By customizing the `getter/setter` interception scheme to convert data, we got this decorator.

## Basic implementation

```ts
export function ObservableInput<
  T = any,
  SK extends keyof T = any,
  K extends keyof T = any
>(propertyKey?: K | boolean, initialValue?: SubjectType<T[SK]>) {
  return (target: T, sPropertyKey: SK) => {
    const symbol = Symbol();

    type ST = SubjectType<T[SK]>;

    type Mixed = T & {
      [symbol]: BehaviorSubject<ST>;
    } & Record<SK, BehaviorSubject<ST>>;

    Object.defineProperty(target, sPropertyKey, {
      enumerable: true,
      configurable: true,
      get(this: Mixed) {
        return (
          this[symbol] || (this[symbol] = new BehaviorSubject<ST>(initialValue))
        );
      },
      set(this: Mixed, value: ST) {
        this[sPropertyKey].next(value);
      },
    });

    if (!propertyKey) {
      return;
    }

    if (propertyKey === true) {
      // Automatically match attribute names and remove the trailing $ symbol.
      propertyKey = (sPropertyKey as string).replace(/\$+$/, '') as K; 
    }

    Object.defineProperty(target, propertyKey, {
      enumerable: true,
      configurable: true,
      get(this: Mixed) {
        return this[sPropertyKey].getValue();
      },
      set(this: Mixed, value: ST) {
        this[sPropertyKey].next(value);
      },
    });
  };
}
```

## Usages

```ts
// Usage one
@Component({})
export class DemoComponent {
  @ObservableInput() // Automatically bind the value of "name", that is, remove the "$" symbol at the end of "name$$".
  @Input('name')
  name$$: Observable<string>;

  name: string;
}

// Usage three
@Component({})
export class DemoComponent {
  @ObservableInput('nameValue') // Automatically bind the value of nameValue Input.
  name$$: Observable<string>;

  @Input()
  nameValue: string;
}
```

## One more thing

You can find out this decorator in [ObservableInput](https://github.com/rx-ts/ngrx/blob/master/src/utils/decorators.ts#L56), or similar decorator in [ngx-observable-input](https://github.com/Futhark/ngx-observable-input).


10. A Useful Custom Decorator in Angular

By customizing the `getter/setter` interception scheme to convert data, we got this decorator.

##...