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Rickard blogs about creating software solutions using ASP.NET and agile practices.
BlogEngine.NET has a widget framework from version 1.4 which are web part like components that can be added and removed, configured and dragged around directly in the page. Simple widgets are very easy to create and plug in to your blog and there are several blog posts explaining how to do that. In this post I'm gonna go through a more advanced example featuring Web services and Ajax.
The Photo Album Widget
Download: PhotoAlbumWidget-0.1.zip (7,96 kb) Unzip to ~/widget/ folder. Requires jQuery library.
First we start with some requirements for the widget we're creating.
The list above is a prioritized backlog and the first version should include items 1 - 4.
We start off with number one, a BlogEngine Widget. To create a widget we add a new folder to the /widget/ folder and we call it PhotoAlbum. Then we create two user controls, one for widget presentation and one for configuration UI. They must be named, by convention, widget.ascx and edit.ascx respectively and should derive from WidgetBase and WidgetEditBase respectively. See more on basic widgetry here by Rtur and here by Mads as well as this, that and whatnot.
That was the easy part.
With the next requirement we first need to get the pictures from somewhere so we skip to the next requirement. Picasa is Google's web album and it has its own REST API as well as a .NET client library. Conveniently there is also a public feed which contains all public photos uploaded to Picasa. (You can of course use your own Picasa album, see the Data API docs for more information). All we need to do is provide a query tag/tags to limit the search or we'll get an error message. The following code retrieves a couple of photos picturing cats:
PicasaService service = new PicasaService("exampleCo-exampleApp-1"); PhotoQuery query = new PhotoQuery("http://picasaweb.google.com/data/feed/api/all"); query.Query = "cat"; query.NumberToRetrieve = 5; PicasaFeed feed = service.Query(query);
foreach (PicasaEntry entry in feed.Entries) { string firstThumbUrl = entry.Media.Thumbnails[0].Attributes["url"] as string; writer.Write("<img src=\"{0}\" alt=\"{1}\" />", firstThumbUrl, entry.Title.Text); }
If we drop the code above into the widget.ascx.cs we affectivally fullfills both requirements 2 and 3 so lets get on with number 4.
Requirement number four implies the Incremental Page Display pattern where the main part of the page is shown quickly and portions of the page that takes longer to load are fetched and displayed asynchronously. To give the user feedback of the loading we place an animated gif image in the widget which is later swapped out when the real pictures arrive. To accomplish this we're using the jQuery JavaScript library which let us get away with very little code for pretty advanced stuff.
First we need to do a little refactoring since the widget needs to do an asynchronous call somewhere to get its content, i.e. the pictures to display. We're gonna go with the simplest possible solution here, just enough to satisfy the requirement. The simplest thing from the widget's point of view is to have the html containing a number of <img> elements returned from the call. Then it's very simple to insert the html into the widget using jQuery. Given that the widget contains a div with the id "photoalbum" this is all it takes:
$("#photoalbum").html(the_html_to_insert);
As it is now, the widget codebehind is rendering the requested html and this is not god. We need to move the code to some place which we can make a http request to and get the html in response. This can be done with a regular aspx page. All we need to do is to put a simple Repeater control on a page and bind it to a list of image URLs which we get from Picasa web album. So, we add a new aspx page to the /widgets/Photo Album/ folder and call it PhotoService.aspx. All we want in response is the <img> tags which should be inserted into the widget so we clear the page from html and put a single Repeater on it like this:
<asp:Repeater ID="pictureRepeater" runat="server"> <ItemTemplate> <asp:Image ID="image" runat="server" ImageUrl="<%# Container.DataItem.ToString() %>" /> </ItemTemplate> </asp:Repeater>
In the codebehind we insert something like before and makes sure we bind to the Repeater control.
// Error checking is omitted for clarity List<string> pictures = new List<string>(); PicasaService service = new PicasaService("exampleCo-testApp-1");
string picasaUri = "http://picasaweb.google.com/data/feed/api/all"; PhotoQuery query = new PhotoQuery(picasaUri); query.Query = "cat"; query.NumberToRetrieve = 6; PicasaFeed feed = service.Query(query);
foreach (PicasaEntry entry in feed.Entries) { string firstThumbUrl = entry.Media.Thumbnails[0] .Attributes["url"] as string; pictures.Add(firstThumbUrl); }
pictureRepeater.DataSource = pictures; pictureRepeater.DataBind();
To test the service we point a web browser to /widgets/Photo Album/PhotoService.aspx which should show a bunch of cats! Everything looks good and all we need to do to get the pictures into the widget are two things. First we need to register the jQuery JavaScript file, either localy or a referer to the jQuery official site. Last we need to add some custom JavaScript that makes the Ajax call and puts the html at the right place.
JavaScript blocks should allways, if possible, be placed as close to the </body> tag as possible. The reason is that the browser won't continue to load the remainder of the page when it hits a JavaScript block but rather wait until it is fully loaded. This is not a problem if we're dealing with short scripts but the jQuery library is relatively large in this context so we need to put it as far down the page as possible. Our own script that does the actual work depends on jQuery, thus it must be loaded last. The problem is that we need to do this in a User control in the middle of the page. To our help to solve this we have the ClientScriptManager that every aspx page has. With the method RegisterStartupScript we get to insert arbitrary scripts just before the </form> tag (which is close enough) of the surrounding page. We make the registrations when the widget is loaded, i.e. in the LoadWidget() method.
With this final step done we have our first version of the Photo Album Widget which is ready for demo.
Before the second iteration I'd like to summarize a couple of things with version one that are not of production quality.
A colleague of mine asked me how to apply a stylesheet to a web page dynamically using jQuery and I had never done such a thing but my first thought was that is must be pretty simple. I've spent a lot of time thinking of so many things other than web et. al. so it was nice to delv into some of that stuff again. Check out the live demo.
As we know stylesheets are defined in the head section of an html file like this
<html> <head> <link rel="stylesheet" href="stylesheet.css" type="text/css" /> </head> <body> ... </html>
Now, say that we want to apply another stylesheet dynamically after the fact, so to speak, triggered by some event. This could be a button click or some other arbitrary event that is triggered. So, what we want to do is simply insert a new <link/> element into the head section of the page DOM. This can be done in a couple of lines of jQuery:
$(document).ready(function() { $("a").click(function() { $('head').append('<link rel="stylesheet" href="style2.css" type="text/css" />'); }); });
The key is at what time we add the link to the style sheet. The first line in the code above repeated here asserts that the DOM is ready for manipulation.
$(document).ready(function() { //... });
The second part repeated here adds a click event to all hyperlinks in the page.
$("a").click(function() { //...
And the very task is performed by the last piece of code where the head element is appended with a new link element.
$('head').append('<link rel="stylesheet" href="style2.css" type="text/css" />');
I thought that I should do a post about the JavaScript resources I've found most useful. There are an endless amount of JavaScript resources out there and and there is only so much time you can spend going through them to find what you need. This is my attempt at gathering some of the best I know. So here goes.
This list became long enough so I will do another post on Ajax resources later on.
What have I missed?