in my project I'm fetching data from an API (trip entities). This is my model:
//Trip.model.ts
export class Trip{
created_at?:Date;
updated_at?:Date;
.... bunch of fields
}
In my component I'm fetching the data and assigning it to the trips variable. However, when I'm trying to access any of the items in the trips array I get 'undefined'. I also can't loop through it, I tried both forEach and for...in/of.
I tried using an interface instead of a class but with no luck. How can I loop through that array of objects in order to use the data in it?
Component's code:
userName:string='';
trips:Trip[]=[];
moment:any=moment;
usersData:any={};
constructor(private auth: AuthService,
private storage: LocalStorageService,
private translate: TranslateService,
private tripService: TripService){}
ngOnInit(): void {
console.log(this.translate.currentLang)
this.userName=localStorage.getItem('username')!;
this.fetchTrips();
this.fetchPics();
}
fetchTrips() {
this.tripService.getTrips().subscribe({
next: data => {
data[0].data.forEach((value) => {
let trip: Trip = plainToInstance(Trip,value);
this.trips.push(trip);
});
}, error: error => {
console.log(error);
}
});
}
//fetchPics because I want to extract
//user's profile pics' urls from the trips array
fetchPics(){
console.log(this.trips);
console.log(this.trips[0]);
this.trips.forEach((trip)=>{
console.log(trip);
});
}
getTrips service method:
getTrips(){
return this.http.get<any>(Api.API+Endpoints.TRIP);
}
This is what shows when I
console.log(this.trips)
after assignment.
Data from the API:
Pictures cropped to make them more readable.
You are trying to get access to this.trips value before you actually have your data on it.This happens becouse you get that data asynchronously, just inside the subscribe of this.tripService.getTrips()
So, in order to solve your problem, you just need to move this invoke:
this.fetchPics();
inside the subscribe of fetchTrips() method, like this:
fetchTrips() {
this.tripService.getTrips().subscribe({
next: data => {
data[0].data.forEach((value) => {
let trip: Trip = plainToInstance(Trip,value);
this.trips.push(trip);
this.fetchPics();
});
}, error: error => {
console.log(error);
}
});
}
The function fetchPics() executes before your getTrips call ends. You need to call the method only after your HTTP call ends, and after you populate your trips array successfully.
fetchTrips() {
this.tripService.getTrips().subscribe({
next: data => {
//Populate your trips array
data[0].data.forEach((value) => {
let trip: Trip = plainToInstance(Trip,value);
this.trips.push(trip);
});
// this is where you need to call
this.fetchPics();
}, error: error => {
console.log(error);
}
});
}
This is happening because of JS is asynchronous. you are making an http request here, that may take some time to get data from server. let's assume that might take 1 minute untill then compiler will not stop it's execution process In that 1 min of time it will execute next statements.because of that your fetchpics() method is being executed before fecthtrips() execution done. To overcome this we can use async, await as like below.
async fetchTrips() {
this.tripService.getTrips().subscribe({
next: data => await {
data[0].data.forEach((value) => {
let trip: Trip = plainToInstance(Trip,value);
this.trips.push(trip);
});
}, error: error => {
console.log(error);
}
});
}
When the page is being loaded for the first time, vue component is not waiting for my custom store file to process it. I thought it might fix it with promises but I am not sure on how to do so on functions that do not really require extra processing time.
I am not including the entire .vue file because I know it surely works just fine. My store includes couple of functions and it is worth mentioning it is not set up using vuex but works very similarly. Since I also tested what causes the issue, I am only adding the function that is related and used in MainComp.
Vue component
import store from "./store";
export default {
name: "MainComp",
data() {
return {
isLoading: true,
storageSetup: store.storage.setupStorage,
cards: Array,
};
},
created() {
this.storageSetup().then(() => {
this.cards= store.state.cards;
});
this.displayData();
},
methods: {
displayData() {
this.isLoading = false;
},
}
My custom store.js file
const STORAGE = chrome.storage.sync;
const state = {
cards: []
};
const storage = {
async setupStorage() {
await STORAGE.get(['cards'], function (data) {
if (Object.keys(data).length === 0) {
storage.addToStorage('ALL');
// else case is the one does not work as required
} else {
data.cards.forEach((elem) => {
// modifies the element locally and then appends it to state.cards
actions.addCard(elem);
});
}
});
}
};
export default {
state,
storage
};
Lastly, please ignore the case in setupStorage() when the length of data is equal to 0. If there is nothing in Chrome's local space, then a cards is added properly(state.cards is an empty array every time the page loads). The problem of displaying the data only occurs when there are existing elements in the browser's storage.
