C＃工厂模式

Question

I am building a search application that has indexed several different data sources. When a query is performed against the search engine index, each search result specifies which data source it came from. I have built a factory pattern that I used to display a different template for each type of search result, but I've realized that this pattern will become more difficult to manage as more and more data sources are indexed by the search engine (ie new code template has to be created for each new data source).

I created the following structure for my factory based off of an article by Granville Barnett over at DotNetSlackers.com

factory pattern http://img11.imageshack.us/img11/8382/factoryi.jpg

In order to make this search application easier to maintain, my thought was to create a set of database tables that can be used to define individual template types that my factory pattern could reference in order to determine which template to construct. I figured that I'd need to have a look up table that would be used to specify the type of template to build based off of the search result data source. I'd then need to have a table(s) to specify which fields to display for that template type. I'd also need a table (or additional columns within the template table) that would be use to define how to render that field (ie Hyperlink, Label, CssClass, etc).

Does anyone have any examples of a pattern like this? Please let me know. Thanks, -Robert

Answer 1

I would offer that this proposed solution is no less maintainable than simply associating a data source to the code template, as you currently have now. In fact, I would even go so far as to say you're going to lose flexibility by pushing the template schema and rendering information to a database, which will make your application harder to maintain.

For example, let's suppose you have these data sources with attributes (if I'm understanding this correctly):

Document { Author, DateModified }
Picture { Size, Caption, Image }
Song { Artist, Length, AlbumCover }

You then may have one of each of these data sources in your search results. Each element is rendered differently (Picture may be rendered with a preview image anchored to the left, or Song could display the album cover, etc.)

Let's just look at the rendering under your proposed design. You're going to query the database for the renderings and then adjust some HTML you are emitting, say because you want a green background for Documents and a blue one for Pictures. For the sake of argument, let's say you realize that you really need three background colors for Songs, two for Pictures, and one for Documents. Now, you're looking at a database schema change, which is promoted and pushed out, in addition to changing the parameterized template you're applying the rendering values to.

Let's say further you decide that the Document result needs a drop-down control, the Picture needs a few buttons, and Songs need a sound player control. Now, each template per data source changes drastically, so you're right back where you started, except now you have a database layer thrown in.

This is how the design breaks, because you've now lost the flexibility to define different templates per data source. The other thing you lose is having your templates versioned in source control.

I would look at how you can re-use common elements/controls in your emitted views, but keep the mapping in the factory between the template and the data source, and keep the templates as separate files per data source. Look at maintaining the rendering via CSS or similar configuration settings. For making it easier to maintain, considering exporting the mappings out as a simple XML file. To deploy a new data source, you simply add a mapping, create the appropriate template and CSS file, and drop them in to expected locations.

Response to comments below:

I meant a simple switch statement should suffice:

switch (resultType)
{
    case (ResultType.Song):
      factory = new SongResultFactory();
      template = factory.BuildResult();
      break;
    // ...

Where you have the logic to output a given template. If you want something more compact than a long switch statement, you can create the mappings in a dictionary, like this:

IDictionary<ResultType, ResultFactory> TemplateMap;
mapping = new Dictionary<ResultType, ResultFactory>();
mapping.Add(ResultType.Song, new SongResultFactory());
// ... for all mappings.

Then, instead of a switch statement, you can do this one-liner:

template = TemplateMap[resultType].CreateTemplate();

My main argument was that at some point you still have to maintain the mappings - either in the database, a big switch statement, or this IDictionary instance that needs to be initialized.

You can take it further and store the mappings in a simple XML file that's read in:

<TemplateMap>
    <Mapping ResultType="Song" ResultFactoryType="SongResultFactory" />
    <!-- ... -->
</TemplateMap>

And use reflection et. al. to populate the IDictionary. You're still maintaining the mappings, but now in an XML file, which might be easier to deploy.