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, but it takes the understanding of $watch and $apply for granted.
What do $apply and $watch do, and how do I use them appropriately?
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 $scopes. 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 $scopes 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:
Make Your Own AngularJS, Part 1: Scopes And Digest
In AngularJS, we update our models, and our views/templates update the DOM "automatically" (via built-in or custom directives).
$apply and $watch, both being Scope methods, are not related to the DOM.
The Concepts page (section "Runtime") has a pretty good explanation of the $digest loop, $apply, the $evalAsync queue and the $watch list. Here's the picture that accompanies the text:
Whatever code has access to a scope – normally controllers and directives (their link functions and/or their controllers) – can set up a "watchExpression" that AngularJS will evaluate against that scope. This evaluation happens whenever AngularJS enters its $digest loop (in particular, the "$watch list" loop). You can watch individual scope properties, you can define a function to watch two properties together, you can watch the length of an array, etc.
When things happen "inside AngularJS" – e.g., you type into a textbox that has AngularJS two-way databinding enabled (i.e., uses ng-model), an $http callback fires, etc. – $apply has already been called, so we're inside the "AngularJS" rectangle in the figure above. All watchExpressions will be evaluated (possibly more than once – until no further changes are detected).
When things happen "outside AngularJS" – e.g., you used bind() in a directive and then that event fires, resulting in your callback being called, or some jQuery registered callback fires – we're still in the "Native" rectangle. If the callback code modifies anything that any $watch is watching, call $apply to get into the AngularJS rectangle, causing the $digest loop to run, and hence AngularJS will notice the change and do its magic.
AngularJS extends this events-loop, creating something called AngularJS context.
$watch()
Every time you bind something in the UI you insert a $watch in a $watch list.
User: <input type="text" ng-model="user" />
Password: <input type="password" ng-model="pass" />
Here we have $scope.user, which is bound to the first input, and we have $scope.pass, which is bound to the second one. Doing this we add two $watches to the $watch list.
When our template is loaded, AKA in the linking phase, the compiler will look for every directive and creates all the $watches that are needed.
AngularJS provides $watch, $watchcollection and $watch(true). Below is a neat diagram explaining all the three taken from watchers in depth.
angular.module('MY_APP', []).controller('MyCtrl', MyCtrl)
function MyCtrl($scope,$timeout) {
$scope.users = [{"name": "vinoth"},{"name":"yusuf"},{"name":"rajini"}];
$scope.$watch("users", function() {
console.log("**** reference checkers $watch ****")
});
$scope.$watchCollection("users", function() {
console.log("**** Collection checkers $watchCollection ****")
});
$scope.$watch("users", function() {
console.log("**** equality checkers with $watch(true) ****")
}, true);
$timeout(function(){
console.log("Triggers All ")
$scope.users = [];
$scope.$digest();
console.log("Triggers $watchCollection and $watch(true)")
$scope.users.push({ name: 'Thalaivar'});
$scope.$digest();
console.log("Triggers $watch(true)")
$scope.users[0].name = 'Superstar';
$scope.$digest();
});
}
http://jsfiddle.net/2Lyn0Lkb/
$digest loop
When the browser receives an event that can be managed by the AngularJS context the $digest loop will be fired. This loop is made from two smaller loops. One processes the $evalAsync queue, and the other one processes the $watch list. The $digest will loop through the list of $watch that we have
app.controller('MainCtrl', function() {
$scope.name = "vinoth";
$scope.changeFoo = function() {
$scope.name = "Thalaivar";
}
});
{{ name }}
<button ng-click="changeFoo()">Change the name</button>
Here we have only one $watch because ng-click doesn’t create any watches.
We press the button.
The browser receives an event which will enter the AngularJS context
The $digest loop will run and will ask every $watch for changes.
Since the $watch which was watching for changes in $scope.name
reports a change, it will force another $digest loop.
The new loop reports nothing.
The browser gets the control back and it will update the DOM
reflecting the new value of $scope.name
The important thing here is that EVERY event that enters the AngularJS context will run a $digest loop. That means that every time we write a letter in an input, the loop will run checking every $watch in this page.
$apply()
If you call $apply when an event is fired, it will go through the angular-context, but if you don’t call it, it will run outside it. It is as easy as that. $apply will call the $digest() loop internally and it will iterate over all the watches to ensure the DOM is updated with the newly updated value.
The $apply() method will trigger watchers on the entire $scope chain whereas the $digest() method will only trigger watchers on the current $scope and its children. When none of the higher-up $scope objects need to know about the local changes, you can use $digest().
