Hydrate snapshotted views lazily to avoid excessive root boot costs

airhorns · scott-rc · commit 48c554202dba · 2024-06-24T17:42:40.000-04:00
[no-changelog-required]
diff --git a/spec/class-model-snapshotted-views.spec.ts b/spec/class-model-snapshotted-views.spec.ts
@@ -1,4 +1,4 @@
-import { observable } from "mobx";
+import { observable, runInAction } from "mobx";
 import { ClassModel, action, snapshottedView, getSnapshot, register, types, onPatch } from "../src";
 import { Apple } from "./fixtures/FruitAisle";
 import { create } from "./helpers";
@@ -85,7 +85,9 @@ describe("class model snapshotted views", () => {
     const instance = MyViewExample.create({ key: "1", name: "Test" });
     onPatch(instance, fn);
 
-    observableArray.push("a");
+    runInAction(() => {
+      observableArray.push("a");
+    });
     expect(fn).toMatchSnapshot();
   });
 
@@ -141,8 +143,7 @@ describe("class model snapshotted views", () => {
     @register
     class HydrateExample extends ClassModel({ url: types.string }) {
       @snapshottedView<URL>({
-        createReadOnly(value, snapshot, node) {
-          expect(snapshot).toBeDefined();
+        createReadOnly(value, node) {
           expect(node).toBeDefined();
           return value ? new URL(value) : undefined;
         },
@@ -178,6 +179,25 @@ describe("class model snapshotted views", () => {
       } as any);
       expect(instance.withoutParams).toEqual(new URL("https://gadget.dev/blog/feature/extra"));
     });
+
+    test("hydrators aren't called eagerly on readonly instances in case they are expensive", () => {
+      const fn = jest.fn().mockReturnValue("whatever");
+      @register
+      class HydrateExampleSpy extends ClassModel({}) {
+        @snapshottedView<URL>({
+          createReadOnly: fn,
+        })
+        get someView() {
+          return "view value";
+        }
+      }
+
+      const instance = HydrateExampleSpy.createReadOnly({ someView: "snapshot value" });
+      expect(fn).not.toHaveBeenCalled();
+
+      expect(instance.someView).toEqual("whatever");
+      expect(fn).toHaveBeenCalledTimes(1);
+    });
   });
 
   describe("references", () => {
diff --git a/src/class-model.ts b/src/class-model.ts
@@ -44,7 +44,7 @@ type ActionMetadata = {
 /** Options that configure a snapshotted view */
 export interface SnapshottedViewOptions<V, T extends IAnyClassModelType> {
   /** A function for converting a stored value in the snapshot back to the rich type for the view to return */
-  createReadOnly?: (value: V | undefined, snapshot: T["InputType"], node: Instance<T>) => V | undefined;
+  createReadOnly?: (value: V | undefined, node: Instance<T>) => V | undefined;
 
   /** A function for converting the view value to a snapshot value */
   createSnapshot?: (value: V) => any;
@@ -195,7 +195,7 @@ export function register<Instance, Klass extends { new (...args: any[]): Instanc
         if (descriptor.get) {
           Object.defineProperty(klass.prototype, property, {
             ...descriptor,
-            get: fastGetters.buildGetter(property, descriptor),
+            get: fastGetters.buildViewGetter(metadata, descriptor),
           });
         }
 
diff --git a/src/fast-getter.ts b/src/fast-getter.ts
@@ -1,4 +1,5 @@
-import type { PropertyMetadata, ViewMetadata } from "./class-model";
+import { snapshotProcessor } from "mobx-state-tree/dist/internal";
+import type { PropertyMetadata, SnapshottedViewMetadata, ViewMetadata } from "./class-model";
 import { getPropertyDescriptor } from "./class-model";
 import { RegistrationError } from "./errors";
 import { $notYetMemoized, $readOnly } from "./symbols";
@@ -28,6 +29,10 @@ export class FastGetBuilder {
     return `mqt/${property}-memo`;
   }
 
+  snapshottedViewInputSymbolName(property: string) {
+    return `mqt/${property}-svi-memo`;
+  }
+
   outerClosureStatements(className: string) {
     return this.memoizableProperties
       .map(
@@ -39,30 +44,67 @@ export class FastGetBuilder {
       .join("\n");
   }
 
