My top level state model looks like this:
{
listOfItems: [], // Item[]
selections: {
itemId: 0
}
}
The items list may contain 10 different shopping items.
When the user clicks on an item, it updates the selection, and my #Selector will rerun.
Action: Set Item Selection
#Action(Item.Select)
setState(
{ setState }: StateContext<ItemsModel>,
{ itemId }: Item.Select
) {
setState(patch({ selections: patch({ itemId }) }));
}
Selector: Select Current Item
#Selector()
static getSelectedItem(state: ItemModel): Item {
return state.itemList.find(i => i.itemId === state.selections.itemId);
}
Problem is: I have up to 20 actions to perform on the selected Item. This results in:
Lots of .find() lookups to find item in the original array (both selector and actions)
Actions to perform on the listOfItems are in the same place as those to perform on a specific Item
I would like to: Keep the array and selection in this state, but separate out the "selected item" into a new substate, where the child state's model can just be Item type. This way I can encapsulate all the actions on Item in a different place to actions on the Items[] array.
I'm not sure how to keep them in sync. I need to keep the 'selectedItem' state up to date when the selection itemId changes in the parent. I also need to make sure any mutations to the selectedItem are reflected in the original array in the parent.
This seems like it might be more of a fundamental problem with how you are trying to represent your application state. Have you thought of normalizing your list of items? Or at least using a key/value lookup object instead of an array? You wouldn't need to use the .find() to do your lookup and could access the key of the object via the unique id you are interested in. Let me know if that is of any help!
Could someone help me out. I am trying to set dynamically the selection in the Project Clarity datagrid component.
https://vmware.github.io/clarity/documentation/v0.11/datagrid/selection
I have a filter which I am fetching from storage and I want to display the selection in the datagrid. Here is the code for populating the selected variable which is string array (string[])
selected: string[] = [];
I am console outputting the selected and it contains the correct values but those are not selected in the datagrid.
private initView() {
Object.entries(this.metadataFilter.metadataTypes).forEach(
([key, value]) => {
this.selected.push(key);
});
console.log('this.selected: ', this.selected);
}
this is what I have in the template:
<clr-datagrid [(clrDgSelected)]="selected">
Here is the population of the rows:
<clr-dg-row *clrDgItems="let meta of metadataTypes | async" (click)="getItemsForMetadataType(meta.name)" [clrDgItem]="meta">
<clr-dg-cell>{{ meta.name }}</clr-dg-cell>
</clr-dg-row>
It works when I am selecting entries from the grid. Those I get populated to a variable but not other way around. Help would be very much appreciated. Am I misunderstanding how this should work ?
There are two things to do here. First, I always recommend to use trackBy so you can be sure that the references are correct. Second, you need to put the whole object, not the key, into the selected array. The internal state of the data grid evaluates equality against references of the object, or if trackBy is used it computes the trackBy internally and evaluates equality between the references (like an ID or some string).
For example, this should initialize the 3rd item to be selected.
this.selected.push(this.metadataFilter.metadataTypes[2]);
Is there anyone who has experience using Polymer?
What I have:
A sqlite database storing an array data with some objects inside.
A polymer app displaying the objects from the database using the dom-repeat web component.
What I need to do:
Displaying from the database only those objects based on a filtering.
In my app I have a "filter" form with some options to be checked, and depending on which option gets checked (they are checkboxes), it should return only those objects from the database that match my selection.
I know you can use the a "filter" element in the dom-repeat component that accepts a value which is a callback function. And in the app to check if an option has been checked we can insert checked="{{hostChecked::change}}". From the Polymer documentation I also know that you can insert observers to look for changes in the app. I got this far, but can't manage to figure out how to put all these findings together.
I'm pretty knew to Polymer and an experience help would be hugely appreciated.
Thanks in advance
Try this:
<my-app>
<my-checkbox checked="{{prop1}}">
<my-checkbox checked="{{prop2}}">
...
</my-app>
then:
static get properties() {
return {
prop1: Boolean,
prop2: Boolean,
filter: {
computed: '_computeFilter(prop1, prop2,...)'
}
};
}
_computeFilter(prop1, prop2,...) {
return function(item) {
return (item.prop1 === prop1 && item.prop2 === prop2 && ...)
}
}
and:
<template is="dom-repeat" items="{{sqldata}}" filter="{{filter}}">
As I understand you're more concerned about changes to the app checkboxes than to sqldata changing at runtime.
In aurelia: I have a string interpulation over object property that works fine in the app.html - it shows number of accounts:
ALL ACCOUNTS (${userAccountsData.length})
In the initial loading, I see that the value changes after few milliseconds from 0 to the actual value (data is retrieving from the service), but - when trying to show aggregate data (count number of active accounts) over the same data in a template (custom element) - the data stays as 0 and not updated as the userAccountsData.length
*When refreshing again after the initial loading - the data is shown as it should be.
This is the custom element instance in the app.html:
<account-status-selection-bar accounts-data.bind="userAccountsData"></account-status-selection-bar>
And this is part of the HTML of the custom element itself:
<template>
<div ref="active"
class="selection">${accountActivationDistribution.numberOfActiveAccounts}
This is the relevant part of the custom element VM:
"use strict";
import { bindable} from 'aurelia-framework';
export class accountStatusSelectionBar {
#bindable accountsData;
constructor() {
this.accounts = [];
this.accountActivationDistribution = { numberOfActiveAccounts: 0,
numberOfInactiveAccounts : 0,
numberOfTotalAccounts : 0
}
get activeAccounts() {
var activeAccounts = this.accounts.filter(function(account) {
return account.IsApproved;
});
return activeAccounts.length;
}
attached()//bind()
{
this.accounts = this.accountsData;
this.accountActivationDistribution.numberOfActiveAccounts =
this.activeAccounts
}
In the app.js I use observerLocator - here is the code related to the working part of userAccountsData.length:
constructor() {
this.userAccountsData = [];
....
this.subscribe = this.observerLocator.getObserver(accounts, "all")
.subscribe((value) => {
if (!value)
return;
this.userAccountsData = value;
**A work around I found (although I'm not sure this is the best way) is to do the aggregation in the app.js (in the observer part) in object and bind the already aggregated object to the custom element - this is working. I'm still looking for the mentioned above solution.
It looks like the problem is you're binding userAccountsData to accountsData on your custom control; then assigning this.accounts = this.accountsData; and finally later you're reassigning userAccountsData in app.js.
Because accounts is not observing or bound to the original userAccountsData, it maintains the reference to the original array (which is set to an empty array) and never gets updated.
There is a race condition on refresh, where some cache probably means that userAccountsData gets the updated value before the binding occurs, which is why it works sometimes.
The solution is to remove some of the reassignment and just bind directly to accounts and forget the intermediate accountsData.
I created a gist here showing the different behaviour.
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.