Ben Nadel
On User Experience (UX) Design, JavaScript, ColdFusion, Node.js, Life, and Love.
Ben Nadel at cf.Objective() 2014 (Bloomington, MN) with: Josh Siok and Megan Siok
Ben Nadel at cf.Objective() 2014 (Bloomington, MN) with: Josh Siok@siok ) and Megan Siok

Implementing ngRepeat Track-By Using A Directive In AngularJS 1.0.8

By Ben Nadel on

For me, one of the most exciting features of AngularJS 1.2 is the "track by" syntax that was added to ngRepeat. This feature allows ngRepeat collections to be updated without the costly overhead of DOM (Document Object Model) node destruction and recreation. Unfortunately, I'm still using a lot of AngularJS 1.0.8 in production. Last night, however, I was laying in bed, thinking about AngularJS, as one often does, and it occurred to me that I might be able to replicate some of the "track by" features using a custom trackBy directive in AngularJS 1.0.8.


 
 
 

 
 
 
 
 

Run this demo in my JavaScript Demos project on GitHub.

Right now, in my AngularJS 1.0.8 applications, I use my hashKeyCopier project in an attempt to decrease the amount of DOM element generation during rendering (and re-rendering). But, this is a complex process and doesn't work in situations where the previous version of a collection isn't readily available. The beauty of the "track by" syntax is that it works independently of the way in which the collections are generated.

To implement this feature using a custom directive, I need to have access to two parts of the "digest": Before ngRepeat renders the collection and after ngRepeat renders the collection. I need pre-rendering access so I can try to copy over $$hashKey values; then, I need post-rendering access so that I can extract newly-assigned $$hashKey values.

I think I have something that is working, though it wasn't heavily tested. So, please think of this as a thought-experiment more than anything else.

The way it works is that the trackBy directive is linked with a higher priority than the ngRepeat directive. This allows it to bind watcher-handlers first, which means it can inspect the given collection before ngRepeat does. Then, it uses the $evalAsync() method to hook back into the $digest phase after ngRepeat has rendered the collection. At that point, I can extract the newly assigned $$hashKeys for use in the next $digest.

  • <!doctype html>
  • <html ng-app="Demo">
  • <head>
  • <meta charset="utf-8" />
  •  
  • <title>
  • Implementing Track-By Using A Directive In AngularJS 1.0.8
  • </title>
  •  
  • <style type="text/css">
  •  
  • a[ ng-click ] {
  • cursor: pointer ;
  • text-decoration: underline ;
  • }
  •  
  • span.size {
  • color: #CCCCCC ;
  • font-style: italic ;
  • }
  •  
  • </style>
  • </head>
  • <body ng-controller="AppController">
  •  
  • <h1>
  • Implementing Track-By Using A Directive In AngularJS 1.0.8
  • </h1>
  •  
  • <h2>
  • Without Track-By
  • </h2>
  •  
  • <ul>
  • <li
  • ng-repeat="friend in friendsOne"
  • bn-log-dom-creation="Without">
  •  
  • {{ friend.id }} &mdash; {{ friend.name }}
  •  
  • <span class="size">( at size {{ friend.size }} )</span>
  •  
  • </li>
  • </ul>
  •  
  • <h2>
  • With Track-By
  • </h2>
  •  
  • <!--
  • This time, we're going to use the same data structure; however, we're going
  • to use a trackBy directive that is intended to mimic (or more like be inspired
  • by) the "track by" syntax provided in AngularJS 1.2+. This will tell AngularJS
  • how to map the objects to the DOM node.
  • -->
  • <ul>
  • <li
  • ng-repeat="friend in friendsTwo" track-by="friend.id"
  • bn-log-dom-creation="With">
  •  
  • {{ friend.id }} &mdash; {{ friend.name }}
  •  
  • <span class="size">( at size {{ friend.size }} )</span>
  •  
  • </li>
  • </ul>
  •  
  • <p>
  • <a ng-click="rebuildFriends()">Rebuild Friends</a>
  • </p>
  •  
  •  
  • <!-- Load scripts. -->
  • <script type="text/javascript" src="../../vendor/jquery/jquery-2.0.3.min.js"></script>
  • <script type="text/javascript" src="../../vendor/angularjs/angular-1.0.8.min.js"></script>
  • <script type="text/javascript">
  •  
  • // Create an application module for our demo.
  • var app = angular.module( "Demo", [] );
  •  
  •  
  • // -------------------------------------------------- //
  • // -------------------------------------------------- //
  •  
  •  
  • // I control the root of the application.
  • app.controller(
  • "AppController",
  • function( $scope ) {
  •  
  • // The size of the friend-collection will grow every time that it is
  • // rebuilt so that we can see how new items interact with the rendered
  • // collection and subsequent DOM creation.
  • var size = 3;
  •  
  • // Set up the initial collections.
  • $scope.friendsOne = getFriends();
  • $scope.friendsTwo = getFriends();
  •  
  •  
  • // ---
  • // PUBLIC METHODS.
  • // ---
  •  
  •  
  • // I rebuild the collections, creating completely new arrays and Friend
  • // object instances. In this way, we can see how AngularJS renders objects
  • // when references change.
  • $scope.rebuildFriends = function() {
  •  
  • console.warn( "Rebuilding..." );
  •  
  • size++;
  •  
  • $scope.friendsOne = getFriends();
  • $scope.friendsTwo = getFriends();
  •  
  • };
  •  
  •  
  • // ---
  • // PRIVATE METHODS.
  • // ---
  •  
  •  
  • // I create a new collection of friends.
  • function getFriends() {
  •  
  • var friends = [
  • {
  • id: 1,
  • name: "Sarah",
  • size: size
  • },
  • {
  • id: 2,
  • name: "Tricia",
  • size: size
  • },
  • {
  • id: 3,
  • name: "Joanna",
  • size: size
  • }
  • ];
  •  
  • while ( friends.length < size ) {
  •  
  • friends.push({
  • id: ( friends.length + 1 ),
  • name: "New friend",
  • size: size
  • });
  •  
  • }
  •  
  • return( friends );
  •  
  • }
  •  
  • }
  • );
  •  
  •  
  • // -------------------------------------------------- //
  • // -------------------------------------------------- //
  •  
  •  
  • // The "track by" syntax for the ngRepeat directive was introduced (stable) in
  • // version 1.2. I attempt to implement a "track by" feature in version of
  • // AngularJS pre-1.2; specifically, 1.0.8. It's a subset of the syntax, using
  • // only direct object references, not $id() syntax).
  • app.directive(
  • "trackBy",
  • function( $parse ) {
  •  
  • // Bind the JavaScript to the local scope.
  • function link( scope, element, attributes ) {
  •  
  • // Make sure there is an ngRepeat attribute to work with.
  • if ( ! attributes.ngRepeat ) {
  •  
  • throw( new Error( "ngRepeat directive required." ) );
  •  
  • }
  •  
  • // Extract the ITEM in COLLECTION parts of the ngRepeat expression.
  • // --
  • // NOTE: For this demo, I'm only supporting the most basic syntax.
  • // No method calls, and definitely no FILTERS! But, really, you
  • // shouldn't be using filters any way - they are bad for performance
  • // and even the AngularJS team has public stated that they are not
  • // meant for production.
  • var parts = attributes.ngRepeat.match( /^\s*(\w+)\s+in\s+([\w.]+)\s*$/i );
  •  
  • if ( ! parts ) {
  •  
  • throw( new Error( "trackBy only supports the most basic ITEM in COLLECTION syntax." ) );
  •  
  • }
  •  
  • // Create accessors for the item and the collection so that we can
  • // get their values without having to hard-code references.
  • var itemAccessor = $parse( parts[ 1 ] );
  • var collectionAccessor = $parse( parts[ 2 ] );
  •  
  • // Create an accessor for the track-by value so that we can extract
  • // it from each collection item context.
  • var trackByAccessor = $parse( attributes.trackBy );
  •  
  • // I map the item identifiers to the generated hashKeys.
  • var hashKeys = {};
  •  
  • // With each digest, we need to look to see if the collection has
