ngIf improvements for Rx

Goals

When writing Rx style code it is often necessary to write code like this

{{ (userObs|async).?lastName}}, {{ (userObs|async).?firstName}}

The issue is that:

1. every async creates a listener and
2. the need to use the .? makes coding cumbersome.

What is needed is to be able to subscribe to the observable once and then refer to it in subsequent location. Additionally we need a way to display an alternative template while the observable has not yet yielded a value.

Proposal

• Allow ngIf to assign data to local value.
• Add an else feature to ngIf so that an alternative template can be shown until the observable yields a value.

<div *ngIf="userObs | async; else loading; let user">
  {{ user.lastName}}, {{ user.firstName }}
</div>
<template #loading>
  loading...
</template>

NOTE: the microsyntax is consistent with existing implementation of the microsyntax. For expanded form see:

<template [ngIf]="userObs | async" [ngIfElse]="loading" let-user>
  {{ user.lastName}}, {{ user.firstName }}
</template>
<template #loading>
  loading...
</template>

[fxck]

This would be useful in many other places, I wouldn’t limit it to ngIf.

[vicb]

@mhevery a little bit off topic but as we have discussed about it, django has a for else

{% for user in users %}
    <li>{{ user.username|e }}</li>
{% else %}
    <li><em>no users found</em></li>
{% endfor %}