How can I prevent vue from assuming cards is not an empty array but instead wait until the the data gets fetched and loaded to state.cards (i.e cards in MainComp)?
Sorry if the problem can be easily solved but I just lost hope of doing it myself. If any more information needs to be provided, please let me know.
Your main issue is that chrome.storage.sync.get is an asynchronous method but it does not return a promise which makes waiting on it difficult.
Try something like the following
const storage = {
setupStorage() {
return new Promise(resolve => { // return a promise
STORAGE.get(["cards"], data => {
if (Object.keys(data).length === 0) {
this.addToStorage("All")
} else {
data.cards.forEach(elem => {
actions.addCard(elem)
})
}
resolve() // resolve the promise so consumers know it's done
})
})
}
}
and in your component...
export default {
name: "MainComp",
data: () => ({
isLoading: true,
cards: [], // initialise as an array, not the Array constructor
}),
async created() {
await store.storage.setupStorage() // wait for the "get" to complete
this.cards = store.state.cards
this.isLoading = false
},
// ...
}
I ran into troubles trying to process data fetched from remote API.
The app is running VueJS with Vuetify, data is formatted with Vuetify's data table component.
This is my code:
export default {
data () {
return {
headers: [
{ text: 'City', value: 'city' },
{ text: '#Citizens', value: 'citizens' },
{ text: '#Schools', value: 'schools' },
{ text: 'Schools per Citizen', value: 'schoolsPerCitizen' },
(...)
API URL is defined as a variable on the root level of the app.
Then, there is this method launched when created() kicks in:
methods: {
loadData() {
axios.get(citiesApiUrl)
.then((response) => {
console.log(response.data) // data displayed correctly
return response.data
})
.catch(error => {console.error(error)})
}
},
created () {
this.loadData()
}
As you noticed in the comment, response.data does display desired values.
Problems start from this point:
computed: {
stats() {
return this.loadData().map(item => {
item.schoolsPerCitizen = (item.schools / item.citizens).toFixed(2)
return item
})
}
}
I get an error: TypeError: Cannot read property 'map' of undefined.
Any ideas what is wrong with my code?
Issues
When loadData is called in created, the axios promise is consumed but nothing happens with the returned data except it's logged and returned to the promise resolver.
When loadData is called in stats (computed), .map is chained off of the return value from loadData, but loadData has no return value.
Even if loadData returned the axios promise, that promise would have to be consumed in stats first before accessing the data (needs .then)
The design is flawed because the computed will make an identical API call every time it recalculates, which is likely unnecessary.
Also, the promise returned by stats wouldn't be resolved by the template render function anyway.
Fix
Create a variable for the loaded data (I'll call it mydata):
data() {
return {
// ...
mydata: []
}
}
Change loadData to:
loadData() {
axios.get(citiesApiUrl).then((response) => {
this.mydata = response.data // <--- Set the data to `mydata`
}).catch(error => {
console.error(error)
})
}
Change stats to:
stats() {
// This is also not designed properly, it's going to mutate `mydata`...
// You should study Vue and learn what the purpose for computeds are before using them
return this.mydata.map(item => { // <-- Once `mydata` is populated, this will recalculate
item.schoolsPerCitizen = (item.schools / item.citizens).toFixed(2)
return item
})
}
loadData does not return any value.
loadData() {
return axios.get(citiesApiUrl)
.then((response) => {
console.log(response.data) // data displayed correctly
return response.data
})
.catch(error => {console.error(error)})
}
I am trying to understand rxjs and got stuck at a debounceTime(n /* ms */) experiment.
public debounceTime(dueTime: number, scheduler: Scheduler): Observable
Emits a value from the source Observable only after a particular time span has passed without another source emission.
source
My code:
function fakeAPI() {
return new Rx.Observable(observer => {
const root = 'https://jsonplaceholder.typicode.com'
$.ajax({
url: root + '/posts/1',
method: 'GET'
}).then(function(data) {
observer.next(data)
}).fail(function(err) {
observer.error(err)
})
return ()=>{
observer.complete()
console.log('unsubscribed!')
