We Keep Coding

Changing a HTML element dynamically

I have a side drawer where I'm showing the current cart products selected by the user. Initially, I have a <p> tag saying the cart is empty. However, I want to remove it if the cart has items inside. I'm using an OOP approach to design this page. See below the class I'm working with.
I tried to use an if statement to condition the <p> tag but this seems the wrong approach. Anyone has a better way to do this. See screenshot of the cart in the UI and code below:
class SideCartDrawer {
cartProducts = [];
constructor() {
this.productInCartEl = document.getElementById('item-cart-template');
}
addToCart(product) {
const updatedProducts = [...this.cartProducts];
updatedProducts.push(product);
this.cartProducts = updatedProducts;
this.renderCart();
}
renderCart() {
const cartListHook = document.getElementById('cart-items-list');
let cartEl = null;
if (this.cartProducts.length === 0) {
cartEl = '<h2>You Cart is Empty</h2>';
} else {
const productInCartElTemplate = document.importNode(
this.productInCartEl.content,
true
);
cartEl = productInCartElTemplate.querySelector('.cart-item');
for (let productInCart of this.cartProducts) {
cartEl.querySelector('h3').textContent = productInCart.productName;
cartEl.querySelector('p').textContent = `£ ${productInCart.price}`;
cartEl.querySelector('span').textContent = 1;
}
}
cartListHook.append(cartEl);
}
}
By the way, the <p> should reappear if the cart is back to empty :) !

With how your code is setup, you would want to reset the list on each render. You would do this by totally clearing out #cart-items-list. Here is a deletion method from this question
while (cartListHook.firstChild) {
cartListHook.removeChild(cartListHook.lastChild);
}
But you could use any method to delete the children of an HTML Node. To reiterate, you would put this right after getting the element by its id.
P.S. You probably want to put more code into your for loop, because it seems like it will only create cart-item element even if there are multiple items in this.cartProducts.

ng2-dragula after adding new item it's getting displayed at the top

I am using ng2-dragula for drag and drop feature. I am seeing issue when I drag and drop first element(or any element) at the end and then try to add new item to the array using addNewItem button, new item is not getting added to the end. If i don't drop element to the end, new item is getting added at the end in UI.
I want new items to be displayed at the bottom in any scenario. Any help is appreciated.
This issue is not reproducible with Angular 7. I see this happening with Angular 9
JS
export class SampleComponent {
items = ['Candlestick','Dagger','Revolver','Rope','Pipe','Wrench'];
constructor(private dragulaService: DragulaService) {
dragulaService.createGroup("bag-items", {
removeOnSpill: false
});
}
public addNewItem() {
this.items.push('New Item');
}
}
HTML
<div class="container" [dragula]='"bag-items"' [(dragulaModel)]='items'>
<div *ngFor="let item of items">{{ item }}</div>
</div>
<button id="addNewItem" (click)="addNewItem()">Add New Item
I edited the stackblitz from the comment to help visualize the issue. This seems to be triggered when a unit is dragged to the bottom of the list. Updated stackblitz : https://stackblitz.com/edit/ng2-dragula-base-ykm8fz?file=src/app/app.component.html
ItemsAddedOutOfOrder

You can try to restore old item position on drop.
constructor(private dragulaService: DragulaService) {
this.subscription = this.dragulaService.drop().subscribe(({ name }) => {
this.dragulaService.find(name).drake.cancel(true);
});
}
Forked Stackblitz
Explanation
There is some difference between how Ivy and ViewEngine insert ViewRef at specific index. They relay on different beforeNode
Ivy always returns ViewContainer host(Comment node)ref if we add item to the end:
export function getBeforeNodeForView(viewIndexInContainer: number, lContainer: LContainer): RNode|
null {
const nextViewIndex = CONTAINER_HEADER_OFFSET + viewIndexInContainer + 1;
if (nextViewIndex < lContainer.length) {
const lView = lContainer[nextViewIndex] as LView;
const firstTNodeOfView = lView[TVIEW].firstChild;
if (firstTNodeOfView !== null) {
return getFirstNativeNode(lView, firstTNodeOfView);
}
}
return lContainer[NATIVE]; <============================= this one
}
ViewEngine returns last rendered node(last <li/> element)ref
function renderAttachEmbeddedView(
elementData: ElementData, prevView: ViewData|null, view: ViewData) {
const prevRenderNode =
prevView ? renderNode(prevView, prevView.def.lastRenderRootNode!) : elementData.renderElement;
...
}
The solution might be reverting the dragged element back to original container so that we can let built-in ngForOf Angular directive to do its smart diffing.
Btw, the same technique is used in Angular material DragDropModule. It remembers position of dragging element and after we drop item it inserts it at its old position in the DOM which is IMPORTANT.

