If different reducers are associated to the same action and are performing changes to the store, are they receiving the same version of it before any edit happens? Is there a priority to consider?
Example:
Lets assume you dispatch an action Update to edit an entity on server side, once data is successfully updated, an effect will dispatch an Update_Success action with an instance of the newly received version of entity as payload. A first classic and logical reducer will use it to update the store:
case ItemActionTypes.UPDATE_ITEM_SUCCESS: {
const item = action.payload;
return {
...adapter.upsertOne(item, state),
loaded: true,
loading: false
};
}
Now lets assume that in a different file, you have a different reducer associated to the same action and needs to compare the newly received entity against the old one in store:
case ItemActionTypes.UPDATE_ITEM_SUCCESS: {
const item = action.payload;
const oldItem = state.entities[item.id];
const changes = getAffectedProperties(item, oldItem);
// ...
}
The question is: Is there any chance oldItem is actually holding the newly received item instead of the old one as the first reducer may have already updated it? Is it a first come first serve case? or is there anything on its implementation that guarantees all reducers are performing changes to the the same version of the store and maybe merging those changes at a further step?
If different reducers are associated to the same action and are
performing changes to the store, are they receiving the same version
of it before any edit happens? Is there a priority to consider?
Reducers only have access to their piece of state. So you're not supposed to really care about that.
Also, when one action is dispatched, all the reducers are called in a synchronous way. Which means that if for the ACTION_A you're modifying your store from different places (/ different reducers), your selectors (or even the view), will only be refreshed once. When all the updates for that action are applied on the store.
Related
I'm an absolute beginner in react-redux and I watched many videos and articles,docs and couldn't understand how the state flows between reducers and between store to reducer.
I am having state for each reducer like this
const initState = {todos:[]}
cont reducer = (state=initState,action) ....
Reducer 2 similarly with different state
initState2 = {todo:" "}
reducer2 = (state=initState2,action) ...
And then I import and combine the reducers. Here I am using two different reducers which have different states and is it the right way of doing things ? If so, How can redux be called single state if each reducer have its own individual state.
Don't we have a single state in the store which is accessed by all the reducers and dispatch actions to change state of store directly instead of changing the reducer's store. Any help is appreciated and it may seems like a silly question but filling the gaps is really important and many beginners have same doubt as mine and please do help. Thank you
You only need 1 reducer to store your todos.
How can redux be called single state if each reducer have its own
individual state.
The application effectively has only 1 global store where all the application state is stored. What the reducer returns is what effectively gets stored in the store.
The configuration of what is stored is a map (key-value) where the key is defined in the root reducer, and the value is what is returned from the reducer function.
The way you have to look at it is that the view is "dumb", in that the only thing it does is tell the app what it wants by dispatching an action. This action is just an event that is marked with some string you give it to identify clearly what it is the view wants. The reducer intercepts this action and updates the state in the store accordingly. This state in turn is accessible to all the components in your app. So it clearly is global.
In your example, the view would just tell the application for example: "Add a todo". The reducer will intercept this message and return an array with the added todo. This returned array is what will be saved in the store.
If you want a seperate "todo", this will probably refer to the currently "active" to do. Marking it as such will make the purpose more expressive.
This is single state because your root reducer will end up with something like:
{
"activeTodo": activeTodoReducer
"todos": todosReducer
}
And you can access these key / values in your components throughout the entire application.
How can redux be called single state if each reducer have its own individual state.
Because the state is not saved in the reducers. The state is only saved in the store and there is only one store. That is why is called single state.
To create the store:
const store = createStore(myBeautifulReducers)
In your case, myBeautifulReducers would be:
const myBeautifulReducers = combineReducers({reducer, reducer2});
myBeautifulReducers will be an object that contains both reducers (reducer and reducer2) and the logic you wrote in each of them (switch statements and so on).
I want to save data of every reducer in localStorage in the form of key value pairs
UserReducer: {isLoding: true, ....}
OrderReducer : {isRecieved: false, ....}
So if there is any changes happen in any of the reducer I want to set the updated data in local storage.
What I am thinking is I can do this store.subscribe but how I could know that which reducer has been changed so I can set the whole reducer data to localStorage.
Need help. Thanks :)
One way to do it is to create a lastAction reducer which will store the value of the last dispatched action. Then in store.subscribe you could do:
let nextState = store.getState();
let dispatchedAction = nextState.lastAction.type;
And when you know the last dispatched action, you can update the local storage depending on that action type.