}
})
}
const fakeObserver = fakeAPI()
$('#buttonText').click(()=>{
fakeObserver
.debounceTime(10000)
.subscribe(() => {
return {
next(item) {
console.log('received: ', item.id)
},
error(err) {
console.log('error:', err)
},
complete() {
console.log('completed!')
}
}
}());
})
My expectation: Even with N number of clicks in the given amount of time, the API call would only be made once. Instead, it seems like it waits for the given time and then all the N clicks result in an API call.
What am I doing wrong?
As per the docs, debounceTime(n) is supposed to discard previous pending delayed emissions if a new value arrives on the source.
Here is a JSBin link
As per the docs, debounceTime(n) is supposed to discard previous pending delayed emissions if a new value arrives on the source.
It's true but on each click:
You create new subscription
It calls api
Api returns result
debounceTime waits 10s (nothing happens because observerable returned by fakeObserver emits only once)
You log the result
You need to convert your clicks in observable to implement what you want:
Rx.Observable.fromEvent(document.getElementById('buttonText'), 'click')
Check jsBin
I've currently got a situation whereby I need Redux Actions to be run consecutively. I've taken a look at various middlewares, such a redux-promise, which seem to be fine if you know what the successive actions are at the point of the root (for lack of a better term) action being triggered.
Essentially, I'd like to maintain a queue of actions that can be added to at any point. Each object has an instance of this queue in its state and dependent actions can be enqueued, processed and dequeued accordingly. I have an implementation, but in doing so I'm accessing state in my action creators, which feels like an anti-pattern.
I'll try and give some context on use case and implementation.
Use Case
Suppose you want to create some lists and persist them on a server. On list creation, the server responds with an id for that list, which is used in subsequent API end points pertaining to the list:
http://my.api.com/v1.0/lists/ // POST returns some id
http://my.api.com/v1.0/lists/<id>/items // API end points include id
Imagine that the client wants to perform optimistic updates on these API points, to enhance UX - nobody likes looking at spinners. So when you create a list, your new list instantly appears, with an option at add items:
+-------------+----------+
| List Name | Actions |
+-------------+----------+
| My New List | Add Item |
+-------------+----------+
Suppose that someone attempts to add an item before the response from the initial create call has made it back. The items API is dependent on the id, so we know we can't call it until we have that data. However, we might want to optimistically show the new item and enqueue a call to the items API so that it triggers once the create call is done.
A Potential Solution
The method I'm using to get around this currently is by giving each list an action queue - that is, a list of Redux actions that will be triggered in succession.
The reducer functionality for a list creation might look something like this:
case ADD_LIST:
return {
id: undefined, // To be filled on server response
name: action.payload.name,
actionQueue: []
}
Then, in an action creator, we'd enqueue an action instead of directly triggering it:
export const createListItem = (name) => {
return (dispatch) => {
dispatch(addList(name)); // Optimistic action
dispatch(enqueueListAction(name, backendCreateListAction(name));
}
}
For brevity, assume the backendCreateListAction function calls a fetch API, which dispatches messages to dequeue from the list on success/failure.
The Problem
What worries me here is the implementation of the enqueueListAction method. This is where I'm accessing state to govern the advancement of the queue. It looks something like this (ignore this matching on name - this actually uses a clientId in reality, but I'm trying to keep the example simple):
const enqueueListAction = (name, asyncAction) => {
return (dispatch, getState) => {
const state = getState();
dispatch(enqueue(name, asyncAction));{
const thisList = state.lists.find((l) => {
return l.name == name;
});
// If there's nothing in the queue then process immediately
if (thisList.actionQueue.length === 0) {
asyncAction(dispatch);
}
}
}
Here, assume that the enqueue method returns a plain action that inserts an async action into the lists actionQueue.
