Angular autoresize textarea directive

Autosize.ts

import {
  Directive,
  ElementRef,
  Input,
  AfterViewInit,
  DoCheck,
  OnDestroy,
  NgZone,
} from '@angular/core';
import {Platform} from '@angular/cdk/platform';
import {auditTime, takeUntil} from 'rxjs/operators';
import {fromEvent, Subject} from 'rxjs';


/** Directive to automatically resize a textarea to fit its content. */
@Directive({
  selector: 'textarea[cdkTextareaAutosize]',
  exportAs: 'cdkTextareaAutosize',
  host: {
    'class': 'cdk-textarea-autosize',
    // Textarea elements that have the directive applied should have a single row by default.
    // Browsers normally show two rows by default and therefore this limits the minRows binding.
    'rows': '1',
    '(input)': '_noopInputHandler()',
  },
})
export class CdkTextareaAutosize implements AfterViewInit, DoCheck, OnDestroy {
  /** Keep track of the previous textarea value to avoid resizing when the value hasn't changed. */
  private _previousValue: string;
  private _initialHeight: string | null;
  private readonly _destroyed = new Subject<void>();

  private _minRows: number;
  private _maxRows: number;
  private _enabled: boolean = true;

  private _textareaElement: HTMLTextAreaElement;

  /** Minimum amount of rows in the textarea. */
  @Input('cdkAutosizeMinRows')
  get minRows(): number { return this._minRows; }
  set minRows(value: number) {
    this._minRows = value;
    this._setMinHeight();
  }

  /** Maximum amount of rows in the textarea. */
  @Input('cdkAutosizeMaxRows')
  get maxRows(): number { return this._maxRows; }
  set maxRows(value: number) {
    this._maxRows = value;
    this._setMaxHeight();
  }

  /** Whether autosizing is enabled or not */
  @Input('cdkTextareaAutosize')
  get enabled(): boolean { return this._enabled; }
  set enabled(value: boolean) {
    value = coerceBooleanProperty(value);

    // Only act if the actual value changed. This specifically helps to not run
    // resizeToFitContent too early (i.e. before ngAfterViewInit)
    if (this._enabled !== value) {
      (this._enabled = value) ? this.resizeToFitContent(true) : this.reset();
    }
  }

  /** Cached height of a textarea with a single row. */
  private _cachedLineHeight: number;

  constructor(
    private _elementRef: ElementRef<HTMLElement>,
    private _platform: Platform,
    private _ngZone: NgZone) {
    this._textareaElement = this._elementRef.nativeElement as HTMLTextAreaElement;
  }

  /** Sets the minimum height of the textarea as determined by minRows. */
  _setMinHeight(): void {
    const minHeight = this.minRows && this._cachedLineHeight ?
        `${this.minRows * this._cachedLineHeight}px` : null;

    if (minHeight)  {
      this._textareaElement.style.minHeight = minHeight;
    }
  }

  /** Sets the maximum height of the textarea as determined by maxRows. */
  _setMaxHeight(): void {
    const maxHeight = this.maxRows && this._cachedLineHeight ?
        `${this.maxRows * this._cachedLineHeight}px` : null;

    if (maxHeight) {
      this._textareaElement.style.maxHeight = maxHeight;
    }
  }

  ngAfterViewInit() {
    if (this._platform.isBrowser) {
      // Remember the height which we started with in case autosizing is disabled
      this._initialHeight = this._textareaElement.style.height;

      this.resizeToFitContent();

      this._ngZone.runOutsideAngular(() => {
        fromEvent(window, 'resize')
          .pipe(auditTime(16), takeUntil(this._destroyed))
          .subscribe(() => this.resizeToFitContent(true));
      });
    }
  }

  ngOnDestroy() {
    this._destroyed.next();
    this._destroyed.complete();
  }

  /**
   * Cache the height of a single-row textarea if it has not already been cached.
   *
   * We need to know how large a single "row" of a textarea is in order to apply minRows and
   * maxRows. For the initial version, we will assume that the height of a single line in the
   * textarea does not ever change.
   */
  private _cacheTextareaLineHeight(): void {
    if (this._cachedLineHeight) {
      return;
    }

    // Use a clone element because we have to override some styles.
    let textareaClone = this._textareaElement.cloneNode(false) as HTMLTextAreaElement;
    textareaClone.rows = 1;

