Correct leaks on Switch and Divider on iOS.

[freakboy3742]

The Switch and Divider widgets on iOS were keeping hard references to the underlying impl/interface, causing leaks.

The same fix should be possible on Box as well... but making the change causes a segfault on OptionContainer and ScrollContainer tests. The fix wasn't obvious; I figured it's better to live with the existing leak for now, and fix the leak where we know it exists. There's likely some overlap with the consequences of beeware/rubicon-objc#256.

Refs #2849.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[samschott]

Thanks!

[samschott]

Out of curiosity, what did you try to fix the leaks on the iOS container widgets? Is the issue there that the parent UIView maintains a strong reference to its children?

[freakboy3742]

I didn't get very far - the object that was causing the segfault wasn't immediately obvious. The manifestation is that switching the implementation of Box from a raw UIView to TogaView causes a bunch of the OptionContainer and ScrollContainer tests to segfault. They all seem to fail in post-test cleanup; and they all seem to be related to tests that replace views during the test run - but because it's a segfault during garbage collection in a test cleanup, it's difficult to narrow down the source (if you've got any hot tips on narrowing down the object - or even the line of code - that causes a segfault, I'd love to hear them...)

Thinking about it with a fresh mind - it might be related to the constraints. There's a specific protection in the cocoa constraints implementation that doesn't exist in the iOS implementation... I'll try to see if porting that fix that works.

[freakboy3742]

Confirmed - it was the constraint handling. PR incoming.