add PointerDeviceKind to ScaleStartDetails

[yakagami]

Adds the PointerDeviceKind of the scale start event to ScaleStartDetails, which is currently missing.

This is a bug according to #115061. Additionally, according to the docs:

Having both a pan gesture recognizer and a scale gesture recognizer is redundant; scale is a superset of pan. Just use the scale gesture recognizer.

See also #135936 for similar issues and the PRs fixing them.

If multiple pointers are contacting the screen at scale start, then the PointerDeviceKind of the first pointer is used. In practice this should be good enough as I don't know of any UI that expects you to use two different pointer kinds at the same time. However, if this is an issue, we could either give a list of pointers with their kinds or a set of all active PointerDeviceKinds. I don't think this is necessary though.

Closes #115061.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[justinmc]

Looks good except for a small docs change. Thanks for jumping in and making this change. I'm sure a lot of people have struggled with not having this value over the years when someone just needed to add it.

[justinmc]

The first sentence should be its own paragraph per the styleguide: https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md#dartdoc-specific-requirements

[yakagami]

done

[yakagami]

I'm sure a lot of people have struggled with not having this value over the years when someone just needed to add it.

Yeah, I was going though the f: gestures issues and noticed I would actually benefit from this myself for my package (https://pub.dev/packages/zoom_view). Right now I use a Listner to get the device kind. The specific reason I need it is because for trackpads you need to disambiguate scaling from panning differently than other pointers. With other pointers you can just check that pointerCount == 1 which is a pan and pointerCount == 2 is a scale. This doesn't work with trackpads because you pan with two fingers on a trackpad. #13102 is related here. I've been thinking if there was a way for the framework to handle that natively and might make another pull request for that.

[justinmc]

LGTM 👍

Please tag me for review if you open the trackpad PR. I've been thinking about better trackpad support recently.

[QuncCccccc]

LGTM:)

[yakagami]

@justinmc Sure. In the meantime I opened an issue to get some ideas on what the API should look like because I can't think of a good one: #166066