The whole thing feels a bit against the grain, but I'm not sure if there's another way to go with it. Additionally, since I need to dispatch in my asyncActions, I need to pass the dispatch method down to them.
There is similar code in the method to dequeue from the list, which triggers the next action should one exist:
const dequeueListAction = (name) => {
return (dispatch, getState) => {
dispatch(dequeue(name));
const state = getState();
const thisList = state.lists.find((l) => {
return l.name === name;
});
// Process next action if exists.
if (thisList.actionQueue.length > 0) {
thisList.actionQueue[0].asyncAction(dispatch);
}
}
Generally speaking, I can live with this, but I'm concerned that it's an anti-pattern and there might be a more concise, idiomatic way of doing this in Redux.
Any help is appreciated.
I have the perfect tool for what you are looking for. When you need a lot of control over redux, (especially anything asynchronous) and you need redux actions to happen sequentially there is no better tool than Redux Sagas. It is built on top of es6 generators giving you a lot of control since you can, in a sense, pause your code at certain points.
The action queue you describe is what is called a saga. Now since it is created to work with redux these sagas can be triggered to run by dispatching in your components.
Since Sagas use generators you can also ensure with certainty that your dispatches occur in a specific order and only happen under certain conditions. Here is an example from their documentation and I will walk you through it to illustrate what I mean:
function* loginFlow() {
while (true) {
const {user, password} = yield take('LOGIN_REQUEST')
const token = yield call(authorize, user, password)
if (token) {
yield call(Api.storeItem, {token})
yield take('LOGOUT')
yield call(Api.clearItem, 'token')
}
}
}
Alright, it looks a little confusing at first but this saga defines the exact order a login sequence needs to happen. The infinite loop is allowed because of the nature of generators. When your code gets to a yield it will stop at that line and wait. It will not continue to the next line until you tell it to. So look where it says yield take('LOGIN_REQUEST'). The saga will yield or wait at this point until you dispatch 'LOGIN_REQUEST' after which the saga will call the authorize method, and go until the next yield. The next method is an asynchronous yield call(Api.storeItem, {token}) so it will not go to the next line until that code resolves.
Now, this is where the magic happens. The saga will stop again at yield take('LOGOUT') until you dispatch LOGOUT in your application. This is crucial since if you were to dispatch LOGIN_REQUEST again before LOGOUT, the login process would not be invoked. Now, if you dispatch LOGOUT it will loop back to the first yield and wait for the application to dispatch LOGIN_REQUEST again.
Redux Sagas are, by far, one of my favorite tools to use with Redux. It gives you so much control over your application and anyone reading your code will thank you since everything now reads one line at a time.
Have a look at this: https://github.com/gaearon/redux-thunk
The id alone shouldn't go through the reducer. In your action creator (thunk), fetch the list id first, and then() perform a second call to add the item to the list. After this, you can dispatch different actions based on whether or not the addition was successful.
You can dispatch multiple actions while doing this, to report when the server interaction has started and finished. This will allow you to show a message or a spinner, in case the operation is heavy and might take a while.
A more in-depth analysis can be found here: http://redux.js.org/docs/advanced/AsyncActions.html
All credit to Dan Abramov
I was facing a similar problem to yours. I needed a queue to guarantee that optimistic actions were committed or eventually committed (in case of network problems) to remote server in same sequential order they were created, or rollback if not possible. I found that with Redux only, fells short for this, basically because I believe it was not designed for this and doing it with promises alone can be really a hard problem to reason with, besides the fact you need to manage your queue state somehow... IMHO.
I think #Pcriulan's suggestion on using redux-saga was a good one. At first sight, redux-saga doesn't provide anything to help you with until you get to channels. This opens you a door to deal with concurrency in other ways other languages do, CSP specifically (see Go or Clojure's async for example), thanks to JS generators. There are even questions on why is named after the Saga pattern and not CSP haha... anyway.
