Added generic MapDecoder.

Author: simphotonics

lib/src/decoder/collection_decoder.dart

@@ -1,19 +1,18 @@
 part of '../reader.dart';
 
-class ListDecoder<T> extends Decoder<List<T>> {
+class ListDecoder<E> extends Decoder<List<E>> {
   const ListDecoder();
 
   @override
-  List<T> read(DartObject obj) =>
-      switch (obj.toListValue()?.map((item) => item.read<T>())) {
-        Iterable<T> iterable => iterable.toList(),
+  List<E> read(DartObject obj) =>
+      switch (obj.toListValue()?.map((item) => item.read<E>())) {
+        Iterable<E> iterable => iterable.toList(),
         _ => throw readError(obj),
       };
 }
 
 class SetDecoder<T> extends Decoder<Set<T>> {
-  const SetDecoder({this.prototype = const {}});
-  final Set<T> prototype;
+  const SetDecoder();
 
   @override
   Set<T> read(DartObject obj) =>
@@ -24,8 +23,7 @@ class SetDecoder<T> extends Decoder<Set<T>> {
 }
 
 class IterableDecoder<T> extends Decoder<Iterable<T>> {
-  const IterableDecoder({this.prototype = const {}});
-  final Set<T> prototype;
+  const IterableDecoder();
 
   @override
   Set<T> read(DartObject obj) =>
@@ -34,3 +32,27 @@ class IterableDecoder<T> extends Decoder<Iterable<T>> {
         _ => throw readError(obj),
       };
 }
+
+class MapDecoder<K, V> extends Decoder<Map<K, V>> {
+  const MapDecoder();
+  @override
+  Map<K, V> read(DartObject obj) {
+    final result = <K, V>{};
+
+    final mapObj = obj.toMapValue();
+    if (mapObj == null) {
+      throw readError(obj);
+    } else {
+      mapObj.forEach((keyObj, valueObj) {
+        final key = keyObj?.read<K>();
+        final value = valueObj?.read<V>();
+
+        if (key is K && value is V) {
+          //TODO will this test ever fail? Should this method throw?
+          result[key] = value;
+        }
+      });
+      return result;
+    }
+  }
+}