[转]Angular 4 *ngIf/Else

本文转自：http://tylerscode.com/2017/03/angular-4-ngifelse/

As you may know it wasn’t that many months ago that Angular 2 left RC and went Full Release(back in August). We are already upon the next big release of Angular with v4. Angular 4.0.0-rc.1 was released in late February with rc.2 hot on it’s heels 6 days later, today, March 2nd. There are lots of improvements including smaller bundle sizes and faster compilation. My favorite new feature at the moment is the new NgIf/Else syntax.

Previously, you may have used something like this:

Now you can use syntax like this:

You can specify another template using 

ng-template

, give it a variable using 

#

 and then reference it in the 

*ngIf

 statement with an 

else

 clause.

You can also use a more explicit syntax with NgIf/Else/Then. It would look something like this:

In my opinion this helps code readability as it makes it more explicit and easier to follow. No more falsy checks with 

!someCondition

 like code.

Also, the 

async

 pipe was added to 

*ngIf

. Previously you may have had a form or page that contained several fields that all independently subscribed to observables using the 

async

 pipe. It may have looked something like this:

Now you can wrap all those observables into a single observable and subscribe to it in the 

*ngIf

statement and assign a local object variable to reference in all your fields like this:

This code, in my opinion, is cleaner because it only subscribes to a single observable once to retrieve data. I hope this feature is as beneficial to others as it is to me.