This request header contains the targeted layer's context, serialized as JSON.
Example
The current layer has a context { lives: 3 }.
When the user updates a fragment, Unpoly automatically includes the following request headers:
X-Up-Context: { "lives": 3 }
X-Up-Target: main
The server may choose to render HTML based on that context, e.g. by including
a live counter in the response. It responds with the following HTTP:
Vary: X-Up-Context
<main>
3 lives left
...
</main>
Note
Request headers that influenced a response should be listed in a Vary response header.
This tells Unpoly to partition its cache for that URL so that each
request header value gets a separate cache entries.
Updating context from the server
The server may update the layer context by sending a X-Up-Context response header with
changed key/value pairs:
Content-Type: text/html
X-Up-Context: { "lives": 2 }
<html>
...
</html>
Important
HTTP headers may only contain US-ASCII (7-bit) characters.
If you have higher code points in a JSON value, you may
encode those characters using Unicode escape sequences.
Upon seeing the response header, Unpoly will merge the server-provided context object into
the layer's context object, adding or replacing keys as needed.
Client-side context keys not mentioned in the response will remain unchanged.
There is no explicit protocol to remove keys from the context, but the server may send a key
with a null value to effectively remove a key.
The frontend will use the server-provided context upates for both successful (HTTP status 200 OK)
and failed (status 4xx or 5xx) responses. If no X-Up-Context response header is set,
the updating layer's context will not be changed.
It is recommended that the server only places changed key/value pairs into the X-Up-Context
response header, and not echo the entire context object. Otherwise any client-side changes made while
the request was in flight will get overridden by the server-provided context.
