Is possible to have an "event interceptor"?

[predofrazao]

I'm creating a machine that constantly receives frames at FRAMES_RECEIVED event and I have four states: detecting, validating, processing, done.

detecting and validating takes the frame and assign values, based on custom logic, to context, that I use in other events.

The "problem" is that these validations refer to the frame received in that moment. When the machine reaches a new state, it will have already another frame that needs to go through these validations to proceed; otherwise the flow must be restarted.

It would be nice if I could create an interceptor for this event that perform these validations before reaching the handler of the state, then I could redirect the machine to the initial state based on certain condition.

This is my first time working with FSM and XState, and although I've been studying it all week, I'm not sure if this idea makes sense for an FSM, but I'm really enjoying XState and would appreciate some help with it.

Thanks in advance.

[rothsandro]

[...] an interceptor for this event that perform these validations before reaching the handler of the state, then I could redirect the machine to the initial state based on certain condition.

I'm not sure I fully understand your requirements 🤔 When transitioning to processing state, a new frame comes in that needs to be validated. I assume the validation is async? What should happen with the previous frame (that is ready for processing) while the new frame is being validated?

[predofrazao]

Hi Sandro, firstly thanks for your time.

On my use-case, specifically when the machine reaches processing state, is when I can stop to listening for new frames and have enough to process the last one received (since it passed successfullly through all validations).

I kept working through the logic over the weekend and end ended with an idea that works for me, I basically created this "interceptor" by defining a global handler for the FRAMES_RECEIVED event that raises a FRAME_UPDATED event to be listen by the current state.

setup({
//...
}).createMachine({
  id: "frameCapture",
  initial: "detecting",
  on: {
    FRAMES_RECEIVED: [ // catch the FRAMES_RECEIVED "globally".
      {
        guard: or([stateIn("processing"), stateIn("done")]), // prevent `raise` when in `processing` or `done` states.
      },
      {
        target: "#frameCapture.detecting", // reset to first event when no frames received.
        actions: {
          type: "resetContext",
        },
        guard: {
          type: "noFramesDetected",
        },
      },
      {
        actions: [
          {
            type: "chooseFrame", // update the context with custom frame selection logic.
          },
          raise({ type: "FRAMES_UPDATED" }), // raise the FRAMES_UPDATED event that will be handled by the current state.
        ],
      },
    ],
  },
  states: {
    detecting: {
      on: {
        FRAMES_UPDATED: {} // handling raised events.
      }
    }
  }
})

[rothsandro]

Ah, now I understand. I'm glad you found a solution.