Monday, July 16

Using HTTP Handler for dynamic image generation

I was working on a project recently that needed bar charts to be created dynamically. We were not using sql reporting for this project. The bar chart needed to be changed/ updated based on certain criteria chosen from date controls, dropdowns etc.

One of the ways to achieve this is to create the image server side, store it on the disk on server and set the ImageUrl property of the image on the page to that saved image and send it to the browser.
This approach works but leads to addditional maintenance tasks, such as cleaning up old images, managing disk space etc...

One of the short comings of the image control Vs the image control in WinForms is that it lacks a property called "Bitmap" which allows you to assign a dynamically generated bitmap to the image control.

The only property exposed is the "ImageUrl".

I heard about http handlers and modules in podcats from polymorphic podcasts where Miguel Castro was explaining the use of HttpHandler to process certain file types.

Here is a formal definition from msdn : "An ASP.NET HTTP handler is the process (frequently referred to as the "endpoint") that runs in response to a request made to an ASP.NET Web application. The most common handler is an ASP.NET page handler that processes .aspx files. When users request an .aspx file, the request is processed by the page via the page handler.

HTTP handlers have access to the application context, including the requesting user's identity (if known), application state, and session information. When an HTTP handler is requested, ASP.NET calls the ProcessRequest method on the appropriate handler. The handler's ProcessRequest method creates a response, which is sent back to the requesting browser. As with any page request, the response goes through any HTTP modules that have subscribed to events that occur after the handler has run."

Scott Hanselman says : "Remember, an HttpHandler is the kind of thing you want to use when an HttpRequest is going to return a file or image to the browser. Basically anything other than a standard page. True, you could use a page, remove all the HTML from the ASPX part, and return stuff in Page_Load, but that's not really what pages are for, right? Also, Pages are themselves an HttpHandler (returned by PageHandlerFactory) so why not write your own HttpHandler?"

The processRequest for our bar chart image handler looks like this.

public void ProcessRequest (HttpContext context)
mPresenter = new ChartsPresenter(this);

When the bitmap generation is completed, we output the bitmap in this fashion

context.Response.ContentType = "image/gif";

We associate the ImageUrl property of the image to the http handler created to output the bar chart. Here is an abstract base class for creating http handlers that return proper error codes in case of problems. Phil Haack wrote it.

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