Only use async modifier when necessary (#54)

floor_generator/lib/model/change_method.dart

@@ -12,8 +12,10 @@ class ChangeMethod {
 
   DartType get returnType => method.returnType;
 
+  /// Whether this method returns an int or a list of ints.
   bool get returnsInt => isInt(_flattenedReturnType);
 
+  /// Whether this methods 'returns' void.
   bool get returnsVoid => _flattenedReturnType.isVoid;
 
   String get name => method.displayName;
@@ -34,6 +36,8 @@ class ChangeMethod {
     return parameters.first;
   }
 
+  /// Whether this method takes a list as parameter and thus
+  /// changes multiple items.
   bool get changesMultipleItems => isList(parameter.type);
 
   ClassElement get flattenedParameterClass {
@@ -53,6 +57,13 @@ class ChangeMethod {
         .any((entity) => entity == _flattenedParameterType.displayName);
   }
 
+  bool get requiresAsyncModifier => returnsVoid || changesMultipleItems;
+
+  bool get returnsList {
+    final type = method.returnType.flattenFutures(method.context.typeSystem);
+    return isList(type);
+  }
+
   DartType get _flattenedParameterType {
     return changesMultipleItems ? flattenList(parameter.type) : parameter.type;
   }
@@ -68,9 +79,4 @@ class ChangeMethod {
     final type = method.returnType.flattenFutures(method.context.typeSystem);
     return returnsList ? flattenList(type) : type;
   }
-
-  bool get returnsList {
-    final type = method.returnType.flattenFutures(method.context.typeSystem);
-    return isList(type);
-  }
 }

floor_generator/lib/writer/change_method_writer.dart

@@ -17,15 +17,19 @@ class ChangeMethodWriter implements Writer {
     return _generateChangeMethod();
   }
 
-  // TODO remove async modifier when returning single int (one id)
   Method _generateChangeMethod() {
-    return Method((builder) => builder
+    final builder = MethodBuilder()
       ..annotations.add(overrideAnnotationExpression)
       ..returns = refer(method.returnType.displayName)
       ..name = method.name
       ..requiredParameters.add(_generateParameter())
-      ..modifier = MethodModifier.async
-      ..body = methodBodyWriter.write());
+      ..body = methodBodyWriter.write();
+
+    if (method.requiresAsyncModifier) {
+      builder..modifier = MethodModifier.async;
+    }
+
+    return builder.build();
   }
 
   Parameter _generateParameter() {