I am new to Redux and Redux toolkit. I learned that createSelector can accept multiple input selectors, which can be provided as separate arguments or as an array. The results from all the input selectors are provided as separate arguments to the output selector.
const selectA = state => state.a;
const selectB = state => state.b;
const selectC = state => state.c;
const selectABC = createSelector(
[selectA, selectB, selectC],
(a, b, c) => {
// do something with a, b, and c, and return a result
return a + b + c;
}
);
my question is, if I only care about one simple state, can I just use useSelector like this
const selectA = state => state.a;
const a = useSelector(selectA)
what's the difference between the two usages?
A "selector" is any function that accepts the Redux state tree as an argument, and returns some extracted or derived data. That includes plain functions like you showed.
In many cases, you want to memoize the calculation of the results, such as mapping over an array of items, so that it's not re-calculated unless the inputs have changed. Reselect's createSelector creates memoized selector functions that only recalculate the output if the inputs change.
For more details, see my post Using Reselect Selectors for Encapsulation and Performance , as well as the "Performance and Normalizing Data" page in the new "Redux Essentials" core docs tutorial.
Use createSelector only if we want to memoise the function which retrieves store value.
Otherwise it's not required.
Related
I am learning Redux Toolkit. From a React POV it seems very intuitive to access whatever part of the state you need from within useSelector using an inline arrow function, and then conduct any calculations. As an example consider a cart item with its data (like item count) in redux store.
function CartItemCounter({ itemId }){
const cart = useSelector(state => state.cart);
const itemInCart = cart.items[itemId];
const count = itemInCart?.count || 0;
return <div>{itemId} - {count} nos</div>
}
But I'm seeing all this information saying you should define your selectors beside your slice, use createSelector and all. What is the right way, and why is it better?
The information that is out there is essentially talking about different levels of optimization while using useSelector.
What you need to understand before anything else, is how useSelector works internally.
How does useSelector work?
When you pass a function to useSelector (obviously inside a react component), it essentially hooks on to the global redux state. Whenever any change happens in any part of the global state (i.e. when dispatch() is called from any part of the app), redux will run all the functions you passed to useSelector in your app, and perform certain checks.
Redux will take the result from each function, and compare it to the value it got the last time it ran the same function.
How does it make this comparison?
It uses a reference equality for this comparison. So if redux has to think that the result of the function hasn't changed, either the value returned from the function has to be a primitive and equal.
4 === 4 // true
'itemA' === 'itemA // true
Or, the value returned must be a derived data type (arrays, objects), with the same reference. So essentially the same object.
const x = { name: 'Shashi' }
const fn1 = () => x;
const fn2 = () => x;
const fn3 = () => { name: 'Shashi' }
fn1() === fn2(); // true
fn1() === fn3(); // false, because the objects are different, with different references
In practice, redux changes the wrapping object if either a key (or key of a nested object) is changed, or you manually change the entire object using a dispatch action (This is related to the immer library integration). This is similar to how you would do in regular React.
/* See how most keys are spread in, and will hence maintain reference equality.
While certain keys like 'first', 'first.second', 'first.second[action.someId]'
are changed with new objects, and so will break reference equality */
function handwrittenReducer(state, action) {
return {
...state,
first: {
...state.first,
second: {
...state.first.second,
[action.someId]: {
...state.first.second[action.someId],
fourth: action.someValue,
},
},
},
}
}
Otherwise it maintains the same objects within its state, and returns the exact same objects with the same references, when you access them. To verify this, if you access your cart twice, its literally going to be the same object.
const cart1 = useSelector(state => state.cart)
const cart2 = useSelector(state => state.cart)
cart1 === cart2; // true
What does it do with this comparison?
If the comparison returns true, i.e. the new value is the same as the old value, Redux tells that instance of useSelector to chill tf out, and not do anything. If it returns false however, it tells that component to re-render. After all, the value you are accessing from state has "changed"(according to Redux laws), so you probably want to show the new value.
