AngularJS $watch vs $watchCollection: which is better for performance?

问题

For watching an object scope variable, is $scope.$watch with objectEquality set to true OR $scope.$watchCollection better?

For a $scope object variable (like 15 attributes, some nested 2 levels deep) updated with input elements and ng-model in the view, how bad is $scope.$watch with objectEquality set to true? Is this a big thing to avoid?

Is $watchCollection a better solution?

I am looking for easy wins to improve performance on my AngularJS App (I'm still stuck on v1.2.2).

  // ctrl scope var
  $scope.filters = {
    name: '',
    info: {test: '', foo: '', bar: ''},
    yep: ''
    // etc ...
  }

  // ctrl watch ?
  $scope.$watch('filters', function(newVal, oldVal) {
    if(newVal !== oldVal) {
      // call with updated filters
    }
  }, true);

  // or ctrl watch collection ?
  $scope.$watchCollection('filters', function(newVal, oldVal) {
    if(newVal !== oldVal) {
      // call with updated filters
    }
  });

  // view input with ng-model
  <input type="text" ng-model="filters.name" />
  <input type="text" ng-model="filters.info.test" />
  <input type="text" ng-model="filters.yep" />
  // etc ...

回答1:

The $watchCollection() function is a sort-of mid-ground between the two $watch() configurations above. It's more in-depth than the vanilla $watch() function; but, it's not nearly as expensive as the deep-equality $watch() function. Like the $watch() function, the $watchCollection() works by comparing physical object references; however, unlike the $watch() function, the $watchCollection() goes one-level deep and performs an additional, shallow reference check of the top level items in the collection.

see this explanation

回答2:

$watch() will be triggered by:

$scope.myArray = [];
$scope.myArray = null;
$scope.myArray = someOtherArray;

$watchCollection() will be triggered by everything above AND:

$scope.myArray.push({}); // add element
$scope.myArray.splice(0, 1); // remove element
$scope.myArray[0] = {}; // assign index to different value

$watch(..., true) will be triggered by EVERYTHING above AND:

$scope.myArray[0].someProperty = "someValue";

JUST ONE MORE THING...

$watch() is the only one that fires when an array is replaced with another with the same exact content. For example:

$scope.myArray = ["Apples", "Bananas", "Orange" ];

var newArray = [];
newArray.push("Apples");
newArray.push("Bananas");
newArray.push("Orange");

$scope.myArray = newArray;

Below is a link to an example JSFiddle that uses all the different watch combinations and outputs log messages to indicate which "watches" were triggered:

http://jsfiddle.net/luisperezphd/2zj9k872/

回答3:

$watchCollection is optimized for vector arrays [] where elements can be push

and $watch is good for associative arrays objects {}

$watchCollection will not watch for depth changes, is like watch with objectEquality set to false.

If you already know to structure of the depth you can optimize like this:

  // ctrl watch ?
  $scope.$watch('filters', function(newVal, oldVal) {
    if(newVal !== oldVal) {
      // call with updated filters
    }
  });

  // ctrl watch ?
  $scope.$watch('filters.info', function(newVal, oldVal) {
    if(newVal !== oldVal) {
      // call with updated filters
    }
  });

来源：https://stackoverflow.com/questions/26535415/angularjs-watch-vs-watchcollection-which-is-better-for-performance

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angularjs

angularjs-scope

angularjs-watch