Awaitable with CancellationToken vs coroutine cancel flag for deadline-bound scene-exit fetch

Answer

Use an Awaitable driven by a CancellationTokenSource (CTS) with CancelAfter, but you must explicitly register a callback to call UnityWebRequest.Abort(). This provides a single, verifiable path that halts the async continuation and terminates the network request. To prevent leaks on IL2CPP, the CTS must be disposed in a finally block. The coroutine approach is less desirable as it requires manual IEnumerator tracking and is more prone to leaks during scene transitions.

Confirmed Facts

• CancellationToken only prevents the Awaitable from resuming; it does not automatically stop a UnityWebRequest or AsyncOperation.
• UnityWebRequest.timeout triggers a network error result rather than throwing an OperationCanceledException.
• CancellationTokenSource.CancelAfter is supported in .NET Standard 2.1 (IL2CPP), but the underlying timer requires manual disposal to avoid memory leaks.
• Awaitable integrates with the Unity player loop, removing the need for the manual state-machine bookkeeping required by StopCoroutine.

Implementation and Mapping

Assumptions: The fetch occurs during a scene-exit phase where accessing destroyed UnityEngine.Object references will cause MissingReferenceException. Uncertainty: The exact error code for timeouts can vary by platform; checking UnityWebRequest.Result is the safest verification method.

1. Initialize a CancellationTokenSource with the required deadline.
2. Use cts.Token.Register(() => request.Abort()) to link the token to the network request.
3. Wrap the await call in a try-catch block for OperationCanceledException.
4. Map UnityWebRequest.Result.ConnectionError (where timeouts surface) to your cancellation logic.
5. Perform a null check on the calling object immediately after the await to ensure the scene has not unloaded.

// Example implementation pattern
using var cts = new CancellationTokenSource(deadlineMs);
var request = UnityWebRequest.Get(url);

// Link token to the actual network abort
using var registration = cts.Token.Register(() => request.Abort());

try {
    await request.SendWebRequest().ToAwaitable(cts.Token);
    
    // Map network timeout/error to cancellation model
    if (request.result != UnityWebRequest.Result.Success) {
        return Result.Canceled;
    }

    // Critical guard for scene-exit
    if (this == null) return Result.Canceled;
    
    return Result.Success;
} catch (OperationCanceledException) {
    return Result.Canceled;
} finally {
    // CTS disposed by 'using' statement to prevent IL2CPP timer leaks
}

Targeting IL2CPP with .NET Standard 2.1 does not change this recommendation, as CancelAfter is available. The primary risk is the timer leak, which is mitigated by the using pattern. Ensure that the Awaitable is not used in a way that triggers aggressive AOT stripping of the async state machine.

Missing Diagnostic: Do you require the request to abort the instant the scene unload begins, or is the deadline timer sufficient? If immediate abort on unload is required, you must call cts.Cancel() within the OnDestroy or OnDisable method of the owning object.