What is a Page Viewer Web Part?
A PVWP allows you to embed in your page another Web Page. Technically, a PVWP doesn’t do much, it is just a wrapper for a html iframe element.
iframes have a bad reputation. One of the main reason is that people often confuse them with frames. An excerpt from the MDN page:
<frame> element is not encouraged because of certain disadvantages such as performance problems and lack of accessibility for users with screen readers. Instead of the
<iframe> may be preferred.
As a matter of fact, iframes are enjoying a surge in popularity in modern Web design. They serve as building blocks in modular pages (several SharePoint Web Parts rely on iframes), and are a key component of application pages (for example on Twitter, Facebook or with Disqus). I would expect them to become even more popular once the new iframe html5 attributes become widely available.
Note: besides linking to Web pages, the PVWP also has options to display folders or file content.
How is this similar to the CEWP?
As explained in my old post, a CEWP allows you to embed, via its content link property, Web content (html, stylesheets or scripts) in your page. A PVWP acts in a similar way, except that the linked file is a whole Web page, instead of a fragment stored in a text file.
But this means two different pages?
Right. With a CEWP, the code fragment becomes part of the host page. With a PVWP, the code remains in its own page. That said, it doesn’t mean that the two pages cannot communicate, within the boundaries allowed by the same origin policy. In particular, an iframe can access and modify its host page. In the basic example shown below, the code in the iframe accesses the parent body to change its background color.
Any real life example to share?
Sure! In the past few months, I published in this blog several demos from my SPELL library. SPELL is built to work in dual mode: either “inline” for direct insertion within a page (CEWP, Master Page, etc.), or “app” for inclusion in an iframe. For example, the process navigation showcased in this blog post is inserted via a PVWP. Same for its sibling featuring a button navigation. Other solutions like the SPELL Tabs or mini-BI work exactly the same way.
Why use a PVWP rather than a CEWP?
Because the iframe loads another page, this provides a clear separation between the host page and the iframe content. The sandboxed environment avoids code conflicts. This can be for example useful:
- for charting apps, to make sure that the styles of the main page don’t interfere with the styles of the chart itself.
- for scripting: if your app requires a specific jQuery version, you can run it within the iframe, without having to worry about the main page running a different version.
One constraint with Web Parts is that they have to live in a page. Picture a SharePoint dashboard: you view it as a whole, with no easy option to isolate a specific chart. With a PVWP, the chart can be viewed either embedded in the dashboard or directly in its underlying page. This scenario is showcased in this post: with read only permissions on my site, you cannot build a SharePoint dashboard page, but you can still interact with the individual chart page.
Even better, standalone pages work great with SharePoint dialogs. Iframing is actually the technique SharePoint relies on with forms: when you open a new, display or edit form in a dialog, you are simply opening newform.aspx, dispform.aspx or editform.aspx in an iframe! Do I have a dialog example with SPELL? Thanks for asking You’ll find it here. Working with standalone pages allows to pile up dialogs and create the drill-down effect.
3/ Asynchronous load
Because the host page and the iframe are two different pages, the browser can load them in parallel, so this “should” result in better performance. With a CEWP, the content becomes part of the host page and execution will follow the page flow.
4/ Links across site collections
The CEWP allows you to link to an external file, but only within a same site collection. An iframe doesn’t have such restrictions, so a single application page stored in SharePoint can be reused across site collections.
5/ Storage of parameters
This is a technique I use with the SPELL library – not really standard but let me explain it anyway.
Because the script running in the iframe can access its own url, this url becomes a place where you can store parameters. For example the URL for the SPELL Tabs will look like this:
For cascading dropdowns, the url would be for example:
You might find the above urls intimidating, but if you look at SharePoint urls they actually work in a similar fashion. The point is: if you used a CEWP, you’d need to store one snippet of code for each cascading dropdown you implement. With the above method, one single page called CascadingSelect.aspx can be reused across all site collections!
Note: In practice the SPELL library includes editors, the above paths are not built manually.
Of course, iframes also have some drawbacks. The asynchronous load complicates the relationship between the iframed content and the parent’s content. The sandbox means that if the same code is needed in both the host page and the iframe, locally you have to load it twice (or find a trick…). Both the CEWP and the PVWP have their place in SharePoint design!