How to ensure that a function that depends on ngOnInit runs after it?

I have a class that when initialized retrieves data from a service and popular one of its attributes, which is an array . This class has a function that sorts this array , filters and returns the result.

When instantiating an object of this class and calling this function I realized that the function is called before finishing the constructor and ngOnInit() execution. I believe this happens because of the use of the asynchronous call to the service, which uses Observables .

How can I make sure this does not happen?

export class BaseChoice implements PickAppraiser, OnInit {
    weight = 0;
    options = new Array<PickQuality>();

    constructor(private championService: ChampionService) {}

    ngOnInit() {
    // Iterates through the list of champions adding them to the current object
    this.championService.getChampions()
        .subscribe(champions => {
            // Iterates through the list of champions adding them to the current object
            Object.keys(champions).map(key => this.options.push(new PickQuality(champions[key], 0)))
        })
    }

    choose(n?: number): PickQuality[] {
        var sorted = this.options.sort((a, b) => a.score - b.score);
        return sorted;
    }
}