refactored inheritance circularity avoidance

HISTORY.md

@@ -17,6 +17,9 @@
     * NEW Slovak translation, thanks, Peter Lukacovic
     * German
 
+### 2019-12-19
+* objects, threads: refactored inheritance circularity avoidance
+
 ### 2019-12-18
 * gui, api: rearranged Snap! API into its own file
 * added API documentation

snap.html

@@ -7,8 +7,8 @@
         <script type="text/javascript" src="src/morphic.js?version=2019-11-12"></script>
         <script type="text/javascript" src="src/widgets.js?version=2019-10-16"></script>
         <script type="text/javascript" src="src/blocks.js?version=2019-12-13"></script>
-        <script type="text/javascript" src="src/threads.js?version=2019-12-18"></script>
-        <script type="text/javascript" src="src/objects.js?version=2019-12-16"></script>
+        <script type="text/javascript" src="src/threads.js?version=2019-12-19"></script>
+        <script type="text/javascript" src="src/objects.js?version=2019-12-19"></script>
         <script type="text/javascript" src="src/gui.js?version=2019-12-18"></script>
         <script type="text/javascript" src="src/paint.js?version=2019-06-27"></script>
         <script type="text/javascript" src="src/lists.js?version=2019-12-08"></script>

src/objects.js

@@ -84,7 +84,7 @@ BlockEditorMorph, BlockDialogMorph, PrototypeHatBlockMorph,  BooleanSlotMorph,
 localize, TableMorph, TableFrameMorph, normalizeCanvas, VectorPaintEditorMorph,
 HandleMorph, AlignmentMorph, Process, XML_Element, WorldMap, copyCanvas*/
 
-modules.objects = '2019-December-15';
+modules.objects = '2019-December-19';
 
 var SpriteMorph;
 var StageMorph;
@@ -6462,8 +6462,20 @@ SpriteMorph.prototype.chooseExemplar = function () {
     menu.popUpAtHand(this.world());
 };
 
-SpriteMorph.prototype.setExemplar = function (another) {
+SpriteMorph.prototype.setExemplar = function (another, enableError) {
     var ide;
+
+    // check for circularity
+    if (another instanceof SpriteMorph &&
+            contains(another.allExemplars(), this)) {
+        if (enableError) {
+            throw new Error(
+                localize('unable to inherit\n(disabled or circular?)')
+            );
+        }
+        return; // silently fail so stored projects can still be loaded
+    }
+
     this.emancipate();
     this.exemplar = another;
     if (another) {

src/threads.js

@@ -61,7 +61,7 @@ StageMorph, SpriteMorph, StagePrompterMorph, Note, modules, isString, copy,
 isNil, WatcherMorph, List, ListWatcherMorph, alert, console, TableMorph, Color,
 TableFrameMorph, ColorSlotMorph, isSnapObject, newCanvas, Symbol, SVG_Costume*/
 
-modules.threads = '2019-December-18';
+modules.threads = '2019-December-19';
 
 var ThreadManager;
 var Process;
@@ -4554,14 +4554,7 @@ Process.prototype.doSet = function (attribute, value) {
     case 'parent':
         this.assertType(rcvr, 'sprite');
         value = value instanceof SpriteMorph ? value : null;
-        // avoid circularity
-        if (value instanceof SpriteMorph &&
-                contains(value.allExemplars(), rcvr)) {
-            throw new Error(
-                localize('unable to inherit\n(disabled or circular?)')
-            );
-        }
-        rcvr.setExemplar(value);
+        rcvr.setExemplar(value, true); // throw an error in case of circularity
         break;
     case 'temporary?':
         this.assertType(rcvr, 'sprite');