Async HostBinding in directive
<p>I'm looking for the best way to handle HostBinding with async value.</p>

<p>Before Angular <strong>v2.1.2</strong> I could use the <code>host</code> property in the <code>@Directive</code> decorator like that :</p>

<pre><code>@Directive({
    selector: 'img[my-directive]',
    host    : {
        '[alt]'  : "alt | async"
    }
})
export class MyDirective {
    alt: Observable&lt;string&gt;;
}
</code></pre>

<p>But it looks like this was not the intended behavior, since version 2.1.2 fixes it. See <a href="https://github.com/angular/angular/commit/867494a" rel="noreferrer">don't access view local variables nor pipes in host expressions</a>.</p>

<p>Now, when compiling with AoT compilation, I get <code>Parser Error: Host binding expression cannot contain pipes in Directive</code>.</p>