With this information, we can make change the kind of function we pass to the useSelector, in order to get certain optimization benefits.
Optimization Levels
Level 0: Accessing slice data inline
const cart = useSelector(state => state.cart)
// extract the information you need from within the cart
const itemInCart = cart.items[itemId];
const count = itemInCart?.count || 0;
This is not a good way to access the data. You actually need a subset of the data from the cart, but you are fetching the whole thing, and doing the calculation outside the selector.
Problems:
When you put stuff like this inline, what happens if you change the shape of your data in the future? You have to go to every place that uses useSelector and manually change it. Not so good.
More importantly, every time any part of the cart changes, the entire cart object actually changes. So Redux sees your component that asks for the cart, and thinks
The cart has changed. This component is asking for the cart. It should probably re-render.
BAM Every single instance of this component rerenders. And for what? The count of the item you're referencing probably didn't change. So ideally there shouldn't have been a re-render.
Level 1: Centralize the selector
An easy optimization is to put the selector function in a centralized location next to your slice. That way, if your data shape changes in the future, you can just change it in one place, and your whole app (wherever it uses that data) will work with the new shape.
// inside or next to the slice file
const selectCart = (state) => state.cart
//...
// somewhere inside a react component
const cart = useSelector(selectCart)
Level 2: Access the relevant data
Since redux is comparing the results of your selector function, if you want to avoid unnecessary rerenders, you want to make sure the results have reference equality (===). So target the exact value you wish to look at, in your selector.
// extract the information you need from within the cart, *within the selector*
const count = useSelector(state => state.cart.items[itemId]?.count || 0)
// You don't have to use a one-liner, a multi-line function is better for readability
When Redux executes these functions, it keeps a record of the value returned from these selector functions, for each individual useSelector. This time the values are going to be the same for every single counter, except the one that actually changed. All those other counters that didn't actually change in value don't have to unnecessarily re-render anymore!
And if any of you folks think this is premature optimization, the answer's no. This is more along the lines of putting a dependancy array on your useEffects to avoid infinite loops.
Not forgetting the Level 1 optimization, we can also extract this function centrally
const selectItemById = (state, itemId) => (state.cart.items[itemId]?.count || 0);
function CartItemCounter({ itemId }){
//...
// somewhere inside a react component
const count = useSelector((state) => selectCart(state, itemId))
//...
}
So that solves all of our problems right?
For now, yes. But what if this selector function has to run some expensive computation.
const selectSomething = (state) => reallyExpensiveFn(state.cart)
//...
// somewhere inside a react component
const cart = useSelector(selectSomething)
You don't want to keep running that do you?
Or what if you have no option but to return new objects from your select function. A common scenario for this case would be returning a subset of data from the state.
const selectFilteredItems = (state) => state.itemsArray.filter(checkCondition) // the filter method will always return a new array
//...
// somewhere inside a react component
const cart = useSelector(selectFilteredItems) // re-renders every time
To solve this you would have to memoize or cache the results from the function call. Essentially you would need to make sure that if the input arguments are the same, the result will maintain reference equality with the previous result. This introduces the need to maintain some kind of cache state.
Level 3: createSelector
Fortunately, Reselect library, which is reexported with Redux Tookit, does this work for you. You can take a look at the redux toolkit for the syntax.
const selectFilteredItems = createSelector(
(state) => state.itemsArray, // the first argument accesses relevant data from global state
(itemsArray) => itemsArray.filter(checkCondition) // the second parameter conducts the transformation
)
//...
// somewhere inside a react component
const cart = useSelector(selectFilteredItems) // re-renders only when needed
Here the second function is called the transformation function, and is where we put the expensive computation, or the function that returns inconsistent references as a result (filter,map etc).
