Angular custom ngIf directive 'as' syntax

I'm creating a custom *ngIf directive to replace content with a placeholder while it's loading. I have everything working as I want and modeled it after the *ngIf directive (https://github.com/angular/angular/blob/master/packages/common/src/directives/ng_if.ts) The only thing not working is the 'as' syntax and I don't see any references to it or where to start.

*myCustomDirective="loading$ | async as result" 

The above does not work, result is undefined when the loading$ observable emits data. The placeholder is shown and replaced with the content as expected however. (Content is giving errors though because of the undefined result)

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