I am learning React and just created a simple todo app using only React. My todo app has the standard structure of having a text input and an "ADD" button next to it. The user would type their todo in the input and every time they click on the "ADD" button next to it, a new ordered list of their inputs would appear underneath the input and "ADD" button.
The user can also delete a todo entry by clicking on the entries individually, like this:
To accomplish this behaviour of deleting entries, I used this delete function:
delete(elem) {
for (var i = 0; i < this.state.listArray.length; i++) {
if (this.state.listArray[i] === elem) {
this.state.listArray.splice(i, 1);
this.setState({
listArray: this.state.listArray
});
break;
}
}
}
My todo app works exactly the way that I want it to work, but as I look at other people's more conventional approach to this delete function, they either just simply use the splice method or the filter method.
For the splice method approach, they apparently just simply "remove" the unwanted entry from the listArray when the user clicks the particular entry. This does not work for me as using this method results in all my entries getting deleted except for the entry that I clicked on, which is the one that I want to delete.
On the other hand, the filter method approach apparently works by comparing the elem, which is the data passed from a child component, with each element in the listArray, and if the element in the for loop does not equal to the elem, then it would be passed onto a new array. This new array would be the one to not be deleted. This approach works better than the simple splice approach, however, one problem that I had encountered with this approach is that if I have more than one entry of the same value, for example, "Feed the dog". I only want one of the "Feed the dog" entries to be deleted, but it deletes both of them.
I thought of an approach to tackle this problem, eventually coming up with the current version of my code, which uses the splice method, but the splice method is used before I set it in the state. As evident here:
this.state.listArray.splice(i, 1);
this.setState({
listArray: this.state.listArray
});
My question can be broken down into three subquestions:
Considering that React states should be immutable, is the first line of the code above mutating my state? Is this approach not okay?
I thought that all React states were only possible to be changed inside a "setState" function, but my first line of code from above is not inside a setState function, yet it changed the state of listArray. How is this possible?
If my approach is mutating the state and is not ideal, how would you go about making the delete function so that it only deletes one entry and not more than one if there are multiple similar entries?
Yes, splice affects the array it acts on so don't use in this way. Instead you need to create a new array of the correct elements:
this.setState({
listArray: this.state.listArray.filter((el, idx) => idx !== i);
});
If you want to remove only the first instance, maybe couple with a findIndex (although indexOf would work in your example as well) first:
delete(elem) {
const idxToFilter = this.state.listArray.findIndex(el => el === elem);
if (idxToFilter < 0) {
return;
}
this.setState({
listArray: this.state.listArray.filter((el, idx) => idx !== idxToFilter);
});
}
This creates a new array without modifying the old which will cause anything that reacts to listArray changing to be notified since the reference has changed.
I have a function that find an object from a JSON that has an id === this.match.mainParticipant.stats.perkSubStyle. This object contains a property called slots that is an array and has 4 elements. Each slot has 3 elements which represent runes from a game. If you iterate over the slots and their elements you get this:
I get the object using this function:
secondaryPerks(){
let perksTree = this.$store.state.summonerRunes.find(value => value.id === this.match.mainParticipant.stats.perkSubStyle);
console.log(perksTree.slots.unshift())
return perksTree
}
and I iterate and display the icons using this:
<div v-for='runes in this.secondaryPerks().slots'>
<div v-for='rune in runes.runes'>
<img :src="'https://ddragon.leagueoflegends.com/cdn/img/' + rune.icon" alt="">
</div>
</div>
Now the problem is that because that perks tree is secondary one, the perks in slot[0] can never be picked because if they were picked, they'd have to be part of the primaryPerks tree. This means there's no point displaying that none of them were selected. For that reason I am trying to remove the first slot[0] element from the array, however, when I try to unshift() it, I get an error:
"You may have an infinite update loop in a component render function"
And I have no clue why. Any advices?
Firstly, I think you mean shift rather than unshift. unshift will try to add items to the array rather than removing them. It doesn't actually matter from the perspective of the infinite loop, either method will have the same effect.
You're creating a dependency on the array and then modifying it. Modifying it will trigger a re-render.
Each time the component re-renders it will shift another item onto/out of the array. Even if the call to shift/unshift doesn't actually change anything it will still count as modifying the array.
Try:
computed: {
secondaryPerkSlots () {
const perksTree = this.$store.state.summonerRunes.find(
value => value.id === this.match.mainParticipant.stats.perkSubStyle
);
return perksTree.slots.slice(1)
}
}
with:
<div v-for='runes in secondaryPerkSlots'>
That will create a new array containing the same elements as the original array, omitting the first element.
