Angular2 - pass ASYNC data to child component

Question

I have issue, with passing async data to child component. I trying to write dynamic form generator. Issue starts when I try to call json via Observable and pass it into child component.

service:

generateSearchFields2(): Observable<any> {
        return this.http
            .get(this.API + 'searchFields')
            .map((res:Response) => {
                res.json().data as any;
                for (var i = 0; i < res.json().data.length; i++) {
                    var searchField = res.json().data[i];
                    switch (searchField.component) {
                        case "TextboxQuestion": 

                            let TXQ: TextboxQuestion = new TextboxQuestion({
                                key: searchField.key,
                                label: searchField.label,
                                value: searchField.value,
                                required: searchField.required,
                                order: searchField.order
                            });
                            this.searchFieldModels.push(TXQ);
                            console.log("TXQ: ", TXQ, this.searchFieldModels);
                            break;
                        case "DropdownQuestion": 

                            let DDQ: DropdownQuestion = new DropdownQuestion({
                                key: searchField.key,
                                label: searchField.label,
                                required: searchField.required,
                                options: searchField.options,
                                order: searchField.order
                            });
                            this.searchFieldModels.push(DDQ);
                            console.log("TXQ: ", DDQ, this.searchFieldModels);
                            break;
                        default:
                            alert("DEFAULT");
                            break;
                    }
                }
                return this.searchFieldModels.sort((a, b) => a.order - b.order);
            })
            .catch(this.handleError);
    }

Component Parent:

generateSearchFields2() {
    this.service.generateSearchFields2()
      .subscribe(res => this.searchFields = res)
  }

Iam passing variable via INPUT directive in parent template to child: [searchFields]="searchFields"

Issue is in child component, where searchField has undefined value. In this child I pass value to another service, to create formContros, but I got undefined there also. Data missing starts here, in child:

@Input() searchFields: SearchBase<any>[] = [];

ngOnInit() {   
    this.form = this.qcs.toFormGroup(this.searchFields);
    console.log("ONINIT DYNAMIC FORM COMPONENT: ", this.searchFields);

  }

Please for hint how I can pass async variable, to not loose data meantime

Answer 1

You can make @Input() searchFields a setter

private _searchFields: SearchBase<any>[] = [];
@Input() set searchFields(value SearchBase<any>[]) {
  if(value != null) {
    this.form = this.qcs.toFormGroup(this.searchFields);
    console.log("ONINIT DYNAMIC FORM COMPONENT: ", this.searchFields);
  }
}

get searchFields() : SearchBase<any>[] {
  return this.searchFields;
}

You can also use ngOnChanges() which is called every time an input is updated, but a setter is usually more convenient except perhaps when the executed code depends on multiple inputs being set.

Answer 2

In the ngOnInit event the data which comes from the parent is not bound yet. So your searchFields is undefined yet. You can use it in NgAfterViewInit component lifecycle event.

@Input() searchFields: SearchBase<any>[] = [];

ngAfterViewInit() {   
    this.form = this.qcs.toFormGroup(this.searchFields);
    console.log("ONINIT DYNAMIC FORM COMPONENT: ", this.searchFields);
}

For other cases you can see Angular2 Component Lifecycle events