Question
I don't understand how to use $scope.$watch
and $scope.$apply
. The
official documentation isn't helpful.
What I don't understand specifically:
- Are they connected to the DOM?
- How can I update DOM changes to the model?
- What is the connection point between them?
I tried [this tutorial](http://css.dzone.com/articles/drag-and-drop-angularjs-
using), but it takes the understanding of $watch
and $apply
for granted.
What do $apply
and $watch
do, and how do I use them appropriately?
Answer
You need to be aware about how AngularJS works in order to understand it.
Digest cycle and $scope
First and foremost, AngularJS defines a concept of a so-called digest
cycle. This cycle can be considered as a loop, during which AngularJS checks
if there are any changes to all the variables watched by all the
$scope
s. So if you have $scope.myVar
defined in your controller and this
variable was marked for being watched , then you are implicitly telling
AngularJS to monitor the changes on myVar
in each iteration of the loop.
A natural follow-up question would be: Is everything attached to $scope
being watched? Fortunately, no. If you would watch for changes to every object
in your $scope
, then quickly a digest loop would take ages to evaluate and
you would quickly run into performance issues. That is why the AngularJS team
gave us two ways of declaring some $scope
variable as being watched (read
below).
$watch helps to listen for $scope changes
There are two ways of declaring a $scope
variable as being watched.
- By using it in your template via the expression
<span>{{myVar}}</span>
- By adding it manually via the
$watch
service
Ad 1) This is the most common scenario and I'm sure you've seen it before, but
you didn't know that this has created a watch in the background. Yes, it had!
Using AngularJS directives (such as ng-repeat
) can also create implicit
watches.
Ad 2) This is how you create your own watches. $watch
service helps you
to run some code when some value attached to the $scope
has changed. It is
rarely used, but sometimes is helpful. For instance, if you want to run some
code each time 'myVar' changes, you could do the following:
function MyController($scope) {
$scope.myVar = 1;
$scope.$watch('myVar', function() {
alert('hey, myVar has changed!');
});
$scope.buttonClicked = function() {
$scope.myVar = 2; // This will trigger $watch expression to kick in
};
}
$apply enables to integrate changes with the digest cycle
You can think of the $apply
function as of an integration mechanism. You
see, each time you change some watched variable attached to the$scope
object directly, AngularJS will know that the change has happened. This is
because AngularJS already knew to monitor those changes. So if it happens in
code managed by the framework, the digest cycle will carry on.
However, sometimes you want to change some value outside of the AngularJS
world and see the changes propagate normally. Consider this - you have a
$scope.myVar
value which will be modified within a jQuery's $.ajax()
handler. This will happen at some point in future. AngularJS can't wait for
this to happen, since it hasn't been instructed to wait on jQuery.
To tackle this, $apply
has been introduced. It lets you start the digestion
cycle explicitly. However, you should only use this to migrate some data to
AngularJS (integration with other frameworks), but never use this method
combined with regular AngularJS code, as AngularJS will throw an error then.
How is all of this related to the DOM?
Well, you should really follow the tutorial again, now that you know all this.
The digest cycle will make sure that the UI and the JavaScript code stay
synchronised, by evaluating every watcher attached to all $scope
s as long as
nothing changes. If no more changes happen in the digest loop, then it's
considered to be finished.
You can attach objects to the $scope
object either explicitly in the
Controller, or by declaring them in {{expression}}
form directly in the
view.
Further readings: