So I understand the unique key: if I have something like:
array.push(<ChildComponent key=some_key />
But if I have something like this:
cols.push(<td key={i}>Some value</td>)>
(While I am creating a table component that has pagination...). I am not sure about that key... That table supports sorting (for columns) so... I believe when we sort, if we use that "i" key, something will be broken. Is in this case, a key necessary? Or it is only necessary in an array that contains child components. In any case, if we don't use a key we get a warning from React that we want to avoid/fix.
Index based keys are sufficient unless:
items are inserted at or removed from anywhere except the end
the order of items in the items can change
items can be replaced with other items
If any of those are true, you should use a key which identifies the items themselves, rather than their position in the array. Otherwise the index sufficiently identifies it.
Using keys correctly has performance improvements when the subtrees are large or involve img/video/other expensive components. It's also a requirement if you want to animate items being added/removed/reordered correctly.
So if I understand you correctly, "i" is some index of the sorted array? If so, that will break down because components should be keyed to something unique to their data (and thus their rendering), not something that is varient like an index.
Because the key isn't consistently tied to the same piece of data, this will simply cause the div's to rerender their contents, as opposed to simply rearranging their order. Try setting key to a hash based on their data!
If you're looking for a nice solution to generating unique keys, I plucked this one from some of the React/Flux examples and it's worked great for my team and I.
uuid: function () {
var i, random;
var uuid = '';
for (i = 0; i < 32; i++) {
random = Math.random() * 16 | 0;
if (i === 8 || i === 12 || i === 16 || i === 20) {
uuid += '-';
}
uuid += (i === 12 ? 4 : (i === 16 ? (random & 3 | 8) : random)).toString(16);
}
return uuid;
}
Keys are about state. By state I mean not justthis.statebut also anything you store directly inthisand need or want to preserve between renders. If your children do not have state, keys do not matter. You could generate a random key every time for each child and that would work. But if some children have state, that approach will create a new child, initialized to its initial state. So if you need to preserve child state between renders the child key cannot change. This will ensure that the child component is preserved rather than recreated.
See Child Reconciliation here: http://facebook.github.io/react/docs/multiple-components.html
Related
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 am building aReact Application.I have a state array which holds array of blogs.
I am rendering each blog using a BlogDisplay component.
Following is my code :
render() {
for( var i = 0;i < this.state.blogData.length;i++){
blogDataToRender.push(<BlogDisplay blogData = {this.state.blogData[i]} onUserTriggerForDetailedView ={this.props.onUserTriggerForDetailedView}/>);
}
return (
<div>
{blogDataToRender}
</div>
);
}
I am getting a warning like this :
index.js:5279 Warning: Each child in an array or iterator should have a unique "key" prop. Check the render method of `BloggerHome`. See https://facebook.github.io/react/docs/lists-and-keys.html#keys for more information.
in BlogDisplay (created by BloggerHome)
in BloggerHome (created by Blogger)
in div (created by Blogger)
in div (created by Blogger)
in Blogger
Is my modelling correct ? How can I correct this?
Thanks a lot.
You need to add a key for each BlogDisplay component:
for( var i = 0;i < this.state.blogData.length;i++){
blogDataToRender.push(<BlogDisplay key={i} blogData = {this.state.blogData[i]} onUserTriggerForDetailedView ={this.props.onUserTriggerForDetailedView}/>);
}
The key is important for child components in an array. Because when one of those child components is under operation, React needs the key to determine which one is operated. Also, when the source array data is changed, React needs the key to check the diff of components.
For in-depth explanation about why keys are necessary, please refer to the official document.
What should be the value of key for each child component?
According to the above document:
The key only has to be unique among its siblings, not globally unique.
Also, blogData[i].id is the best choice if there is a unique id in the source data. Indexes as in the above example is not recommended, if the child components would reorder (performance issue).
Each element in React needs to have a unique key, which React uses to identify it. When you just render an element overtly, React automatically assigns one. However, when you assign an ARRAY of elements, each element in that array isn't assigned a key by default, so you need to. Something like this:
blogDataToRender.push(<BlogDisplay key={i} blogData={this.state.blogData[i]} onUserTriggerForDetailedView={this.props.onUserTriggerForDetailedView}/>);
Check out Chris' reply on Understanding unique keys for array children in React.js.
