Display loading while using Observable with Async pipe in template

Question

Situation: I am using FirebaseObjectObservable to populate my Ionic 2 (rc0) template. Template code:

<ion-card-content>
  <p>{{(course | async)?.description}}</p>
  <br>
  <h2>Learning Objectives</h2>
  <ul>
    <li *ngFor = "let objective of (course | async)?.objectives">{{objective.text}}</li>
  </ul>
  <h2>Takeaway</h2>
  <ul>
    <li *ngFor = "let takeaway of (course | async)?.takeaways">{{takeaway.text}}</li>
  </ul>
</ion-card-content>

TS code:

this.course = this.af.database.object('/bbwLocations/courses/' + courseId); 

this.course is a Firebase Object Observable. Everything works! But whenever I come into the template, there is a flash of empty no data. Then all the data jump out! Very not UX friendly. So I want to use some kind of pre-loading strategy. But since there is not TS logic here. Everything is controlled in template level with async pipe. How would I add loading in this situation?

Solution 1

Maybe a little late but in case someone else is wondering how to manage this... What about using a template?

you could for example use something like:

<ion-card-content *ngIf='(course$ | async) as course; else loading'>
  <p>{{course.description}}</p>
  <br>
  <h2>Learning Objectives</h2>
  <ul>
     <li *ngFor = "let objective of course.objectives"> 
         {{objective.text}}</li>
  </ul>
  <h2>Takeaway</h2>
  <ul>
     <li *ngFor = "let takeaway of course.takeaways"> 
         {{takeaway.text}}</li>
  </ul>
</ion-card-content>

<ng-template #loading>
  Loading stuff...
</ng-template>

so your ion-card-content will be hidden, showing the #template, until the async pipe is loaded.

Solution 2

You could do something like this:

<style>
  pre {
   color: orange;
   // or whatever you want
  }
</style>
<ion-card-content>
  <p>{{(course | async)?.description}}</p>
  <br>
  <h2>Learning Objectives</h2>
  <pre *ngIf="!(course | async)">loading objectives...</pre>
  <ul>
    <li *ngFor = "let objective of (course | async)?.objectives">{{objective.text}}</li>
  </ul>
  <h2>Takeaway</h2>
  <pre *ngIf="!(course | async)">loading takeaways...</pre>
  <ul>
    <li *ngFor = "let takeaway of (course | async)?.takeaways">{{takeaway.text}}</li>
  </ul>
</ion-card-content>

Solution 3

Lets say we have an observable meals$. We need to show loader when the observable is getting resolved(i.e fetching data). Below is the solution.

<div *ngIf="meal$ | async as meal; else loading;" >
   //use the meal variable to show some data
</div>
<ng-template #loading>
   //Show your loader here
</ng-template>

Comments

[Hugh Hou]

This works. But if I want exit animation, like fade out the loader when ngIf become false. I won't be able to do that. Also, is that possible to tap into the observable and use Ionic Loader component for this?

[Sasxa]

If you want to use ngIf I believe Animations are the way to go. You could use ngClass instead of ngIf and do animations/transitions with CSS. transition: 1s opacity 0.5s linear, and such...

[Hugh Hou]

Could you update your solution and give me an example how to easily use animations to do a fadeout of the pre element? Thank you!

[André Roggeri Campos]

But this make lots of unecessary requests... :(

[Birchlabs]

If it's duplicating your request: you are giving observers access to a cold observable. You should use share() or similar to setup course as a hot observable. this.course = this.httpClient.get('whatever').pipe(shareReplay(1)). this will store the most recent result, so that if you have multiple subscribers: they can observe a replay of the result instead of initiating a new HTTP request.