The createSelector caches
a) the arguments to the transformation function
b) the result of the transformation function
of the previous call to the selectFilteredItems function. If the arguments are the same, it skips executing the transformation function, and gives you the result you got the last time it was executed.
So when useSelector looks at the result, it gets reference equality. Hence the re-render is skipped!
One little caveat here is that createSelector only caches the very previous result. This makes sense if you think about a single component. In a single component you are only concerned about differences in values and results compared to the previous render. But in practice, you are likely to share selectors across multiple components. If this happens, you have a single cache location, and multiple components using this cache. i.e. Stuff breaks.
Level 4: createSelector factory function
Since the logic for your selector is the same, what you need to do is run createSelector for each component that uses it. This creates a cache for each component, giving us the desired behaviour. In order to do this, we use a factory function.
const makeSelectFilteredItems = () => createSelector(
(state) => state.itemsArray, // the first argument accesses relevant data from global state
(itemsArray) => itemsArray.filter(checkCondition) // the second parameter conducts the transformation
)
//...
// somewhere inside a react component
const selectFilteredItems = useMemo(makeSelectFilteredItems,[]); // make a new selector for each component, when it mounts
const cart = useSelector(selectFilteredItems) // re-renders only when needed
You intend to make a new selector (and by extension, cache) for each new component that mounts. So you put it inside the actual component function and not on the module scope. But this will re-run makeFilteredSelector for each render, and hence create a new selector for each render, and hence eliminate the cache. This is why you need to wrap the function in a useMemo with an empty dependency array. It runs on every mount.
And voila!
You now know where, why and how to use selectors in Redux. I personally feel that the createSelector syntaxes are slightly contrived. There is some discussion on changing cache sizes going on. But for now I feel that sticking to the docs should get you through most situations.
But I'm seeing all this information saying you should define your selectors beside your slice, use createSelector and all.
That's the way to go if you're deriving something from the state, which ends up being an expensive computation or something that's reused often throughout your app. Imagine, for example, your state.cart can contain 50.000 items and you need to sort them from most expensive item to least expensive. You don't want to re-calculate this all the time because it slows your app down. So you cache/memoize the result.
What is the right way, and why is it better?
The right way is to use memoization helpers like createSelector when/if you want to avoid expensive computation. Most people optimize prematurely, so I'd just stick to useSelector and keep it simple if in doubt.
createSelector mentioned at https://github.com/reduxjs/reselect
createSelector API, which generates memoized selector functions.
createSelector accepts one or more "input" selectors, which extract
values from arguments, and an "output" selector that receives the
extracted values and should return a derived value. If the generated
selector is called multiple times, the output will only be
recalculated when the extracted values have changed.
createDraftSafeSelector mentioned at https://redux-toolkit.js.org/api/createSelector
createDraftSafeSelector allows to create selectors that can
safely be used inside of createReducer and createSlice reducers with
Immer-powered mutable logic. When used with plain state values, the
selector will still memoize normally based on the inputs. But, when
used with Immer draft values, the selector will err on the side of
recalculating the results, just to be safe.
I am new to react and redux design patterns so could not understand the difference and purpose of createDraftSafeSelector.
What is the difference in both API? What can be an example to understand the difference?
A selector created with createDraftSafeSelector can be used safely within createReducer or createSlice reducers, which is not possible with createSelector, as that one solely relies on object reference equality.
A selector created with createSelector would always return the same result for one reducer call even if you modified the state in-between.
Most people never use selectors in reducers, so you will probably never need it.
See the real-world usage of the draft safe selector, interactionsBucketsSelectors.selectById(state.interactionsBuckets, convPk)
We can pass the draft state of immer.js in the draft safe selector in RTK case reducer.
The selectById draft safe selector created in getSelectors().
createDraftSafeSelector is a wrapper for createSelector, it not only supports the general JS object state that createSelector can handle, but also supports the draft State of immerjs, see source code:
import { current, isDraft } from 'immer'
import { createSelector } from 'reselect'
/**
* "Draft-Safe" version of `reselect`'s `createSelector`:
* If an `immer`-drafted object is passed into the resulting selector's first argument,
* the selector will act on the current draft value, instead of returning a cached value
* that might be possibly outdated if the draft has been modified since.