I found very in-depth videos which cover $watch, $apply, $digest and digest cycles in:
AngularJS - Understanding Watcher, $watch, $watchGroup, $watchCollection, ng-change
AngularJS - Understanding digest cycle (digest phase or digest process or digest loop)
AngularJS Tutorial - Understanding $apply and $digest (in depth)
Following are a couple of slides used in those videos to explain the concepts (just in case, if the above links are removed/not working).
In the above image, "$scope.c" is not being watched as it is not used in any of the data bindings (in markup). The other two ($scope.a and $scope.b) will be watched.
From the above image: Based on the respective browser event, AngularJS captures the event, performs digest cycle (goes through all the watches for changes), execute watch functions and update the DOM. If not browser events, the digest cycle can be manually triggered using $apply or $digest.
More about $apply and $digest:
There are $watchGroup and $watchCollection as well. Specifically, $watchGroup is really helpful if you want to call a function to update an object which has multiple properties in a view that is not dom object, for e.g. another view in canvas, WebGL or server request.
Here, the documentation link.
Just finish reading ALL the above, boring and sleepy (sorry but is true). Very technical, in-depth, detailed, and dry.
Why am I writing? Because AngularJS is massive, lots of inter-connected concepts can turn anyone going nuts. I often asked myself, am I not smart enough to understand them? No! It's because so few can explain the tech in a for-dummie language w/o all the terminologies!
Okay, let me try:
1) They are all event-driven things. (I hear the laugh, but read on)
If you don't know what event-driven is Then
think you place a button
on the page, hook it up w/ a function using "on-click", waiting for
users to click on it to trigger the actions you plant inside the
function. Or think of "trigger" of SQL Server / Oracle.
2) $watch is "on-click".
What's special about is it takes 2 functions as parameters, first one
gives the value from the event, second one takes the value into
consideration...
3) $digest is the boss who checks around tirelessly,
bla-bla-bla but a good boss.
4) $apply gives you the way when you want to do it manually, like a fail-proof (in case on-click doesn't kick in, you force it to run.)
Now, let's make it visual. Picture this to make it even more easy to
grab the idea:
In a restaurant,
- WAITERS
are supposed to take orders from customers, this is
$watch(
function(){return orders;},
function(){Kitchen make it;}
);
- MANAGER running around to make sure all waiters are awake, responsive to any sign of changes from customers. This is $digest()
- OWNER has the ultimate power to drive everyone upon request, this is $apply()
I have defined a function on scope. And when I call it from the view inside {{}} it executes X number of times.
Controller
function testCtrl($scope) {
$scope.myFunc = function(name) {
return "Hello " + name;
}
}
HTML
<div>{{myFunc('Joe')}}</div>
You can see it in this example: http://jsfiddle.net/rbRvD/2/
Or with Plunker: http://plnkr.co/edit/LLQ7cKs2fEoBwv0C5XPE
I guess this is done the wrong way, but why is it executed so many times?
Your function is run 10 times. Why 10? Why not 100?
The answer is in the docs:
The watch listener may change the model, which may trigger other
listeners to fire. This is achieved by rerunning the watchers until no
changes are detected. The rerun iteration limit is 10 to prevent an
infinite loop deadlock.
When you see this happening, it means you are changing the model in such a way that Angular has to rerun the digest and fire the watches again. In your particular case you are calling a function that updates a counter, which is displayed on the page. When the counter value changes it runs the digest again, which calls the function that updates the counter, etc, etc.
Angular expects you (and indeed encourages you) to change the model and let the view respond to those changes, rather than the other way around.
This is expected behaviour. Angular expressions ({{expression}}) are re-evaluated on each $digest loop (sometimes multiple times per loop). This means that expressions should stay light in computational terms.
For that matter, expression evaluation should not result in an AJAX call or some other intensive or asynchronous operation, or if it has to, than you should be caching the results.
AngularJs does not suggest you to change scope's model on rendering. If you want to change your scope's model, do it in Controller or Directive.
Think of view as a place to display data (scope values in this case) only, all the modifying of the data should be in Controller or Directive.
I just want to know how to use $digest. Inside a controller the following code works fine and it updates the DOM after 3 seconds:
setTimeout(function(){
$scope.$apply(function(){
$scope.name = 'Alice';
});
}, 3000);
However by using
setTimeout(function(){
$scope.$digest(function(){
$scope.name = 'Alice';
});
}, 3000);
nothing happens...
I thought that they do the same thing. Am I wrong?
