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:
1 2 3 4 5 6 7 | <div *ngIf="someCondition"> <h1>Condition Passed!</h1></div><div *ngIf="!someCondition"> <h1>Condition Failed!</h1></div> |
Now you can use syntax like this:
1 2 3 4 5 6 7 | <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:
1 2 3 4 5 6 7 8 9 | <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:
1 2 3 | <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:
1 2 3 4 5 6 7 | <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.