Lately, I've been incorporating $scope.$apply() into my Angular applications to refresh the bindings for my models when new data is received via websockets. It seems to be effective, but I'm curious about its actual functionality and why it's necessary to invoke it in order for the updates to take effect.
When you invoke $apply, the provided code will run in the angular-context where you can leverage all that Angular has to offer.
As stated on this page:
Angular alters the usual JavaScript flow by introducing its own event processing loop. This separates the JavaScript into classical and Angular execution contexts. Only operations carried out in the Angular execution context will benefit from Angular's features like data-binding, exception handling, property watching, and more...
You can also utilize $apply() to transition into the Angular execution context from JavaScript. It's important to note that in most scenarios (such as controllers or services), $apply is automatically called for you by the directive handling the event. An explicit $apply call is only necessary when implementing custom event callbacks or dealing with callbacks from third-party libraries.
Quoted directly from the official Angular documentation:
The $apply() function serves to execute an expression within the angular framework but triggered from outside of it. This could be from various sources such as browser DOM events, setTimeout, XHR requests, or external libraries. As we are interfacing with the Angular framework externally, it is necessary to manage the scope life-cycle, handle exceptions, and run watches appropriately.
Furthermore, a pseudo-code representation of $apply is provided:
function $apply(expr) {
try {
return $eval(expr);
} catch (e) {
$exceptionHandler(e);
} finally {
$root.$digest();
}
}
In essence, the $apply method evaluates an expression and initiates a digest cycle, prompting Angular to execute all watch listeners and update any bound views accordingly.
To clarify, although you mentioned using $apply for updating model bindings, this is primarily essential when updates originate externally to Angular. Typically, manual invocation is not required in most scenarios.
To put it simply:
If you're interested, I've also written a blog post discussing the functions of $apply, $digest, and $watch and how they work together here.
I hope this information is helpful to you.
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