Make JSON encoder take extra function argument to use instead of toJson calls.

sdk/lib/json/json.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Utilities for encoding and decoding JSON (JavaScript Object Notation) data.
  */
 
 @deprecated
 library dart.json;
 
 import "dart:convert";
 import "dart:_collection-dev" show deprecated;
+import "dart:collection" show HashSet;
 export "dart:convert" show JsonUnsupportedObjectError, JsonCyclicError;
 
 // JSON parsing and serialization.
 
 /**
  * *DEPRECATED* Use `dart:convert JSON.decode` instead.
  *
  * Parses [json] and build the corresponding parsed JSON value.
  *
  * Parsed JSON values are of the types [num], [String], [bool], [Null],
@@ skipping 20 matching lines @@
 /**
  * *DEPRECATED* Use `dart:convert JSON.encode` instead.
  *
  * Serializes [object] into a JSON string.
  *
  * Directly serializable values are [num], [String], [bool], and [Null], as well
  * as some [List] and [Map] values.
  * For [List], the elements must all be serializable.
  * For [Map], the keys must be [String] and the values must be serializable.
  *
- * If a value is any other type is attempted serialized, a "toJson()" method
- * is invoked on the object and the result, which must be a directly
- * serializable value, is serialized instead of the original value.
+ * If a value is any other type is attempted serialized, the [toEncodable]
+ * method is called with the object as argument, and the result, which must be a
+ * directly serializable value, is serialized instead of the original value.
+ * If [toEncodable] is omitted, the default is to call `object.toJson()` on the
+ * object.
  *
- * If the object does not support this method, throws, or returns a
- * value that is not directly serializable, a [JsonUnsupportedObjectError]
- * exception is thrown. If the call throws (including the case where there
- * is no nullary "toJson" method, the error is caught and stored in the
+ * If the conversion throws, or returns a value that is not directly
+ * serializable, a [JsonUnsupportedObjectError] exception is thrown.
+ * If the call throws (including the case where there
+ * is no nullary "toJson" method), the error is caught and stored in the
  * [JsonUnsupportedObjectError]'s [:cause:] field.
  *
  * If a [List] or [Map] contains a reference to itself, directly or through
  * other lists or maps, it cannot be serialized and a [JsonCyclicError] is
  * thrown.
  *
- * Json Objects should not change during serialization.
+ * The objects being serialized should not change during serialization.
  * If an object is serialized more than once, [stringify] is allowed to cache
  * the JSON text for it. I.e., if an object changes after it is first
  * serialized, the new values may or may not be reflected in the result.
  */
 @deprecated
-String stringify(Object object) {
-  return _JsonStringifier.stringify(object);
+String stringify(Object object, [toEncodable(object)]) {
+  if (toEncodable == null) toEncodable = _defaultToEncodable;
+  return _JsonStringifier.stringify(object, toEncodable);
 }
 
 /**
  * *DEPRECATED* Use `package:json/json.dart` or `dart:convert` instead.
  *
  * Serializes [object] into [output] stream.
  *
  * Performs the same operations as [stringify] but outputs the resulting
  * string to an existing [StringSink] instead of creating a new [String].
  *
  * If serialization fails by throwing, some data might have been added to
  * [output], but it won't contain valid JSON text.
  */
 @deprecated
-void printOn(Object object, StringSink output) {
-  return _JsonStringifier.printOn(object, output);
+void printOn(Object object, StringSink output, [ toEncodable(object) ]) {
+  if (toEncodable == null) toEncodable = _defaultToEncodable;
+  return _JsonStringifier.printOn(object, output, toEncodable);
 }
 
 //// Implementation ///////////////////////////////////////////////////////////
 
 // Simple API for JSON parsing.
 
