There's no documentation for how to call a VM service extension method

[nex3]

dart:developer supports a means for developers to register extensions to the VM service, but there's no documentation in the service protocol for how those extension handlers should be invoked. The registerExtension documentation says "users of extensions must always specify a target isolate", but there's no information about how to specify that or what RPC call to use.

[johnmccutchan]

You invoke an extension in the same way you invoke any other service protocol RPC.

You target a specific isolate by specifying an 'isolateId' as a parameter, like any other service protocol RPC.

[nex3]

Does this mean extension RPCs can shadow built-in RPCs?

[johnmccutchan]

@nex3 No, extensions have lowest priority and cannot replace existing RPCs. In other words, you can register a 'getObject' extension but it will never be called.

[nex3]

Doesn't that mean that any new built-in RPC is now a breaking change, since it might step on an extension that worked in previous SDK versions?

[johnmccutchan]

@nex3 We haven't gotten to the point where this is a practical problem. Perhaps we will require extensions to be namespaced or start with a specific prefix.

[johnmccutchan]

@nex3 There is also the problem of two Dart libraries that register the same extension method name.

[nex3]

Unfortuantely, changing the registerExtension() method call itself is also a breaking change at this point. But it would probably be good to add a note to its documentation saying something like "For compatibility across packages and future versions of the VM service protocol, extensions should begin with the name of the registering package followed by a period."