Alternatively you could put the slice(1) directly in the template:
<div v-for='runes in secondaryPerks().slots.slice(1)'>
Either way I suggest changing the method to a computed property instead. You should also drop the this in your template.
I had the same problem a few months ago.
I think the main issue is that you perform logic such as arr.unshift()(which will cause the template to re-render in this case) in your computed property.
So, imagine this:
const arr1 = [/* ... */];
// This is different
const computedArr = () => {
return arr.filter(() => { /* ... */ });
};
// Than this
const computedArr = () => {
const newArr = arr.filter(() => { /* ... */ });
// Vue cannot allow this without a re-render!
newArr.unshift();
return newArr;
};
The latter will cause the template to re-render;
EDIT
Check the first comment!
I have a series of buttons that apply filters to an object using a function similar to this:
isNew(type) {
//need to reset filter first
this.results = this.results.filter(function(type) {
return type.has_user_viewed === true
})
console.log('isNew');
}
The problem I have is if one filter is applied by the user clicking, and then another filter applied, the filtered array is filtered again. What I need to do with the above is reset the object to it's original state before applying a new filter. Not sure how to "reset" a filter here?
Just save the unfiltered data to a variable. filter creates a new array object (shallow copying elements) each time, so your code overwrites the original data with the filtered data:
this.data = [...whatever the source is]
isNew(type) {
//need to reset filter first
this.results = this.data.filter(function(type) {
return type.has_user_viewed === true
})
console.log('isNew');
}
You'll probably need to store the result of the filter somewhere other than this.results if you want to access the original list of results for additional filtering later. Note that this change will likely force you to change other code.
An example of what I'm recommending:
this.filteredResults = this.results.filter(function(type) {
return type.has_user_viewed === true
})
I would suggest that you google immutability, as understanding it will help you greatly when you need to solve similar problems in the future.
In ExtJS panel I need to set value of all items (e.g. textfield, pathfield) to blank. I don't want to set value of each individual item to blank but of whole panel in one go.
I am able to get list of items
function getAllChildren (panel) {
/*Get children of passed panel or an empty array if it doesn't have thems.*/
var children = panel.items ? panel.items.items : [];
/*For each child get their children and concatenate to result.*/
CQ.Ext.each(children, function (child) {
children = children.concat(getAllChildren(child));
});
return children;
}
but how to set to blank for whole panel? Please suggest what need to be done in this case.
Actually, it's not possible to do it with one liner - all at the same time. What your method returns is purely an array of objects. In fact if such syntax existed, it would iterate over all fields anyway.
Though clearing all fields, having the method you've proposed is very trivial to do. Just iterate over them all and call reset method. Mind some (especially custom) widgets might not handle it.
var fields = getAllChildren(panel);
CQ.Ext.each(fields, function(field) {
if (child.reset) {
child.reset();
}
});
You've got similar loop in your getAllChildren code - you might reset field at the same place.
The method is defined in Field type which is usually a supertype of each dialog widget. You can read more here.
I'm trying to test out Firebase to allow users to post comments using push. I want to display the data I retrieve with the following;
fbl.child('sell').limit(20).on("value", function(fbdata) {
// handle data display here
}
The problem is the data is returned in order of oldest to newest - I want it in reversed order. Can Firebase do this?
Since this answer was written, Firebase has added a feature that allows ordering by any child or by value. So there are now four ways to order data: by key, by value, by priority, or by the value of any named child. See this blog post that introduces the new ordering capabilities.
The basic approaches remain the same though:
1. Add a child property with the inverted timestamp and then order on that.
2. Read the children in ascending order and then invert them on the client.
Firebase supports retrieving child nodes of a collection in two ways:
by name
by priority
What you're getting now is by name, which happens to be chronological. That's no coincidence btw: when you push an item into a collection, the name is generated to ensure the children are ordered in this way. To quote the Firebase documentation for push:
The unique name generated by push() is prefixed with a client-generated timestamp so that the resulting list will be chronologically-sorted.
The Firebase guide on ordered data has this to say on the topic:
How Data is Ordered
By default, children at a Firebase node are sorted lexicographically by name. Using push() can generate child names that naturally sort chronologically, but many applications require their data to be sorted in other ways. Firebase lets developers specify the ordering of items in a list by specifying a custom priority for each item.
