Add previousValue method

src/babel-plugin-factories.json

@@ -17,6 +17,7 @@
     "patronum/once",
     "patronum/or",
     "patronum/pending",
+    "patronum/previous-value",
     "patronum/reset",
     "patronum/reshape",
     "patronum/snapshot",
@@ -44,6 +45,7 @@
     "once": "once",
     "or": "or",
     "pending": "pending",
+    "previousValue": "previous-value",
     "reset": "reset",
     "reshape": "reshape",
     "snapshot": "snapshot",
@@ -54,4 +56,4 @@
     "throttle": "throttle",
     "time": "time"
   }
-}
+}

src/previous-value/index.ts

@@ -0,0 +1,27 @@
+import { Node, Store, createStore, launch, step } from 'effector';
+
+export function previousValue<State>(store: Store<State>): Store<State | null>;
+export function previousValue<State, Init>(
+  store: Store<State>,
+  initialValue: Init,
+): Store<State | Init>;
+export function previousValue<State, Init = null>(
+  store: Store<State>,
+  initialValue: Init | null = null,
+) {
+  const $prevValue = createStore<State | Init | null>(initialValue);
+  const storeNode: Node = (store as any).graphite;
+  storeNode.seq.push(
+    step.compute({
+      fn(upd, _, stack) {
+        launch({
+          target: $prevValue,
+          params: stack.a,
+          defer: true,
+        });
+        return upd;
+      },
+    }),
+  );
+  return $prevValue;
+}

src/previous-value/previous-value.fork.test.ts

@@ -0,0 +1,55 @@
+import { allSettled, createEvent, createStore, fork, restore } from 'effector';
+
+import { previousValue } from './index';
+
+it('has null when store is not changed', () => {
+  const scope = fork();
+  const $initalStore = createStore(10);
+  const $prevValue = previousValue($initalStore);
+
+  expect(scope.getState($prevValue)).toBe(null);
+});
+
+it('has initial value when defined', () => {
+  const scope = fork();
+  const $initalStore = createStore(10);
+  const $prevValue = previousValue($initalStore, 0);
+
+  expect(scope.getState($prevValue)).toBe(0);
+});
+
+it('has first value on update', async () => {
+  const scope = fork();
+  const changeInitialStore = createEvent<number>();
+  const $initalStore = restore(changeInitialStore, 10);
+
+  const $prevValue = previousValue($initalStore);
+
+  await allSettled(changeInitialStore, { scope, params: 20 });
+
+  expect(scope.getState($prevValue)).toBe(10);
+});
+
+it('has previous value on multiple updates', async () => {
+  const scope = fork();
+  const changeInitialStore = createEvent<number>();
+  const $initalStore = restore(changeInitialStore, 10);
+
+  const $prevValue = previousValue($initalStore);
+
+  await allSettled(changeInitialStore, { scope, params: 20 });
+  await allSettled(changeInitialStore, { scope, params: 30 });
+  await allSettled(changeInitialStore, { scope, params: 40 });
+
+  expect(scope.getState($prevValue)).toBe(30);
+});
+
+it('has first scope value after first update', async () => {
+  const inc = createEvent();
+  const $initalStore = createStore(0);
+  const $prevValue = previousValue($initalStore, -1);
+  $initalStore.on(inc, (x) => x + 1);
+  const scope = fork({ values: [[$initalStore, 10]] });
+  await allSettled(inc, { scope });
+  expect(scope.getState($prevValue)).toBe(10);
+});

src/previous-value/previous-value.test.ts

@@ -0,0 +1,41 @@
+import { createEvent, createStore, restore } from 'effector';
+
+import { previousValue } from './index';
+
+it('has null when store is not changed', () => {
+  const $initalStore = createStore(10);
+  const $prevValue = previousValue($initalStore);
+
+  expect($prevValue.getState()).toBe(null);
+});
+
+it('has initial value when defined', () => {
+  const $initalStore = createStore(10);
+  const $prevValue = previousValue($initalStore, 0);
+
+  expect($prevValue.getState()).toBe(0);
+});
+
+it('has first value on update', () => {
+  const changeInitialStore = createEvent<number>();
+  const $initalStore = restore(changeInitialStore, 10);
+
+  const $prevValue = previousValue($initalStore);
+
+  changeInitialStore(20);
+
+  expect($prevValue.getState()).toBe(10);
+});
+
+it('has previous value on multiple updates', () => {
+  const changeInitialStore = createEvent<number>();
+  const $initalStore = restore(changeInitialStore, 10);
+
+  const $prevValue = previousValue($initalStore);
+
+  changeInitialStore(20);
+  changeInitialStore(30);
+  changeInitialStore(40);
+
+  expect($prevValue.getState()).toBe(30);
+});

src/previous-value/readme.md

@@ -0,0 +1,65 @@
+# previousValue
+
+:::note since
+patronum 2.1.0
+:::
+
+```ts
+import { previousValue } from 'patronum';
+// or
+import { previousValue } from 'patronum/previous-value';
+```
+
+### Motivation
+
+The method allows to get previous value of given store. Usually need for analytics
+
+### Formulae
+
+```ts
+$target = previousValue($source);
+$target = previousValue($source, 'initial value');
+```
+
+### Arguments
+
+1. `$source` ([_`Store`_]) - source store
+2. `defaultValue` (_optional_) - default value for `$target` store, if not passed, `null` will be used
+
+### Returns
+
+- `$target` ([_`Store`_]) - new store that contain previous value of `$source` after first update and null or default value (if passed) before that
+
+### Example
+
+Push analytics with route transition:
+
+```ts
+import { createStore, createEvent, createEffect, sample } from 'effector';
+import { previousValue } from 'patronum';
+
+const openNewRoute = createEvent<string>();
+const $currentRoute = createStore('main_page');
+const $previousRoute = previousValue($currentRoute);
+
+const sendRouteTransitionFx = createEffect(async ({ prevRoute, nextRoute }) => {
+  console.log(prevRoute, '->', newRoute)
+  await fetch(...)
+});
+
+sample({clock: openNewRoute, target: $currentRoute});
+
+sample({
+  clock: openNewRoute,
+  source: {
+    prevRoute: $previousRoute,
+    nextRoute: $currentRoute,
+  },
+  target: sendRouteTransitionFx,
+});
+
+openNewRoute('messages');
+// main_page -> messages
+openNewRoute('chats');
+// messages -> chats
+```