[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:

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<div *ngIf="someCondition">   <h1>Condition Passed!</h1> </div>   <div *ngIf="!someCondition">   <h1>Condition Failed!</h1> </div>

Now you can use syntax like this:

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<div *ngIf="someCondition; else falsyTemplate">   <h1>Condition Passed!</h1> </div>   <ng-template #falsyTemplate>   <h1>Condition Failed!</h1> </ng-template>

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:

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<div *ngIf="someCondition; then truthyTemplate else falsyTemplate"></div>   <ng-template #truthyTemplate >   <h1>Condition Passed!</h1> </ng-template>   <ng-template #falsyTemplate>   <h1>Condition Failed!</h1> </ng-template>

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:

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<p>{{someObservableOne | async}}</p> <p>{{someObservableTwo | async}}</p> <p>{{someObservableThree | async}}</p>

Now you can wrap all those observables into a single observable and subscribe to it in the *ngIfstatement and assign a local object variable to reference in all your fields like this:

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<div *ngIf="someObservable | async; else loadingScreen; let myObject">   <p>{{myObject.propertyOne}}</p>   <p>{{myObject.propertyTwo}}</p>   <p>{{myObject.propertyThree}}</p> </div>   <ng-template #loadingScreen>loading...</ng-template>

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.