The simplest way to get the behavior you want is to also specify an always-decreasing priority when you add the item:
var ref = new Firebase('https://your.firebaseio.com/sell');
var item = ref.push();
item.setWithPriority(yourObject, 0 - Date.now());
Update
You'll also have to retrieve the children differently:
fbl.child('sell').startAt().limitToLast(20).on('child_added', function(fbdata) {
console.log(fbdata.exportVal());
})
In my test using on('child_added' ensures that the last few children added are returned in reverse chronological order. Using on('value' on the other hand, returns them in the order of their name.
Be sure to read the section "Reading ordered data", which explains the usage of the child_* events to retrieve (ordered) children.
A bin to demonstrate this: http://jsbin.com/nonawe/3/watch?js,console
Since firebase 2.0.x you can use limitLast() to achieve that:
fbl.child('sell').orderByValue().limitLast(20).on("value", function(fbdataSnapshot) {
// fbdataSnapshot is returned in the ascending order
// you will still need to order these 20 items in
// in a descending order
}
Here's a link to the announcement: More querying capabilities in Firebase
To augment Frank's answer, it's also possible to grab the most recent records--even if you haven't bothered to order them using priorities--by simply using endAt().limit(x) like this demo:
var fb = new Firebase(URL);
// listen for all changes and update
fb.endAt().limit(100).on('value', update);
// print the output of our array
function update(snap) {
var list = [];
snap.forEach(function(ss) {
var data = ss.val();
data['.priority'] = ss.getPriority();
data['.name'] = ss.name();
list.unshift(data);
});
// print/process the results...
}
Note that this is quite performant even up to perhaps a thousand records (assuming the payloads are small). For more robust usages, Frank's answer is authoritative and much more scalable.
This brute force can also be optimized to work with bigger data or more records by doing things like monitoring child_added/child_removed/child_moved events in lieu of value, and using a debounce to apply DOM updates in bulk instead of individually.
DOM updates, naturally, are a stinker regardless of the approach, once you get into the hundreds of elements, so the debounce approach (or a React.js solution, which is essentially an uber debounce) is a great tool to have.
There is really no way but seems we have the recyclerview we can have this
query=mCommentsReference.orderByChild("date_added");
query.keepSynced(true);
// Initialize Views
mRecyclerView = (RecyclerView) view.findViewById(R.id.recyclerView);
mManager = new LinearLayoutManager(getContext());
// mManager.setReverseLayout(false);
mManager.setReverseLayout(true);
mManager.setStackFromEnd(true);
mRecyclerView.setHasFixedSize(true);
mRecyclerView.setLayoutManager(mManager);
I have a date variable (long) and wanted to keep the newest items on top of the list. So what I did was:
Add a new long field 'dateInverse'
Add a new method called 'getDateInverse', which just returns: Long.MAX_VALUE - date;
Create my query with: .orderByChild("dateInverse")
Presto! :p
You are searching limitTolast(Int x) .This will give you the last "x" higher elements of your database (they are in ascending order) but they are the "x" higher elements
if you got in your database {10,300,150,240,2,24,220}
this method:
myFirebaseRef.orderByChild("highScore").limitToLast(4)
will retrive you : {150,220,240,300}
In Android there is a way to actually reverse the data in an Arraylist of objects through the Adapter. In my case I could not use the LayoutManager to reverse the results in descending order since I was using a horizontal Recyclerview to display the data. Setting the following parameters to the recyclerview messed up my UI experience:
llManager.setReverseLayout(true);
llManager.setStackFromEnd(true);
The only working way I found around this was through the BindViewHolder method of the RecyclerView adapter:
#Override
public void onBindViewHolder(final RecyclerView.ViewHolder holder, int position) {
final SuperPost superPost = superList.get(getItemCount() - position - 1);
}
Hope this answer will help all the devs out there who are struggling with this issue in Firebase.
Firebase: How to display a thread of items in reverse order with a limit for each request and an indicator for a "load more" button.
This will get the last 10 items of the list
FBRef.child("childName")
.limitToLast(loadMoreLimit) // loadMoreLimit = 10 for example
This will get the last 10 items. Grab the id of the last record in the list and save for the load more functionality. Next, convert the collection of objects into and an array and do a list.reverse().
LOAD MORE Functionality: The next call will do two things, it will get the next sequence of list items based on the reference id from the first request and give you an indicator if you need to display the "load more" button.
this.FBRef
.child("childName")
.endAt(null, lastThreadId) // Get this from the previous step
.limitToLast(loadMoreLimit+2)
You will need to strip the first and last item of this object collection. The first item is the reference to get this list. The last item is an indicator for the show more button.
