PROTOCOL

Protocol
========

The native messaging as used in Chrome and Firefox WebExtensions
uses length-prefixed JSON-serialized messages for communication.
Dotjs-NH uses this channel as described below.

The extension sends, as request to the native host, a single string value
containing the URL of the page where content might need to be injected.

    ==> "https://example.org/"

The native host shall respond with an array, first element of which
(if exists and not falsy) shall be an array of strings representing
scripts to inject, and second of which (if exists and not falsy) shall
be an array of strings representing stylesheets to inject.
The array may be longer or shorter than 2 elements, missing or falsy
elements should be handled identically to empty arrays.

    <== [["alert(\"Hello, world!\")"],["*{color:red}"]]

Messages are processed sequentially and each request message shall
result in exactly one reply message.

Rationale
---------

The primary goal is simplicity of both the protocol and the code of
native host which runs in privileged environment (i.e. with access to
the file system); what's needed on the extension side is not a concern.

Sending just an URL as a string is the simplest thing that can be done,
and since it is required to be a JSON value, future extensibility is
trivial as anything else will just use a value of different type.

To keep the protocol simple, the reply could also be a single string as
it's possible to convert stylesheets to JavaScript and to concatenate
multiple scripts, but this would put additional burden on the native host
to implement another quoting and safe concatenation of scripts properly.
Since there's already a JSON encoder required and it's usually available
as a well-tested library, there's no need to do anything more in native
host and any post-processing can be done on the extension side.

Finally, arrays are usually more efficient than dictionaries/objects
and, as long there is only JavaScript and CSS to consider, using object
value for replies wouldn't really make any difference for readability
or maintainability.