颤动定位元素弯曲它，这个日志主要是什么？

Question

I get this log when I would the Widget Header Fixed on the Window, I don't why he get this. that is my log:

I/flutter ( 9657): The following assertion was thrown during performLayout():
I/flutter ( 9657): RenderFlex children have non-zero flex but incoming width constraints are unbounded.
I/flutter ( 9657): When a row is in a parent that does not provide a finite width constraint, for example if it is in a
I/flutter ( 9657): horizontal scrollable, it will try to shrink-wrap its children along the horizontal axis. Setting a
I/flutter ( 9657): flex on a child (e.g. using Expanded) indicates that the child is to expand to fill the remaining
I/flutter ( 9657): space in the horizontal direction.
I/flutter ( 9657): These two directives are mutually exclusive. If a parent is to shrink-wrap its child, the child
I/flutter ( 9657): cannot simultaneously expand to fit its parent.
I/flutter ( 9657): Consider setting mainAxisSize to MainAxisSize.min and using FlexFit.loose fits for the flexible
I/flutter ( 9657): children (using Flexible rather than Expanded). This will allow the flexible children to size
I/flutter ( 9657): themselves to less than the infinite remaining space they would otherwise be forced to take, and
I/flutter ( 9657): then will cause the RenderFlex to shrink-wrap the children rather than expanding to fit the maximum
I/flutter ( 9657): constraints provided by the parent.
I/flutter ( 9657): The affected RenderFlex is:
I/flutter ( 9657):   RenderFlex#b7f40 relayoutBoundary=up9 NEEDS-LAYOUT NEEDS-PAINT
I/flutter ( 9657): The creator information is set to:
I/flutter ( 9657):   Row ← Padding ← Container ← Padding ← DecoratedBox ← ConstrainedBox ← Padding ← Container ← Align
I/flutter ( 9657):   ← ClipRect ← Header ← Container ← ⋯
I/flutter ( 9657): The nearest ancestor providing an unbounded width constraint is:
I/flutter ( 9657):   RenderStack#9cf16 relayoutBoundary=up1 NEEDS-LAYOUT NEEDS-PAINT
I/flutter ( 9657):   creator: Stack ← Container ← MediaQuery ← LayoutId-[<_ScaffoldSlot.body>] ← CustomMultiChildLayout
I/flutter ( 9657):   ← AnimatedBuilder ← DefaultTextStyle ← AnimatedDefaultTextStyle ← _InkFeatures-[GlobalKey#ccab9
I/flutter ( 9657):   ink renderer] ← NotificationListener<LayoutChangedNotification> ← PhysicalModel ←
I/flutter ( 9657):   AnimatedPhysicalModel ← ⋯
I/flutter ( 9657):   parentData: offset=Offset(0.0, 0.0); id=_ScaffoldSlot.body (can use size)
I/flutter ( 9657):   constraints: BoxConstraints(0.0<=w<=411.4, 0.0<=h<=774.9)
I/flutter ( 9657):   size: Size(411.4, 774.9)
I/flutter ( 9657):   alignment: AlignmentDirectional.topStart
I/flutter ( 9657):   textDirection: ltr
I/flutter ( 9657):   fit: loose
I/flutter ( 9657):   overflow: clip
I/flutter ( 9657): See also: https://flutter.io/layout/
I/flutter ( 9657): If this message did not help you determine the problem, consider using debugDumpRenderTree():
I/flutter ( 9657):   https://flutter.io/debugging/#rendering-layer
I/flutter ( 9657):   http://docs.flutter.io/flutter/rendering/debugDumpRenderTree.html
I/flutter ( 9657): If none of the above helps enough to fix this problem, please don't hesitate to file a bug:
I/flutter ( 9657):   https://github.com/flutter/flutter/issues/new?template=BUG.md

