Initial snap offset support for ScrollableWidgetList (and ScrollableList<T>) and ScrollableMixedWidgetList. If a ```toSnapOffset(scrollOffset)``` function is provided, fling Scrolls will coast to the returned value. If ```alignmentOffset``` is specified then fling scrolls conclude when toSnapOffset's value lines up with the Scrollable widget's origin + alignmentOffset. For example if the Scrollable widget's height was 200.0, and alignmentOffset:100.0 was specified, then fling scrolls would end with the value returned by toSnapOffset() lined up with the center of the Scrollable.
This approach to Scrollable snapping assumes that the layout of whatever the Scrollable contains is known at the outset. This is often true however a ScrollableMixedWidgetList may not know its items' sizes until they've been reached by scrolling.
This is a first cut at snapping support. Among the things that remain to be done:
- Scrolling limits trump snapping. Snapping should probably trump scrolling limits.
- Drag scrolls aren't snapped. This may be desirable so perhaps the feature should be controlled with a flag.
- Specifying alignmentOffset as a percentage would probably be more convenient.
- It would be nice if one could wrap items in a SnapOffset value like: ```new SnapOffset(0.5, child: myItem)``` to snap to the center of the item.
Updated the CardCollection example: snapping and fixed size items can be turned on/off with Drawer checkboxes.
This patch makes a number of changes:
1) buildDirtyComponents now prevents all calls to setState, not just those
higher in the tree. This change is necessary for consistency with
MixedViewport and HomogeneousViewport because those widgets already build
subwidgets with that restriction. If the "normal" build didn't enforce that
rule, then some widgets would break when put inside a mixed or homogeneous
viewport.
2) We now notify global key listeners in a microtask after beginFrame. That
means setState is legal in these callbacks and that we'll produce another
frame if someone calls setState in such a callback.
Also:
- Make Dismissable report when it starts squashing, since otherwise we
don't invalidate the list early enough and it gets mad that it wasn't
told one of its children had changed size.
- Have Dismissable check that it gets removed after it's dismissed, to
avoid having lots of redundant widgets around.
- Start tracking the height of each child of a MixedViewport, so that we
don't accumulate floating point errors when a child jiggles up and down.
- Have _childOffsets reuse its storage space rather than newing up a new
copy each time we reset the cache.
- Avoid double-updating child sizes when handling mixed viewport invalidations.
