, or Patch;
• excessive callbacks -> output updates multiple times per frame;
• stale builder handle -> graph rebuilt/versioned;
• live changes absent -> not auditioning with supported Live Updates or wrong builder instance.