Add convenience accessor for primaryFocus

[gspencergoog]

Description

This adds accessors for WidgetsBinding.instance.focusManager and WidgetsBinding.instance.focusManager.primaryFocus so that they can be more easily found in IDEs and accessed.

This adds a top level getter for WidgetsBinding.instance.focusManager.primaryFocus called primaryFocus, and a static accessor FocusManager.instance that returns WidgetsBinding.instance.focusManager.

Tests

• Modified tests to use the accessors.

Breaking Change

• No, this is not a breaking change.

[darrenaustin]

Other than my concern about using the global namespace, this LGTM.

[darrenaustin]

Looks good. Did you consider using a static method on FocusManager instead of a top-level getter here? Do we have other top-level globals like this? Is there any concern about name clashing with existing app code for either of these?

[gspencergoog]

Well, I did consider a static, but it would have had to have a different name (because primaryFocus is taken), and it's pretty useful at the top level. There are other top level globals like these: imageCache, and defaultTargetPlatform to name a couple. Like any new name, It could clash, of course, but I don't think it's very likely to clash.

However, I think I will convert the focusManager one to a static FocusManager.instance so that it appears as the singleton that it is (but still get it internally from the WidgetsBinding.instance.focusManager). That way it's one less thing to clash.

[darrenaustin]

If we already have a precedence for top level names, this is probably not a big deal. Pragmatically speaking I doubt there would be much collision. I prefer not using global namespaces, but this case maybe useful enough to do so.

Or given that you already changed it to FocusManager.instance while I was typing this :), never-mind.