The compile function of angularjs' directive has two functions: preLink and postLink.
Pre-linking function
Executed before the child elements are linked. Not safe to do DOM transformation since the compiler linking function will fail to locate the correct elements for linking.
Post-linking function
Executed after the child elements are linked. It is safe to do DOM transformation in the post-linking function.
It tells what we should not do in preLink, I wonder what and when should I use preLink? For most of time I just used postLink. Is there any case that we must use it?
You need almost never use preLink. Viable cases for it are when you need to manipulate data in scope, but not the DOM, before link functions (also of other directives) are executed.
As jacob commented, you can always do that from a controller too, but sometimes it's more appropriate to have the code in the directive itself.
There is an excellent article about how directives work where linking order is explained well to: http://www.jvandemo.com/the-nitty-gritty-of-compile-and-link-functions-inside-angularjs-directives/
If you need a good example of why pre-linking is sometimes necessary, I recommend you look at the code of angular directives themselves. For example https://github.com/angular/angular.js/blob/master/src/ng/directive/ngModel.js
A preLink function is used when the directive wants to put something into a shared scope so that it's ready to be used by other directives in their postLink functions.
Angular's form directive, e.g., creates an object that contains entries for all inputs. A custom directive could safely access this object in a postLink function.
I've had to use preLink when creating custom directives which include other directives. In my case, my directive included a template which applied Angular UI Bootstrap's Typeahead directive to some of its elements and used its own scope variables to initialize Typeahead features.
For example:
...
template:
"<select ng-show='dropdown' class='form-control' ng-model='ngModel' ng-options='s for s in suggestions'></select>"
+ "<textarea ng-show='!dropdown' class='form-control' ng-model='ngModel' typeahead='s for s in suggestions |filter:$viewValue' typeahead-min-length='0' typeahead-editable='{{editable}}'></textarea>",
...
In that case, Angular links the child directives before the parent, so I needed to use preLink to setup the typeahead. When I initialized the $scope.dropdown and $scope.editable variables in the directives postLink function, I found they were not initialized when the typeahead directives were linked and I had to move their initialization into the preLink to make this directive work correctly.
Related
I have read that DOM manipulation in angular should be done in the compile function of a directive, and not in the pre-link/post-link/controller. The compile function does not have access to the scope.
My problem is that I want to do DOM manipulation that is dependent on scope variables. For example, I have a list that I am passing into the directive. Within the directive, I am creating a custom select with the list items within it. Where is the right place to manipulate the DOM in this case?
Note that I am not using ng-repeat - I have found it very slow when the list becomes large.
I don't know where you have read "DOM manipulation in angular should be done in the compile function of a directive". That contradicts the advice of the AngularJS team.
Creating a Directive that Manipulates the DOM
Directives that want to modify the DOM typically use the link option to register DOM listeners as well as update the DOM.
-- AngularJS Developer Guide - Directives - DOM Manipulation
The built-in directives, ng-repeat, ng-if, ng-when, etc. all do their DOM manipulation in the link function.
compile
The compile function deals with transforming the template DOM. Since most directives do not do template transformation, it is not used often.
-- AngularJS $compile Service API Reference -- compile
Some places seem to use the controller function for directive logic and others use link. The tabs example on the angular homepage uses controller for one and link for another directive. What is the difference between the two?
I'm going to expand your question a bit and also include the compile function.
compile function - use for template DOM manipulation (i.e., manipulation of tElement = template element), hence manipulations that apply to all DOM clones of the template associated with the directive. (If you also need a link function (or pre and post link functions), and you defined a compile function, the compile function must return the link function(s) because the 'link' attribute is ignored if the 'compile' attribute is defined.)
link function - normally use for registering listener callbacks (i.e., $watch expressions on the scope) as well as updating the DOM (i.e., manipulation of iElement = individual instance element). It is executed after the template has been cloned. E.g., inside an <li ng-repeat...>, the link function is executed after the <li> template (tElement) has been cloned (into an iElement) for that particular <li> element. A $watch allows a directive to be notified of scope property changes (a scope is associated with each instance), which allows the directive to render an updated instance value to the DOM.
controller function - must be used when another directive needs to interact with this directive. E.g., on the AngularJS home page, the pane directive needs to add itself to the scope maintained by the tabs directive, hence the tabs directive needs to define a controller method (think API) that the pane directive can access/call. For a more in-depth explanation of the tabs and pane directives, and why the tabs directive creates a function on its controller using this (rather than on $scope), please see 'this' vs $scope in AngularJS controllers.
In general, you can put methods, $watches, etc. into either the directive's controller or link function. The controller will run first, which sometimes matters (see this fiddle which logs when the ctrl and link functions run with two nested directives). As Josh mentioned in a comment, you may want to put scope-manipulation functions inside a controller just for consistency with the rest of the framework.
As complement to Mark's answer, the compile function does not have access to scope, but the link function does.
I really recommend this video; Writing Directives by Misko Hevery (the father of AngularJS), where he describes differences and some techniques. (Difference between compile function and link function at 14:41 mark in the video).
running code before Compilation : use controller
running code after Compilation : use Link
Angular convention : write business logic in controller and DOM manipulation in link.
Apart from this you can call one controller function from link function of another directive.For example you have 3 custom directives
<animal>
<panther>
<leopard></leopard>
</panther>
</animal>
and you want to access animal from inside of "leopard" directive.
http://egghead.io/lessons/angularjs-directive-communication will be helpful to know about inter-directive communication
compile function -
is called before the controller and link function.
In compile function, you have the original template DOM so you can make changes on original DOM before AngularJS creates an instance of it and before a scope is created
ng-repeat is perfect example - original syntax is template element, the repeated elements in HTML are instances
There can be multiple element instances and only one template element
Scope is not available yet
Compile function can return function and object
returning a (post-link) function - is equivalent to registering the linking function via the link property of the config object when the compile function is empty.
returning an object with function(s) registered via pre and post properties - allows you to control when a linking function should be called during the linking phase. See info about pre-linking and post-linking functions below.
syntax
function compile(tElement, tAttrs, transclude) { ... }
controller
called after the compile function
scope is available here
can be accessed by other directives (see require attribute)
pre - link
The link function is responsible for registering DOM listeners as well as updating the DOM. It is executed after the template has been cloned. This is where most of the directive logic will be put.
You can update the dom in the controller using angular.element but this is not recommended as the element is provided in the link function
Pre-link function is used to implement logic that runs when angular js has already compiled the child elements but before any of the child element's post link have been called
post-link
directive that only has link function, angular treats the function as a post link
post will be executed after compile, controller and pre-link funciton, so that's why this is considered the safest and default place to add your directive logic
Some places seem to use the controller function for directive logic and others use link. The tabs example on the angular homepage uses controller for one and link for another directive. What is the difference between the two?
I'm going to expand your question a bit and also include the compile function.
compile function - use for template DOM manipulation (i.e., manipulation of tElement = template element), hence manipulations that apply to all DOM clones of the template associated with the directive. (If you also need a link function (or pre and post link functions), and you defined a compile function, the compile function must return the link function(s) because the 'link' attribute is ignored if the 'compile' attribute is defined.)
link function - normally use for registering listener callbacks (i.e., $watch expressions on the scope) as well as updating the DOM (i.e., manipulation of iElement = individual instance element). It is executed after the template has been cloned. E.g., inside an <li ng-repeat...>, the link function is executed after the <li> template (tElement) has been cloned (into an iElement) for that particular <li> element. A $watch allows a directive to be notified of scope property changes (a scope is associated with each instance), which allows the directive to render an updated instance value to the DOM.
controller function - must be used when another directive needs to interact with this directive. E.g., on the AngularJS home page, the pane directive needs to add itself to the scope maintained by the tabs directive, hence the tabs directive needs to define a controller method (think API) that the pane directive can access/call. For a more in-depth explanation of the tabs and pane directives, and why the tabs directive creates a function on its controller using this (rather than on $scope), please see 'this' vs $scope in AngularJS controllers.
In general, you can put methods, $watches, etc. into either the directive's controller or link function. The controller will run first, which sometimes matters (see this fiddle which logs when the ctrl and link functions run with two nested directives). As Josh mentioned in a comment, you may want to put scope-manipulation functions inside a controller just for consistency with the rest of the framework.
As complement to Mark's answer, the compile function does not have access to scope, but the link function does.
I really recommend this video; Writing Directives by Misko Hevery (the father of AngularJS), where he describes differences and some techniques. (Difference between compile function and link function at 14:41 mark in the video).
running code before Compilation : use controller
running code after Compilation : use Link
Angular convention : write business logic in controller and DOM manipulation in link.
Apart from this you can call one controller function from link function of another directive.For example you have 3 custom directives
<animal>
<panther>
<leopard></leopard>
</panther>
</animal>
and you want to access animal from inside of "leopard" directive.
http://egghead.io/lessons/angularjs-directive-communication will be helpful to know about inter-directive communication
compile function -
is called before the controller and link function.
In compile function, you have the original template DOM so you can make changes on original DOM before AngularJS creates an instance of it and before a scope is created
ng-repeat is perfect example - original syntax is template element, the repeated elements in HTML are instances
There can be multiple element instances and only one template element
Scope is not available yet
Compile function can return function and object
returning a (post-link) function - is equivalent to registering the linking function via the link property of the config object when the compile function is empty.
returning an object with function(s) registered via pre and post properties - allows you to control when a linking function should be called during the linking phase. See info about pre-linking and post-linking functions below.
syntax
function compile(tElement, tAttrs, transclude) { ... }
controller
called after the compile function
scope is available here
can be accessed by other directives (see require attribute)
pre - link
The link function is responsible for registering DOM listeners as well as updating the DOM. It is executed after the template has been cloned. This is where most of the directive logic will be put.
You can update the dom in the controller using angular.element but this is not recommended as the element is provided in the link function
Pre-link function is used to implement logic that runs when angular js has already compiled the child elements but before any of the child element's post link have been called
post-link
directive that only has link function, angular treats the function as a post link
post will be executed after compile, controller and pre-link funciton, so that's why this is considered the safest and default place to add your directive logic
As outlined here:
http://docs.angularjs.org/guide/directive
Angular js directives take two different types of link functions:
Pre-linking function
Executed before the child elements are linked. Not safe to do DOM transformation since the compiler linking function will fail to locate the correct elements for linking.
Post-linking function
Executed after the child elements are linked. It is safe to do DOM transformation in the post-linking function.
Additionally, it appears that the default key of link will bind to postLink if given an anonymous function.
When and why would I ever want to use a pre link function?
The only time you'd want to use a pre link is when you need to perform some preparation on the scope before any child elements compile.
My team has used it when writing a grid directive to define the grid object on the scope and setup some of its properties that are needed before any of the child row and cell objects are compiled.
Hope that helps!
I'm fairly new to Javascript programming and I have only touched upon AngularJS. In order to evaluate it I decided to write a simple note application. The model is really simple, a list of notes where each note has a label, a text and a list of tags. However I ran into problem passing data between isolated scopes of nested directives.
I have three directives, notes, note and tagger (defining new elements with the same names). Each of them using an isolated scope.
The notes directive uses ng-repeat to "render" each one of its notes with the note element.
The note directive uses the tagger element to "render" the list of tags.
The note directive defines scope: { getNote: "&", ... } in order to pass a note instance from the list of notes to the note controller/directive. The getNote(index) function is called in the link function of the note directive. This works fine!
The tagger directive defines scope: { getTags: "&", ... } in order to pass a list of tags for a given note to the tagger controller/directive. The getTags function is called in the link function of the tagger directive. This does not work!
As I understand it, the problem is that the link functions of the directives are called in an inconsistent order. Debugging the application shows that the link functions are called in the following order:
link function in the notes directive
(adding the getNote function to the notes scope)
link function in the tagger directive of the first note
(calling getTags in the parent note scope) function
link function in the first note directive
(adding the getTags to the scope)
(calling getNote in the parent notes scope)
link function in the tagger directive of the second note
(calling getTags in the parent note scope) function
link function in the second note directive
(adding the getTags to the scope)
(calling getNote in the parent notes scope)
This will not work since in #2 the scope of the first note has not yet an getTags function.
A simplistic example can be found in Plunker.
Hence, my question boils down to: What determines the order in which link functions are called in nested directives.
(I solved to the problem using $watch on getTags in the tagger directive...)
regards
Quoting Josh D. Miller who had kindly responded to a similar question I had :
" Just a couple of technical notes. Assume that you have this markup:
<div directive1>
<div directive2>
<!-- ... -->
</div>
</div>
Now AngularJS will create the directives by running directive functions in a certain order:
directive1: compile
directive2: compile
directive1: controller
directive1: pre-link
directive2: controller
directive2: pre-link
directive2: post-link
directive1: post-link
By default a straight "link" function is a post-link, so your outer directive1's link function will not run until after the inner directive2's link function has ran. That's why we say that it's only safe to do DOM manipulation in the post-link. "
On a single element the order of the linking functions is determined by the order of the Compile Functions which in turn is ordered according to the priority property of the directive definition object.
Source : http://docs.angularjs.org/guide/directive
On multiple elements with transclusion : the inner directives are evaluated before outer directives. Reason: Nature of transclusion.
On multiple elements with siblings : Executed in order top to bottom. Reason: Parsing logic of $compile.