-  buildGetter(property: string, descriptor: PropertyDescriptor) {
+  buildViewGetter(metadata: ViewMetadata | SnapshottedViewMetadata, descriptor: PropertyDescriptor) {
+    const property = metadata.property;
     const $memo = Symbol.for(this.memoSymbolName(property));
-    const source = `
-      (
-        function build({ $readOnly, $memo, $notYetMemoized, getValue }) {
-          return function get${property}(model, imports) {
-            if (!this[$readOnly]) return getValue.call(this);
-            let value = this[$memo];
-            if (value !== $notYetMemoized) {
+
+    let source;
+    let args;
+
+    if (metadata.type === "snapshotted-view" && metadata.options.createReadOnly) {
+      const $snapshotValue = Symbol.for(this.snapshottedViewInputSymbolName(property));
+
+      // this snapshotted view has a hydrator, so we need a special view function for readonly instances that lazily hydrates the snapshotted value
+      source = `
+        (
+          function build({ $readOnly, $memo, $notYetMemoized, $snapshotValue, getValue, hydrate }) {
+            return function get${property}(model, imports) {
+              if (!this[$readOnly]) return getValue.call(this);
+              let value = this[$memo];
+              if (value !== $notYetMemoized) {
+                return value;
+              }
+
+              const dehydratedValue = this[$snapshotValue];
+              if (typeof dehydratedValue !== "undefined") {
+                value = hydrate(dehydratedValue, this);
+              } else {
+                value = getValue.call(this);
+              }
+
+              this[$memo] = value;
               return value;
             }
+          }
+        )
+        //# sourceURL=mqt-eval/dynamic/${this.klass.name}-${property}-get.js
+      `;
+      args = { $readOnly, $memo, $snapshotValue, $notYetMemoized, hydrate: metadata.options.createReadOnly, getValue: descriptor.get };
+    } else {
+      source = `
+        (
+          function build({ $readOnly, $memo, $notYetMemoized, getValue }) {
+            return function get${property}(model, imports) {
+              if (!this[$readOnly]) return getValue.call(this);
+              let value = this[$memo];
+              if (value !== $notYetMemoized) {
+                return value;
+              }
 
-            value = getValue.call(this);
-            this[$memo] = value;
-            return value;
+              value = getValue.call(this);
+              this[$memo] = value;
+              return value;
+            }
           }
-        }
-      )
-      //# sourceURL=mqt-eval/dynamic/${this.klass.name}-${property}-get.js
-    `;
+        )
+        //# sourceURL=mqt-eval/dynamic/${this.klass.name}-${property}-get.js
+      `;
+      args = { $readOnly, $memo, $notYetMemoized, getValue: descriptor.get };
+    }
 
     try {
       const builder = eval(source);
-      return builder({ $readOnly, $memo, $notYetMemoized, getValue: descriptor.get });
+      return builder(args);
     } catch (error) {
       console.error(`Error building getter for ${this.klass.name}#${property}`);
       console.error(`Compiled source:\n${source}`);
diff --git a/src/fast-instantiator.ts b/src/fast-instantiator.ts
@@ -79,8 +79,8 @@ export class InstantiatorBuilder<T extends IClassModelType<Record<string, IAnyTy
     `);
     }
 
-    for (const [index, snapshottedView] of this.model.snapshottedViews.entries()) {
-      segments.push(this.assignSnapshottedViewExpression(snapshottedView, index));
+    for (const snapshottedView of this.model.snapshottedViews) {
+      segments.push(this.assignSnapshottedViewExpression(snapshottedView));
     }
 
     let className = this.model.name;
@@ -332,21 +332,24 @@ export class InstantiatorBuilder<T extends IClassModelType<Record<string, IAnyTy
       }`;
   }
 
-  private assignSnapshottedViewExpression(snapshottedView: SnapshottedViewMetadata, index: number) {
+  private assignSnapshottedViewExpression(snapshottedView: SnapshottedViewMetadata) {
     const varName = `view${snapshottedView.property}`;
 
-    let valueExpression = `snapshot?.["${snapshottedView.property}"]`;
+    let destinationProp;
     if (snapshottedView.options.createReadOnly) {
-      const alias = this.alias(`snapshottedViews[${index}].options.createReadOnly`);
-      valueExpression = `${alias}(${valueExpression}, snapshot, this)`;
+      // we're using a hydrator, so we don't store it right at the memo, and instead stash it where we'll lazily hydrate it in the getter
+      destinationProp = this.alias(`Symbol.for("${this.getters.snapshottedViewInputSymbolName(snapshottedView.property)}")`);
+    } else {
+      // we're not using a hydrator, so we can stash the snapshotted value right into the memoized spot
+      destinationProp = this.alias(`Symbol.for("${this.getters.memoSymbolName(snapshottedView.property)}")`);
     }
-    const memoSymbolAlias = this.alias(`Symbol.for("${this.getters.memoSymbolName(snapshottedView.property)}")`);
 
+    const valueExpression = `snapshot?.["${snapshottedView.property}"]`;
     return `
       // setup snapshotted view for ${snapshottedView.property}
       const ${varName} = ${valueExpression};
       if (typeof ${varName} != "undefined") {
-        this[${memoSymbolAlias}] = ${varName};
+        this[${destinationProp}] = ${varName};
       }
     `;
   }
