Research possible ways to implement ActivityStreams extensions

[warriordog]

Extensions are a critical and actively used part of the ActivityPub spec, but they're also remarkably difficult to implement in C#. This poses a research challenge - we need to find an implementation that meets all these requirements simultaneously:

Deserialization:

• Can extract a subset of unmapped properties from incoming JSON
• Shape is defined by code that won't exist at library build time (but will exist when linked with user code)
• Conversion ONLY takes place if the appropriate context is included
• User code should not need to pass around multiple objects. That is, extension data must be directly linked to the base object type.
• Properties may be remapped at runtime by an alias in an inline or external context. currently out of scope - will be handled in a separate issue
• Property values can be remapped by an extension. currently out of scope - will be handled in a separate issue

Serialization:

• Extensions must be linked to and serialized along with the base object - users should not need to make multiple calls
• Extension context must be promoted to @context property
• Multiple extensions may define overlapping properties, and they must be aliased or expanded to resolve conflicts.

For more information, see previous discussions in #2.

[warriordog]

This is the most viable solution so far: #2 (comment)

[warriordog]

This is the most viable solution so far: #2 (comment)

Expansion of this - register extensions at runtime (like the types). Then we can remove the JSON properties and make the extension map read-only. Much safer!

[warriordog]

We should wait until #39 is done - the implementation for that will impact this. Extension types would be more natural if integrated through that system. Will probably need to integrate @context into the type resolution process.

[warriordog]

TypeMap seems to be the best solution so far. Closing in favor of #71.