angular-once

One time bindings for AngularJS.

Why?

AngularJS is a great framework with many superb features, but when is used to display large amount of data, it can become quite slow due to it's binding mechanism.

When you bind data via {{ }} or ng-bind directives, angular sets up watch under the cover which is being executed every time angular event loop triggers (for example after $http request, or keypress). It's fine when number of bindings (pieces of dynamic information you want to display) is relatively small up to few hundreds, but when that number increases page can become unresponsive expecially on low-end devices (it was Surface RT in my case, and reason to create that project, it was painfully slow even for 200 bindings).

There isn't much you can do about it, except displaying less data (via paging for example), but it's not always the case. Should we wait till browsers implement Object.observe and AngularJS will used it so we get 40x speed-up? We could, but when your data is readonly (and in many cases is) you can use this project which sets up one time bindings and doesn't create any watchers so is incredibly fast!

bottom line: If you use AngularJS, have performance issues and need to display lots of readonly data, this project is for you!

Installation

$ bower install angular-once

or copy once.js file.

Usage

Prerequisites

• reference once.js file
• add once module as dependency to your app (angular.module('yourApp', ['once']))

Lets look at this standard AngularJS code snippet:

<ul>
	<li ng-repeat="user in users">
	  <a ng-href="{{ user.profileUrl }}">{{ user.name }}</a>
		<a ng-href="{{ user.profileUrl }}"><img ng-src="{{ user.avatarUrl }}"></a>
		<div ng-class="{'formatted': user.description}" ng-bind="user.description"></div>
	</li>
</ul>

Now given 100 users it's 600 watchers and list is not only information on page in most cases. If users data needs to be only displayed not edited inline, we don't need to set up watchers in ng-repeat directive, especially as user goes back and forward within the app many times list is being refreshed on each display as controller in angular are transient.

Let's look at the same example using angular-once:

<ul>
	<li ng-repeat="user in users">
	  <a once-href="user.profileUrl" once-text="user.name"></a>
		<a once-href="user.profileUrl"><img once-src="user.avatarUrl"></a>
		<div once-class="{'formatted': user.description}" once-bind="user.description"></div>
	</li>
</ul>

Number of watchers? 0 (actually 1 as angular uses separate watch for ng-repeat directive itself).

IMPORTANT: Build in angular ng-href and ng-src directives support interpolation ({{ }} notation), angular-once doesn't due to performance reasons (avoid setting up watchers).

API

List below contains comparison of angular-once directives ( one time bindings ) with build in angular directives ( two-way bindings ).

angular-once	native angular equivalent	example usage
once-text="value"	ng-bind or {{ }}	<ANY once-text="user.name"></ANY>
once-html="value"	ng-bind-html	<ANY once-html="user.description"></ANY>
once-src="value"	ng-src	<img once-src="user.avatarUrl" />
once-href="value"	ng-href	<a once-href="user.websiteUrl" ></a>
once-title="value"	ng-attr-title	<ANY once-title="user.name"></ANY>
once-alt="value"	ng-attr-alt	<img once-alt="user.name" />
once-id="value"	ng-attr-id	<ANY once-id="user.name" /></ANY>
once-if="condition"	ng-if	<ANY once-if="user.isAdmin" /></ANY>
once-class="name:cond"	ng-class	<ANY once-class="{'p': user.admin}" /></ANY>
once-style="value"	ng-style	<ANY once-style="{color:blue}" /></ANY>
once-show="condition"	ng-show	<ANY once-show="user.isAdmin" /></ANY>
once-hide="condition"	ng-hide	<ANY once-hide="user.isAdmin" /></ANY>
once once-attr-*="value"	ng-attr-*	<ANY once once-attr-tooltip="user.name" /></ANY>

Important:

One important thing to note is that when using angular-once and bound data is undefined it creates watch which waits till data is available (promise is resolved) and then remove that watch itself. Reason for that is to be able to use it with data which is not yet available, but still readonly.

In case bound data contains static and dynamic part, for example once-src="'http://placekitten.com/'+ kitty.size" and kitty-size isn't immediately available, you can use once-wait-for directive to wait till kitten.size is fetched, so it will look like: once-src="'http://placekitten.com/'+ kitty.size" once-wait-for='kitty.size'

Credits

Thanks both to @Pasvaz for bindonce and @abourget for $watch fighters, as both modules were inspiration and starting point for creating this module.

Similar projects

• bindonce - similar, but requires addtional bindonce directive which wraps other dindonce directives.
• watch fighters - similar, but doesn't handle case when there is no data yet to bind, simply binds nothing, so doesn't work with promises.

License

WTFPL