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* Copyright (c) Siemens 2016 - 2025
* SPDX-License-Identifier: MIT
*/
import { isPlatformBrowser } from '@angular/common';
import { inject, Injectable, NgZone, PLATFORM_ID } from '@angular/core';
import { Observable, Subscriber } from 'rxjs';
export interface ElementDimensions {
width: number;
height: number;
}
interface ResizeSubscriber {
sub: Subscriber<ElementDimensions>;
dim?: ElementDimensions;
throttle: number;
blocked: boolean;
emitImmediate?: boolean;
}
interface Listener {
element: Element;
subscribers: ResizeSubscriber[];
}
interface QueueEntry {
element: Element;
subscriber: ResizeSubscriber;
unblock: boolean;
force: boolean;
}
/**
* A service wrapping `ResizeObserver`. This is a service for those reasons:
* - only one `ResizeObserver` should be used for performance reason.
* - For Angular change detection to work, explicit `ngZone` calls are necessary
* - Observable stream
*/
@Injectable({
providedIn: 'root'
})
export class ResizeObserverService {
private listeners = new Map<Element, Listener>();
private resizeObserver?: ResizeObserver;
private timerQueue = new Map<number, QueueEntry[]>();
private zone = inject(NgZone);
constructor() {
const isBrowser = isPlatformBrowser(inject(PLATFORM_ID));
Iif (!isBrowser || !ResizeObserver) {
return;
}
this.resizeObserver = new ResizeObserver((entries: ResizeObserverEntry[]) =>
entries.forEach(entry => this.handleElement(entry.target))
);
}
/**
* Observe the size of an element. Returns an observable with the changes.
* @param element - The element to observe
* @param throttle - Throttle time in ms. Will emit this time after the resize
* @param emitInitial - Emit the initial size after subscribe?
* @param emitImmediate - Emit an event immediately after the size changes. Useful e.g. for visibility checks.
*/
observe(
element: Element,
throttle: number,
emitInitial?: boolean,
emitImmediate?: boolean
): Observable<ElementDimensions> {
let entry = this.listeners.get(element);
if (!entry) {
entry = { element, subscribers: [] };
this.listeners.set(element, entry);
}
return new Observable<ElementDimensions>(subscriber => {
const sub: ResizeSubscriber = {
sub: subscriber,
dim: undefined,
throttle,
blocked: false,
emitImmediate
};
this.subscriberAdded(entry!, sub, emitInitial);
return () => this.subscriberRemoved(entry!, sub);
});
}
private subscriberAdded(
entry: Listener,
subscriber: ResizeSubscriber,
emitInitial?: boolean
): void {
entry.subscribers.push(subscriber);
if (entry.subscribers.length === 1) {
this.resizeObserver?.observe(entry.element);
}
if (emitInitial) {
this.schedule(0, entry.element, subscriber, false, true);
}
}
private subscriberRemoved(entry: Listener, subscriber: ResizeSubscriber): void {
const index = entry.subscribers.indexOf(subscriber);
if (index >= 0) {
entry.subscribers.splice(index, 1);
}
if (entry.subscribers.length === 0) {
// no more subscribers, tear down everything
this.resizeObserver?.unobserve(entry.element);
this.listeners.delete(entry.element);
}
this.unschedule(subscriber);
// close down, no re-subscription possible
subscriber.sub.complete();
}
private handleElement(element: Element): void {
const entry = this.listeners.get(element);
Iif (!entry) {
this.resizeObserver?.unobserve(element);
return;
}
entry.subscribers.forEach(sub => this.handleResizeSubscriber(element, sub));
}
private handleResizeSubscriber(element: Element, entry: ResizeSubscriber): void {
if (entry.blocked) {
return;
}
if (entry.emitImmediate) {
this.schedule(0, element, entry, false);
}
this.schedule(entry.throttle, element, entry, true);
}
private emitSize(element: Element, entry: ResizeSubscriber, force = false): void {
const dimensions = { width: element.clientWidth, height: element.clientHeight };
if (
!force &&
entry.dim?.width === dimensions.width &&
entry.dim?.height === dimensions.height
) {
// Prevent spurious emissions. Subpixels and all..
return;
}
entry.dim = dimensions;
entry.sub.next(dimensions);
}
private schedule(
timeout: number,
element: Element,
subscriber: ResizeSubscriber,
unblock: boolean,
force = false
): void {
if (unblock) {
subscriber.blocked = true;
}
let queue = this.timerQueue.get(timeout);
if (!queue) {
queue = [];
this.timerQueue.set(timeout, queue);
setTimeout(() => {
this.timerQueue.delete(timeout);
this.processQueue(queue!);
}, timeout);
}
queue.push({ element, subscriber, unblock, force });
}
private unschedule(entry: ResizeSubscriber): void {
const queued = this.timerQueue.get(entry.throttle);
if (queued) {
const index = queued.findIndex(q => q.subscriber === entry);
if (index > -1) {
queued.splice(index, 1);
}
}
}
private processQueue(queue: QueueEntry[]): void {
this.zone.run(() => {
queue?.forEach(q => {
if (q.unblock) {
q.subscriber.blocked = false;
}
this.emitSize(q.element, q.subscriber, q.force);
});
});
}
/**
* check size on all observed elements. Only use in testing!
*/
// eslint-disable-next-line @typescript-eslint/naming-convention
_checkAll(): void {
this.listeners.forEach(entry => this.handleElement(entry.element));
}
}
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