How do I creat a new plug-in?To summarize: You just have to add a corresponding action for the swiki which you want to create the plug-in for.
Using the swiki browser, check out the existing plug-ins found in refs :: actions (page) :: plug-ins and documented here. For example, take a look at since-plugin. That plug-in tells you how much time has past since a certain day.
To create a new plug-in, simply create a new page action which ends with -plugin (it can be in any category, but keeping it in a separate plug-ins category might be useful). Let's say you call your new plugin new-plugin, then, when <?newone?> is called in the text of the page, it will insert the results of that page action. The request will contain a setting called options with a dictionary of all fields added to the plug-in. From there, you can process it how you want.
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