  • // changed; and if so, track the generated hashKeys.
  • scope.$watch( trackByWatch, handleCollectionChange );
  •  
  • // When the scope is destroyed, try to clean up variable references
  • // to help with proper garbage collection of "closed over" variables.
  • scope.$on(
  • "$destroy",
  • function handleDestroyEvent() {
  •  
  • // Clear variables.
  • scope = element = attributes = parts = itemAccessor = collectionAccessor = trackByAccessor = hashKeys = null;
  •  
  • // Clear methods.
  • handleCollectionChange = trackByWatch = updateIdsAfterRenderingNgRepeat = null;
  •  
  • }
  • );
  •  
  •  
  • // ---
  • // PRIVATE METHODS.
  • // ---
  •  
  •  
  • // I handle changes in the ngRepeat collection.
  • function handleCollectionChange( newValue, oldValue ) {
  •  
  • // If the collection has changed, the change will only be in-
  • // memory so far. We need to give ngRepeat a chance to the link
  • // the new collection items to the DOM and assign $$hashKey
  • // values. As such, we'll use $evalAsync() to re-examine the
  • // collection after the ngRepeat has rendered it.
  • scope.$evalAsync( updateIdsAfterRenderingNgRepeat );
  •  
  • }
  •  
  •  
  • // I execute on every $digest and check to see if any values items
  • // have been added to the ngRepeat collection. If either the length
  • // of the collection changes, or the number of new items (ie, items
  • // we haven't seen before) changes, the watch-handler will be invoked.
  • function trackByWatch() {
  •  
  • // As we get and set values, we need to use a temporary "context"
  • // scope for the compiled accessors.
  • var context = {};
  •  
  • // Keep track of how many new items we encounter in the collection.
  • var newItemCount = 0;
  •  
  • // Get the collection out of the current scope.
  • var collection = collectionAccessor( scope );
  •  
  • for ( var i = 0, length = collection.length ; i < length ; i++ ) {
  •  
  • // If the item already has a hashKey, we've already looked at
  • // it - no need to examine this object any further.
  • if ( collection[ i ].$$hashKey ) {
  •  
  • continue;
  •  
  • }
  •  
  • // Assign the "item" to the temporary scope so we can
  • // evaluate the track-by value.
  • itemAccessor.assign( context, collection[ i ] );
  •  
  • // Extract the track-by value from the temporary scope.
  • var itemID = trackByAccessor( context );
  •  
  • // If we already have a hashKey associated with this unique
  • // identifier, inject it into the new object reference (so
  • // that AngularJS won't have to rebuild the DOM element).
  • if ( hashKeys[ itemID ] ) {
  •  
  • collection[ i ].$$hashKey = hashKeys[ itemID ];
  •  
  • // If we don't have a hashKey associated with this item
  • // identifier, then this is a new object. Track the change
  • // so that we can extract generated hashKey in the subsequent
  • // watch-handler.
  • } else {
  •  
  • newItemCount++;
  •  
  • }
  •  
  • }
  •  
  • return( collection.length + newItemCount );
  •  
  • }
  •  
  •  
  • // After the ngRepeat has rendered the collection, it has injected
  • // $$hashKey values into the rendered items. This allows it to
  • // associate each object with a DOM element so that it doesn't have
  • // to rebuild DOM elements unnecessarily. Once this has happened, we
  • // need to map the $$hashKey values to the item identifiers so that
  • // we can manually inject them into the collection if it changes.
  • function updateIdsAfterRenderingNgRepeat() {
  •  
  • // As we get and set values, we need to use a temporary "context"
  • // scope for the compiled accessors.
  • var context = {};
  •  
  • // Get the collection out of the current scope.
  • var collection = collectionAccessor( scope );
  •  
  • for ( var i = 0, length = collection.length ; i < length ; i++ ) {
  •  
  • // Assign the "item" to the temporary scope so we can
  • // evaluate the track-by value.
  • itemAccessor.assign( context, collection[ i ] );
  •  
  • // Extract the track-by value from the temporary scope and
  • // the injected $$hashKey from the item that ngRepeat has
  • // assigned (and associated with a DOM element).
  • var itemID = trackByAccessor( context );
  • var hashKey = collection[ i ].$$hashKey;
  •  
  • // Map the hashKey for the next "watch" iteration.
  • hashKeys[ itemID ] = hashKey;
  •  
  • }
  •  
  • }
  •  
  • }
  •  
  •  
  • // Return the directive configuration.
  • // --
  • // NOTE: This needs to run at a slightly higher priority than the ngRepeat
  • // directive so that it can attach $watch() bindings to the scope before
  • // the ngRepeat does.
  • return({
  • link: link,
  • priority: 1001,
  • restrict: "A"
  • });
  •  
  • }
  • );
  •  
  •  
  • // -------------------------------------------------- //
  • // -------------------------------------------------- //
  •  
  •  
  • // I simply log the creation / linking of a DOM node to illustrate the way the
  • // DOM nodes are created with the various tracking approaches.
  • app.directive(
  • "bnLogDomCreation",
  • function() {
  •  
  • // I bind the UI to the $scope.
  • function link( $scope, element, attributes ) {
  •  
  • console.log( attributes.bnLogDomCreation, $scope.$index );
  •  
  • }
  •  
  •  
  • // Return the directive configuration.
  • return({
  • link: link,
  • restrict: "A"
  • });
  •  
  • }
  • );
  •  
  • </script>
  •  
  • </body>
  • </html>

