Non-memoized / ref'd reducer runs into double dispatch issue when used in custom hook

useEffectReducer

A React hook for managing side-effects in your reducers.

Inspired by the useReducerWithEmitEffect hook idea by Sophie Alpert.

If you know how to useReducer, you already know how to useEffectReducer.

💻 CodeSandbox example: Dog Fetcher with useEffectReducer

Installation
Quick Start
Named Effects
Effect Implementations
Initial Effects
Effect Entities
Effect Cleanup
Replacing Effects
String Events
API
exec.replace(entity, effect)
TypeScript

Installation

Install it:

npm install use-effect-reducer

Import it:

import { useEffectReducer } from 'use-effect-reducer';

Create an effect reducer:

const someEffectReducer = (state, event, exec) => {
  // execute effects like this:
  exec(() => {/* ... */});

  // or parameterized (better):
  exec({ type: 'fetchUser', user: event.user });

  // and treat this like a normal reducer!
  // ...

  return state;
};

Use it:

// ...
const [state, dispatch] = useEffectReducer(someEffectReducer, initialState, {
  // implementation of effects
});

// Just like useReducer:
dispatch({ type: 'FETCH', user: 'Sophie' });

Isn't this unsafe?

No - internally, useEffectReducer (as the name implies) is abstracting this pattern:

// pseudocode
const myReducer = ([state], event) => {
  const effects = [];
  const exec = (effect) => effects.push(effect);
  
  const nextState = // calculate next state
  
  return [nextState, effects];
}

// in your component
const [[state, effects], dispatch] = useReducer(myReducer);

useEffect(() => {
  effects.forEach(effect => {
    // execute the effect
  });
}, [effects]);

Instead of being implicit about which effects are executed and when they are executed, you make this explicit in the "effect reducer" with the helper exec function. Then, the useEffectReducer hook will take the pending effects and properly execute them within a useEffect() hook.

Quick Start

An "effect reducer" takes 3 arguments:

state - the current state
event - the event that was dispatched to the reducer
exec - a function that captures effects to be executed and returns an effect entity that allows you to control the effect

import { useEffectReducer } from 'use-effect-reducer';

// I know, I know, yet another counter example
const countReducer = (state, event, exec) => {
  switch (event.type) {
    case 'INC':
      exec(() => {
        // "Execute" a side-effect here
        console.log('Going up!');
      });

      return {
        ...state,
        count: state.count + 1,
      };

    default:
      return state;
  }
};

const App = () => {
  const [state, dispatch] = useEffectReducer(countReducer, { count: 0 });

  return (
    <div>
      <output>Count: {state.count}</output>
      <button onClick={() => dispatch('INC')}>Increment</button>
    </div>
  );
};

Named Effects

A better way to make reusable effect reducers is to have effects that are named and parameterized. This is done by running exec(...) an effect object (instead of a function) and specifying that named effect's implementation as the 3rd argument to useEffectReducer(reducer, initial, effectMap).

const fetchEffectReducer = (state, event, exec) => {
  switch (event.type) {
    case 'FETCH':
      // Capture a named effect to be executed
      exec({ type: 'fetchFromAPI', user: event.user });

      return {
        ...state,
        status: 'fetching',
      };
    case 'RESOLVE':
      return {
        status: 'fulfilled',
        user: event.data,
      };
    default:
      return state;
  }
};

const initialState = { status: 'idle', user: undefined };

const fetchFromAPIEffect = (_, effect, dispatch) => {
  fetch(`/api/users/${effect.user}`)
    .then(res => res.json())
    .then(data => {
      dispatch({
        type: 'RESOLVE',
        data,
      });
    });
};

const Fetcher = () => {
  const [state, dispatch] = useEffectReducer(fetchEffectReducer, initialState, {
    // Specify how effects are implemented
    fetchFromAPI: fetchFromAPIEffect,
  });

  return (
    <button
      onClick={() => {
        dispatch({ type: 'FETCH', user: 42 });
      }}
    >
      Fetch user
    </div>
  );
};

Effect Implementations

An effect implementation is a function that takes 3 arguments:

The state at the time the effect was executed with exec(effect)
The event object that triggered the effect
The effect reducer's dispatch function to dispatch events back to it. This enables dispatching within effects in the effectMap if it is written outside of the scope of your component. If your effects require access to variables and functions in the scope of your component, write your effectMap there.