Ionic: change ion-list items on click

Lets say I have 3 lists
list: 1 ) bus , plane
list: 2 ) [related to bus] slow , can't fly
list: 3) [related to plane] fast, can fly
In my Ionic Angular project I have successfully made the 1st ion-list. But how can I change the whole ion-list by clicking on the item inside it?
[I get it, its something to do with (click) function, but how I can affect the whole list using typescript]

Edit: I get what you want to achieve. You can do this by creating an intermediary list and using that list in your ngFor. That way you can just simply change the reference of the intermediary list to whatever list you like onClick
export class ListPage {
transportationTypes: string[] = ['bus', 'plane'];
busSpecs: string[] = ['slow', "can't fly"];
planeSpecs: string[] = ['fast', 'can fly'];
currentList: string[] = this.transportationTypes;
itemClicked(type): void {
if (type === 'bus') {
this.currentList = this.busSpecs;
} else if(type === 'plane') {
this.currentList = this.planeSpecs;
} else {
this.currentList = this.transportationTypes;
}
}
}
And in your HTML just call the itemClicked function
<ion-list *ngIf="currentList">
<ion-item *ngFor="let item of currentList" (click)="itemClicked(item)">
{{item}}
</ion-item>
</ion-list>

Take N values from Observable until its complete based on an event. Lazy loading a multi select list

I'm new to rxjs, and I'm developing an angular multiselect list component that should render a long list of values (500+).
I'm rendering the list based on an UL, I'm iterating over an observable that will render the LI's.
I'm thinking about my options to avoid impacting the performance by rendering all the elements at once. But I don't know whether this is possible or not, and if it's possible what is the best operator to use.
The proposed solution:
On init I load all the data into an Observable. (src) and I'll take 100 elements from it and will put them on the target observable (The one that will be used to render the list)
Everytime that the user reaches the end of the list (the scrollEnd event fires) I'll load 100 elements more, until there are no more values in the src observable.
The emission of new values in the target observable will be triggered by the scrollEnd event.
Find my code below, I still need to implement the proposed solution, but I'm stuck at this point.
EDIT: I'm implementing #martin solution, but I'm still not able to make it work in my code. My first step was to replicate it in the code, to get the logged values, but the observable is completing immediately without producing any values.
Instead of triggering an event, I've added a subject. Everytime the scrollindEnd output emits, I will push a new value to the subject. The template has been modified to support this.
multiselect.component.ts
import { Component, AfterViewInit } from '#angular/core';
import { zip, Observable, fromEvent, range } from 'rxjs';
import { map, bufferCount, startWith, scan } from 'rxjs/operators';
import { MultiSelectService, ProductCategory } from './multiselect.service';
#Component({
selector: 'multiselect',
templateUrl: './multiselect.component.html',
styleUrls: ['./multiselect.component.scss']
})
export class MultiselectComponent implements AfterViewInit {
SLICE_SIZE = 100;
loadMore$: Observable<Event>;
numbers$ = range(450);
constructor() {}
ngAfterViewInit() {
this.loadMore$ = fromEvent(document.getElementsByTagName('button')[0], 'click');
zip(
this.numbers$.pipe(bufferCount(this.SLICE_SIZE)),
this.loadMore$.pipe(),
).pipe(
map(results => console.log(results)),
).subscribe({
next: v => console.log(v),
complete: () => console.log('complete ...'),
});
}
}
multiselect.component.html
<form action="#" class="multiselect-form">
<h3>Categories</h3>
<input type="text" placeholder="Search..." class="multiselect-form--search" tabindex="0"/>
<multiselect-list [categories]="categories$ | async" (scrollingFinished)="lazySubject.next($event)">
</multiselect-list>
<button class="btn-primary--large">Proceed</button>
</form>
multiselect-list.component.ts
import { Component, Input, Output, EventEmitter } from '#angular/core';
#Component({
selector: 'multiselect-list',
templateUrl: './multiselect-list.component.html'
})
export class MultiselectListComponent {
#Output() scrollingFinished = new EventEmitter<any>();
#Input() categories: Array<string> = [];
constructor() {}
onScrollingFinished() {
this.scrollingFinished.emit(null);
}
}
multiselect-list.component.html
<ul class="multiselect-list" (scrollingFinished)="onScrollingFinished($event)">
<li *ngFor="let category of categories; let idx=index" scrollTracker class="multiselect-list--option">
<input type="checkbox" id="{{ category }}" tabindex="{{ idx + 1 }}"/>
<label for="{{ category }}">{{ category }}</label>
</li>
</ul>
NOTE: The scrollingFinished event is being triggered by the scrollTracker directive that holds the tracking logic. I'm bubbling the event from multiselect-list to the multiselect component.
Thanks in advance!

