AudioQueueRef intercept audio data to build a music visualiser (or AVAudioEngine or AudioEngine from AudioKit)

Question

I want to integrate the FFTView from AudioKit into my app to show this cool visualiser.

The problem I am facing is that the audio that I am playing comes from a 2014 library (TritonSDK) which streams the audio from somewhere to provide you a radio station to listen to. So I have no control over the audio whatsoever, everything is handled by the SDK.

The SDK however, is exposing a AudioQueueRef property, but from what I see in Apple's docs, there is no way to subscribe to audio data to it, if you're not controlling the creation of the queue.

So I thought, plan B: use AVAudioEngine to pick up the sound that is already playing, but unfortunately I don't get any data for the visualiser (I get back 0 value array of Floats)

Any idea what I might be doing wrong, or is this even possible what I'm trying? Thanks in advance!

My setup:

import AudioKit
import AVFAudio

final class AudioCapture: ObservableObject {
    @Published private(set) var node: Node

    private let engine = AVAudioEngine()

    init() {
        self.node = NodeWrapper(avAudioNode: engine.mainMixerNode)
    }

    func start() {
        do {
            try engine.start()
        } catch {
            print("Error starting AVAudioEngine: \(error.localizedDescription)")
        }
        if node.avAudioNode != engine.mainMixerNode {
            node = NodeWrapper(avAudioNode: engine.mainMixerNode)
        }
    }

    func stop() {
        engine.stop()
    }
}

// MARK: - Utility

private final class NodeWrapper: Node {
    var connections: [Node] = []
    let avAudioNode: AVAudioNode

    init(avAudioNode: AVAudioNode) {
        self.avAudioNode = avAudioNode
    }
}

import SwiftUI
import AudioKitUI

struct RadioView: View {
    @ObservedObject var viewModel: RadioViewModel
    @Environment(\.safeAreaInsets) private var safeAreaInsets: EdgeInsets

    var body: some View {
        FFTView(viewModel.audioCapture.node)
    }
}