Using observables in an Angular 2 template with *ngFor

问题

I am unable to access a property of an object provided by the async pipe if I build the access chain using *ngFor.

In the example below, suppose that Parking in the test line and the ?.[filter.propName] two lines beneath represent the same key on the same object. The test line will evaluate to true, but the checked property does not. Why? How do I access the propery when stamping these out with *ngFor?

For example, if {{(compModel | async)?.Parking?.Garage}} === true I would expect {{(compModel | async)?.[filter.propName].instance}} === true as well, but this isn't the case.

The syntax below doesn't work. I'm using it to demonstrate intended functionality.

<div *ngFor="let filter of filters | async">
...
<fieldset class="filter-category-title">
      <legend>{{filter.name}}</legend>
        <label *ngFor="let instance of filter.options">
          test: {{(compModel | async)?.Parking.instance}}
          <input type="checkbox"
                 checked={{(compModel | async)?.[filter.propName].instance}}
                 (click)="amenityEmit({category: filter.propName, filter: instance, resetCategory: false})">
                 {{instance}}
        </label>
    </fieldset>
</div>

The filter data I get from the server in the format below. I use this to build a comparison model, also below, which is how I manage the state of my search results page. ([key: string] is always a propName from a Filter).

export interface Filter {
  base: boolean,
  filter: string,
  propName: string,
  unionType: string,
  inputType: string,
  options: {
    options: string[],
    sectionTitle: string
  }[] | string[]
}

compModel interface:

export interface CompModel {
  [key: string]: {
    touched: boolean
    unionType: string,
    options: {
      options: string[],
      sectionTitle: string
    }[] | string[],
    updateSelf(x: CompModel, y: Action): void
  }
}

回答1:

The safe-navigation operator ?. does not work with []. There is no ?[] operator and also no ?.[] or .[] operator, therefore this can't work.

You can try

{{(compModel | async) && (compModel | async)[filter.propName].instance}} === true

otherwise you'll need to move some code to the components class

for example

this.compModel = someService.getSomeObservable()
.filter(val => !!val)

to ensure there are no null values

来源：https://stackoverflow.com/questions/42057483/using-observables-in-an-angular-2-template-with-ngfor