静态JSON对象上异步数据的Angular2问题

Question

I have a service set up with some static objects that I am using in my UI.

fetchRulesVariables()

fetchRuleVariables() {

    let variables = [

        {
            name: 'Credit Funding Type',
            id: 1,
            multiple: false,
            hasOperators: true,
            availableOperators: this.fetchOperators('inResults'),
            inputType: 'single',
            placeholder: 'Select a funding type',
            availableValues: this.fetchFundingInstraments()
        }, {
            name: 'Device Trust Score',
            id: 2,
            multiple: false,
            hasOperators: true,
            availableOperators: this.fetchOperators('gtltResults'),
            inputType: 'text',
            placeholder: 'Enter a value between 0 - 100',
            availableValues: ''
        }
    ]
 return variables;
}

This object runs another function which adds another array of data to it such as availableOperators and availableValues . Those too are just static json objects.

In my component, I have a function that is fetching this data by ID using lodash .

fetchVariableData(id, type) {

    let data = [];

    switch (type) {
        case 'operators':
            data = _.find(this.variables, {id}).availableOperators;
            break;
    }

    return data;

}

In my component HTML, I am running an ngFor using my function to fetch the results.

<li *ngFor="let x of fetchVariableData(1, 'operators')"></li>

The issue here is that I am running into some async issues I believe. When I run this, I get an error that availableOperators is undefined. If I hardcode the operators into my fetchVariableData() function however, it works fine.

How can I handle this async issue where it appears that it is trying to use the availableOperators when it isn't ready?

This is all static JSON , no HTTP calls.

Edit1

Component Code:

export class AddRuleComponent implements OnInit {
    variables: any;

    ngOnInit() {
        this.loadVars();
        this.renderAddRuleForm();
    }


    loadVars() {
        this.outcomes = this._mserv.fetchOutcomeTypes();
        this.variables = this._mserv.fetchRuleVariables();
    }

    fetchVariableData(id, type) {

        let data: any;

        switch (type) {
            case 'operators':
                data = _.find(this.variables, {
                    id
                }).availableOperators;
                break;
        }
        return data;
    }

}

Service Code:

fetchRuleVariables() {

    let variables = [

        {
            name: 'Credit Funding Type',
            id: 1,
            multiple: false,
            hasOperators: true,
            availableOperators: this.fetchOperators('inResults'),
            inputType: 'single',
            placeholder: 'Select a funding type',
            availableValues: this.fetchFundingInstraments()
        }, {
            name: 'Device Trust Score',
            id: 2,
            multiple: false,
            hasOperators: true,
            availableOperators: this.fetchOperators('gtltResults'),
            inputType: 'text',
            placeholder: 'Enter a value between 0 - 100',
            availableValues: ''
        }
    ]
 return variables;
}

fetchOperators(type) {

    let operators = [];

    // Based on the set of operators we want
    switch (type) {

        // In Results
        case 'inResults':

            operators.push({
                name: 'In List',
                id: 1,
            }, {
                name: 'Not In List',
                id: 2,
            });

            break;

            // Greater than & Less than
        case 'gtltResults':

            operators.push({
                name: 'Greater Than <',
                id: 3,
            }, {
                name: 'Less Than >',
                id: 4,
            });

            break;

            // Is / is not
        case 'isisnot':

            operators.push({
                name: 'Is',
                id: 5,
            }, {
                name: 'Is Not',
                id: 6,
            });

            break;
    }

    return operators;
}

HTML Code:

 <select class="form-control input-sm" formControlName="operator" [attr.id]="'operator'+i">
   <option value="">Select an Operator</option>
   <option *ngFor="let o of fetchVariableData(1, 'operators') | values" value="{{ o.id }}">{{ o.name }}</option>
 </select>

Values Pipe:

import { Pipe, PipeTransform } from '@angular/core';

@Pipe({ name: 'values',  pure: false })
export class ValuesPipe implements PipeTransform {
  transform(value: any, args: any[] = null): any {
    return Object.keys(value).map(key => value[key]);
  }
}

Answer 1

Check the lifecycle of your component;

As explained here in the official guide your AddRuleComponent calls ngOnInit() to fetch and initialize the data for the component.

As stated in the docs, the order is constructor , ngOnChanges() , ngOnInit() and so on, so by the time ngOnInit() fetches the values, your component already checked if the properties has been initialized/mutated, that's why fetchVariableData() could be called with invalid params (happening from the ngOnChanges() ) resulting in the undefined error because _.find(...) is not returning the variables object as you expect.

I'd try to initialize first:

variables: any = [];

and then do an *ngIf="variables.length" on the container, or move the initialization to the constructor.