The effect implementation should return a disposal function that cleans up the effect:

// Effect defined inline
exec(() => {
  const id = setTimeout(() => {
    // do some delayed side-effect
  }, 1000);

  // disposal function
  return () => {
    clearTimeout(id);
  };
});

// Parameterized effect implementation
// (in the effect reducer)
exec({ type: 'doDelayedEffect' });

// ...

// (in the component)
const [state, dispatch] = useEffectReducer(someReducer, initialState, {
  doDelayedEffect: () => {
    const id = setTimeout(() => {
      // do some delayed side-effect
    }, 1000);

    // disposal function
    return () => {
      clearTimeout(id);
    };
  },
});

Initial Effects

The 2nd argument to useEffectReducer(state, initialState) can either be a static initialState or a function that takes in an effect exec function and returns the initialState:

const fetchReducer = (state, event) => {
  if (event.type === 'RESOLVE') {
    return {
      ...state,
      data: event.data,
    };
  }

  return state;
};

const getInitialState = exec => {
  exec({ type: 'fetchData', someQuery: '*' });

  return { data: null };
};

// (in the component)
const [state, dispatch] = useEffectReducer(fetchReducer, getInitialState, {
  fetchData(_, { someQuery }) {
    fetch(`/some/api?${someQuery}`)
      .then(res => res.json())
      .then(data => {
        dispatch({
          type: 'RESOLVE',
          data,
        });
      });
  },
});

Effect Entities

The exec(effect) function returns an effect entity, which is a special object that represents the running effect. These objects can be stored directly in the reducer's state:

const someReducer = (state, event, exec) => {
  // ...

  return {
    ...state,
    // state.someEffect is now an effect entity
    someEffect: exec(() => {
      /* ... */
    }),
  };
};

The advantage of having a reference to the effect (via the returned effect entity) is that you can explicitly stop those effects:

const someReducer = (state, event, exec) => {
  // ...

  // Stop an effect entity
  exec.stop(state.someEffect);

  return {
    ...state,
    // state.someEffect is no longer needed
    someEffect: undefined,
  };
};

Effect Cleanup

Instead of implicitly relying on arbitrary values in a dependency array changing to stop an effect (as you would with useEffect), effects can be explicitly stopped using exec.stop(entity), where entity is the effect entity returned from initially calling exec(effect):

const timerReducer = (state, event, exec) => {
  if (event.type === 'START') {
    return {
      ...state,
      timer: exec(() => {
        const id = setTimeout(() => {
          // Do some delayed effect
        }, 1000);

        // Disposal function - will be called when
        // effect entity is stopped
        return () => {
          clearTimeout(id);
        };
      }),
    };
  } else if (event.type === 'STOP') {
    // Stop the effect entity
    exec.stop(state.timer);

    return state;
  }

  return state;
};

All running effect entities will automatically be stopped when the component unmounts.

Replacing Effects

If you want to replace an effect with another (likely similar) effect, instead of calling exec.stop(entity) and calling exec(effect) to manually replace an effect, you can call exec.replace(entity, effect) as a shorthand:

const doSomeDelay = () => {
  const id = setTimeout(() => {
    // do some delayed effect
  }, delay);

  return () => {
    clearTimeout(id);
  };
};

const timerReducer = (state, event, exec) => {
  if (event.type === 'START') {
    return {
      ...state,
      timer: exec(() => doSomeDelay()),
    };
  } else if (event.type === 'LAP') {
    // Replace the currently running effect represented by `state.timer`
    // with a new effect
    return {
      ...state,
      timer: exec.replace(state.timer, () => doSomeDelay()),
    };
  } else if (event.type === 'STOP') {
    // Stop the effect entity
    exec.stop(state.timer);

    return state;
  }

  return state;
};

