feat(createGlobalState)!: Align API with React.useState

This commit adds support for functional updates to align with the React.useState API. Type definitions are updated to match useState. This might break existing code that expects functions to be held in the global state, rather than applied as an update. This change may also cause new type errors.
streamich · Mar 5, 2020 · c10ad48 · c10ad48
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diff --git a/docs/createGlobalState.md b/docs/createGlobalState.md
@@ -4,28 +4,59 @@ A React hook which creates a globally shared state.
 
 ## Usage
 
+
+```ts
+const useGlobalState = createGlobalState<T>(defaultValue: T)
+...
+const [ value, setValue ] = useGlobalState(initialValue?: T)
+```
+
+Here you can see that there is both a `defaultValue` and an `initialValue`. Together, they dictate what the starting value will be.
+* `defaultValue` is the value returned on the first use of the `useGlobalState()` hook when no `initialValue` is provided, and allows you to ensure that there will always be valid value available.
+* `initialValue` allows overriding the `defaultValue` on the very first rendering of the hook within the app. This also allows the hook to be a drop in replacement for `React.useState`. If no `initialValue` is provided, the `defaultValue` will be used.
+
+### Example:
+
+In the following example, the starting value with be `1`, because it overrides the `defaultValue`, and because that invocation is rendered before the `useGlobalValue(2)`, and `useGlobalValue(3)`, whose `initialValue` parameters are ignored because they are not the first invocations.
+
 ```tsx
-const useGlobalValue = createGlobalState<number>(0);
+const useMyGlobalState = createGlobalState<number>(0);
 
-const CompA: FC = () => {
-  const [value, setValue] = useGlobalValue();
+const SetValueDirectly: FC = () => {
+  const [value, setValue] = useMyGlobalState(2);
 
-  return <button onClick={() => setValue(value + 1)}>+</button>;
+  return (
+    <div>
+      <p>{value}</p>
+      <div>
+        <button onClick={() => setValue(value + 1)}>+</button>
+        <button onClick={() => setValue(value - 1)}>-</button>
+      </div>
+    </div>
+  );
 };
 
-const CompB: FC = () => {
-  const [value, setValue] = useGlobalValue();
+const SetValueWithFunctionalUpdate: FC = () => {
+  const [value, setValue] = useMyGlobalState(1);
 
-  return <button onClick={() => setValue(value - 1)}>-</button>;
+  return (
+    <div>
+      <p>{value}</p>
+      <div>
+        <button onClick={() => setValue(val => val + 1)}>+</button>
+        <button onClick={() => setValue(val => val - 1)}>-</button>
+      </div>
+    </div>
+  );
 };
 
 const Demo: FC = () => {
-  const [value] = useGlobalValue();
+  const [value] = useGlobalValue(1);
   return (
     <div>
       <p>{value}</p>
-      <CompA />
-      <CompB />
+      <SetValueDirectly />
+      <SetValueWithFunctionalUpdate />
     </div>
   );
 };

diff --git a/src/createGlobalState.ts b/src/createGlobalState.ts
@@ -2,18 +2,37 @@
 import { useLayoutEffect, useState } from 'react';
 import useEffectOnce from './useEffectOnce';
 
-export function createGlobalState<S = any>(initialState?: S) {
-  const store: { state: S | undefined; setState: (state: S) => void; setters: any[] } = {
-    state: initialState,
-    setState(state: S) {
-      store.state = state;
+export type useStateReturnType<S> = [ S , React.Dispatch<React.SetStateAction<S>> ]
+interface StateStore<S> {
+  initialized: boolean,
+  state: S
+  setState: React.Dispatch<React.SetStateAction<S>>
+  setters: React.Dispatch<React.SetStateAction<S>>[]
+}
+export type useGlobalStateType<S> = (initialState?: S | (() => S)) => useStateReturnType<S>
+export function createGlobalState<S = any>(defaultState: S): useGlobalStateType<S> {
+  const store: StateStore<S> = {
+    initialized: false,
+    state: defaultState,
+    setState(newState) {
+      store.state = newState instanceof Function
+        ? newState(store.state)
+        : newState;
       store.setters.forEach(setter => setter(store.state));
     },
     setters: [],
   };
 
-  return (): [S | undefined, (state: S) => void] => {
-    const [globalState, stateSetter] = useState<S | undefined>(store.state);
+  // unlike in useState, here initialState is optional because there will always
+  // be a valid state provided by defaultState
+  return function(initialState?: S | (() => S)): useStateReturnType<S> {
+    // Prevent clobbering defaultState or existing state if no argument was passed
+    if (!store.initialized && arguments.length > 0) {
