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>
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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>
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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>
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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>
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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>
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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.