So here is how a saga could help you with your queue:
export default function* watchRequests() {
while (true) {
// 1- Create a channel for request actions
const requestChan = yield actionChannel('ASYNC_ACTION');
let resetChannel = false;
while (!resetChannel) {
// 2- take from the channel
const action = yield take(requestChan);
// 3- Note that we're using a blocking call
resetChannel = yield call(handleRequest, action);
}
}
}
function* handleRequest({ asyncAction, payload }) {
while (true) {
try {
// Perform action
yield call(asyncAction, payload);
return false;
} catch(e) {
if(e instanceof ConflictError) {
// Could be a rollback or syncing again with server?
yield put({ type: 'ROLLBACK', payload });
// Store is out of consistency so
// don't let waiting actions come through
return true;
} else if(e instanceof ConnectionError) {
// try again
yield call(delay, 2000);
}
}
}
}
So the interesting part here is how the channel acts as a buffer (a queue) which keeps "listening" for incoming actions but won't proceed with future actions until it finish with the current one. You might need to go over their documentation in order to grasp the code better, but I think it's worth it. The resetting channel part might or not work for your needs :thinking:
Hope it helps!
This is how I would tackle this problem:
Make sure each local list have an unique identifier. I'm not talking about the backend id here. Name is probably not enough to identify a list? An "optimistic" list not yet persisted should be uniquely identifiable, and user may try to create 2 lists with the same name, even if it's an edge case.
On list creation, add a promise of backend id to a cache
CreatedListIdPromiseCache[localListId] = createBackendList({...}).then(list => list.id);
On item add, try to get the backend id from Redux store. If it does not exist, then try to get it from CreatedListIdCache. The returned id must be async because CreatedListIdCache returns a promise.
const getListIdPromise = (localListId,state) => {
// Get id from already created list
if ( state.lists[localListId] ) {
return Promise.resolve(state.lists[localListId].id)
}
// Get id from pending list creations
else if ( CreatedListIdPromiseCache[localListId] ) {
return CreatedListIdPromiseCache[localListId];
}
// Unexpected error
else {
return Promise.reject(new Error("Unable to find backend list id for list with local id = " + localListId));
}
}
Use this method in your addItem, so that your addItem will be delayed automatically until the backend id is available
// Create item, but do not attempt creation until we are sure to get a backend id
const backendListItemPromise = getListIdPromise(localListId,reduxState).then(backendListId => {
return createBackendListItem(backendListId, itemData);
})
// Provide user optimistic feedback even if the item is not yet added to the list
dispatch(addListItemOptimistic());
backendListItemPromise.then(
backendListItem => dispatch(addListItemCommit()),
error => dispatch(addListItemRollback())
);
You may want to clean the CreatedListIdPromiseCache, but it's probably not very important for most apps unless you have very strict memory usage requirements.
Another option would be that the backend id is computed on frontend, with something like UUID. Your backend just need to verify the unicity of this id. Thus you would always have a valid backend id for all optimistically created lists, even if backend didn't reply yet.
You don't have to deal with queuing actions. It will hide the data flow and it will make your app more tedious to debug.
I suggest you to use some temporary ids when creating a list or an item and then update those ids when you actually receive the real ones from the store.
Something like this maybe ? (don't tested but you get the id) :
EDIT : I didn't understand at first that the items need to be automatically saved when the list is saved. I edited the createList action creator.