+      store.state = initialState instanceof Function
+        ? initialState()
+        : initialState as S
+    }
+    const [globalState, stateSetter] = useState<S>(store.state);
 
     useEffectOnce(() => () => {
       store.setters = store.setters.filter(setter => setter !== stateSetter);
@@ -25,6 +44,7 @@ export function createGlobalState<S = any>(initialState?: S) {
       }
     });
 
+    store.initialized = true
     return [globalState, store.setState];
   };
 }

diff --git a/stories/createGlobalState.story.tsx b/stories/createGlobalState.story.tsx
@@ -3,27 +3,43 @@ import React, { FC } from "react";
 import { createGlobalState } from "../src";
 import ShowDocs from "./util/ShowDocs";
 
-const useGlobalValue = createGlobalState<number>(0);
+const useMyGlobalState = createGlobalState<number>(0);
 
-const CompA: FC = () => {
-  const [value, setValue] = useGlobalValue();
+const SetValueDirectly: FC = () => {
+  const [value, setValue] = useMyGlobalState(2);
 
-  return <button onClick={() => setValue(value + 1)}>+</button>;
+  return (
+    <div>
+      <p>{value}</p>
+      <div>
+        <button onClick={() => setValue(value + 1)}>+</button>
+        <button onClick={() => setValue(value - 1)}>-</button>
+      </div>
+    </div>
+  );
 };
 
-const CompB: FC = () => {
-  const [value, setValue] = useGlobalValue();
+const SetValueWithFunctionalUpdate: FC = () => {
+  const [value, setValue] = useMyGlobalState(3);
 
-  return <button onClick={() => setValue(value - 1)}>-</button>;
+  return (
+    <div>
+      <p>{value}</p>
+      <div>
+        <button onClick={() => setValue(val => val + 1)}>+</button>
+        <button onClick={() => setValue(val => val - 1)}>-</button>
+      </div>
+    </div>
+  );
 };
 
 const Demo: FC = () => {
-  const [value] = useGlobalValue();
+  const [value] = useMyGlobalState(1);
   return (
     <div>
       <p>{value}</p>
-      <CompA />
-      <CompB />
+      <SetValueDirectly />
+      <SetValueWithFunctionalUpdate />
     </div>
   );
 };

diff --git a/tests/createGlobalState.test.ts b/tests/createGlobalState.test.ts
@@ -6,16 +6,61 @@ describe('useGlobalState', () => {
     expect(createGlobalState).toBeDefined();
   });
 
-  it('both components should be updated', () => {
+  it('both components should be updated directly', () => {
     const useGlobalValue = createGlobalState(0);
     const { result: result1 } = renderHook(() => useGlobalValue());
     const { result: result2 } = renderHook(() => useGlobalValue());
-    expect(result1.current[0] === 0);
-    expect(result2.current[0] === 0);
+    let [ state1, setState1 ] = result1.current;
+    let [ state2 ] = result2.current;
+    expect(state1 === 0);
+    expect(state2 === 0);
     act(() => {
-      result1.current[1](1);
+      setState1(1);
     });
-    expect(result1.current[0] === 1);
-    expect(result2.current[0] === 1);
+    [ state1, setState1 ] = result1.current;
+    [ state2 ] = result2.current;
+    expect(state1 === 1);
+    expect(state2 === 1);
   });
+
+  it('both components should be updated via update function', () => {
+    const useGlobalValue = createGlobalState(0);
+    const { result: result1 } = renderHook(() => useGlobalValue());
+    const { result: result2 } = renderHook(() => useGlobalValue());
+    let [ state1, setState1 ] = result1.current;
+    let [ state2 ] = result2.current;
+    expect(state1 === 0);
+    expect(state2 === 0);
+    act(() => {
+      setState1(x => x + 1);
+    });
+    [ state1, setState1 ] = result1.current;
+    [ state2 ] = result2.current;
+    expect(state1 === 1);
+    expect(state2 === 1);
+  });
+
+  it('passing initialState should override defaultState on first use', () => {
+    const useGlobalValue = createGlobalState(0);
+    const { result } = renderHook(() => useGlobalValue(1));
+    let [ state ] = result.current;
+    expect(state === 1)
+  })
+
+  it('passing initialState should be ignored after first use', () => {
+    const useGlobalValue = createGlobalState(1);
+    const { result: result1 } = renderHook(() => useGlobalValue(2));
+    const { result: result2 } = renderHook(() => useGlobalValue(3));
+    let [ state1 ] = result1.current;
+    let [ state2 ] = result2.current;
+    expect(state1 === 2);
+    expect(state2 === 2);
+  })
+
+  it('an unpassed initialState should not clobber defaultState', () => {
+    const useGlobalValue = createGlobalState(1);
+    const { result } = renderHook(() => useGlobalValue());
+    let [ state ] = result.current;
+    expect(state === 1)
+  })
 });