String Events

The events handled by the effect reducers are intended to be event objects with a type property; e.g., { type: 'FETCH', other: 'data' }. For events without payload, you can dispatch the event type alone, which will be converted to an event object inside the effect reducer:

// dispatched as `{ type: 'INC' }`
// and is the same as `dispatch({ type: 'INC' })`
dispatch('INC');

API

`useEffectReducer` hook

The useEffectReducer hook takes the same first 2 arguments as the built-in useReducer hook, and returns the current state returned from the effect reducer, as well as a dispatch function for sending events to the reducer.

const SomeComponent = () => {
  const [state, dispatch] = useEffectReducer(someEffectReducer, initialState);

  // ...
};

The 2nd argument to useEffectReducer(...) can either be a static initialState or a function that takes in exec and returns an initialState (with executed initial effects). See Initial Effects for more information.

const SomeComponent = () => {
  const [state, dispatch] = useEffectReducer(
    someEffectReducer,
    exec => {
      exec({ type: 'someEffect' });
      return someInitialState;
    },
    {
      someEffect(state, effect) {
        // ...
      },
    }
  );

  // ...
};

Additionally, the useEffectReducer hook takes a 3rd argument, which is the implementation details for named effects:

const SomeComponent = () => {
  const [state, dispatch] = useEffectReducer(someEffectReducer, initialState, {
    log: (state, effect, dispatch) => {
      console.log(state);
    },
  });

  // ...
};

`exec(effect)`

Used in an effect reducer, exec(effect) queues the effect for execution and returns an effect entity.

The effect can either be an effect object:

// ...
const entity = exec({
  type: 'alert',
  message: 'hello',
});

Or it can be an inline effect implementation:

// ...
const entity = exec(() => {
  alert('hello');
});

`exec.stop(entity)`

Used in an effect reducer, exec.stop(entity) stops the effect represented by the entity. Returns void.

// Queues the effect entity for disposal
exec.stop(someEntity);

`exec.replace(entity, effect)`

Used in an effect reducer, exec.replace(entity, effect) does two things:

Queues the entity for disposal (same as calling exec.stop(entity))
Returns a new effect entity that represents the effect that replaces the previous entity.

TypeScript

The effect reducer can be specified as an EffectReducer<TState, TEvent, TEffect>, where the generic types are:

The state type returned from the reducer
The event object type that can be dispatched to the reducer
The effect object type that can be executed

import { useEffectReducer, EffectReducer } from 'use-effect-reducer';

interface User {
  name: string;
}

type FetchState =
  | {
      status: 'idle';
      user: undefined;
    }
  | {
      status: 'fetching';
      user: User | undefined;
    }
  | {
      status: 'fulfilled';
      user: User;
    };

type FetchEvent =
  | {
      type: 'FETCH';
      user: string;
    }
  | {
      type: 'RESOLVE';
      data: User;
    };

type FetchEffect = {
  type: 'fetchFromAPI';
  user: string;
};

const fetchEffectReducer: EffectReducer<FetchState, FetchEvent, FetchEffect> = (
  state,
  event,
  exec
) => {
  switch (event.type) {
    case 'FETCH':
    // State, event, and effect types will be inferred!

    // Also you should probably switch on
    // `state.status` first ;-)

    // ...

    default:
      return state;
  }
};

	if (event === flushEffectsSymbol) {
	// Record that effects have already been executed
	return [state, []];
	}

	useEffect(() => {
	if (stateEffectTuples.length) {
	stateEffectTuples.forEach(([stateForEffect, effects]) => {

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useeffectreducer's Introduction

useEffectReducer

Installation

Isn't this unsafe?

Quick Start

Named Effects

Effect Implementations

Initial Effects

Effect Entities

Effect Cleanup

Replacing Effects

String Events

API

useEffectReducer hook

exec(effect)

exec.stop(entity)

exec.replace(entity, effect)

TypeScript

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`useEffectReducer` hook

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