Async function not working as intended when called at onInit lifecycle hook

问题

In my project I have a service that should load local database when the app is started. For that the function GetData() is used. I try to use it by calling at the ngOnInit() lifecycle hook. It logs the result in the console, but the property datum appears to be unchanged.

However, if I add GetData() method to a button, the property changes and data is displayed in the console as intended.

All sources that I looked through suggest that the right way to load the DB if I need it right away is to use ngOnInit() hook, that's why I don't wat to call GetData() when some event from the DOM is fired.

Component

export class AppComponent implements OnInit {
  datum;
  constructor(private searchService: SearchService){ }
  getData(){
    this.searchService.getData().subscribe(
      data => {
        this.datum = data[0];
        console.log(this.datum);},
      err => console.log("E", err)
    );
  }

  ngOnInit(){
    this.getData();
  }
  title = 'app';
}

HTML

<div style="text-align:center">
  <h1>
    Welcome to {{title}}!!
    Data: {{datum | json}}
  </h1>
  <button (click)="getData()">Click Me</button>

App loaded

Button clicked

EDIT Implementation of getData() with setTimeout() as suggested by AlexKhymenko

  getData(){
    this.searchService.getData().subscribe(
      data => {
        setTimeout(() =>{
          this.datum = data[0];
        });
        console.log(this.datum);},
      err => console.log("E", err)
    );
  }

回答1:

What you need to do is to run change Detection manually for example by calling setTimeout. I have created a Plunker.

export class AppComponent implements OnInit {
  datum;
  title = 'app';

  constructor(private searchService: SearchService){ }
       }

 ngOnInit(){
   this.getData();
 }
  getData(){
      this.searchService.getData().subscribe(
  data => {
   setTimeout(() => {
            this.datum = data[0];
   })
    console.log(this.datum);},
  err => console.log("E", err)
);

}

回答2:

Because in subscribe function you get an observable object (not array).

export class AppComponent implements OnInit {
   datum;
   constructor(private searchService: SearchService){ }
   getData(){
this.searchService.getData().subscribe(
  data => {
    //remove brackets
    this.datum = data;
    console.log(this.datum);},
  err => console.log("E", err)
 );
}

ngOnInit(){
   this.getData();
}
title = 'app';
}

回答3:

Try in HTML

<h1 *ngIf="datum !== null && datum !== undefined && datum !== ''">
Welcome to {{title}}!!
Data: {{datum | json}}
</h1>

Your datum variable was bind two-way data binding. When datum was not equal null or undefined or blank that time show that HTML tag.

Hope that help you.

来源：https://stackoverflow.com/questions/46189445/async-function-not-working-as-intended-when-called-at-oninit-lifecycle-hook

标签

javascript

angular

observable