 /// *DEPRECATED* Use `package:json/json.dart` instead.
 @deprecated
 abstract class JsonListener {
   void handleString(String value) {}
@@ skipping 534 matching lines @@
     int sliceEnd = position + 20;
     if (sliceEnd > source.length) {
       slice = "'${source.substring(position)}'";
     } else {
       slice = "'${source.substring(position, sliceEnd)}...'";
     }
     throw new FormatException("Unexpected character at $position: $slice");
   }
 }
 
+Object _defaultToEncodable(object) => object.toJson();
 
 class _JsonStringifier {
-  StringSink sink;
-  List<Object> seen;  // TODO: that should be identity set.
+  final Function toEncodable;
+  final StringSink sink;
+  final Set<Object> seen;
 
-  _JsonStringifier(this.sink) : seen = [];
+  _JsonStringifier(this.sink, this.toEncodable)
+      : this.seen = new HashSet.identity();
 
-  static String stringify(final object) {
+  static String stringify(final object, toEncodable(object)) {
     StringBuffer output = new StringBuffer();
-    _JsonStringifier stringifier = new _JsonStringifier(output);
+    _JsonStringifier stringifier = new _JsonStringifier(output, toEncodable);
     stringifier.stringifyValue(object);
     return output.toString();
   }
 
-  static void printOn(final object, StringSink output) {
-    _JsonStringifier stringifier = new _JsonStringifier(output);
+  static void printOn(final object, StringSink output, toEncodable(object)) {
+    _JsonStringifier stringifier = new _JsonStringifier(output, toEncodable);
     stringifier.stringifyValue(object);
   }
 
   static String numberToString(num x) {
     return x.toString();
   }
 
   // ('0' + x) or ('a' + x - 10)
   static int hexDigit(int x) => x < 10 ? 48 + x : 87 + x;
 
@@ skipping 36 matching lines @@
         charCodes.add(JsonParser.BACKSLASH);
         charCodes.add(charCode);
       } else {
         charCodes.add(charCode);
       }
     }
     sb.write(needsEscape ? new String.fromCharCodes(charCodes) : s);
   }
 
   void checkCycle(final object) {
-    // TODO: use Iterables.
-    for (int i = 0; i < seen.length; i++) {
-      if (identical(seen[i], object)) {
-        throw new JsonCyclicError(object);
-      }
+    if (seen.contains(object)) {
+      throw new JsonCyclicError(object);
     }
     seen.add(object);
   }
 
   void stringifyValue(final object) {
     // Tries stringifying object directly. If it's not a simple value, List or
     // Map, call toJson() to get a custom representation and try serializing
     // that.
     if (!stringifyJsonValue(object)) {
       checkCycle(object);
       try {
-        var customJson = object.toJson();
+        var customJson = toEncodable(object);
         if (!stringifyJsonValue(customJson)) {
           throw new JsonUnsupportedObjectError(object);
         }
-        seen.removeLast();
+        seen.remove(object);
       } catch (e) {
         throw new JsonUnsupportedObjectError(object, cause: e);
       }
     }
   }
 
   /**
    * Serializes a [num], [String], [bool], [Null], [List] or [Map] value.
    *
    * Returns true if the value is one of these types, and false if not.
@@ skipping 24 matching lines @@
       sink.write('[');
       if (a.length > 0) {
         stringifyValue(a[0]);
         // TODO: switch to Iterables.
         for (int i = 1; i < a.length; i++) {
           sink.write(',');
           stringifyValue(a[i]);
         }
       }
       sink.write(']');
-      seen.removeLast();
+      seen.remove(object);
       return true;
     } else if (object is Map) {
       checkCycle(object);
       Map<String, Object> m = object;
       sink.write('{');
       bool first = true;
       m.forEach((String key, Object value) {
         if (!first) {
           sink.write(',"');
         } else {
           sink.write('"');
         }
         escape(sink, key);
         sink.write('":');
         stringifyValue(value);
         first = false;
       });
       sink.write('}');
-      seen.removeLast();
+      seen.remove(object);
       return true;
     } else {
       return false;
     }
   }
 }