correcly apply client-side JSON patch

Author: rmorshea

src/client/packages/idom-client-react/src/utils.js

@@ -2,31 +2,67 @@ import React from "react";
 import jsonpatch from "fast-json-patch";
 
 export function useJsonPatchCallback(initial) {
-  const model = React.useRef(initial);
+  const doc = React.useRef(initial);
   const forceUpdate = useForceUpdate();
 
   const applyPatch = React.useCallback(
-    (pathPrefix, patch) => {
-      if (pathPrefix) {
-        patch = patch.map((op) =>
-          Object.assign({}, op, { path: pathPrefix + op.path })
-        );
+    (path, patch) => {
+      let patchResult;
+      if (!path) {
+        // We CANNOT mutate the part of the document because React checks some
+        // attributes of the model (e.g. model.attributes.style is checked for
+        // identity).
+        patchResult = applyNonMutativePatch(doc, patch, false, false, true);
+      } else {
+        // We CAN mutate the document here though because we know that nothing above
+        // The patch `path` is changing. Thus, maintaining the identity for that section
+        // of the model is accurate.
+        patchResult = applyMutativePatch(doc.current, [
+          {
+            op: "replace",
+            path: path,
+            // We CANNOT mutate the part of the document where the actual patch is being
+            // applied. Instead we create a copy because React checks some attributes of
+            // the model (e.g. model.attributes.style is checked for identity). The part
+            // of the document above the `path` can be mutated though because we know it
+            // has not changed.
+            value: applyNonMutativePatch(
+              jsonpatch.getValueByPointer(doc.current, path),
+              patch
+            ),
+          },
+        ]);
       }
-      // Always return a newDocument because React checks some attributes of the model
-      // (e.g. model.attributes.style is checked for identity)
-      model.current = jsonpatch.applyPatch(
-        model.current,
-        patch,
-        false,
-        false,
-        true
-      ).newDocument;
+
+      doc.current = patchResult.newDocument;
       forceUpdate();
     },
-    [model]
+    [doc]
   );
 
-  return [model.current, applyPatch];
+  return [doc.current, applyPatch];
+}
+
+function applyNonMutativePatch(doc, patch) {
+  return jsonpatch.applyPatch(
+    doc,
+    patch,
+    false,
+
+    false,
+    true
+  ).newDocument;
+}
+
+function applyMutativePatch(doc, patch) {
+  return jsonpatch.applyPatch(
+    doc,
+    patch,
+    false,
+
+    true,
+    true
+  ).newDocument;
 }
 
 function useForceUpdate() {