FFM-12129 Add onFlagNotFound to FFContextProviderProps

[erdirowlands]

What

Adds a new optional prop called onFlagNotFound, which is a callback that gets triggered whenever a variation is requested and the flag cannot be found. This prop receives two arguments:

flagNotFound : A typeDefaultVariationEventPayload from the Javascript SDK containing the flag identifier and the default variation that was served.

loading: A boolean that indicates whether the SDK was still initializing at the time the variation call was made and the flag was not found.

The mechanism for achieving this is listening to the new Javascript SDK event (as of 1.29.0) ERROR_DEFAULT_VARIATION_RETURNED which is fired by the Javascript SDK when a call to variation has returned the default variation.

Why

Inform users when the SDK has served a default variation, for example, due to a flag typo, misconfiguration, or the SDK not being fully initialized. For cases where the SDK is not initialized, the user may choose not to care as this is the default behaviour in async mode, and the loading argument can be used to filter those out.

Testing

Added new test suite for FFContext

Manual
Using useFeatureFlag hook:

• async disabled
  • Non-existent flag: onFlagNotFound executes when the SDK has finished initialising
  • Flag exists: onFlagNotFound does not execute after the SDK initialises
• async enabled
  • Non-existent flag: onFlagNotFound executes immediately, and again when the SDK initializes.
  • Existing flag: onFlagNotFound executes immediately, but does not execute after the SDK initiailises

[erdirowlands]

I tried using the loading state and passing it to the new onFlagNotFound option. But it would still be true and not false when the SDK initialised and onFlagNotFound was called. I think this is due to the state being updated async, so this is what I went with as an alternative.

[knagurski]

The reason is that your handler is set in stone at the point it's created and so has the value of the state. This is because it's being defined in the useEffect, which isn't being recalled when the loading state changes. The ref is a cleaver way of getting the right value without triggering renders 👍🏻

[knagurski]

Just some minor stuff

[knagurski]

The reason is that your handler is set in stone at the point it's created and so has the value of the state. This is because it's being defined in the useEffect, which isn't being recalled when the loading state changes. The ref is a cleaver way of getting the right value without triggering renders 👍🏻