You can create a redux middleware, it will get fired everytime an action is dispatched. Within the middleware you can put a conditional check to see if prev and next store are same. If not, you can update your local storage.
I found this helpful for creating a middleware. And it's fairly easy.
I think you could not know the changes from the store.subscribe.
The last place when you know the changes are the actions.
You can put the persisting logic to the reducers. But maybe you should not want to introduce side effect in your reducer.
So basically you have two options if you rule out the first one.
Persist the whole store as a single object to local storage. It is the easier option. I would try this solution first and I would measure the overhead with a worst case scenario.
Make a redux middleware and capture the individual actions. You can copy the reducer pattern to persist only the object that changed.
You can use subscribe for option 2. You can use middleware for option 2 and option 3.
I came up with one more solution.
You can do the persisting in your action creator with Redux Thunk or with Redux Saga.
You can use React Component for that. Connect it to the Store, and use it to write to localStorage instead of rendering something.
Imho, it would be easier to write than middleware.
You can use redux-watch to listen to the changes in redux store
for e.g.
// ... other imports/requires
import watch from 'redux-watch'
// assuming you have an admin reducer / state slice
console.log(store.getState().admin.name) // 'JP'
// store is THE redux store
let w = watch(store.getState, 'admin.name')
store.subscribe(w((newVal, oldVal, objectPath) => {
console.log('%s changed from %s to %s', objectPath, oldVal, newVal)
// admin.name changed from JP to JOE
}))
// somewhere else, admin reducer handles ADMIN_UPDATE
store.dispatch({ type: 'ADMIN_UPDATE', payload: { name: 'JOE' }})
this may help you.
I want to extend the approach 3 that #PeterAmbruzs shared. Actually the problem which I faced was whenever there was an action dispatched, store wasn't updated immediately but I still wanted to persist it so I imported reducer and passed state and action into it to get the required next state
import reducer from './reducer'
const someMiddleware = store => next => action => {
const state = store.getState(),
const nextState = reducer(state, action)
// Save it to Local Storage
// Or Make an API Call to save it to server
next(action)
}
I'm pretty new to Redux and i'm encountering some problems with it.
I'm creating a list of items in a component, sending it to redux state and then i want to read from that redux state and display the items in a different list component.
The creation part works as i can console.log and getState() without problems (i am seeing the changes in Redux State).
My problem is that my component state does not change, nor does it re-render.
And now some code ->
this.state = {
initialItems: this.props.SharepointItems,
}
And at the end
const mapStateToProps = (state) => {
return {
SharepointItems: state.listItems,
}
}
export default connect(mapStateToProps)(SharePointList);
I even tried something like this in my componentDidMount() ->
store.subscribe(() => {
this.setState({ initialItems: this.props.SharepointItems });
console.log("updating state");
});
From what i've read i shouldnt need to update the state manually while using redux, or am i wrong?
EDIT: Since my list doesnt throw an error if i console.log i can see that the array is empty (which is what i defined in the Redux state). Is there something that i should be doing to get the new state ? Seems like its getting the immutable state or something like that (the empty array).
Edit2: Found the problem (or part of it). It appears as my state is 1 event behind. So redux contains the array with 4 items, the component state is empty. If i do a dispatch from the browser i get 5 items (as expected) in redux, but 4 in state (it finally shows not empty).
Also my code was a bit bugged (i was passing the entire array instead of items in the array).
I've changed it to
result.map((item) => {
store.dispatch(addListItem(item));
});
And it started rendering. The problem is it displays items from 0 to 2 (4 in array), but the last one is left behind. Once again if i do another dispatch from the browser i get item 3 rendered, but 4 (the last one added) is only in redux state and does not update the list state.
Also...is it a good idea to do it like this? My list might have 1000 items in the future and i'm not sure that dispatch is a good solution (i need to make an API call to get the items first, which is why i'm using dispatch to populate redux).
Updated with reducer ->
const rootReducer = (state = initialState, action) => {
switch (action.type) {
case ADD_LISTITEM:
return { ...state, listItems: [...state.listItems, action.payload] };
case ADD_SPOTOKEN:
return { ...state, spoToken: action.payload };
default:
return state;
}
};
export default rootReducer;
Found something else thats a bit weird. I am also using React Router. As i said my list displays only 3 of the 4 items in my Redux State array. If i navigate to a different page and then back to the list page it actually renders all 4 of the items.
