AngularJS : Binding a variable in template to increase performances

问题

Here is a code sample I use:

<div ng-class="{alert: ((elements|filter:{hasAlert: true}).length / elements.length) > maxPercentAlerts}">

{{(elements|filter:{hasAlert: true}).length}}

({{Math.floor((elements|filter:{hasAlert: true}).length * 100 / elements.length)}} %)

</div>

As you see, I need to filter my 'elements' array 3 times. I would like to use this kind of following code to increase perfs: (this is just an example of what I need, not real code)

<div some-ng-prop="alertCount=(elements|filter:{hasAlert: true}).length"
    <div ng-class="{alert: (alertCount / elements.length) > maxPercentAlerts}">

    {{alertCount}}

    ({{Math.floor(alertCount * 100 / elements.length)}} %)

</div>

I've tried to handle it with the 'ng-init' attribute: it worked great... But when my model changes, the values are not updated.

Is there a way to do that ?

I've tried to be clear, but please ask for details if you don't understand what I mean.

回答1:

Here is a plunker: http://plnkr.co/edit/bNjSnee5UwVjp6wHE6RK

I created a directive:

I use $parse rather than $eval for optimizations.
You provide a collection to watch and an expression to run on each change.
It works like ngInit only it updates when the collection is dirty.
I chose general names for attributes, you can change it to what you like.

directive:

app.directive('watchCollection', function($parse){
  return {
    compile: function(tElm,tAttrs){

      if(! tAttrs.assign) return;

      var assignFn = $parse(tAttrs.assign)

      return function(scope,elm,attrs){      
        scope.$watchCollection(tAttrs.watchCollection , function(val){
          assignFn(scope);
        })
      }
    }
  }
})

html:

<div watch-collection="elements" 
     assign="alertCount=(elements|filter:{hasAlert: true}).length">

回答2:

Stewie suggested it was a duplicate, I think it's not but it made me try the following code which works great :

<div ng-class="{alert: (alertCount / elements.length) > maxPercentAlerts}">

{{alertCount = (elements|filter:{hasAlert: true}).length}}

({{Math.floor(alertCount * 100 / elements.length)}} %)

</div>

来源：https://stackoverflow.com/questions/21408274/angularjs-binding-a-variable-in-template-to-increase-performances

标签

angularjs

templates

variables

binding