* #public
*/
export const createDraftSafeSelector: typeof createSelector = (
...args: unknown[]
) => {
const selector = (createSelector as any)(...args)
const wrappedSelector = (value: unknown, ...rest: unknown[]) =>
selector(isDraft(value) ? current(value) : value, ...rest)
return wrappedSelector as any
}
You may also want to see these two test cases about the difference between createSelector and createDraftSafeSelector.
If you don't pass the draft state of immerjs to selectors, you don't need to use createDraftSafeSelector.
I'm trying to use some kind of memoization in my workflow with React, and I'm searching for the best and most importantly the "easiest" solution to integrate with my workflow that includes React and Redux.
I came across many articles talking about memoization in general and some demonstrate the use of "memoize-one" and brace it as the fastest and easiest to get up and running with, and others don't even mention it and talk about "reselect".
I just want to know which is better and easiest and which should I invest in.
Both libraries return a function which accepts a given numbers of arguments and returns a value:
getA(arg1, arg2, arg3) // Returns a value
The difference lays in what happens under the hoods when the function is called.
memoize-one
collect provided arguments
compare arguments with the ones provided in previous call (===)
arguments are the same: return cached result
arguments are NOT the same: re-evaluate result function and return
reselect
collect provided arguments
run a set of inputSelectors function providing them with the collected arguments
collect inputSelectors return values
compare inputSelectors return values with the ones obtained in previous call (===)
values are the same: return cached result
values are NOT the same: re-evaluate result function and return
Conclusions
memoize-one is a value-based memoize utility: memoization is performed over the value of provided arguments.
reselect adds a further evaluation layer on top of it: memoization is NOT performed over arguments values BUT over the results of a set inputSelectors functions fed with those initial arguments.
It means that reselect selectors are easily composable since each inputSelectors can be another reselect selector.
I haven't used reselect, but memoize-one works great for me when I want to calculate something from props inside render. This is a great pattern for doing an expensive operation, like mapping a large array, on props that may change over time but also may not change on some re-renders. It ensures an expensive operation used in render is re-computed only when the inputs change. It also avoids having to use lifecycle methods like getDerivedStateFromProps (if it can be calculated from props, it probably shouldn't be on state).
import memoize from 'memoize-one'
class Example extends Component {
mapList = memoize(
(list) => list.map(item => ({text: item.text}))
)
render() {
// if this.props.list hasn't changed since the last render
// memoize-one will re-use the last return value from cache
const mappedList = this.mapList(this.props.list)
return (
...
)
}
}
Keep in mind, in most cases, you’ll want to attach the memoized function to a component instance vs. using a static class variable. This prevents multiple instances of a component from resetting each other’s memoized keys.
react memoization reactjs
I suggest to use reselect, since it was specifically designed to use with React/Redux. memoize-one is more like a general purpose memoization library.
It's really easy to use reselect, it just wraps your selectors:
import { createSelector } from 'reselect';
const shopItemsSelector = state => state.shop.items;
// without reselect
const subtotalSelector = state => {
const items = shopItemsSelector(state);
return items.reduce((acc, item) => acc + item.value, 0);
}
// with reselect
const subtotalSelector = createSelector(
shopItemsSelector, // list the selectors you need
items => items.reduce((acc, item) => acc + item.value, 0) // the last argument is actual selector
)
Following the example on the reselect docs:
import { createSelector } from 'reselect'
const shopItemsSelector = state => state.shop.items
const subtotalSelector = createSelector(
shopItemsSelector,
items => items.reduce((acc, item) => acc + item.value, 0)
)
In a typical redux app, subtotalSelector will recompute if a user updates item.name, even though this has no impact on the result. Is there a way to avoid this?
