google_maps_flutter for iOS should allow pre-caching GMSServices

[jonahwilliams]

According to the documentation of https://developers.google.com/maps/documentation/ios-sdk/reference/interface_g_m_s_services , this object can be created ahead of map creation to reduce the initial map creation cost. From several profiling runs, this is expensive in debug/profile - though significantly less so in release mode.

Nevertheless, it would be nice to have especially if it could reduce the jank encountered while developing.

[jmagman]

I spotted this in a trace:

The +sharedServices doc says:

It also may be useful to take this object in advance of the first map creation, to reduce initial map creation performance cost.

[jmagman]

I was looking at GM code, +sharedServices must be called on the main thread, but internally preps the service on a background thread. It should be retained to speed up initialization of the app.

Since it handles all the backgrounding, perhaps we should be call and retain it during registerWithRegistrar.

[jmagman]

flutter/plugins#6211 will cache +[GMSServices sharedServices] the first time a map is created after app launch so the initialization work can be shared over multiple maps (includes when the map is scrolled away and then comes back). This won't solve the initial jank though. @stuartmorgan recommends adding a pre-cache API so the developer can decide when to take that hit.