Add ConstantReader.peek. (#243)

* Add ConstantReader.peek.

* Update constants_test.dart

Author: matanlurey

CHANGELOG.md

@@ -13,9 +13,19 @@
     `package:json_serializable`.
   * Removed `lib/builder.dart`. Import through `source_gen.dart` instead.
   * Removed `OutputFormatter` typedef.
+  
 * Add `LibraryReader.allElements` - a utility to iterate across all `Element`
   instances contained in Dart library.
 * Add `LibraryReader.element` to get back to the `LibraryElement` instance.
+* Add `ConstantReader.peek` to read a value that returns `null` if not found:
+
+```dart
+// Tries to read the field "token" first, then "_token".
+findTokenField(DartObject o) {
+  final reader = new ConstantReader(o);
+  final token = o.peek('token') ?? o.read('_token');
+}
+```
 
 ## 0.6.1+1
 

lib/src/constants.dart

@@ -152,6 +152,11 @@ abstract class ConstantReader {
   /// [FormatException].
   ConstantReader read(String field);
 
+  /// Reads [field] from the constant as another constant value.
+  ///
+  /// Unlike [read], returns `null` if the field is not found.
+  ConstantReader peek(String field);
+
   /// Returns as a revived meta class.
   ///
   /// This is appropriate for cases where the underlying object is not a literal
@@ -224,6 +229,9 @@ class _NullConstant implements ConstantReader {
   @override
   ConstantReader read(_) => throw new UnsupportedError('Null');
 
+  @override
+  ConstantReader peek(_) => null;
+
   @override
   Revivable revive() => throw new UnsupportedError('Null');
 
@@ -307,14 +315,24 @@ class _Constant implements ConstantReader {
       checker.isAssignableFromType(_object.type);
 
   @override
-  ConstantReader read(String field) {
+  ConstantReader peek(String field) {
     final constant = new ConstantReader(_getFieldRecursive(_object, field));
     if (constant.isNull) {
-      _assertHasField(_object?.type?.element, field);
+      return null;
     }
     return constant;
   }
 
+  @override
+  ConstantReader read(String field) {
+    final reader = peek(field);
+    if (reader == null) {
+      _assertHasField(_object?.type?.element, field);
+      return const _NullConstant();
+    }
+    return reader;
+  }
+
   @override
   Revivable revive() => reviveInstance(_object);
 

test/constants_test.dart

@@ -99,7 +99,7 @@ void main() {
 
       final nested = constant.read('nested');
       expect(nested.isNull, isFalse, reason: '$nested');
-      expect(nested.read('aString').isNull, isTrue);
+      expect(nested.read('aString').isNull, isTrue, reason: '$nested');
       expect(nested.read('aInt').isNull, isTrue);
       expect(nested.read('aBool').isNull, isTrue);
     });
@@ -150,6 +150,12 @@ void main() {
       expect(() => $null.read('foo'), throwsUnsupportedError);
     });
 
+    test('should not fail reading from `null` when using peek', () {
+      final $null = constants[3];
+      expect($null.isNull, isTrue, reason: '${$null}');
+      expect($null.peek('foo'), isNull);
+    });
+
     test('should fail reading a missing field', () {
       final $super = constants[5];
       expect(() => $super.read('foo'), throwsFormatException);