AngularJS remembers the value and compares it to a previous value. This is basic dirty-checking. If there is a change in value, then it fires the change event.
The $apply()
method, which is what you call when you are transitioning from a non-AngularJS world into an AngularJS world, calls $digest()
. A digest is just plain old dirty-checking. It works on all browsers and is totally predictable.
To contrast dirty-checking (AngularJS) vs change listeners (KnockoutJS and Backbone.js): While dirty-checking may seem simple, and even inefficient (I will address that later), it turns out that it is semantically correct all the time, while change listeners have lots of weird corner cases and need things like dependency tracking to make it more semantically correct. KnockoutJS dependency tracking is a clever feature for a problem which AngularJS does not have.
Issues with change listeners:
- The syntax is atrocious, since browsers do not support it natively. Yes, there are proxies, but they are not semantically correct in all cases, and of course there are no proxies on old browsers. The bottom line is that dirty-checking allows you to do POJO, whereas KnockoutJS and Backbone.js force you to inherit from their classes, and access your data through accessors.
- Change coalescence. Suppose you have an array of items. Say you want to add items into an array, as you are looping to add, each time you add you are firing events on change, which is rendering the UI. This is very bad for performance. What you want is to update the UI only once, at the end. The change events are too fine-grained.
- Change listeners fire immediately on a setter, which is a problem, since the change listener can further change data, which fires more change events. This is bad since on your stack you may have several change events happening at once. Suppose you have two arrays which need to be kept in sync for whatever reason. You can only add to one or the other, but each time you add you fire a change event, which now has an inconsistent view of the world. This is a very similar problem to thread locking, which JavaScript avoids since each callback executes exclusively and to completion. Change events break this since setters can have far-reaching consequences which are not intended and non obvious, which creates the thread problem all over again. It turns out that what you want to do is to delay the listener execution, and guarantee, that only one listener runs at a time, hence any code is free to change data, and it knows that no other code runs while it is doing so.
What about performance?
So it may seem that we are slow, since dirty-checking is inefficient. This is where we need to look at real numbers rather than just have theoretical arguments, but first let's define some constraints.
Humans are:
Slow — Anything faster than 50 ms is imperceptible to humans and thus can be considered as "instant".
Limited — You can't really show more than about 2000 pieces of information to a human on a single page. Anything more than that is really bad UI, and humans can't process this anyway.
So the real question is this: How many comparisons can you do on a browser in 50 ms? This is a hard question to answer as many factors come into play, but here is a test case: http://jsperf.com/angularjs-digest/6 which creates 10,000 watchers. On a modern browser this takes just under 6 ms. On Internet Explorer 8 it takes about 40 ms. As you can see, this is not an issue even on slow browsers these days. There is a caveat: The comparisons need to be simple to fit into the time limit... Unfortunately it is way too easy to add a slow comparison into AngularJS, so it is easy to build slow applications when you don't know what you are doing. But we hope to have an answer by providing an instrumentation module, which would show you which are the slow comparisons.
It turns out that video games and GPUs use the dirty-checking approach, specifically because it is consistent. As long as they get over the monitor refresh rate (typically 50-60 Hz, or every 16.6-20 ms), any performance over that is a waste, so you're better off drawing more stuff, than getting FPS higher.
Use PushState and Precomposition
The current (2015) way to do this is using the JavaScript pushState method.
PushState changes the URL in the top browser bar without reloading the page. Say you have a page containing tabs. The tabs hide and show content, and the content is inserted dynamically, either using AJAX or by simply setting display:none and display:block to hide and show the correct tab content.
When the tabs are clicked, use pushState to update the url in the address bar. When the page is rendered, use the value in the address bar to determine which tab to show. Angular routing will do this for you automatically.
Precomposition
There are two ways to hit a PushState Single Page App (SPA)
- Via PushState, where the user clicks a PushState link and the content is AJAXed in.
- By hitting the URL directly.
The initial hit on the site will involve hitting the URL directly. Subsequent hits will simply AJAX in content as the PushState updates the URL.
Crawlers harvest links from a page then add them to a queue for later processing. This means that for a crawler, every hit on the server is a direct hit, they don't navigate via Pushstate.
Precomposition bundles the initial payload into the first response from the server, possibly as a JSON object. This allows the Search Engine to render the page without executing the AJAX call.
There is some evidence to suggest that Google might not execute AJAX requests. More on this here:
https://web.archive.org/web/20160318211223/http://www.analog-ni.co/precomposing-a-spa-may-become-the-holy-grail-to-seo
Search Engines can read and execute JavaScript
Google has been able to parse JavaScript for some time now, it's why they originally developed Chrome, to act as a full featured headless browser for the Google spider. If a link has a valid href attribute, the new URL can be indexed. There's nothing more to do.
If clicking a link in addition triggers a pushState call, the site can be navigated by the user via PushState.
Search Engine Support for PushState URLs
PushState is currently supported by Google and Bing.
Google
Here's Matt Cutts responding to Paul Irish's question about PushState for SEO:
http://youtu.be/yiAF9VdvRPw
Here is Google announcing full JavaScript support for the spider:
http://googlewebmastercentral.blogspot.de/2014/05/understanding-web-pages-better.html
The upshot is that Google supports PushState and will index PushState URLs.
See also Google webmaster tools' fetch as Googlebot. You will see your JavaScript (including Angular) is executed.
Bing
Here is Bing's announcement of support for pretty PushState URLs dated March 2013:
http://blogs.bing.com/webmaster/2013/03/21/search-engine-optimization-best-practices-for-ajax-urls/
Don't use HashBangs #!
Hashbang urls were an ugly stopgap requiring the developer to provide a pre-rendered version of the site at a special location. They still work, but you don't need to use them.
Hashbang URLs look like this:
domain.com/#!path/to/resource
This would be paired with a metatag like this:
<meta name="fragment" content="!">
Google will not index them in this form, but will instead pull a static version of the site from the _escaped_fragments_ URL and index that.
Pushstate URLs look like any ordinary URL:
domain.com/path/to/resource
The difference is that Angular handles them for you by intercepting the change to document.location dealing with it in JavaScript.
If you want to use PushState URLs (and you probably do) take out all the old hash style URLs and metatags and simply enable HTML5 mode in your config block.
Testing your site
Google Webmaster tools now contains a tool which will allow you to fetch a URL as google, and render JavaScript as Google renders it.
https://www.google.com/webmasters/tools/googlebot-fetch
Generating PushState URLs in Angular
To generate real URLs in Angular, rather than # prefixed ones, set HTML5 mode on your $locationProvider object.
$locationProvider.html5Mode(true);
Server Side
Since you are using real URLs, you will need to ensure the same template (plus some precomposed content) gets shipped by your server for all valid URLs. How you do this will vary depending on your server architecture.
Sitemap
Your app may use unusual forms of navigation, for example hover or scroll. To ensure Google is able to drive your app, I would probably suggest creating a sitemap, a simple list of all the urls your app responds to. You can place this at the default location (/sitemap or /sitemap.xml), or tell Google about it using webmaster tools.
It's a good idea to have a sitemap anyway.
Browser support
Pushstate works in IE10. In older browsers, Angular will automatically fall back to hash style URLs
A demo page
The following content is rendered using a pushstate URL with precomposition:
http://html5.gingerhost.com/london
As can be verified, at this link, the content is indexed and is appearing in Google.
Serving 404 and 301 Header status codes
Because the search engine will always hit your server for every request, you can serve header status codes from your server and expect Google to see them.
Best Answer
I made a small directive to listen for file input changes.
View JSFiddle
view.html:
controller.js:
directive.js: