Class SelectionModel

src/cdk/collections/selection-model.ts:14–275

 * Class to be used to power selecting one or more options from a list.
 */
export class SelectionModel<T> {
  /** Currently-selected values. */
  private _selection = new Set<T>();

  /** Keeps track of the deselected options that haven't been emitted by the change event. */
  private _deselectedToEmit: T[] = [];

  /** Keeps track of the selected options that haven't been emitted by the change event. */
  private _selectedToEmit: T[] = [];

  /** Cache for the array value of the selected items. */
  private _selected: T[] | null = null;

  /** Selected values. */
  get selected(): T[] {
    if (!this._selected) {
      this._selected = Array.from(this._selection.values());
    }

    return this._selected;
  }

  /** Event emitted when the value has changed. */
  readonly changed = new Subject<SelectionChange<T>>();

  /**
   * Exposes selection/deselection methods that work on array of values and don't expect a spread.
   * This is useful in the cases where you may have a large collection of items that can't be
   * easily spread into the existing methods without hitting browser limits.
   */
  readonly bulk: Readonly<{
    select: (values: T[]) => boolean;
    deselect: (values: T[]) => boolean;
    setSelection: (values: T[]) => boolean;
  }> = {
    select: values => this._select(values),
    deselect: values => this._deselect(values),
    setSelection: values => this._setSelection(values),
  };

  constructor(
    private _multiple = false,
    initiallySelectedValues?: T[],
    private _emitChanges = true,
    public compareWith?: (o1: T, o2: T) => boolean,
  ) {
    if (initiallySelectedValues && initiallySelectedValues.length) {
      if (_multiple) {
        initiallySelectedValues.forEach(value => this._markSelected(value));
      } else {
        this._markSelected(initiallySelectedValues[0]);
      }

      // Clear the array in order to avoid firing the change event for preselected values.
      this._selectedToEmit.length = 0;
    }
  }