Detecting forceSimulation termination without a stopped event

Direct answer

Use a single setTimeout loop that polls simulation.alpha() and simulation.alphaTarget(); resolve a Promise when alpha <= alphaMin and alphaTarget === 0. Call simulation.stop() in cleanup to stop immediately, and reset alpha to 1 before restart() if you need to resume.

Confirmed facts

• The simulation ends when alpha falls below alphaMin (default 0.001) and alphaTarget is 0; no stopped event is emitted.
• simulation.stop() forces alpha to 0 instantly and halts the tick loop.
• React, Vue, or Svelte components should invoke simulation.stop() in their unmount or cleanup hooks to prevent dangling listeners.
• A custom event emitter can be attached to the tick callback; when the stop condition is met it emits a user‑defined stopped event and removes the tick listener.
• Restarting after a stop requires resetting alpha to 1 with simulation.alpha(1) before calling restart().

Likely explanation

Because alpha is mutated only through the provided setter methods, the only reliable way to know when the internal cooling process has completed is to observe its value and the target value. A polling loop runs once per animation frame (≈16 ms) and checks the two conditions; this adds negligible overhead compared to attaching a listener to every tick. The loop resolves a Promise, allowing asynchronous cleanup to await the termination. If the simulation is stopped programmatically, stop() already satisfies the condition, so no polling is needed. When alphaTarget is temporarily raised during drag, the loop naturally waits until the target returns to 0 and alpha drops below the minimum before resolving.

Missing diagnostic detail

Is the simulation ever restarted after it stops? Knowing this helps decide whether you must reset alpha to 1 before restart() to avoid a frozen state.