New 'shared engine' message type when sending JSX to photoshop

[mcilroyc]

The current Photoshop interface completely refreshes the extendscript engine each time generator executes a script. This adds significant overhead that is especially noticeable during startup while generator is initializing itself. A recent change in photoshop allows use of a new message type, supplied in the message header, that causes photoshop to use a shared extendscript engine.

This change is not in the shipping version of photoshop.

This PR provides an amended API for specifying the shared-engine-compatibililty of a given JSX string or file, and uses the new PS command message type when appropriate. The existing methods will continue to use a non-shared engine by default, but an additional, optional parameter is now accepted.

• Fix existing JSX within this repo to be safe for shared engine execution; including removing instances of const declarations, and situations where vars could be declared but unassigned.
• Update internal JSX evaluations to use shared engine
• Add public 'sharedSafe' versions of JSX evaluation methods

Note: the other known instances of "unsafe" JSX scripts are in svgomg. Work has been started to update those scripts, but it is not strictly necessary.

[pineapplespatula]

@mcilroyc 🔂

[Jolg42]

Nice! @mcilroyc is it gonna be available in the next big release of Photoshop? (2018?)

[joelrbrandt]

Neat-o

[pineapplespatula]

Thanks!

[mcilroyc]

@Jolg42 I can't commit to that, but we will communicate release notes ahead of the next release.

[Jolg42]

I understand 😉 Thanks!