Add shouldTrackChanges option to @snapshottedView

spec/class-model-snapshotted-views.spec.ts

@@ -92,6 +92,28 @@ describe("class model snapshotted views", () => {
     expect(fn).toMatchSnapshot();
   });
 
+  test("an observable instance's doesn't emit a patch when the shouldTrackChanges option returns false", () => {
+    const observableArray = observable.array<string>([]);
+
+    @register
+    class MyViewExample extends ClassModel({ key: types.identifier, name: types.string }) {
+      @snapshottedView({ shouldTrackChanges: () => false })
+      get arrayLength() {
+        return observableArray.length;
+      }
+    }
+
+    const fn = jest.fn();
+    const instance = MyViewExample.create({ key: "1", name: "Test" });
+    onPatch(instance, fn);
+
+    runInAction(() => {
+      observableArray.push("a");
+    });
+
+    expect(fn).not.toHaveBeenCalled();
+  });
+
   test("an observable instance's snapshot includes the snapshotted views epoch", () => {
     const instance = ViewExample.create({ key: "1", name: "Test" });
     expect(getSnapshot(instance)).toEqual({ __snapshottedViewsEpoch: 0, key: "1", name: "Test" });

src/class-model.ts

@@ -49,6 +49,9 @@ export interface SnapshottedViewOptions<V, T extends IAnyClassModelType> {
   /** A function for converting the view value to a snapshot value */
   createSnapshot?: (value: V) => any;
 
+  /** A function that determines whether the view should be tracked for changes. */
+  shouldTrackChanges?: (node: Instance<T>) => boolean;
+
   /** A function that will be called when the view's reaction throws an error. */
   onError?: (error: any) => void;
 }
@@ -303,6 +306,10 @@ export function register<Instance, Klass extends { new (...args: any[]): Instanc
         return {
           afterCreate() {
             for (const view of klass.snapshottedViews) {
+              if (view.options.shouldTrackChanges && !view.options.shouldTrackChanges(self)) {
+                continue;
+              }
+
               reactions.push(
                 reaction(
                   () => {