Centralize logic for walking type hierarchy

Author: kevmoo

json_serializable/lib/src/shared_checkers.dart

@@ -59,22 +59,21 @@ String asStatement(DartType type) {
   return ' as $type';
 }
 
-DartType _getImplementationType(DartType type, TypeChecker checker) {
-  if (checker.isExactlyType(type)) return type;
+/// Returns all of the [DartType] types that [type] implements, mixes-in, and
+/// extends, starting with [type] itself.
+Iterable<DartType> typeImplementations(DartType type) sync* {
+  yield type;
 
   if (type is InterfaceType) {
-    var match = type.interfaces
-        .followedBy(type.mixins)
-        .map((type) => _getImplementationType(type, checker))
-        .firstWhere((value) => value != null, orElse: () => null);
-
-    if (match != null) {
-      return match;
-    }
+    yield* type.interfaces.expand(typeImplementations);
+    yield* type.mixins.expand(typeImplementations);
 
     if (type.superclass != null) {
-      return _getImplementationType(type.superclass, checker);
+      yield* typeImplementations(type.superclass);
     }
   }
-  return null;
 }
+
+DartType _getImplementationType(DartType type, TypeChecker checker) =>
+    typeImplementations(type)
+        .firstWhere(checker.isExactlyType, orElse: () => null);

json_serializable/lib/src/type_helpers/json_helper.dart

@@ -67,7 +67,7 @@ class JsonHelper extends TypeHelper {
 
 bool _canSerialize(DartType type) {
   if (type is InterfaceType) {
-    var toJsonMethod = _getMethod(type, 'toJson');
+    var toJsonMethod = _toJsonMethod(type);
 
     if (toJsonMethod != null) {
       // TODO: validate there are no required parameters
@@ -95,25 +95,6 @@ JsonSerializable _annotation(InterfaceType source) {
   return valueForAnnotation(new ConstantReader(annotations.single));
 }
 
-MethodElement _getMethod(DartType type, String methodName) {
-  if (type is InterfaceType) {
-    var method = type.element.getMethod(methodName);
-    if (method != null) {
-      return method;
-    }
-
-    var match = type.interfaces
-        .followedBy(type.mixins)
-        .map((type) => _getMethod(type, methodName))
-        .firstWhere((value) => value != null, orElse: () => null);
-
-    if (match != null) {
-      return match;
-    }
-
-    if (type.superclass != null) {
-      return _getMethod(type.superclass, methodName);
-    }
-  }
-  return null;
-}
+MethodElement _toJsonMethod(DartType type) => typeImplementations(type)
+    .map((dt) => dt is InterfaceType ? dt.getMethod('toJson') : null)
+    .firstWhere((me) => me != null, orElse: () => null);