Populate extension point's element attribute during runtime - eclipse

Question

Here is my use case. I have an Editor that different plugins can use to display their data. These plugins can create (with intervention from user - wizards) different files that Editor can accept: .p1, .p2, etc. Each plugin contributes file extensions of files that it produces and this data is available via custom extension point. I need a way, if possible to "inject" this data into org.eclipse.ui.editors extension point's extensions attribute.
One way I was thinking about doing this is in a plugin that is called early enough collect file extensions from all plugins that use my extension point and write these values into plugin.properties file's key like supportedFileExtension and extension point org.eclipse.ui.editors will in turn consume this file:
<extension
point="org.eclipse.ui.editors">
<editor
class="MyEditor"
contributorClass="MyActionBarContributor"
default="false"
extensions="%supportedFileExtensions"
id="my.com.editor"
name="My Editor">
</editor>
I also saw class ExtensionParameterValues which is something I can possibly use, but for developer who might want to reuse this little mechanism, it might not be obvious enough what's going, especially when extensions attribute would be empty when using ExtensionParameterValues class. Of course I can put this in documentation, but who reads that, right? :)
Perhaps I am overlooking something simple and there is an easier way to do what I am trying to accomplish?

Answer 1

Rather than using extensions directly you can Eclipse Content Types to do this. First define a base content type:
<extension
point="org.eclipse.core.contenttype.contentTypes">
<content-type
id="contenttype.base"
name="Base content type"/>
Then your different plugins can define a content type for their file extensions using a a content type derived from the base type:
<content-type
base-type="contenttype.base"
file-extensions="p1"
id="contenttype.p1"
name="P1 content type"/>
<content-type
base-type="contenttype.base"
file-extensions="p2"
id="contenttype.p2"
name="P2 content type"/>
For your editor don't specify any extensions, instead use the contentTypeBinding for the base type:
<extension
point="org.eclipse.ui.editors">
<editor
class="MyEditor"
contributorClass="MyActionBarContributor"
default="false"
id="my.com.editor"
name="My Editor">
<contentTypeBinding
contentTypeId="contenttype.base"/>
</editor>
The editor will now be used for all content types based on the base content type.