Avoid evaluating input properties on Angular directives after init

I have a very simple Angular attribute directive that looks something like this:

import { Directive, Input, OnInit } from "@angular/core";

@Directive({
  selector: "[foo]",
})
export class FooDirective implements OnInit {
  @Input() public foo: string;

  public ngOnInit(): void {
    // Do something here once
  }
}

It will be used like this:

<div [foo]="bar"></div>

bar might be a simple string, but could also be a string getter that is relatively expensive to run. I would therefore like to read the value once on initialization, but ignore it afterwards. I know that the ngOnInit is only called once, but change detection will cause the bar property to be called repeatedly (depending on how the containing component works).

Using @Attributes would have been nice, since they only get evaluated once. But it doesn't seem like @Attribute works for directives (with the same name?):

import { Directive, OnInit, Attribute } from "@angular/core";

@Directive({
  selector: "[foo]",
})
export class FooDirective implements OnInit {
  public constructor(
    @Attribute("foo")
    private readonly foo: string
  ) {}

  public ngOnInit(): void {
    // Do something with this.foo here
  }
}

How do I avoid that the input property is evaluated more than once? I would prefer to unsubscribe from any changes, so to speak.

Directives don't seem to support ChangeDetectionStrategy.OnPush, but something similar could be nice.