    // Use `position: absolute` so that this doesn't cause a browser layout and use
    // `visibility: hidden` so that nothing is rendered. Clear any other styles that
    // would affect the height.
    textareaClone.style.position = 'absolute';
    textareaClone.style.visibility = 'hidden';
    textareaClone.style.border = 'none';
    textareaClone.style.padding = '0';
    textareaClone.style.height = '';
    textareaClone.style.minHeight = '';
    textareaClone.style.maxHeight = '';

    // In Firefox it happens that textarea elements are always bigger than the specified amount
    // of rows. This is because Firefox tries to add extra space for the horizontal scrollbar.
    // As a workaround that removes the extra space for the scrollbar, we can just set overflow
    // to hidden. This ensures that there is no invalid calculation of the line height.
    // See Firefox bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=33654
    textareaClone.style.overflow = 'hidden';

    this._textareaElement.parentNode!.appendChild(textareaClone);
    this._cachedLineHeight = textareaClone.clientHeight;
    this._textareaElement.parentNode!.removeChild(textareaClone);

    // Min and max heights have to be re-calculated if the cached line height changes
    this._setMinHeight();
    this._setMaxHeight();
  }

  ngDoCheck() {
    if (this._platform.isBrowser) {
      this.resizeToFitContent();
    }
  }

  /**
   * Resize the textarea to fit its content.
   * @param force Whether to force a height recalculation. By default the height will be
   *    recalculated only if the value changed since the last call.
   */
  resizeToFitContent(force: boolean = false) {
    // If autosizing is disabled, just skip everything else
    if (!this._enabled) {
      return;
    }

    this._cacheTextareaLineHeight();

    // If we haven't determined the line-height yet, we know we're still hidden and there's no point
    // in checking the height of the textarea.
    if (!this._cachedLineHeight) {
      return;
    }

    const textarea = this._elementRef.nativeElement as HTMLTextAreaElement;
    const value = textarea.value;

    // Only resize of the value changed since these calculations can be expensive.
    if (value === this._previousValue && !force) {
      return;
    }

    const placeholderText = textarea.placeholder;

    // Reset the textarea height to auto in order to shrink back to its default size.
    // Also temporarily force overflow:hidden, so scroll bars do not interfere with calculations.
    // Long placeholders that are wider than the textarea width may lead to a bigger scrollHeight
    // value. To ensure that the scrollHeight is not bigger than the content, the placeholders
    // need to be removed temporarily.
    textarea.classList.add('cdk-textarea-autosize-measuring');
    textarea.placeholder = '';

    // The cdk-textarea-autosize-measuring class includes a 2px padding to workaround an issue with
    // Chrome, so we account for that extra space here by subtracting 4 (2px top + 2px bottom).
    const height = textarea.scrollHeight - 4;

    // Use the scrollHeight to know how large the textarea *would* be if fit its entire value.
    textarea.style.height = `${height}px`;
    textarea.classList.remove('cdk-textarea-autosize-measuring');
    textarea.placeholder = placeholderText;

    // On Firefox resizing the textarea will prevent it from scrolling to the caret position.
    // We need to re-set the selection in order for it to scroll to the proper position.
    if (typeof requestAnimationFrame !== 'undefined') {
      this._ngZone.runOutsideAngular(() => requestAnimationFrame(() => {
        const {selectionStart, selectionEnd} = textarea;

        // IE will throw an "Unspecified error" if we try to set the selection range after the
        // element has been removed from the DOM. Assert that the directive hasn't been destroyed
        // between the time we requested the animation frame and when it was executed.
        // Also note that we have to assert that the textarea is focused before we set the
        // selection range. Setting the selection range on a non-focused textarea will cause
        // it to receive focus on IE and Edge.
        if (!this._destroyed.isStopped && document.activeElement === textarea) {
          textarea.setSelectionRange(selectionStart, selectionEnd);
        }
      }));
    }

    this._previousValue = value;
  }

  /**
   * Resets the textarea to it's original size
   */
  reset() {
    // Do not try to change the textarea, if the initialHeight has not been determined yet
    // This might potentially remove styles when reset() is called before ngAfterViewInit
    if (this._initialHeight === undefined) {
      return;
    }
    this._textareaElement.style.height = this._initialHeight;
  }

  _noopInputHandler() {
    // no-op handler that ensures we're running change detection on input events.
  }
}