The generic type of ParentDataWidget changed to ParentData
Summary
#The generic type of ParentDataWidget
has changed from
RenderObjectWidget
to ParentData
.
Context
#Prior to this change, a ParentDataWidget
was bound
to a specific RenderObjectWidget
type as ancestor.
For example, a Positioned
widget could only be used
within a Stack
widget. With this change,
a ParentDataWidget
can be used with any
RenderObjectWidget
type as ancestor as long as
the RenderObject
of said RenderObjectWidget
sets up the correct ParentData
type. In this new world,
the Positioned
widget can be reused with a hypothetical
new SuperStack
widget.
Description of change
#The generic type argument of ParentDataWidget
has changed from RenderObjectWidget
to ParentData
,
and a new debug property, debugTypicalAncestorWidgetClass
,
is added to ParentDataWidget
.
The latter is used for error messages to give users a
better idea of the context a given ParentDataWidget
is supposed to be used in.
Migration guide
#You must migrate your code as described in this section
if you're subclassing or implementing ParentDataWidget
.
If you do, the analyzer shows the following warnings when you
upgrade to the Flutter version that includes this change:
error • Missing concrete implementation of 'getter ParentDataWidget.debugTypicalAncestorWidgetClass' • lib/main.dart:114:7 • non_abstract_class_inherits_abstract_member
error • 'FrogJar' doesn't extend 'ParentData' • lib/main.dart:114:41 • type_argument_not_matching_bounds
Code before migration:
class FrogSize extends ParentDataWidget<FrogJar> {
FrogSize({
Key key,
required this.size,
required Widget child,
}) : assert(child != null),
assert(size != null),
super(key: key, child: child);
final Size size;
@override
void applyParentData(RenderObject renderObject) {
final FrogJarParentData parentData = renderObject.parentData;
if (parentData.size != size) {
parentData.size = size;
final RenderFrogJar targetParent = renderObject.parent;
targetParent.markNeedsLayout();
}
}
}
class FrogJarParentData extends ParentData {
Size size;
}
class FrogJar extends RenderObjectWidget {
// ...
}
Code after migration:
class FrogSize extends ParentDataWidget<FrogJarParentData> { // FrogJar changed to FrogJarParentData
FrogSize({
Key key,
required this.size,
required Widget child,
}) : assert(child != null),
assert(size != null),
super(key: key, child: child);
final Size size;
@override
void applyParentData(RenderObject renderObject) {
final FrogJarParentData parentData = renderObject.parentData;
if (parentData.size != size) {
parentData.size = size;
final RenderFrogJar targetParent = renderObject.parent;
targetParent.markNeedsLayout();
}
}
@override
Type get debugTypicalAncestorWidgetClass => FrogJar; // Newly added
}
The generic type of the ParentDataWidget
superclass
changes from FrogJar
(a RenderObjectWidget
) to
FrogJarParentData
(the ParentData
type that
FrogSize.applyParentData
wants to operate on).
Additionally, the new debugTypicalAncestorWidgetClass
is implemented for this ParentDataWidget
subclass.
It returns the type of a typical ancestor RenderObjectWidget
for this ParentDataWidget
. Most of the time,
you just want to return the old generic type here
(FrogJar
in this example).
Timeline
#Landed in version: 1.16.3
In stable release: 1.17
References
#API documentation:
Relevant PR:
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