Assert that runApp is called in the same zone as binding.ensureInitialized

[Hixie]

Fixes#94123

[Hixie]

Previously landed as #117113.
Previously reverted as #122830.

[christopherfujino]

LGTM

[christopherfujino]

For posterity, the failures this was reverted for were actually fixed in 6711411

[mit-mit]

@Hixie a few folks have requested that should be documented with migration steps as a breaking change.

[gabrimatic]

@Hixie a few folks have requested that should be documented with migration steps as a breaking change.

@mit-mit You're true! I am getting this error after updating to Flutter 3.10:

════════ Exception caught by Flutter framework ══════════════════
The following assertion was thrown during runApp:
Zone mismatch.
The Flutter bindings were initialized in a different zone than is now being used. This will likely cause confusion and bugs as any zone-specific configuration will inconsistently use the configuration of the original binding initialization zone or this zone based on hard-to-predict factors such as which zone was active when a particular callback was set.
It is important to use the same zone when calling `ensureInitialized` on the binding as when calling `runApp` later.
To make this warning fatal, set BindingBase.debugZoneErrorsAreFatal to true before the bindings are initialized (i.e. as the first statement in `void main() { }`).
When the exception was thrown, this was the stack
#0 BindingBase.debugCheckZone.<anonymous closure>
binding.dart:497
#1 BindingBase.debugCheckZone
binding.dart:502
#2 runApp
binding.dart:1080
#3 main.<anonymous closure>.<anonymous closure>
main.dart:60
<asynchronous suspension>

[Hixie]

Sure, I'll see what I can whip up.

[Hixie]

proposed migration guide: flutter/website#8683

[filiph]

Hi @Hixie, I need guidance on another use of Zone than is not mentioned in your migration guide.

Historically, when using Firebase Crashlytics, Zones were used to catch all errors: https://firebase.flutter.dev/docs/crashlytics/usage/#zoned-errors — even those in various callbacks. For something like Crashlytics, this is useful, because you do want to have reports about all your errors.

I used this approach in the game_template sample, among other places. But now I see this is wrong through the warning discussed here (flutter/samples#1818).

I was going to ask if there is any new guidance on cases like this, when you want to run your app in a Zone (i.e. runApp is called from a non-root zone), but you also need to send the errors from that zone to a plugin (i.e. ensureInitialized needs to be called before the zone is created).

Newer Crashlytics documentation does not mention this case at all.

cc @domesticmouse

[Hixie]

Create the zone, call initialize binding in the zone, then configure the plugin, then call runApp. You'll need to do something like have the zone error handler call a configurable callback that you can set up from inside the zone.