I believe your problem stems from the fact that you have "forked" SharepointItems off of props and set it to the component's local this.state. Unless you really need to, I'd recommend not doing that. Just use this.props. Dan Abramov (author of Redux) recommends this as a general principle too.
By forking props onto state, you create two sources of truth regarding the state of SharepointItems, namely, this.state.initialItems and this.props.SharepointItems. It then becomes your responsibility to keep this.state and this.props in sync by implementing componentDidUpdate (that is why you're not seeing it update). You can avoid all the extra work by just using the data that flows in from props.
The connect function will re-render your component with new props whenever you update the redux store. So in your render method, just refer to this.props.SharepointItems and not this.state.initialItems. Then you should be good to go, that is, assuming you've implemented your reducer(s) and store configuration properly.
I actually fixed this by mistake. I was planning on leaving it for the end and find a workaround for it, but i somehow fixed it.
I was importing the list component (which had the redux state props) in a Page (react-router). The page wasn't connected since it wasn't ready yet. Apparently after connecting the page (parent component which holds the list) everything works fine and i can see all 4 of my items (instead of seeing 3 without having the parent connected).
I wonder if this is intended...
I am trying my hands on ngrx library for managing the state of my application. I have gone through many ngrx documents and git pages. I understand that there are three important concept:
Store
Reducer and
Action
Store is the single source of data for our application. So any modification or retrieval of data is done through Actions. My question here is what exactly happens when an action is dispatched to the store? How does it know which reducers is to be invoked? Does it parses all the reducers registered to the store? There can be multiple actions with the same name in that case what happens?
Thanks in advance.
A picture is worth a thousand words...
Source: Building a Redux Application with Angular2
Example Code: ngrx-todo-app
Demo: Todo App using #ngrx/store and #ngrx/effects
My question here is what exactly happens when an action is dispatched to the store?
All of the registered reducers get a chance to handle the action
How does it know which reducers is to be invoked?
All of the registered reducers get invoked. Try putting console.logs into all the reducers and you can see for yourself.
Does it parses all the reducers registered to the store?
Yes
There can be multiple actions with the same name in that case what happens?
If you have multiple actions with the same name they would be treated the same. For example if I dispatched type "ADD" with payload 3 and then dispatched a different action called type "ADD" with payload 3 it would be the same thing.
Ngrx isn't that smart. Let's say we have the following reducers:
const reducers = {
blog: BlogReducer,
post: PostReducer,
comment: CommentReducer
}
Say I dispatch 'ADD_COMMENT'. Basically BlogReducer will first try to handle it, followed by PostReducer, and finally by CommentReducer. The ordering is determined by how you specified the reducers object above. So if I did this:
const reducers = {
comment: CommentReducer,
blog: BlogReducer,
post: PostReducer
}
CommentReducer would be the first one to try and handle the 'ADD_COMMENT'.
Redux documentations says I should make actions and action creators, like this:
function addTodo(filter) {
return {
type: SET_VISIBILITY_FILTER,
filter
}
}
Then write reducers, like this:
function todoApp(state = initialState, action) {
switch (action.type) {
case SET_VISIBILITY_FILTER:
return Object.assign({}, state, {
visibilityFilter: action.filter
});
}
}
I then invoke the action using dispatch:
store.dispatch(addTodo("Ask question on stackoverflow"));
It seems there's a one-one correspondence between actions and reducers; the sole purpose of the action is to select a reducer and provide input data for that reducer.
Why don't we skip the middle man and identify actions with reducers and action creators with functions producing reducers? Then dispatch would take a single argument, a reducer/action of type State => State:
// Action/reducer. (Parametrised state transformer, really.)
const addTodo = text => state => {
return Object.assign({}, state, {
visibilityFilter: action.filter
});
}
// Dispatch takes as a argument the action/reducer
store.dispatch(addTodo("Ask question on stackoverflow"));
You'd lose the ability to serialise actions, but otherwise, it seems you'd get rid of boilerplate action creators and express more clearly the connection between actions and reducers. If you're in Typescript, you also get typechecking of the data in actions, which is difficult to express otherwise.
So what reasons for having actions as data am I missing?
The main purpose of action in Redux is to reduce the state.
