Tuesday, October 21, 2008

Creating Objects on the fly in Ruby

I'm writing a file parser where I want to plug in different parsing modules depending on the kind of file I need to parse.

In order to do this without having to change code, I'm storing the configuration in a YAML file, like this:

id: 2
match_string: "home_page"
measurement: "total_clicks"
clazz_name: "DummyParser"
date_comparision: 1

I'm specifying that for pages that are categorized as 'home_page_total_clicks', that I want to instantiate a class named "DummyParser". I'm thinking that in the future I could allow someone to specify an arbitrary parser to an abitrary file type.

The way Ruby allows you do instantiate classes from specified strings relys on the fact that all classes are constants, that you can retrieve:

clazz = Kernel.const_get(file_process_data.clazz_name)
processor = clazz.new(file_process_data)

and tada! a new processor -- note how I've assumed that a processor takes a file_process_data as an input. This will fail processors that don't have initialize methods that dont take file_process_data.

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