Fix #70: Add @Volatile to DefaultNetworkListener fields

core/src/main/java/io/github/madeeye/meow/net/DefaultNetworkListener.kt

@@ -0,0 +1,35 @@
+package io.github.madeye.meow.net
+
+import android.net.ConnectivityManager
+import android.net.Network
+import android.net.NetworkCapabilities
+import android.net.NetworkRequest
+import kotlinx.coroutines.CoroutineScope
+import timber.log.Timber
+
+object DefaultNetworkListener {
+    @Volatile private var callback: ConnectivityManager.NetworkCallback? = null
+    @Volatile private var connectivityManager: ConnectivityManager? = null
+
+    fun start(service: android.net.VpnService, onNetworkChanged: (Network?) -> Unit) {
+        val cm = service.getSystemService(ConnectivityManager::class.java)
+        connectivityManager = cm
+        val cb = object : ConnectivityManager.NetworkCallback() {
+            override fun onAvailable(network: Network) { onNetworkChanged(network) }
+            override fun onLost(network: Network) { onNetworkChanged(null) }
+            override fun onCapabilitiesChanged(network: Network, nc: NetworkCapabilities) { onNetworkChanged(network) }
+        }
+        callback = cb
+        val request = NetworkRequest.Builder()
+            .addCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET)
+            .addCapability(NetworkCapabilities.NET_CAPABILITY_NOT_RESTRICTED)
+            .build()
+        cm.registerNetworkCallback(request, cb)
+    }
+
+    fun stop(scope: CoroutineScope) {
+        try { callback?.let { connectivityManager?.unregisterNetworkCallback(it) } }
+        catch (e: Exception) { Timber.w(e) }
+        callback = null
+    }
+}