Friday, 24 October 2014

AngularJS ng-if and ng-show

We use AngularJS to do a lot of the front end on, at some point I may do some posts on the pros and cons of AngularJS, but for now it's enough to say that -in conjunction with Bootstrap it makes producing a front end pretty easy for a back end developer like me.

Anyhoo one of the joys of Angular is how easy it is to produce a display that varies depending on the data provided to it (even dynamically -although that's not the subject of this post). When we started the way to do this was via ng-show and ng-hide, which basically twiddles the CSS display property for the element to show or hide it. This works very well, but the element would always be fetched from the server, it just wouldn't appear on the screen.

Now we have ng-if, this will remove the element from the DOM altogether if the expression evaluates to false, and so, assuming it is some some of resource it won't fetch it from the server, thus speeding up the page.

Quick Example

ng-show :
<img ng-show="job.user_id && !job.system.logo_url" ng-src="/logo/{{job.user_id}}/{{job_id}}" class="img-responsive">

ng-if :
<img ng-if="job.user_id && !job.system.logo_url" ng-src="/logo/{{job.user_id}}/{{job_id}}" class="img-responsive">

The ng-show in this case has a particularly bad effect in that it will load, although hide, an image that isn't there, i.e. one with no user_id!

References :

ng-if in Angular Docs
Stackexchange -you may want to read this if unexpected things start happening with ng-if, like some other Angular commands it has a habit of creating child scopes.

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