refactor: Use threadsafe data structures to avoid any race conditions

Author: thomson-t

android/src/main/kotlin/com/mparticle/mparticle_flutter_sdk/MparticleFlutterSdkPlugin.kt

@@ -3,6 +3,7 @@ package com.mparticle.mparticle_flutter_sdk
 import android.app.Activity
 import android.content.Context
 import android.graphics.Typeface
+import android.os.Build
 import androidx.annotation.NonNull
 
 import io.flutter.embedding.engine.plugins.FlutterPlugin
@@ -60,7 +61,9 @@ class MparticleFlutterSdkPlugin: FlutterPlugin, MethodCallHandler, ActivityAware
         VIEW_TYPE,
         layoutFactory,
     )
-    roktEventHandler = RoktEventHandler(flutterPluginBinding.binaryMessenger)
+      if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
+          roktEventHandler = RoktEventHandler(flutterPluginBinding.binaryMessenger)
+      }
   }
 
   override fun onMethodCall(@NonNull call: MethodCall, @NonNull result: Result) {
@@ -750,12 +753,14 @@ class MparticleFlutterSdkPlugin: FlutterPlugin, MethodCallHandler, ActivityAware
       }
 
       MParticle.getInstance()?.let { instance ->
-        activity?.let {
-          roktEventHandler?.subscribeToEvents(
-            events = instance.Rokt().events(placementId),
-            activity = it,
-            identifier = placementId,
-          )
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
+          activity?.let {
+            roktEventHandler?.subscribeToEvents(
+              events = instance.Rokt().events(placementId),
+              activity = it,
+              identifier = placementId,
+            )
+          }
         }
 
         instance.Rokt().selectPlacements(placementId, stringAttributes, null, placeHolders.takeIf { it.isNotEmpty() }, customFonts, config)

android/src/main/kotlin/com/mparticle/mparticle_flutter_sdk/RoktEventHandler.kt

@@ -1,6 +1,8 @@
 package com.mparticle.mparticle_flutter_sdk
 
 import android.app.Activity
+import android.os.Build
+import androidx.annotation.RequiresApi
 import androidx.lifecycle.Lifecycle
 import androidx.lifecycle.LifecycleOwner
 import androidx.lifecycle.lifecycleScope
@@ -11,11 +13,13 @@ import io.flutter.plugin.common.EventChannel
 import kotlinx.coroutines.Job
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.launch
-import java.util.ArrayDeque
+import java.util.concurrent.ConcurrentHashMap
+import java.util.concurrent.ConcurrentLinkedDeque
 
+@RequiresApi(Build.VERSION_CODES.LOLLIPOP)
 class RoktEventHandler(private val messenger: BinaryMessenger) {
 
-    private val eventListeners = mutableMapOf<Any?, ArrayDeque<EventChannel.EventSink>>()
+    private val eventListeners = ConcurrentHashMap<Any?, ConcurrentLinkedDeque<EventChannel.EventSink>>()
     private val eventSubscriptions = mutableMapOf<String, Job?>()
 
     init {
@@ -75,18 +79,18 @@ class RoktEventHandler(private val messenger: BinaryMessenger) {
                     sink: EventChannel.EventSink?,
                 ) {
                     sink?.let {
-                        val sinks = eventListeners.getOrPut(arguments) { ArrayDeque() }
+                        val sinks = eventListeners.getOrPut(arguments ?: "") { ConcurrentLinkedDeque() }
                         sinks.addLast(it)
                     }
                 }
 
                 override fun onCancel(arguments: Any?) {
-                    val sinks = eventListeners[arguments]
+                    val sinks = eventListeners[arguments ?: ""]
                     if (sinks?.isNotEmpty() == true) {
                         sinks.removeLast()
                     }
                     if (sinks?.isEmpty() == true) {
-                        eventListeners.remove(arguments)
+                        eventListeners.remove(arguments ?: "")
                     }
                 }
             },