$apply() and $digest() have some similarities and differences. They are similar in that they both check what's changed and update the UI and fire any watchers.
One difference between the two is how they are called. $digest() gets called without any arguments. $apply() takes a function that it will execute before doing any updates.
The other difference is what they affect. $digest() will update the current scope and any child scopes. $apply() will update every scope. So most of the time $digest() will be what you want and more efficient.
The final difference which explains why $apply() takes a function is how they handle exceptions in watchers. $apply() will pass the exceptions to $exceptionHandler (uses try-catch block internally), while $digest() will require you handle the exceptions yourself.
I think you must go through documents $apply
$apply() is used to execute an expression in angular from outside of
the angular framework
Usually, you don't call $digest() directly in controllers or in
directives. Instead, you should call $apply() (typically from within a
directive), which will force a $digest().
Also as suggest by Jorg, use $timeout
In angularjs $scope object is having different functions like $watch(), $digest() and $apply() and we will call these functions as central functions.
The angularjs central functions $watch(), $digest() and $apply are used to bind data to variables in view and observe changes happening in variables.
Generally in angularjs we use $scope object to bind data to variables and use that variables values wherever we required in application. In angularjs whatever the variables we assigned with $scope object will be added to watch list by using $scope.$watch() function.
In angularjs once variables added to watch list $scope.digest() function will iterates through the watch list variables and check if any changes made for that variables or not. In case if any changes found for that watch list variables immediately corresponding event listener function will call and update respective variable values with new value in view of application.
The $scope.$apply() function in angularjs is used whenever we want to integrate any other code with angularjs. Generally the $scope.$apply() function will execute custom code and then it will call $scope.$digest() function forcefully to check all watch list variables and update variable values in view if any changes found for watch list variables.
In most of the time angularjs will use $scope.watch() and $scope.digest() functions to check and update values based on the changes in variable values.
A better understanding of $watch, $apply, and $digest is provided in this article.
Simply,
$watch that is used to detect changes in the UI
$apply would tell the $digest loop what changes should be applied
$digest loop would run and will ask every $watch for changes, the DOMs would change accordingly for what has what has been applied
You should call $apply only.
what is difference between in angularjs
$apply(), $digest() and $watch() ?
Please use below tool to explain if it is possible
http://plnkr.co/edit/dgCQ4tC6ZsrxLgxs8KO9?p=preview
Basically these three are different.
When You want to watch a variable. It means if you want, On change of a variable some code should be executed.then you will use watch.
Syntax $watch('variableName',function());
For example, On your page you have a variable which have some value and on change on that value from server or any other source you want to update it then you can apply watch on it. In your function you can write the code which will update the value.
$digest :- Angular internally uses this method. $digest will iterate all the watches and if its find the values of any variable is changed then it will execute the respective function. we can also call this in our code.
$apply :- when we want some code is executed and after that $apply should be executed then we can call $apply.This will execute the code n internally when $apply will be excute then it will be call $digest.
I have defined a function on scope. And when I call it from the view inside {{}} it executes X number of times.
Controller
function testCtrl($scope) {
$scope.myFunc = function(name) {
return "Hello " + name;
}
}
HTML
<div>{{myFunc('Joe')}}</div>
You can see it in this example: http://jsfiddle.net/rbRvD/2/
Or with Plunker: http://plnkr.co/edit/LLQ7cKs2fEoBwv0C5XPE
I guess this is done the wrong way, but why is it executed so many times?
Your function is run 10 times. Why 10? Why not 100?
The answer is in the docs:
The watch listener may change the model, which may trigger other
listeners to fire. This is achieved by rerunning the watchers until no
changes are detected. The rerun iteration limit is 10 to prevent an
infinite loop deadlock.
When you see this happening, it means you are changing the model in such a way that Angular has to rerun the digest and fire the watches again. In your particular case you are calling a function that updates a counter, which is displayed on the page. When the counter value changes it runs the digest again, which calls the function that updates the counter, etc, etc.
Angular expects you (and indeed encourages you) to change the model and let the view respond to those changes, rather than the other way around.
This is expected behaviour. Angular expressions ({{expression}}) are re-evaluated on each $digest loop (sometimes multiple times per loop). This means that expressions should stay light in computational terms.
For that matter, expression evaluation should not result in an AJAX call or some other intensive or asynchronous operation, or if it has to, than you should be caching the results.
AngularJs does not suggest you to change scope's model on rendering. If you want to change your scope's model, do it in Controller or Directive.
Think of view as a place to display data (scope values in this case) only, all the modifying of the data should be in Controller or Directive.