Reduce method will be called on array of actions (thats why it called a reducer). Example:
import reducer from './reducer';
const actions = [
{type: 'INIT'},
{type: 'SOME_ACTION', params: {...}},
{type: 'RECEIVE_DATA', data: [...]},
{type: 'SOME_ANOTHER_ACTION', params: {...}},
{type: 'RECEIVE_DATA', data: [...]},
...
];
const finalState = actions.reduce(reducer, undefined);
Action creators is a function that can create actions. It is not necessary that action creator will create only one action.
Actually, if your reducer is able to receive functions instead of objects - your actions will be functions and it will do the main purpose, but you can loose some benefits of Redux functional abilities.
In that case the reducer will be implemented like this:
function reducer(state, action) {
return action(state);
}
The reasons why you may create actions as {type: 'ACTION_NAME'} format:
Redux DevTools expects this format.
You need to store sequence of actions.
Reducer makes state transformations on worker.
Everybody in Redux ecosystem use this format. It's a kind of convention.
Hot reloading abilities (your stored functions will not be reloaded).
You need to send actions as is on server.
Debugging benefits - to see the stack of actions with actions names.
Writing unit tests for reducer: assert.equal(finalState, expectedState).
More declarative code - action name and parameters are about "what to do" and not about "how to do" (but addTodo('Ask question') is declarative too).
Note about coupling between action creators and state changes
Just compare two notations:
First:
function someActionCreator() {
return {
type: "ADD_TODO",
text: "Ask question on stackoverflow"
}; // returns object
}
Second:
function someActionCreator() {
return addTodo("Ask question on stackoverflow"); // returns function
}
"In both cases we see that code is declarative and action creator is decoupled from state change. You can still reuse addTodo or dispatch two addTodo's or use middleware or dispatch compose(addTodo('One'), addTodo('Two')). The main difference is that we created Object and Function and place in code where state changes.
There is NOT a one-to-one mapping between actions and reducers. Per Dan Abramov's comments at https://github.com/Pitzcarraldo/reduxible/issues/8 :
It reinforces a very common misconception about Redux: namely that action creators and reducers are one-to-one mapping.
This is only true in trivial examples, but it is extremely limiting in real applications. Beginners exposed to this pattern couple reducers and action creators, and fail to realize they're meant to be many-to-many and decoupled.
Many reducers may handle one action. One reducer may handle many actions. Putting them together negates many benefits of how Flux and Redux application scale. This leads to code bloat and unnecessary coupling. You lose the flexibility of reacting to the same action from different places, and your action creators start to act like “setters”, coupled to a specific state shape, thus coupling the components to it as well.
As for actions and the "type" parameter, the other answers are right. That's deliberately how Redux was designed, and that was intended to give the benefits of serialization for debugging purposes.
Good question.
Separating actions from state changes is really a Flux pattern, rather than a specifically Redux thing. (Though I will answer the question with reference to Redux.) It's an example of loose coupling.
In a simple app, tight coupling between actions and state changes might be fine. But in a larger app, this could be a headache. For instance, your addTodo action might trigger changes in several parts of the state. Splitting actions from state changes - the latter performed in reducers - allows you to write smaller functions, which are easier to reason about and more testable.
Additionally, decoupling your actions and state changes allows your reducer logic to be more reusable. e.g. Action X might trigger state changes A and B, whilst action Y only triggers state change A.
Furthermore, this decoupling gives rise to a Redux feature called middleware. Middleware listens to action dispatches. It doesn't change the state of the app, but it can read the current state and the next state, as well as the action information. Middleware is useful for functionality extraneous from the core application logic, e.g. logging and tracking (dev tools were mentioned in a previous answer).
[UPDATE] But why have actions as objects?
If it were simply a matter of functions calling other functions, that decoupling would be much less explicit. Indeed it may even get lost; since most actions only enact one state change, developers might tire of a single function calling a single function and do away with the separation entirely.
The other thing is the Flux data flow model. One-way data flow is very important to the Flux/React paradigm. A typical Redux/React goes something like this: Store state -> Higher order React components -> Lower order React components -> DOM. The action is the aberration in the model; it's the backwards arrow transmitting data from the view to the store. It makes sense to make the action something as loud and emphatic as possible. Not a mere function call, but a dispatched object. It's as if your app were announcing Hey! Something important happened here!