Dry layout support for RenderBox
Summary
#A new method named computeDryLayout
was added to the RenderBox
protocol.
Subclasses of RenderBox
are expected to implement it to correctly report
their desired size given a set of BoxConstraints
during intrinsic
calculations. Subclasses that implement computeDryLayout
no longer need to
override performResize
.
Context
#A new method, computeDryLayout
, was added to the RenderBox
protocol to
correctly calculate the intrinsic sizes of a RenderParagraph
with WidgetSpan
children and a RenderWrap
. The method receives a set of BoxConstraints
and
is expected to calculate the resulting size of the RenderBox
without changing
any internal state. It's essentially a dry run of performLayout
that only
calculates the resulting size and doesn't place the children. The
computeDryLayout
method is part of the intrinsics protocol (see also
RenderBox.computeMinIntrinsicWidth
and friends).
Description of change
#Subclasses of RenderBox
need to override the new computeDryLayout
method
if they are used as a descendant of a RenderObject
that may query the intrinsic
size of its children. Examples of widgets that do this are IntrinsicHeight
and IntrinsicWidth
.
The default implementation of RenderBox.performResize
also uses the size
computed by computeDryLayout
to perform the resize. Overriding performResize
is therefore no longer necessary.
Migration guide
#Subclasses that already override performResize
can be migrated by simply
changing the function signature from void performResize()
to
Size computeDryLayout(BoxConstraints constraints)
and by returning the
calculated size instead of assigning it to the size
setter. The old
implementation of performResize
can be removed.
Code before migration:
@override
void performResize() {
size = constraints.biggest;
}
Code after migration:
// This replaces the old performResize method.
@override
Size computeDryLayout(BoxConstraints constraints) {
return constraints.biggest;
}
If the subclass doesn't override performResize
, the implementation of
computeDryLayout
has to be extracted from the performLayout
method.
Basically, computeDryLayout
needs to do all the work performLayout
is doing
to figure out the size of the RenderBox
. However, instead of assigning it
to the size
setter, it returns the computed size. If computeDryLayout
needs to know the size of its children, it must obtain that size by calling
getDryLayout
on the child instead of calling layout
.
If for some reason it is impossible to calculate the dry layout, computeDryLayout
must call debugCannotComputeDryLayout
from within an assert and return a dummy
size of const Size(0, 0)
. Calculating a dry layout is, for example, impossible
if the size of a RenderBox
depends on the baseline metrics of its children.
@override
Size computeDryLayout(BoxConstraints constraints) {
assert(debugCannotComputeDryLayout(
reason: 'Layout requires baseline metrics, which are only available after a full layout.'
));
return const Size(0, 0);
}
Timeline
#Landed in version: 1.25.0-4.0.pre
In stable release: 2.0.0
References
#API documentation:
RenderBox
computeMinInstrinsicWidth
computeDryLayout
getDryLayout
performResize
RenderWrap
RenderParagraph
Relevant issues:
Relevant PRs:
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