Mirrored variables read a cached value

A ConsoleMirror registers the variables of a console served on a MessagePort (remote consoles). VariableDef.get() is synchronous, and the console calls it in four places: reading a variable, the hint of the highlighted suggestion, browse mode and value completion. A port only answers asynchronously.

The mirror keeps the value of each variable from the latest snapshot, and its get() returns that value. The server sends a snapshot after every command or write it runs, and an active mirror asks for one when it becomes active and when the console emits opened. VariableDef.set() may return a promise, so a write waits for the page, prints the value the page reads back and still reports a rejection.

Considered Options

• An asynchronous get(). Every hint, completion and browse row would wait on the page, and each of them would need a pending state.
• The page pushes every value change. Variables have no change event, and adding one would make every registrant notify the console.

Consequences

• A value changed on the page by something other than the console, such as a brush slider, reads stale in an open console until the next command, write or reopening.
• set returning a promise is public API. A local variable may use it too; the echo stays pending until it settles.