(These are changes cherry-picked from in-flight branches since they are
more independent and could be helpful even without those changes.)
- Change RouteBuilder's signature to take a single argument in which the
other fields are placed, so that we can keep iterating on those
arguments without having to break compatibility each time. Also, this
makes defining route builders much simpler (only one argument to
ignore rather than a variable number).
- Expose the next performance to RouteBuilders, since sometimes the
route itself might not be where it's used.
- Allow BuildContext to be used to walk children, just like it can for
ancestors
- Allow BuildContext to be used to get the Widget of the current
BuildContext
- Allow StatefulComponentElement to be referenced with a type
specialisation so that you don't have to cast when you know what the
type you're dealing with actually is.
Also:
- don't mark a node as from the new generation if it is dirty, since we
know it still has to be built.
- establish the rule that you can't call setState() during initState()
or build().
- make syncChild() return early for unchanged children.
- update the tests, including adding a new one.
If it's a StatefulComponent, then it's ok to reuse it so long as it
hasn't been initialised.
If it's a regular Component or a TagNode, then it's always ok to reuse.
If it's a RenderObjectWrapper, then it's ok to reuse so long as it
doesn't have a renderObject.
To put it another way, this changes how we prevent the following
nonsensical situations from arising:
- Sync two stateful StatefulComponents together
- Sync two RenderObjectWrappers with RenderObjects together
When either of those cases happen, we just drop the old one on the
ground and use the new one unchanged.