Two solutions:
Let it be. Unless you have a large number of items, the browser's computing capacity is well enough to handle the recompute.
Seperate prices from the item object. That is, you have state.shop.items.itemNames(containing id-name pairs) and state.shop.items.itemValues(containing id-value pairs). Then only the itemValues is passed to the selector.
I have a similar problem and I have found a sort of hack to get arround it.
I have a complex set of filters, and a huge number of items to filter through. Part of the filter state includes display state. I want to ignore changes in the display state so I don't filter a huge list all the time. This is an easy-ish solution:
const getFilters = createSelector(
state => state.filters,
filters => {
const filtersWithoutDisplay = {};
const ignoreObj = { collapsed: null };
for (let filterGroup in filters) {
filtersWithoutDisplay[filterGroup] = Object.assign({}, filters[filterGroup], ignoreObj);
}
// We create a new object every time, so this cannot be memoized properly unless we stringify.
return JSON.stringify(filtersWithoutDisplay);
}
);
It returns a JSON string that has to be parsed, but it's a primitive so as an input to another selector it doesn't trigger a recomputation if the actual contents don't change. That's kind of a hack.
You could also define an object outside of the selector function and always keep the same reference, change the insides according to this same patter, and then use a custom deep equality check by pulling in createSelectorCreator, as explained here https://github.com/reactjs/reselect#q-why-is-my-selector-recomputing-when-the-input-state-stays-the-same . This is probably a better way to go, but as it says:
Always check that the cost of an alternative equalityCheck function or deep equality check in the state update function is not greater than the cost of recomputing every time.
That goes for the JSON.stringify hack as well. I wouldn't do it for the huge list, but for the filters, sure.
In my situation, it's probably better to refactor my state because the filter values may be a separate concern from the filter display settings, and this may not be the only time I want them separate.
I'm having a hard time understanding redux middleware and exactly how it gets configured within the Redux store. For example, I have the following examples
const store = compose(
resetRedux('LOGOUT_USER'),
applyMiddleware(...middlewares),
)(createStore)(rootReducer, initialState);
resetRedux is a middleware that basically resets the entire redux store when the string LOGOUT_USER is dispatched. This works great, however, if I put a console.log within the resetRedux middleware, it only gets called once, which is strange, considering I'd imagine that action needs to be checked every time in order to be able to decide whether to reset the store.
export default function resetMiddleware(types = ['RESET']) {
return (next) => (reducer, initialState) => {
const resetTypes = Array.isArray(types) ? types : [types];
console.log('THIS IS ONLY CALLED ONCE??!')
const enhanceReducer = (state, action) => {
if (~resetTypes.indexOf(action.type)) {
state = undefined;
}
return reducer(state, action);
};
return next(enhanceReducer, initialState);
}
};
So, I'm curious how this is working when the console.log is only called once.
You're confusing and conflating "middleware" and "store enhancers". The resetRedux piece that you've written is actually a "store enhancer", not a middleware.
A real middleware would have its main body executed for every action that is dispatched. The store enhancer itself is evaluated/executed once, at the time of store creation. The reducer you're returning will be executed for every action, because you're creating a function that wraps around whatever the "real" reducer is that's provided to createStore.
As an addendum, the Redux Tutorials#Implementation Walkthroughs section of my React/Redux links list has several articles that explain how middleware work in Redux.
Thats what compose does, it takes your passed functions and create a new function that "compose" them.
Arguments
(arguments): The functions to compose. Each function is expected to
accept a single parameter. Its return value will be provided as an
argument to the function standing to the left, and so on. The
exception is the right-most argument which can accept multiple
parameters, as it will provide the signature for the resulting
composed function. Returns
Returns (Function): The final function obtained by composing the given
functions from right to left.
You can check the source code here
Its a typical utility in functional programming, e.g. lodash's also provide it, underscore as well