If I run this page and then start rebuilding the friends collection, we'll get the following console output:

Without 0
Without 1
Without 2
With 0
With 1
With 2
Rebuilding...
Without 0
Without 1
Without 2
Without 3
With 3
Rebuilding...
Without 0
Without 1
Without 2
Without 3
Without 4
With 4
Rebuilding...
Without 0
Without 1
Without 2
Without 3
Without 4
Without 5
With 5

What you can see here is that with each rebuilding of the Friends collection, the plain ngRepeat recreates all of the DOM nodes while the ngRepeat with the trackBy directive only builds the newest item; the already-existing items were simply updated, not recreated.

I can't wait to upgrade to AngularJS 1.2 (or 1.3 at this point). But, for now, this thought experiment has a lot of promise. I still need to integrate it in a real application and look at the performance considerations and make sure there aren't any hidden bugs that a white-page demo isn't exposing. But, if nothing else, this was a lot of fun to try to put together.




Reader Comments

@László,

I was having trouble finding the video that I was looking for. I can't remember if this is the one - I don't think it is. But, in this one, Misko Hevery does say that you should create an intermediary model to hold the filtered data:

http://youtu.be/ZhfUv0spHCY?t=49m34s

In this other post, I demonstrate how often filters fire... a lot!

http://www.bennadel.com/blog/2489-how-often-do-filters-execute-in-angularjs.htm

Hope some of that helps.

Reply to this Comment

I love your articles Ben, and I especially loved this one. I have a question though. How would you remove an item from the repeater when using track by friend.id? Do you mind writing a short example? Thanks!

Reply to this Comment

@Alex,

Thanks for the kind words! In order to remove an item from the repeater, you don't have to do anything special. If you just remove it from the "friends" collection, in this case, the core AngularJS directive ngRepeat will automatically remove it form the DOM. The "Track By" isn't actually doing any of the DOM manipulation - it's just trying to copy the $$hashKey from one digest to the next.

Reply to this Comment

TypeError: Cannot read property 'length' of undefined
at Object.trackByWatch

Hi ben i'm getting above error because

on line

var collection = collectionAccessor( scope );

Its unable to initialize collection.

Can you help ??

Reply to this Comment

@Priya,

Is it possible that you are not using an array in the ngRepeat directive? When I wrote this, I think I only ever expected the "collection" to be an array, as in:

ng-repeat="item in ARRAY"

From the error, it sounds like it the collection doesn't have a "length" property, which makes me think it's not an array. But, that is just my best guess.

Reply to this Comment

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