feat: implement dedicated handler thread for IO operations

This prevents accidental simultaneous calls to the Android NFC methods,
as well as performing better due to the lack of overhead from spawning
new threads on each call.

Author: knthm

android/src/main/kotlin/im/nfc/flutter_nfc_kit/FlutterNfcKitPlugin.kt

@@ -8,6 +8,7 @@ import android.nfc.NfcAdapter
 import android.nfc.NfcAdapter.*
 import android.nfc.tech.*
 import android.os.Handler
+import android.os.HandlerThread
 import android.os.Looper
 import im.nfc.flutter_nfc_kit.ByteUtils.canonicalizeData
 import im.nfc.flutter_nfc_kit.ByteUtils.hexToBytes
@@ -30,7 +31,6 @@ import java.lang.ref.WeakReference
 import java.lang.reflect.InvocationTargetException
 import java.util.*
 import kotlin.concurrent.schedule
-import kotlin.concurrent.thread
 
 
 class FlutterNfcKitPlugin : FlutterPlugin, MethodCallHandler, ActivityAware {
@@ -43,6 +43,9 @@ class FlutterNfcKitPlugin : FlutterPlugin, MethodCallHandler, ActivityAware {
         private var ndefTechnology: Ndef? = null
         private var mifareInfo: MifareInfo? = null
 
+        private lateinit var nfcHandlerThread: HandlerThread
+        private lateinit var nfcHandler: Handler
+
         private fun TagTechnology.transceive(data: ByteArray, timeout: Int?): ByteArray {
             if (timeout != null) {
                 try {
@@ -53,13 +56,27 @@ class FlutterNfcKitPlugin : FlutterPlugin, MethodCallHandler, ActivityAware {
             val transceiveMethod = this.javaClass.getMethod("transceive", ByteArray::class.java)
             return transceiveMethod.invoke(this, data) as ByteArray
         }
+
+        private fun runOnNfcThread(result: Result, fn: () -> Unit) {
+            if (!nfcHandler.post(fn)) {
+                result.error("500", "Failed to post job to NFC Handler thread.", null)
+            }
+        }
     }
 
     override fun onAttachedToEngine(flutterPluginBinding: FlutterPlugin.FlutterPluginBinding) {
+        nfcHandlerThread = HandlerThread("NfcHandlerThread")
+        nfcHandlerThread.start()
+        nfcHandler = Handler(nfcHandlerThread.looper)
+
         val channel = MethodChannel(flutterPluginBinding.binaryMessenger, "flutter_nfc_kit")
         channel.setMethodCallHandler(this)
     }
 
+    override fun onDetachedFromEngine(binding: FlutterPlugin.FlutterPluginBinding) {
+        nfcHandlerThread.quitSafely()
+    }
+
     override fun onMethodCall(call: MethodCall, result: Result) {
         handleMethodCall(call, MethodResultWrapper(result))
     }
@@ -113,14 +130,14 @@ class FlutterNfcKitPlugin : FlutterPlugin, MethodCallHandler, ActivityAware {
                 val timeout = call.argument<Int>("timeout")!!
                 // technology and option bits are set in Dart code
                 val technologies = call.argument<Int>("technologies")!!
-                thread {
+                runOnNfcThread(result) {
                     pollTag(nfcAdapter, result, timeout, technologies)
                 }
             }
 
             "finish" -> {
                 pollingTimeoutTask?.cancel()
-                thread {
+                runOnNfcThread(result) {
                     try {
                         val tagTech = tagTechnology
                         if (tagTech != null && tagTech.isConnected) {
@@ -155,7 +172,7 @@ class FlutterNfcKitPlugin : FlutterPlugin, MethodCallHandler, ActivityAware {
                 }
                 val (sendingBytes, sendingHex) = canonicalizeData(data)
 
