Add existing to tracked

node_modules/@cesium/engine/Source/DataSources/createPropertyDescriptor.js

@@ -0,0 +1,71 @@
+import defined from "../Core/defined.js";
+import ConstantProperty from "./ConstantProperty.js";
+
+function createProperty(
+  name,
+  privateName,
+  subscriptionName,
+  configurable,
+  createPropertyCallback,
+) {
+  return {
+    configurable: configurable,
+    get: function () {
+      return this[privateName];
+    },
+    set: function (value) {
+      const oldValue = this[privateName];
+      const subscription = this[subscriptionName];
+      if (defined(subscription)) {
+        subscription();
+        this[subscriptionName] = undefined;
+      }
+
+      const hasValue = value !== undefined;
+      if (
+        hasValue &&
+        (!defined(value) || !defined(value.getValue)) &&
+        defined(createPropertyCallback)
+      ) {
+        value = createPropertyCallback(value);
+      }
+
+      if (oldValue !== value) {
+        this[privateName] = value;
+        this._definitionChanged.raiseEvent(this, name, value, oldValue);
+      }
+
+      if (defined(value) && defined(value.definitionChanged)) {
+        this[subscriptionName] = value.definitionChanged.addEventListener(
+          function () {
+            this._definitionChanged.raiseEvent(this, name, value, value);
+          },
+          this,
+        );
+      }
+    },
+  };
+}
+
+function createConstantProperty(value) {
+  return new ConstantProperty(value);
+}
+
+/**
+ * Used to consistently define all DataSources graphics objects.
+ * This is broken into two functions because the Chrome profiler does a better
+ * job of optimizing lookups if it notices that the string is constant throughout the function.
+ * @private
+ */
+function createPropertyDescriptor(name, configurable, createPropertyCallback) {
+  //Safari 8.0.3 has a JavaScript bug that causes it to confuse two variables and treat them as the same.
+  //The two extra toString calls work around the issue.
+  return createProperty(
+    name,
+    `_${name.toString()}`,
+    `_${name.toString()}Subscription`,
+    configurable ?? false,
+    createPropertyCallback ?? createConstantProperty,
+  );
+}
+export default createPropertyDescriptor;