Add machine.value with single snapshot of data

Author: RoyalIcing

src/index.test.ts

@@ -76,24 +76,45 @@ describe("Machine with entry and exit actions", () => {
 
     test("sending events", async () => {
       const loader = start(Loader);
-      expect(loader.current).toEqual("idle");
+      expect(loader.value).toMatchObject({
+        change: 0,
+        state: "idle",
+        actions: []
+      });
+      expect(loader.value).toBe(loader.value);
       expect(loader.changeCount).toEqual(0);
+      expect(loader.current).toEqual("idle");
+
+      const valueA = loader.value;
 
       loader.next("NOOP");
       expect(loader.current).toEqual("idle");
       expect(loader.changeCount).toEqual(0);
+      expect(loader.value).toBe(valueA);
 
       const transitionResult = loader.next("FETCH");
       expect(fetch).toHaveBeenCalledWith("https://example.org/");
-      expect(transitionResult.actions).toEqual([
-        { type: "entry", f: fetchData },
-      ]);
+      expect(transitionResult.value).toMatchObject({
+        change: 1,
+        state: "loading",
+        actions: [
+          { type: "entry", f: fetchData }
+        ]
+      });
+      expect(loader.value).toMatchObject({
+        change: 1,
+        state: "loading",
+        actions: [
+          { type: "entry", f: fetchData }
+        ]
+      });
+      expect(loader.value).not.toBe(valueA);
       expect(loader.current).toEqual("loading");
       expect(loader.changeCount).toEqual(1);
       expect(finishedLoading).toHaveBeenCalledTimes(0);
 
-      await expect(loader.results).resolves.toEqual({ fetchData: 42 });
-      await expect(Promise.resolve(transitionResult)).resolves.toEqual({
+      await expect(loader.value.results).resolves.toEqual({ fetchData: 42 });
+      await expect(Promise.resolve(transitionResult.value.results)).resolves.toEqual({
         fetchData: 42,
       });
       expect(finishedLoading).toHaveBeenCalledTimes(1);
@@ -118,19 +139,28 @@ describe("Machine with entry and exit actions", () => {
 
     test("sending events", async () => {
       const loader = start(Loader);
-      expect(loader.current).toEqual("idle");
+      expect(loader.value).toMatchObject({
+        change: 0,
+        state: "idle",
+        actions: []
+      });
 
       const transitionResult = loader.next("FETCH");
       expect(fetch).toHaveBeenCalledTimes(1);
       expect(fetch).toHaveBeenLastCalledWith("https://example.org/");
-      expect(transitionResult.actions).toEqual([
+      expect(transitionResult.value.actions).toEqual([
         { type: "entry", f: fetchData },
       ]);
-      expect(loader.current).toEqual("loading");
-      expect(loader.changeCount).toEqual(1);
+      expect(loader.value).toMatchObject({
+        change: 1,
+        state: "loading",
+        actions: [
+          { type: "entry", f: fetchData },
+        ]
+      });
 
-      await expect(loader.results).rejects.toEqual(new Error("Failed!"));
-      await expect(Promise.resolve(transitionResult)).rejects.toEqual(
+      await expect(loader.value.results).rejects.toEqual(new Error("Failed!"));
+      await expect(Promise.resolve(transitionResult.value.results)).rejects.toEqual(
         new Error("Failed!")
       );
       expect(loader.changeCount).toEqual(2);
@@ -211,7 +241,7 @@ describe.skip("Fetch with abort signal", () => {
 
       const transitionResult = loader.next("FETCH");
       expect(fetch).toHaveBeenCalledWith("https://example.org/");
-      expect(transitionResult.actions).toEqual([
+      expect(transitionResult.value.actions).toEqual([
         { type: "entry", f: fetchData },
       ]);
       expect(loader.current).toEqual("loading");
@@ -249,7 +279,7 @@ describe.skip("Fetch with abort signal", () => {
       const transitionResult = loader.next("FETCH");
       expect(fetch).toHaveBeenCalledTimes(1);
       expect(fetch).toHaveBeenLastCalledWith("https://example.org/");
-      expect(transitionResult.actions).toEqual([
+      expect(transitionResult.value.actions).toEqual([
         { type: "entry", f: fetchData },
       ]);
       expect(loader.current).toEqual("loading");

src/index.ts

@@ -123,17 +123,22 @@ export function readContext(contextName: string | symbol): ReadContext {
   return { type: "readContext", contextName };
 }
 
-export interface MachineInstance extends Iterator<null | string | Record<string, string>, void, string | symbol> {
+interface MachineValue {
+  readonly change: number;
+  readonly state: null | string | Record<string, string>;
+  readonly actions: Array<EntryAction | ExitAction>;
+  readonly results: null | Promise<unknown>;
+}
+
+export interface MachineInstance extends Iterator<MachineValue, void, string | symbol> {
+  readonly value: MachineValue;
   readonly changeCount: number;
   readonly current: null | string | Record<string, string>;
   readonly results: null | Promise<unknown>;
   readonly accumulations: Map<symbol | string, Array<symbol | string | Event>>;
   readonly done: boolean;
   readonly signal: AbortSignal;
-  next(arg: string | symbol): IteratorResult<null | string | Record<string, string>> &
-    PromiseLike<any> & {
-      actions: Array<EntryAction | ExitAction>;
-    };
+  next(arg: string | symbol): IteratorResult<MachineValue>;
   abort(): void;
 }
 
@@ -539,10 +544,34 @@ export function start(
     const current = instance.current;
     return current !== null ? current[rootName] : null;
   }
+
+  let _cachedValue: undefined | MachineValue;
+  function getValue() {
+    if (_cachedValue?.change === _changeCount) {
+      return _cachedValue;
+    }
+
+    let resultCache: undefined | Promise<unknown> = undefined;
+    _cachedValue = Object.freeze({
+      change: _changeCount,
+      state: getCurrent(),
+      actions: instance.actions,
+      get results() {
+        if (resultCache === undefined) {
+          resultCache = instance.results;
+        }
+        return resultCache;
+      }
+    });
+    return _cachedValue;
+  }
 
   _changeCount = 0;
 
   return {
+    get value() {
+      return getValue();
+    },
     get changeCount() {
       return _changeCount;
     },
@@ -560,11 +589,8 @@ export function start(
     },
     next(event: string | symbol) {
       instance.receive(event);
-      const promise = instance.results;
       return {
-        value: instance.current !== null ? instance.current[rootName] : null,
-        actions: instance.actions,
-        then: promise?.then.bind(promise),
+        value: getValue(),
         done: false,
       };
     },