-                thread {
+                runOnNfcThread(result) {
                     try {
                         switchTechnology(tagTech, ndefTechnology)
                         val timeout = call.argument<Int>("timeout")
@@ -187,7 +204,7 @@ class FlutterNfcKitPlugin : FlutterPlugin, MethodCallHandler, ActivityAware {
             "readNDEF" -> {
                 if (!ensureNDEF()) return
                 val ndef = ndefTechnology!!
-                thread {
+                runOnNfcThread(result) {
                     try {
                         switchTechnology(ndef, tagTechnology)
                         // read NDEF message
@@ -236,7 +253,7 @@ class FlutterNfcKitPlugin : FlutterPlugin, MethodCallHandler, ActivityAware {
                     result.error("405", "Tag not writable", null)
                     return
                 }
-                thread {
+                runOnNfcThread(result) {
                     try {
                         switchTechnology(ndef, tagTechnology)
                         // generate NDEF message
@@ -283,7 +300,7 @@ class FlutterNfcKitPlugin : FlutterPlugin, MethodCallHandler, ActivityAware {
                     result.error("405", "Tag not writable", null)
                     return
                 }
-                thread {
+                runOnNfcThread(result) {
                     try {
                         switchTechnology(ndef, tagTechnology)
                         if (ndef.makeReadOnly()) {
@@ -316,22 +333,22 @@ class FlutterNfcKitPlugin : FlutterPlugin, MethodCallHandler, ActivityAware {
                 }
                 val keyA = call.argument<Any>("keyA")
                 val keyB = call.argument<Any>("keyB")
-                thread {
+                runOnNfcThread(result) {
                     try {
                         val tag = tagTech as MifareClassic
                         switchTechnology(tagTech, ndefTechnology)
                         // key A takes precedence if present
-                        val success = if (keyA != null) {
+                        if (keyA != null) {
                             val (key, _) = canonicalizeData(keyA)
-                            tag.authenticateSectorWithKeyA(index, key)
+                            val authStatus = tag.authenticateSectorWithKeyA(index, key)
+                            result.success(authStatus)
                         } else if (keyB != null) {
                             val (key, _) = canonicalizeData(keyB)
-                            tag.authenticateSectorWithKeyB(index, key)
+                            val authStatus = tag.authenticateSectorWithKeyB(index, key)
+                            result.success(authStatus)
                         } else {
                             result.error("400", "No keys provided", null)
-                            return@thread
                         }
-                        result.success(success)
                     } catch (ex: IOException) {
                         Log.e(TAG, "Authenticate block error", ex)
                         result.error("500", "Authentication error", ex.localizedMessage)
@@ -351,7 +368,7 @@ class FlutterNfcKitPlugin : FlutterPlugin, MethodCallHandler, ActivityAware {
                     result.error("400", "Invalid block/page index $index, should be in (0, $maxBlock)", null)
                     return
                 }
-                thread {
+                runOnNfcThread(result) {
                     try {
                         switchTechnology(tagTech, ndefTechnology)
                         tagTech.readBlock(index, result)
@@ -374,7 +391,7 @@ class FlutterNfcKitPlugin : FlutterPlugin, MethodCallHandler, ActivityAware {
                     result.error("400", "Invalid sector index $index, should be in (0, $maxSector)", null)
                     return
                 }
-                thread {
+                runOnNfcThread(result) {
                     try {
                         val tag = tagTech as MifareClassic
                         switchTechnology(tagTech, ndefTechnology)
@@ -407,7 +424,7 @@ class FlutterNfcKitPlugin : FlutterPlugin, MethodCallHandler, ActivityAware {
                     result.error("400", "Invalid data size ${bytes.size}, should be ${mifareInfo!!.blockSize}", null)
                     return
                 }
-                thread {
+                runOnNfcThread(result) {
                     try {
                         switchTechnology(tagTech, ndefTechnology)
                         tagTech.writeBlock(index, bytes, result)
@@ -421,9 +438,6 @@ class FlutterNfcKitPlugin : FlutterPlugin, MethodCallHandler, ActivityAware {
         }
     }
 
-
-    override fun onDetachedFromEngine(binding: FlutterPlugin.FlutterPluginBinding) {}
-
     override fun onAttachedToActivity(binding: ActivityPluginBinding) {
         activity = WeakReference(binding.activity)
     }