To fix this, you must set a unique id for each of your elements and then add a key prop to each element. You can read more about how React uses keys to identify elements in lists in the official documentation.
This is how your render() method should look:
render() {
return (
<div>
{this.state.blogData.map((item) => {
return (<BlogDisplay key={item.id} blogData={item} onUserTriggerForDetailedView={this.props.onUserTriggerForDetailedView} />)
})}
</div>
);
}
To answer your question quickly all you need to do is read the error and fix it! Add a unique key value to each component like so:
...
for( var i = 0;i < this.state.blogData.length;i++){
blogDataToRender.push(
<BlogDisplay
key={this.state.blogData[i].id}
blogData={this.state.blogData[i]}
onUserTriggerForDetailedView{this.props.onUserTriggerForDetailedView}
/>
);
}
...
In this case I assume that each one of your blog entries has a unique id, the thing you need to be aware of here is that the key value needs to be unique and not depending on the position of that element inside the array. Every time you render these elements that key needs to be the same, this value is used by React to optimize re rendering and avoid rendering items that did not change. If you want to understand why this is necessary here is a good link. Next time I recommend reading the docs because they are very well done and you can learn a lot from it.
Now the way I would do it is using map:
...
const blogDataToRender = this.state.blogData.map(blogData =>
<BlogDisplay
key={blogData.id}
blogData={blogData}
onUserTriggerForDetailedView{this.props.onUserTriggerForDetailedView}
/>
);
...
This helps readability and leverages an iterator so you don't need to keep track of the index. It is a widely used pattern when developing in React.
I am trying to solve a problem I am seeing when rendering a list of items in my ui that is coming out of a es6 class I have created. The model is working great, however I am using animations that are listening to (in react) mount, onEnter, and onLeave of the items.
When I apply my filters and sorting via the model and spit back the new list of items via the getter, the animations do not apply to some items because the list is just being re sorted, not necessarily changed.
So my getter just grabs this.products of the class and returns it and applies a sort order to it. And if filters are applied (which are tracked by this._checkedList in the class), the this.products is reduced based on which filters are selected then sorted. So that getter looks like so :
get productList() {
if (this._checkedList.length > 0) {
const filteredProducts = _.reduce(this.filterMap, reduceFilters, []);
const deDuped = _.uniq(filteredProducts, 'id');
return this.applySort(deDuped);
}
const deDuped = _.uniq(this.products, 'id');
return this.applySort(deDuped);
}
What I am trying to figure out, is a way to to temporarily send back an empty array while the filters or sorting run. The reason being the ui would receive an empty array (even if for a split second) and react would register the new sorted/filtered list as a new list and fire the enter/leave/mount animations again.
My attempt was to set a local property of the class like -
this._tempReturn = false;
then in the functions where the sort or filter happen, I set it to true, then back to false when the function is done like this -
toggleFilter(args) {
this._tempReturn = true;
...toggle logic
this._tempReturn = false;
}
Then changed the getter to check for that property before i do anything else, and if it's true, send back an empty array -
get productList() {
if (this._tempReturn) {
return [];
}
...
}
However, this does not seem to work. Even putting a console.log in the if (this._tempReturn) { didn't show any logs.
I also tried sending back a new list with lodash's _.cloneDeep like so :
get productList() {
if (this._checkedList.length > 0) {
const filteredProducts = _.reduce(this.filterMap, reduceFilters, []);
const deDuped = _.uniq(filteredProducts, 'id');
return _.cloneDeep(this.applySort(deDuped));
}
const deDuped = _.uniq(this.products, 'id');
return _.cloneDeep(this.applySort(deDuped));
}
this did not work either. So it seems the empty array return might be a better approach.
I am wondering if there is some way to achieve this - I would like to have the array be return empty for a second perhaps while the filters and sort are applying.
Very stuck on how to achieve, perhaps I am even looking at this problem from the wrong angle and there is a much better way to solve this. Any advice would be welcomed, thanks for reading!
In order to force a re-render of items in a list when updating them you just need to make sure that each items has a unique key property.
Instead of rendering the list, then rendering it as empty, then re-rendering a changed list make sure each child has a unique key. Changing the key property on a child in an array will always cause it to re-render.
my array:
tempListArray = "[{"id":"12","value":false},{"id":"10","value":false},{"id":"9","value":false},{"id":"8","value":false}]";
To check if an element exists I would do this:
for (var i in tempListArray) {
//check flag
if (tempListArray[i].id == Id) {
flagExistsLoop = 1;
break;
}
}
Is there anyway, I can check if an Id exists without looping through the whole array. Basically I am worried about performance if say I have a 100 elements.
Thanks
No, without using custom dictionary objects (which you seriously don't want to for this) there's no faster way than doing a 'full scan' of all contained objects.
As a general rule of thumb, don't worry about performance in any language or any situation until the total number of iterations hits 5 digits, most often 6 or 7. Scanning a table of 100 elements should be a few milliseconds at worst. Worrying about performance impact before you have noticed performance impact is one of the worst kinds of premature optimization.
No, you can't know that without iterating the array.
However, note for...in loops are a bad way of iterating arrays:
There is no warranty that it will iterate the array with order
It will also iterate (enumerable) non-numeric own properties
It will also iterate (enumerable) properties that come from the prototype, i.e., defined in Array.prototype and Object.protoype.
I would use one of these:
for loop with a numeric index:
for (var i=0; i<tempListArray.length; ++i) {
if (tempListArray[i].id == Id) {
flagExistsLoop = 1;
break;
}
}
Array.prototype.some (EcmaScript 5):
var flagExistsLoop = tempListArray.some(function(item) {
return item.id == Id;
});
Note it may be slower than the other ones because it calls a function at each step.
for...of loop (EcmaScript 6):
for (var item of tempListArray) {
if (item.id == Id) {
flagExistsLoop = 1;
break;
}
}
Depending on your scenario, you may be able to use Array.indexOf() which will return -1 if the item is not present.
Granted it is probably iterating behind the scenes, but the code is much cleaner. Also note how object comparisons are done in javascript, where two objects are not equal even though their values may be equal. See below:
var tempListArray = [{"id":"12","value":false},{"id":"10","value":false},{"id":"9","value":false},{"id":"8","value":false}];
var check1 = tempListArray[2];
var check2 = {"id":"9","value":false};
doCheck(tempListArray, check1);
doCheck(tempListArray, check2);
function doCheck(array, item) {
var index = array.indexOf(item);
if (index === -1)
document.write("not in array<br/>");
else
document.write("exists at index " + index + "<br/>");
}
try to use php.js it may help while you can use same php function names and it has some useful functionalities
There is no way without iterating through the elements (that would be magic).
But, you could consider using an object instead of an array. The object would use the (presumably unique) id value as the key, and the value could have the same structure you have now (or without the redundant id property). This way, you can efficiently determine if the id already exists.
There is a possible cheat for limited cases :) and it is magic...cough cough (math)
imagine you have 3 elements:
1
2
3
and you want to know if one of these is in an array without iterating it...
we could make a number that contains a numerical flavor of the array. we do this by assigning prime numbers to the elements:
1 - 2
2 - 3
3 - 5
the array so when we add item 2 we check that the array doesn't already contain the prime associated to that item by checking (if Flavor!=0 && (Flavor%3)!=0) then adding the prime Flavor*=3;
now we can tell that the second element is in the array by looking at the number.
if Flavor!=0 && (Flavor%3)==0 // its There!
Of course this is limited to the numerical representation that can be handled by the computer. and for small array sizes (1-3 elements) it might still be faster to scan. but it's just one idea.
but the basis is pretty sound. However, this method becomes unusable if you cannot correlate elements one to one with a set of primes. You'll want to have the primes calculated in advance. and verify that the product of those is less numerical max numerical representation. (also be careful with floating-point. because they might not be able to represent the number at the higher values due to the gaps between representable values.) You probably have the best luck with an unsigned integer type.
This method will probably be too limiting. And there is something else you can do to possibly speed up your system if you don't want to iterate the entire array.
Use different structures:
dictionaries/maps/trees etc.
if your attached to the array another method can be a bloom filter. This will let you know if an element is not in your set, which can be just as useful.
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.