This example generates an array of 450 items and then splits them into chunks of 100. It first dumps the first 100 items and after every button click it takes another 100 and appends it to the previous results. This chain properly completes after loading all data.
I think you should be able to take this and use to for your problem. Just instead of button clicks use a Subject that will emit every time user scrolls to the bottom:
import { fromEvent, range, zip } from 'rxjs';
import { map, bufferCount, startWith, scan } from 'rxjs/operators';
const SLICE_SIZE = 100;
const loadMore$ = fromEvent(document.getElementsByTagName('button')[0], 'click');
const data$ = range(450);
zip(
data$.pipe(bufferCount(SLICE_SIZE)),
loadMore$.pipe(startWith(0)),
).pipe(
map(results => results[0]),
scan((acc, chunk) => [...acc, ...chunk], []),
).subscribe({
next: v => console.log(v),
complete: () => console.log('complete'),
});
Live demo: https://stackblitz.com/edit/rxjs-au9pt7?file=index.ts
If you're concerned about performance you should use trackBy for *ngFor to avoid re-rendering existing DOM elements but I guess you already know that.

Here is a live demo on Stackblitz.
If your component subscribes to an observable holding the whole list to be displayed, your service will have to hold this whole list and send a new one every time an item is added. Here is an implementation using this pattern. Since lists are passed by reference, each list pushed in the observable is simply a reference and not a copy of the list, so sending a new list is not a costly operation.
For the service, use a BehaviorSubject to inject your new items in your observable. You can get an observable from it using its asObservable() method. Use another property to hold your current list. Each time loadMore() is called, push the new items in your list, and then push this list in the subject, which will push it in the observable as well, and your components will rerender.
Here I am starting with a list holding all items (allCategories), every time loadMore() is called, a block of 100 items if taken and placed on the current list using Array.splice():
#Injectable({
providedIn: 'root'
})
export class MultiSelectService {
private categoriesSubject = new BehaviorSubject<Array<string>>([]);
categories$ = this.categoriesSubject.asObservable();
categories: Array<string> = [];
allCategories: Array<string> = Array.from({ length: 1000 }, (_, i) => `item #${i}`);
constructor() {
this.getNextItems();
this.categoriesSubject.next(this.categories);
}
loadMore(): void {
if (this.getNextItems()) {
this.categoriesSubject.next(this.categories);
}
}
getNextItems(): boolean {
if (this.categories.length >= this.allCategories.length) {
return false;
}
const remainingLength = Math.min(100, this.allCategories.length - this.categories.length);
this.categories.push(...this.allCategories.slice(this.categories.length, this.categories.length + remainingLength));
return true;
}
}
Then call the loadMore() method on your service from your multiselect component when the bottom is reached:
export class MultiselectComponent {
categories$: Observable<Array<string>>;
constructor(private dataService: MultiSelectService) {
this.categories$ = dataService.categories$;
}
onScrollingFinished() {
console.log('load more');
this.dataService.loadMore();
}
}
In your multiselect-list component, place the scrollTracker directive on the containing ul and not on the li:
<ul class="multiselect-list" scrollTracker (scrollingFinished)="onScrollingFinished()">
<li *ngFor="let category of categories; let idx=index" class="multiselect-list--option">
<input type="checkbox" id="{{ category }}" tabindex="{{ idx + 1 }}"/>
<label for="{{ category }}">{{ category }}</label>
</li>
</ul>
In order to detect a scroll to bottom and fire the event only once, use this logic to implement your scrollTracker directive:
#Directive({
selector: '[scrollTracker]'
})
export class ScrollTrackerDirective {
#Output() scrollingFinished = new EventEmitter<void>();
emitted = false;
#HostListener("window:scroll", [])
onScroll(): void {
if ((window.innerHeight + window.scrollY) >= document.body.offsetHeight && !this.emitted) {
this.emitted = true;
this.scrollingFinished.emit();
} else if ((window.innerHeight + window.scrollY) < document.body.offsetHeight) {
this.emitted = false;
}
}
}
Hope that helps!

