alfresco-ng2-components/lib/core/directives/infinite-select-scroll.directive.ts

/*!
 * @license
 * Copyright 2019 Alfresco Software, Ltd.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import { Inject, AfterViewInit, Directive, EventEmitter, OnDestroy, Output } from '@angular/core';
import { MatSelect, SELECT_ITEM_HEIGHT_EM } from '@angular/material/select';
import { Subject } from 'rxjs';
import { takeUntil } from 'rxjs/operators';

@Directive({
    selector: '[adf-infinite-select-scroll]'
})
export class InfiniteSelectScrollDirective implements AfterViewInit, OnDestroy {
    static readonly MAX_ITEMS = 50;

    /** Emitted when scroll reaches the last item. */
    @Output() scrollEnd = new EventEmitter<Event>();

    private onDestroy$ = new Subject<boolean>();
    private itemHeightToWaitBeforeLoadNext = 0;

    constructor(@Inject(MatSelect) private matSelect: MatSelect) {}

    ngAfterViewInit() {
        this.matSelect.openedChange
            .pipe(takeUntil(this.onDestroy$))
            .subscribe((opened: boolean) => {
                if (opened) {
                    this.itemHeightToWaitBeforeLoadNext = (this.getItemHeight() * (InfiniteSelectScrollDirective.MAX_ITEMS / 2));
                    this.matSelect.panel.nativeElement.addEventListener('scroll', (event: Event) => this.handleScrollEvent(event));
                }
            });
    }

    ngOnDestroy() {
        this.onDestroy$.next(true);
        this.onDestroy$.complete();
    }

    private handleScrollEvent(event: Event) {
        if (this.isScrollInNextFetchArea(event)) {
            this.scrollEnd.emit(event);
        }
    }

    private isScrollInNextFetchArea(event: Event): boolean {
        const target = event.target as HTMLElement;
        return target.scrollTop >= (target.scrollHeight - target.offsetHeight - this.itemHeightToWaitBeforeLoadNext);
    }

    private getItemHeight(): number {
        return parseFloat(getComputedStyle(this.matSelect.panel.nativeElement).fontSize || '0') * SELECT_ITEM_HEIGHT_EM;
    }
}