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Part 8::What is $Watch and $digest() in angularJs?

The AngularJS $scope capacities $watch(), $digest() and $apply() are a portion of the focal capacities in AngularJS. Understanding $watch(), $digest() and $apply() is key with a specific end goal to comprehend AngularJS. 

When you make an information tying from some place in your perspective to a variable on the $scope object, AngularJS makes a "watch" inside. A watch implies that AngularJS watches changes in the variable on the $scope object. The system is "viewing" the variable. Watches are made utilizing the $scope.$watch() capacity which I will cover later in this content. 

At key focuses in your application AngularJS calls the $scope.$digest() capacity. This capacity emphasizes through all watches and checks if any of the watched variables have changed. On the off chance that a watched variable has changed, a relating audience capacity is called. The audience capacity does whatever work it needs to do, for occurrence changing a HTML content to mirror the new estimation of the watched variable. Accordingly, the $digest() capacity is the thing that triggers the information tying to upgrade. 

More often than not AngularJS will call the $scope.$watch() and $scope.$digest() capacities for you, however in a few circumstances you might need to call them yourself. Accordingly it is better than average to know how they function. 

The $scope.$apply() capacity is utilized to execute some code, and afterward call $scope.$digest() after that, so all watches are checked and the relating watch audience capacities are called. The $apply() capacity is valuable when incorporating AngularJS with other code. 

I will dive into more insight about the $watch(), $digest() and $apply() works in the rest of this content.

$watch()

The $scope.watch() capacity makes a watch off some variable. When you enlist a watch you pass two capacities as parameters to the $watch() capacity: 

A worth capacity 

An audience capacity 

Here is an illustration:-
$scope.$watch(function() {},
capacity() {}
);

$digest()

The $scope.$digest() capacity emphasizes through every one of the watches in the $scope article, and its tyke $scope objects (in the event that it has any). Whenever $digest() emphasizes over the watches, it calls the worth capacity for every watch. In the event that the worth returned by the quality capacity is not quite the same as the quality it gave back the last time it was called, the audience capacity for that watch is called. 

The $digest() capacity is called at whatever point AngularJS supposes it is fundamental. Case in point, after a catch click handler has been executed, or after an AJAX call returns (after the done()/come up short() callback capacity has been executed). 

You might experience some corner situations where AngularJS does not call the $digest() capacity for you. You will for the most part recognize that by seeing that the information ties don't upate the showed values. All things considered, call $scope.$digest() and it ought to work. Alternately, you can maybe utilize $scope.$apply() rather which I will clarify in the following area.
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Author: Bikesh Srivastava - Saturday, 30 January 2016

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