Sensible promise handling and middleware for redux
redux-pack is a library that introduces promise-based middleware that allows async actions based on the lifecycle of a promise to be declarative.
Async actions in redux are often done using redux-thunk or other middlewares. The problem with this approach is that it makes it too easy to use dispatch sequentially, and dispatch multiple "actions" as the result of the same interaction/event, where they probably should have just been a single action dispatch.
This can be problematic because we are treating several dispatches as all part of a single transaction, but in reality, each dispatch causes a separate rerender of the entire component tree, where we not only pay a huge performance penalty, but also risk the redux store being in an inconsistent state.
redux-pack helps prevent us from making these mistakes, as it doesn't give us the power of a dispatch function, but allows us to do all of the things we were doing before.
To give you some more context into the changes, here are some examples/information about the old way and new way of doing things below:
Ready to use it? Jump straight to the How-To and API doc
Before this change, you would create individual action constants for each lifecycle of the promise, and use redux-thunk to dispatch before the promise, and when it resolves/rejects.
// types.js
export const LOAD_FOO_STARTED = 'LOAD_FOO_STARTED';
export const LOAD_FOO_SUCCESS = 'LOAD_FOO_SUCCESS';
export const LOAD_FOO_FAILED = 'LOAD_FOO_FAILED';// actions.js
export function loadFoo(id) {
return dispatch => {
dispatch({ type: LOAD_FOO_STARTED, payload: id });
return Api.getFoo(id).then(foo => {
dispatch({ type: LOAD_FOO_SUCCESS, payload: foo });
}).catch(error => {
dispatch({ type: LOAD_FOO_FAILED, error: true, payload: error });
});
};
}In the reducer, you would handle each action individually in your reducer:
// reducer.js
export function fooReducer(state = initialState, action) {
const { type, payload } = action;
switch (type) {
case LOAD_FOO_STARTED:
return {
...state,
isLoading: true,
fooError: null
};
case LOAD_FOO_SUCCESS:
return {
...state,
isLoading: false,
foo: payload
};
case LOAD_FOO_FAILED:
return {
...state,
isLoading: false,
fooError: payload
};
default:
return state;
}
}Note: The example uses { ...state } syntax that is called Object rest spread properties. If you'd prefer the API of Immutable.js, you could write code like the following:
switch (type) {
case LOAD_FOO_STARTED:
return state
.set('isLoading', true)
.set('fooError', null);
case LOAD_FOO_SUCCESS:
// ...
}With redux-pack, we only need to define a single action constant for the entire promise lifecycle, and then return the promise directly with a promise namespace specified:
// types.js
export const LOAD_FOO = 'LOAD_FOO';// actions.js
export function loadFoo(id) {
return {
type: LOAD_FOO,
promise: Api.getFoo(id),
};
}In the reducer, you handle the action with redux-pack's handle function, where you can specify several smaller "reducer" functions for each lifecycle. finish is called for both resolving/rejecting, start is called at the beginning, success is called on resolve, failure is called on reject, and always is called for all of them.
// reducer.js
import { handle } from 'redux-pack';
export function fooReducer(state = initialState, action) {
const { type, payload } = action;
switch (type) {
case LOAD_FOO:
return handle(state, action, {
start: prevState => ({
...prevState,
isLoading: true,
fooError: null
}),
finish: prevState => ({ ...prevState, isLoading: false }),
failure: prevState => ({ ...prevState, fooError: payload }),
success: prevState => ({ ...prevState, foo: payload }),
});
default:
return state;
}
}Often times we want to log whether an action succeeded or not etc. We are able to handle this now using the onSuccess or onFailure meta options:
Before:
// actions.js
export function loadFoo(id) {
return dispatch => {
dispatch(loadFooStart());
Api.getFoo(id).then(response => {
dispatch(loadFooSucceeded(response);
logSuccess(response);
}).catch(error => dispatch(loadFooFailed(error)));
};
}After:
// actions.js
export function loadFoo(id) {
return {
type: LOAD_FOO,
promise: Api.getFoo(id),
meta: {
onSuccess: (response) => logSuccess(response)
},
};
}The first step is to add redux-pack in your project
npm install -S redux-pack
# or
yarn add redux-packThe redux-pack middleware is the heart of redux-pack. As the following example shows, it installs like most middlewares:
import { createStore, applyMiddleware } from 'redux'
import { middleware as reduxPackMiddleware } from 'redux-pack'
import thunk from 'redux-thunk'
import createLogger from 'redux-logger'
import rootReducer from './reducer'
const logger = createLogger()
const store = createStore(
rootReducer,
applyMiddleware(thunk, reduxPackMiddleware, logger)
)Note that it should probably be one of the first middleware to run, here it would run just after thunk and before logger.
Let's start with the function signature: handle(state, action, handlers) → newState
As you can see, it takes 3 arguments:
- state: the current state in your reducer
- action: the action that should be handled
- handlers: a object mapping the promise lifecycle steps to reducer functions
- the steps names are:
start,finish,failure,successandalways - every handler function should be of the form:
state => state
Here is a minimalist example:
import { handle } from 'redux-pack';
import { getFoo } from '../api/foo';
const LOAD_FOO = 'LOAD_FOO';
const initialState = {
isLoading: false,
error: null,
foo: null,
};
export function fooReducer(state = initialState, action) {
const { type, payload } = action;
switch (type) {
case LOAD_FOO:
return handle(state, action, {
start: prevState => ({ ...prevState, isLoading: true, error: null, foo: null }),
finish: prevState => ({ ...prevState, isLoading: false }),
failure: prevState => ({ ...prevState, error: payload }),
success: prevState => ({ ...prevState, foo: payload }),
always: prevState => prevState, // unnecessary, for the sake of example
});
default:
return state;
}
}
export function loadFoo() {
return {
type: LOAD_FOO,
promise: getFoo(),
}
}Note: The example uses { ...state } syntax that is called Object rest spread properties.
You might want to add side effects (like sending analytics events or navigate to different views) based on promise results.
redux-pack lets you do that through event hooks functions. These are functions attached to the meta attribute of the original action. They are called with two parameters:
- the matching step payload (varies based on the step, details below)
- the
getStatefunction
Here are the available hooks and their associated payload:
onStart, called with the initial actionpayloadvalueonFinish, called withtrueif the promise resolved,falseotherwiseonSuccess, called with the promise resolution valueonFailure, called with the promise error
Here is an example usage to send analytics event when the user doesFoo:
import { sendAnalytics } from '../analytics';
import { doFoo } from '../api/foo';
export function userDoesFoo() {
return {
type: DO_FOO,
promise: doFoo(),
meta: {
onSuccess: (result, getState) => {
const userId = getState().currentUser.id;
const fooId = result.id;
sendAnalytics('USER_DID_FOO', {
userId,
fooId,
});
}
}
}
}At the moment, testing reducers and action creators with redux-pack does
require understanding a little bit about its implementation. The handle
method uses a special KEY.LIFECYCLE property on the meta object on the
action that denotes the lifecycle of the promise being handled.
Right now it is suggested to make a simple helper method to make testing easier. Simple test code might look something like this:
import { LIFECYCLE, KEY } from 'redux-pack';
import FooReducer from '../path/to/FooReducer';
// this utility method will make an action that redux pack understands
function makePackAction(lifecycle, { type, payload, meta={} }) {
return {
type,
payload,
meta: {
...meta,
[KEY.LIFECYCLE]: lifecycle,
},
}
}
// your test code would then look something like...
const initialState = { ... };
const expectedEndState = { ... };
const action = makePackAction(LIFECYCLE.START, { type: 'FOO', payload: { ... } });
const endState = FooReducer(initialState, action);
assertDeepEqual(endState, expectedEndState);
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