We were traversing all children to only perform calculations/changes to
flexible children in order to avoid new allocations during layout. This
diff ensures we only visit flexible children during layout calculations
if any are present. We accomplish this by keeping a private linked list
of flexible children.
There's no need to go through all absolute children at the end of the
layout calculation if the node at hand doesn't have any. This also
ensures only absolutely positioned children are traversed in the final
loop.
There's no need to go through all children before starting the main line loop
as we'll visit all children in the former loop anyway. This diff merges the
pre-fill loop into the main line one to avoid an extraneous traversal on the
node's children.
There's no need to set the trailing position on left-to-right layouts
as the nodes will already have what we need (x, y, width, and height).
This means we still have an extra cost for reversed layout directions
but they are not as common as LTR ones.
Store immutable values from the node being laid out to avoid unnecessary
method invocations during layout calculation. This gives us a 3%-5%
performance boost in my benchmarks on Android.
The JavaScript implementation of isDimDefined contains a check to ensure
that the dimension value is positive; the C and Java versions did not
have this check. As a result, a negative style value for 'width' (such
as that used by the "should layout node with negative width" test) would
have different layout under the C/Java implementation to the JavaScript
implementation.
This was hidden because the C/Java transpilers filtered out any negative
instantiation values from the test suite. In effect, the negative value
tests weren't running on the C/Java implementation.
This patch removes the negative value filter from the transpiler, and
makes the isDimDefined definition consistent between the three
implementations.
The way the algorithm work is that you first layout fixed children on the main axis, then compute all the dimensions so that you can layout flexible children. This separation doesn't work anymore if we add the other axis. The solution here is a hacky (but working!) attempt at fixing the issue. We start by doing a pass to set the children dimensions if they are stretch.
