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fiddle

I have a list, and I'd like to be notified whenever it changes. A simple $watch expression isn't working, which I'm guessing is because angular is checking for referential equality, not structural equality. 

<html ng-app>
    <head>
    </head>
    <body ng-controller="Root">
        <h1>Base Angular Fiddle</h1>
        Times changed: {{timesChanged}}
        <ul>
            <li ng-repeat="name in names">{{name}}</li>
            <li><button ng-click="names.push('another name')">Add</button></li>
        </ul>
    </body>
</html>
​

​function Root($scope) {
    $scope.timesChanged = 0;
    $scope.names = ['foo', 'bar', 'baz'];
    $scope.$watch('names', function() {
        $scope.timesChanged++;
    });
}​

What I'm hoping will happen is that the callback for 'names' is called each time that I call names.push(). Is there a workaround that's remended for this? Or am I just not using $watch correctly?

fiddle

I have a list, and I'd like to be notified whenever it changes. A simple $watch expression isn't working, which I'm guessing is because angular is checking for referential equality, not structural equality. 

<html ng-app>
    <head>
    </head>
    <body ng-controller="Root">
        <h1>Base Angular Fiddle</h1>
        Times changed: {{timesChanged}}
        <ul>
            <li ng-repeat="name in names">{{name}}</li>
            <li><button ng-click="names.push('another name')">Add</button></li>
        </ul>
    </body>
</html>
​

​function Root($scope) {
    $scope.timesChanged = 0;
    $scope.names = ['foo', 'bar', 'baz'];
    $scope.$watch('names', function() {
        $scope.timesChanged++;
    });
}​

What I'm hoping will happen is that the callback for 'names' is called each time that I call names.push(). Is there a workaround that's remended for this? Or am I just not using $watch correctly?

  • javascript
  • angularjs
Share edited Dec 11, 2012 at 23:14 Nick Heiner asked Dec 11, 2012 at 22:39 Nick HeinerNick Heiner 123k192 gold badges486 silver badges704 bronze badges 3
  • You may have posted the wrong fiddle version. There is no code in the fiddle you posted. – Kyeotic Commented Dec 11, 2012 at 22:50
  • Your fiddle doesn't have this code - can you update? – Roy Truelove Commented Dec 11, 2012 at 22:51
  • Yep, I definitely posted the wrong version - sorry about that. – Nick Heiner Commented Dec 11, 2012 at 23:12
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2 Answers 2

Reset to default 9

You are right, angular checks for reference by default, for perfomance reasons. The usage of reference vs equality depends on the value of the $watch function's third argument, as you can see in the docs. Just set it to true (it is false by default) and your fiddle will work - here it is, working.

the change: 

$scope.$watch('names', function() {
        $scope.timesChanged++;
    }, true);

EDIT - Performance implications:

Comparing two arrays/objects can obviously be very inefficient (hence the check by reference angular makes by default), depending on the size and plexity of your variables. There is an alternate method - watching the length of the array. This has obvious limitations - changing one element no longer triggers the $watch function - but can be immensely faster. Here is an updated fiddle!

Just a quick addition:

If you want to watch for more special things you can do it like so:

$scope.$watch(function() {
    // Define what you are watching for
    // For example convert your array into a string
    return String($scope.names);
}, function(newValue, oldValue) {
    // Do something cool.
});

This is definitely not as fast as names.length but you could play around and find a sophisticating solution.

If anyone is interested I played around a bit with fast hashing that uses a modified Duff's device: http://jsfiddle/flek/mfKT6/1/ If you are dealing with massive data this hashing method could also be updated to be error responsive (like hash only every 2nd or 3rd character)

本文标签： javascriptHow to watch for changes to a listStack Overflow

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