Add support for addListener on iOS

ios/Plugin/TextToSpeech.swift

@@ -9,6 +9,7 @@ enum QUEUE_STRATEGY: Int {
     let synthesizer = AVSpeechSynthesizer()
     var calls: [CAPPluginCall] = []
     let queue = DispatchQueue(label: "backgroundAudioSetup", qos: .userInitiated, attributes: [], autoreleaseFrequency: .inherit, target: nil)
+    private weak var plugin: TextToSpeechPlugin?
 
     override init() {
         super.init()
@@ -25,6 +26,22 @@ enum QUEUE_STRATEGY: Int {
         }
     }
 
+    init(plugin: TextToSpeechPlugin) {
+        self.plugin = plugin
+        super.init()
+        self.synthesizer.delegate = self
+        // set session in background to avoid UI hangs.
+        queue.async {
+            do {
+                let avAudioSessionCategory: AVAudioSession.Category = .playback
+                try AVAudioSession.sharedInstance().setCategory(avAudioSessionCategory, mode: .default, options: .duckOthers)
+                try AVAudioSession.sharedInstance().setActive(true)
+            } catch {
+                print("Error setting up AVAudioSession: \(error)")
+            }
+        }
+    }
+
     public func speechSynthesizer(_ synthesizer: AVSpeechSynthesizer, didCancel utterance: AVSpeechUtterance) {
         self.resolveCurrentCall()
     }
@@ -33,6 +50,24 @@ enum QUEUE_STRATEGY: Int {
         self.resolveCurrentCall()
     }
 
+    public func speechSynthesizer(_ synthesizer: AVSpeechSynthesizer, willSpeakRangeOfSpeechString characterRange: NSRange, utterance: AVSpeechUtterance) {
+        guard let plugin = self.plugin else { return }
+
+        let start = characterRange.location
+        let end = characterRange.location + characterRange.length
+
+        if let range = Range(characterRange, in: utterance.speechString) {
+            let spokenWord = String(utterance.speechString[range])
+            let ret = [
+                "start": start,
+                "end": end,
+                "spokenWord": spokenWord
+            ] as [String: Any]
+
+            plugin.notifyListeners("onRangeStart", data: ret)
+        }
+    }
+
     @objc public func speak(_ text: String, _ lang: String, _ rate: Float, _ pitch: Float, _ category: String, _ volume: Float, _ voice: Int, _ queueStrategy: Int, _ call: CAPPluginCall) throws {
         if queueStrategy == QUEUE_STRATEGY.QUEUE_FLUSH.rawValue {
             self.synthesizer.stopSpeaking(at: .immediate)

ios/Plugin/TextToSpeechPlugin.swift

@@ -10,7 +10,11 @@ import AVFoundation
 public class TextToSpeechPlugin: CAPPlugin {
     private static let errorUnsupportedLanguage = "This language is not supported."
 
-    private let implementation = TextToSpeech()
+    private var implementation: TextToSpeech!
+
+    public override func load() {
+        implementation = TextToSpeech(plugin: self)
+    }
 
     @objc public func speak(_ call: CAPPluginCall) {
         let text = call.getString("text") ?? ""