cache signature runner

lib/src/interpreter.dart

@@ -31,6 +31,7 @@ import 'tensor.dart';
 /// TensorFlowLite interpreter for running inference on a model.
 class Interpreter {
   final Pointer<TfLiteInterpreter> _interpreter;
+  final Map<String, Pointer<TfLiteSignatureRunner>> _signatureRunners = {};
   bool _deleted = false;
   bool _allocated = false;
   int _lastNativeInferenceDurationMicroSeconds = 0;
@@ -327,6 +328,10 @@ class Interpreter {
   void resetVariableTensors() {
     checkState(_deleted,
         message: 'Should not access delegate after it has been closed.');
+    // loop through signatureRunners and delete them
+    _signatureRunners.forEach((key, value) {
+      _deleteSignatureRunner(value);
+    });
     tfliteBinding.TfLiteInterpreterResetVariableTensors(_interpreter);
   }
 
@@ -413,12 +418,12 @@ class Interpreter {
 
     final Pointer<TfLiteSignatureRunner> signatureRunner;
 
-    try {
-      // TODO: Should we cache the signature runner?
+    // check if signature key exists
+    if (!_signatureRunners.containsKey(signatureKey)) {
       signatureRunner = getSignatureRunner(signatureKey);
-      _allocated = false;
-    } catch (e) {
-      throw ArgumentError('Input error: Signature key is not valid.');
+      _signatureRunners[signatureKey] = signatureRunner;
+    } else {
+      signatureRunner = _signatureRunners[signatureKey]!;
     }
 
     inputs.forEach((key, value) {
@@ -454,10 +459,17 @@ class Interpreter {
   }
 
   /// Delete signature runner. Should be called before interpreter is deleted.
-  void deleteSignatureRunner(Pointer<TfLiteSignatureRunner> signatureRunner) {
+  void _deleteSignatureRunner(Pointer<TfLiteSignatureRunner> signatureRunner) {
     tfliteBinding.TfLiteSignatureRunnerDelete(signatureRunner);
   }
 
+  void deleteSignatureRunner(String signatureKey) {
+    if (_signatureRunners.containsKey(signatureKey)) {
+      _deleteSignatureRunner(_signatureRunners[signatureKey]!);
+      _signatureRunners.remove(signatureKey);
+    }
+  }
+
   /// Returns the address to the interpreter
   int get address => _interpreter.address;