@Directive({
  selector: '[ngModel]:not([formControlName]):not([formControl])',
  providers: [formControlBinding],
  exportAs: 'ngModel'
})
export class NgModel extends NgControl implements OnChanges,
    OnDestroy {
  public readonly control: FormControl = new FormControl();
  /** @internal */
  _registered = false;

  /**
   * @description
   * Internal reference to the view model value.
   */
  viewModel: any;

  @Input() name !: string;

  @Input('disabled') isDisabled !: boolean;

  /**
   * @description
   * Tracks the value bound to this directive.
   */
  // 根据注释，这里即指令要跟踪的值
  @Input('ngModel') model: any;

  @Input('ngModelOptions')
  options !: {name?: string, standalone?: boolean, updateOn?: FormHooks};

  @Output('ngModelChange') update = new EventEmitter();

  constructor(@Optional() @Host() parent: ControlContainer,
              @Optional() @Self() @Inject(NG_VALIDATORS) validators: Array<Validator|ValidatorFn>,
              @Optional() @Self() @Inject(NG_ASYNC_VALIDATORS) asyncValidators: Array<AsyncValidator|AsyncValidatorFn>,
              @Optional() @Self() @Inject(NG_VALUE_ACCESSOR)
              valueAccessors: ControlValueAccessor[]) {
                super();
                this._parent = parent;
                this._rawValidators = validators || [];
                this._rawAsyncValidators = asyncValidators || [];
                this.valueAccessor = selectValueAccessor(this, valueAccessors);
              }

              /**
               * @description
               * A lifecycle method called when the directive's inputs change. For internal use
               * only.
               *
               * @param changes A object of key/value pairs for the set of changed inputs.
               */
              // 定义 ngOnChange( Angular 对 change 事件的封装) 的 handler
              ngOnChanges(changes: SimpleChanges) {
                this._checkForErrors();
                if (!this._registered) this._setUpControl();
                if ('isDisabled' in changes) {
                  this._updateDisabled(changes);
                }

                if (isPropertyUpdated(changes, this.viewModel)) {
                  // 调用 _updateValue 来应用新的值
                  this._updateValue(this.model);
                  this.viewModel = this.model;
                }
              }

              ngOnDestroy(): void { this.formDirective && this.formDirective.removeControl(this); }

              get path(): string[] {
                return this._parent ? controlPath(this.name, this._parent) : [this.name];
              }

              get formDirective(): any { return this._parent ? this._parent.formDirective : null; }

              get validator(): ValidatorFn|null { return composeValidators(this._rawValidators); }

              get asyncValidator(): AsyncValidatorFn|null {
                return composeAsyncValidators(this._rawAsyncValidators);
              }

              viewToModelUpdate(newValue: any): void {
                this.viewModel = newValue;
                this.update.emit(newValue);
              }

              private _setUpControl(): void {
                this._setUpdateStrategy();
                this._isStandalone() ? this._setUpStandalone() :
                                       this.formDirective.addControl(this);
                this._registered = true;
              }

              private _setUpdateStrategy(): void {
                if (this.options && this.options.updateOn != null) {
                  this.control._updateOn = this.options.updateOn;
                }
              }

              private _isStandalone(): boolean {
                return !this._parent || !!(this.options && this.options.standalone);
              }

              private _setUpStandalone(): void {
                setUpControl(this.control, this);
                this.control.updateValueAndValidity({emitEvent: false});
              }

              private _checkForErrors(): void {
                if (!this._isStandalone()) {
                  this._checkParentType();
                }
                this._checkName();
              }

              private _checkParentType(): void {
                if (!(this._parent instanceof NgModelGroup) &&
                    this._parent instanceof AbstractFormGroupDirective) {
                  TemplateDrivenErrors.formGroupNameException();
                } else if (
                    !(this._parent instanceof NgModelGroup) && !(this._parent instanceof NgForm)) {
                  TemplateDrivenErrors.modelParentException();
                }
              }

              private _checkName(): void {
                if (this.options && this.options.name) this.name = this.options.name;

                if (!this._isStandalone() && !this.name) {
                  TemplateDrivenErrors.missingNameException();
                }
              }

              // 用 control.setValue 给绑定的属性赋值
              private _updateValue(value: any): void {
                resolvedPromise.then(
                    () => { this.control.setValue(value, {emitViewToModelChange: false}); });
              }

              private _updateDisabled(changes: SimpleChanges) {
                const disabledValue = changes['isDisabled'].currentValue;

                const isDisabled =
                    disabledValue === '' || (disabledValue && disabledValue !== 'false');

                resolvedPromise.then(() => {
                  if (isDisabled && !this.control.disabled) {
                    this.control.disable();
                  } else if (!isDisabled && this.control.disabled) {
                    this.control.enable();
                  }
                });
              }
}