How do I target all items in a list, when a change occurs in Vue.js?

I'm building a site that uses Vue for to power the majority of the UI. The main component is a list of videos that is updated whenever a certain URL pattern is matched.
The main (video-list) component looks largely like this:
let VideoList = Vue.component( 'video-list', {
data: () => ({ singlePost: '' }),
props: ['posts', 'categorySlug'],
template: `
<div>
<transition-group tag="ul">
<li v-for="(post, index) in filterPostsByCategory( posts )">
<div #click.prevent="showPost( post )">
<img :src="post.video_cover" />
/* ... */
</div>
</li>
</transition-group>
</div>`,
methods: {
orderPostsInCategory: function ( inputArray, currentCategory ) {
let outputArray = [];
for (let i = 0; i < inputArray.length; i++) {
let currentCategoryObj = inputArray[i].video_categories.find( (category) => {
return category.slug === currentCategory;
});
let positionInCategory = currentCategoryObj.category_post_order;
outputArray[positionInCategory] = inputArray[i];
}
return outputArray;
},
filterPostsByCategory: function ( posts ) {
let categorySlug = this.categorySlug,
filteredPosts = posts.filter( (post) => {
return post.video_categories.some( (category) => {
return category.slug === categorySlug;
})
});
return this.orderPostsInCategory( filteredPosts, categorySlug );
}
}
});
The filterPostsByCategory() method does its job switching between the various possible categories, and instantly updating the list, according to the routes below:
let router = new VueRouter({
mode: 'history',
linkActiveClass: 'active',
routes: [
{ path: '/', component: VideoList, props: {categorySlug: 'home-page'} },
{ path: '/category/:categorySlug', component: VideoList, props: true }
]
});
The difficulty I'm having is transitioning the list in the way that I'd like. Ideally, when new category is selected all currently visible list items would fade out and the new list items would then fade in. I've looked at the vue transitions documentation, but haven't been able to get the effect I'm after.
The issue is that some items have more than one category, and when switching between these categories, those items are never affected by whatever transition I try to apply (I assume because Vue is just trying to be efficient and update as few nodes as possible). It's also possible that two or more categories contain the exact same list items, and in these instances enter and leave methods don't seem to fire at all.
So the question is, what would be a good way to ensure that I can target all current items (regardless of whether they're still be visible after the route change) whenever the route patterns above are matched?

Have you noticed the special key attribute in the documentation?
Vue.js is really focused on performance, because of that, when you modify lists used with v-for, vue tries to update as few DOM nodes as possible. Sometimes it only updates text content of the nodes instead of removing the whole node and then append a newly created one. Using :key you tell vue that this node is specifically related to the given key/id, and you force vue to completely update the DOM when the list/array is modified and as a result the key is changed. In your case is appropriate to bind the key attribute to some info related to the post and the category filter itself, so that whenever the list is modified or the category is changed the whole list may be rerendered and thus apply the animation on all items:
<li v-for="(post, index) in filterPostsByCategory( posts )" :key="post.id + categorySlug">
<div #click.prevent="showPost( post )">
<img :src="post.video_cover" />
/* ... */
</div>
</li>