Finds the intrinsic size of a widget and provides it in a convenient builder.
This is a small but very useful package for those cases when you need to know the intrinsic size of some widget, prior to adding it in the widget tree.
What is intrinsic size?
It's the size a widget gets when it can determine its size itself. As opposed to the case where the parent restricts the sizing.
Note
You may allow the parent to restrict the size thought, through the constrainedAxis parameter of IntrinsicSizeBuilder.
One very good usage is when you want a SliverAppBar with dynamically sized flexible space/ bottom. Checkout the examples.
Trivial case:
IntrinsicSizeBuilder(
subject: const Text(
'Hello world',
style: TextStyle(fontSize: 25),
),
builder: (context, subjectSize, subject) => Center(
child: Column(
mainAxisSize: MainAxisSize.min,
children: [
subject,
Text('The size of subject is $subjectSize'),
],
),
),
)Under the hood, the builder puts the subject (the widget which you need the size for) in the widget tree for just 1 frame with zero opacity, during which time the size is determined. After that the builder function is called, with the subject and its size, and the real widget is displayed.
Does this mean glitchy ui?
If you find the initial (transparent) 1 frame problematic, then use the firstFrameWidget parameter. You may for example prolong your loading screen 1 frame to completely eliminate any glitch. Illustrated in this example.
What about when the subject changes size?
When the subject's intrinsic dimensions change you need to inform the IntrinsicSizeBuilder to redo the size evaluation. You do this simply by calling the static method IntrinsicSizeBuilder.refresh(context). It will refresh all IntrinsicSizeBuilders above context. There's no 1 frame glitch in this case - the currently displayed widget is preserved during the 1-frame evaluation.