/* REDUCERS & ACTIONS */
// this "thunk" action creator is responsible for :
// - creating the temporary list item in the store with some
// generated unique id
// - dispatching the action to tell the store that a temporary list
// has been created (optimistic update)
// - triggering a POST request to save the list in the database
// - dispatching an action to tell the store the list is correctly
// saved
// - triggering a POST request for saving items related to the old
// list id and triggering the correspondant receiveCreatedItem
// action
const createList = (name) => {
const tempList = {
id: uniqueId(),
name
}
return (dispatch, getState) => {
dispatch(tempListCreated(tempList))
FakeListAPI
.post(tempList)
.then(list => {
dispatch(receiveCreatedList(tempList.id, list))
// when the list is saved we can now safely
// save the related items since the API
// certainly need a real list ID to correctly
// save an item
const itemsToSave = getState().items.filter(item => item.listId === tempList.id)
for (let tempItem of itemsToSave) {
FakeListItemAPI
.post(tempItem)
.then(item => dispatch(receiveCreatedItem(tempItem.id, item)))
}
)
}
}
const tempListCreated = (list) => ({
type: 'TEMP_LIST_CREATED',
payload: {
list
}
})
const receiveCreatedList = (oldId, list) => ({
type: 'RECEIVE_CREATED_LIST',
payload: {
list
},
meta: {
oldId
}
})
const createItem = (name, listId) => {
const tempItem = {
id: uniqueId(),
name,
listId
}
return (dispatch) => {
dispatch(tempItemCreated(tempItem))
}
}
const tempItemCreated = (item) => ({
type: 'TEMP_ITEM_CREATED',
payload: {
item
}
})
const receiveCreatedItem = (oldId, item) => ({
type: 'RECEIVE_CREATED_ITEM',
payload: {
item
},
meta: {
oldId
}
})
/* given this state shape :
state = {
lists: {
ids: [ 'list1ID', 'list2ID' ],
byId: {
'list1ID': {
id: 'list1ID',
name: 'list1'
},
'list2ID': {
id: 'list2ID',
name: 'list2'
},
}
...
},
items: {
ids: [ 'item1ID','item2ID' ],
byId: {
'item1ID': {
id: 'item1ID',
name: 'item1',
listID: 'list1ID'
},
'item2ID': {
id: 'item2ID',
name: 'item2',
listID: 'list2ID'
}
}
}
}
*/
// Here i'm using a immediately invoked function just
// to isolate ids and byId variable to avoid duplicate
// declaration issue since we need them for both
// lists and items reducers
const lists = (() => {
const ids = (ids = [], action = {}) => ({
switch (action.type) {
// when receiving the temporary list
// we need to add the temporary id
// in the ids list
case 'TEMP_LIST_CREATED':
return [...ids, action.payload.list.id]
// when receiving the real list
// we need to remove the old temporary id
// and add the real id instead
case 'RECEIVE_CREATED_LIST':
return ids
.filter(id => id !== action.meta.oldId)
.concat([action.payload.list.id])
default:
return ids
}
})
const byId = (byId = {}, action = {}) => ({
switch (action.type) {
// same as above, when the the temp list
// gets created we store it indexed by
// its temp id
case 'TEMP_LIST_CREATED':
return {
...byId,
[action.payload.list.id]: action.payload.list
}
// when we receive the real list we first
// need to remove the old one before
// adding the real list
case 'RECEIVE_CREATED_LIST': {
const {
[action.meta.oldId]: oldList,
...otherLists
} = byId
return {
...otherLists,
[action.payload.list.id]: action.payload.list
}
}
}
})
return combineReducers({
ids,
byId
})
})()
const items = (() => {
const ids = (ids = [], action = {}) => ({
switch (action.type) {
case 'TEMP_ITEM_CREATED':
return [...ids, action.payload.item.id]
case 'RECEIVE_CREATED_ITEM':
return ids
.filter(id => id !== action.meta.oldId)
.concat([action.payload.item.id])
default:
return ids
}
})
const byId = (byId = {}, action = {}) => ({
switch (action.type) {
case 'TEMP_ITEM_CREATED':
return {
...byId,
[action.payload.item.id]: action.payload.item
}
case 'RECEIVE_CREATED_ITEM': {
const {
[action.meta.oldId]: oldList,
...otherItems
} = byId
return {
...otherItems,
[action.payload.item.id]: action.payload.item
}
}
// when we receive a real list
// we need to reappropriate all
// the items that are referring to
// the old listId to the new one
case 'RECEIVE_CREATED_LIST': {
const oldListId = action.meta.oldId
const newListId = action.payload.list.id
const _byId = {}
for (let id of Object.keys(byId)) {
let item = byId[id]
_byId[id] = {
...item,
listId: item.listId === oldListId ? newListId : item.listId
}
}
return _byId
}
}
})
return combineReducers({
ids,
byId
})
})()
const reducer = combineReducers({
lists,
items
})
/* REDUCERS & ACTIONS */