I have a bunch of other logic that will keep everything clean. You will need to add this code only for the load more functionality.
list = snapObjectAsArray; // The list is an array from snapObject
lastItemId = key; // get the first key of the list
if (list.length < loadMoreLimit+1) {
lastItemId = false;
}
if (list.length > loadMoreLimit+1) {
list.pop();
}
if (list.length > loadMoreLimit) {
list.shift();
}
// Return the list.reverse() and lastItemId
// If lastItemId is an ID, it will be used for the next reference and a flag to show the "load more" button.
}
I'm using ReactFire for easy Firebase integration.
Basically, it helps me storing the datas into the component state, as an array. Then, all I have to use is the reverse() function (read more)
Here is how I achieve this :
import React, { Component, PropTypes } from 'react';
import ReactMixin from 'react-mixin';
import ReactFireMixin from 'reactfire';
import Firebase from '../../../utils/firebaseUtils'; // Firebase.initializeApp(config);
#ReactMixin.decorate(ReactFireMixin)
export default class Add extends Component {
constructor(args) {
super(args);
this.state = {
articles: []
};
}
componentWillMount() {
let ref = Firebase.database().ref('articles').orderByChild('insertDate').limitToLast(10);
this.bindAsArray(ref, 'articles'); // bind retrieved data to this.state.articles
}
render() {
return (
<div>
{
this.state.articles.reverse().map(function(article) {
return <div>{article.title}</div>
})
}
</div>
);
}
}
There is a better way. You should order by negative server timestamp. How to get negative server timestamp even offline? There is an hidden field which helps. Related snippet from documentation:
var offsetRef = new Firebase("https://<YOUR-FIREBASE-APP>.firebaseio.com/.info/serverTimeOffset");
offsetRef.on("value", function(snap) {
var offset = snap.val();
var estimatedServerTimeMs = new Date().getTime() + offset;
});
To add to Dave Vávra's answer, I use a negative timestamp as my sort_key like so
Setting
const timestamp = new Date().getTime();
const data = {
name: 'John Doe',
city: 'New York',
sort_key: timestamp * -1 // Gets the negative value of the timestamp
}
Getting
const ref = firebase.database().ref('business-images').child(id);
const query = ref.orderByChild('sort_key');
return $firebaseArray(query); // AngularFire function
This fetches all objects from newest to oldest. You can also $indexOn the sortKey to make it run even faster
I had this problem too, I found a very simple solution to this that doesn't involved manipulating the data in anyway. If you are rending the result to the DOM, in a list of some sort. You can use flexbox and setup a class to reverse the elements in their container.
.reverse {
display: flex;
flex-direction: column-reverse;
}
myarray.reverse(); or this.myitems = items.map(item => item).reverse();
I did this by prepend.
query.orderByChild('sell').limitToLast(4).on("value", function(snapshot){
snapshot.forEach(function (childSnapshot) {
// PREPEND
});
});
Someone has pointed out that there are 2 ways to do this:
Manipulate the data client-side
Make a query that will order the data
The easiest way that I have found to do this is to use option 1, but through a LinkedList. I just append each of the objects to the front of the stack. It is flexible enough to still allow the list to be used in a ListView or RecyclerView. This way even though they come in order oldest to newest, you can still view, or retrieve, newest to oldest.
You can add a column named orderColumn where you save time as
Long refrenceTime = "large future time";
Long currentTime = "currentTime";
Long order = refrenceTime - currentTime;
now save Long order in column named orderColumn and when you retrieve data
as orderBy(orderColumn) you will get what you need.
just use reverse() on the array , suppose if you are storing the values to an array items[] then do a this.items.reverse()
ref.subscribe(snapshots => {
this.loading.dismiss();
this.items = [];
snapshots.forEach(snapshot => {
this.items.push(snapshot);
});
**this.items.reverse();**
},
For me it was limitToLast that worked. I also found out that limitLast is NOT a function:)
const query = messagesRef.orderBy('createdAt', 'asc').limitToLast(25);
The above is what worked for me.
PRINT in reverse order
Let's think outside the box... If your information will be printed directly into user's screen (without any content that needs to be modified in a consecutive order, like a sum or something), simply print from bottom to top.
So, instead of inserting each new block of content to the end of the print space (A += B), add that block to the beginning (A = B+A).
If you'll include the elements as a consecutive ordered list, the DOM can put the numbers for you if you insert each element as a List Item (<li>) inside an Ordered Lists (<ol>).
This way you save space from your database, avoiding unnecesary reversed data.