NG_VALUE_ACCESSOR
用于为表单控件提供 ControlValueAccessor
Used to provide a ControlValueAccessor for form controls.
const NG_VALUE_ACCESSOR: InjectionToken<ReadonlyArray<ControlValueAccessor>>;
说明
有关如何实现的信息，请参见 DefaultValueAccessor
See DefaultValueAccessor for how to implement one.