Performing hot reload...
Syncing files to device Android SDK built for x86...
I/flutter ( 9657): ══╡ EXCEPTION CAUGHT BY RENDERING LIBRARY ╞═════════════════════════════════════════════════════════
I/flutter ( 9657): The following assertion was thrown during performLayout():
I/flutter ( 9657): RenderFlex children have non-zero flex but incoming width constraints are unbounded.
I/flutter ( 9657): When a row is in a parent that does not provide a finite width constraint, for example if it is in a
I/flutter ( 9657): horizontal scrollable, it will try to shrink-wrap its children along the horizontal axis. Setting a
I/flutter ( 9657): flex on a child (e.g. using Expanded) indicates that the child is to expand to fill the remaining
I/flutter ( 9657): space in the horizontal direction.
I/flutter ( 9657): These two directives are mutually exclusive. If a parent is to shrink-wrap its child, the child
I/flutter ( 9657): cannot simultaneously expand to fit its parent.
I/flutter ( 9657): Consider setting mainAxisSize to MainAxisSize.min and using FlexFit.loose fits for the flexible
I/flutter ( 9657): children (using Flexible rather than Expanded). This will allow the flexible children to size
I/flutter ( 9657): themselves to less than the infinite remaining space they would otherwise be forced to take, and
I/flutter ( 9657): then will cause the RenderFlex to shrink-wrap the children rather than expanding to fit the maximum
I/flutter ( 9657): constraints provided by the parent.
I/flutter ( 9657): The affected RenderFlex is:
I/flutter ( 9657):   RenderFlex#b7f40 relayoutBoundary=up9 NEEDS-LAYOUT NEEDS-PAINT
I/flutter ( 9657): The creator information is set to:
I/flutter ( 9657):   Row ← Padding ← Container ← Padding ← DecoratedBox ← ConstrainedBox ← Padding ← Container ← Align
I/flutter ( 9657):   ← ClipRect ← Header ← Container ← ⋯
I/flutter ( 9657): The nearest ancestor providing an unbounded width constraint is:
I/flutter ( 9657):   RenderStack#9cf16 relayoutBoundary=up1 NEEDS-LAYOUT NEEDS-PAINT
I/flutter ( 9657):   creator: Stack ← Container ← MediaQuery ← LayoutId-[<_ScaffoldSlot.body>] ← CustomMultiChildLayout
I/flutter ( 9657):   ← AnimatedBuilder ← DefaultTextStyle ← AnimatedDefaultTextStyle ← _InkFeatures-[GlobalKey#ccab9
I/flutter ( 9657):   ink renderer] ← NotificationListener<LayoutChangedNotification> ← PhysicalModel ←
I/flutter ( 9657):   AnimatedPhysicalModel ← ⋯
I/flutter ( 9657):   parentData: offset=Offset(0.0, 0.0); id=_ScaffoldSlot.body (can use size)
I/flutter ( 9657):   constraints: BoxConstraints(0.0<=w<=411.4, 0.0<=h<=774.9)
I/flutter ( 9657):   size: Size(411.4, 774.9)
I/flutter ( 9657):   alignment: AlignmentDirectional.topStart
I/flutter ( 9657):   textDirection: ltr
I/flutter ( 9657):   fit: loose
I/flutter ( 9657):   overflow: clip
I/flutter ( 9657): See also: https://flutter.io/layout/
I/flutter ( 9657): If this message did not help you determine the problem, consider using debugDumpRenderTree():
I/flutter ( 9657):   https://flutter.io/debugging/#rendering-layer
I/flutter ( 9657):   http://docs.flutter.io/flutter/rendering/debugDumpRenderTree.html
I/flutter ( 9657): If none of the above helps enough to fix this problem, please don't hesitate to file a bug:
I/flutter ( 9657):   https://github.com/flutter/flutter/issues/new?template=BUG.md

  @override
  Widget build(BuildContext context) {
    //final Size screenSize = MediaQuery.of(context).size;
    return WillPopScope(
      onWillPop: () {
        print('Back button pressed');
        Navigator.pop(context, false);
        return Future.value(false);
      },
      child: Scaffold(
        body: Container(
          child: Stack(children: <Widget>[
            Positioned(
              top: 0.0,
              left: 0.0,
              child: Container(child: Header(),),
            ),
            ListView(children: <Widget>[
              Container(
                margin: EdgeInsets.all(20),
                child: ...
              ),
            ]),
          ]),
        ),
      ),
    );
  }
}

Answer 1

I found the answer, I should only add the height and the width from the device screen width.

I hope that helps others, that learn flutter like me :)

  @override
  Widget build(BuildContext context) {
    final Size screenSize = MediaQuery.of(context).size;
    return WillPopScope(
      onWillPop: () {
        print('Back button pressed');
        Navigator.pop(context, false);
        return Future.value(false);
      },
      child: Scaffold(
        body: Stack(children: <Widget>[
          ListView(children: <Widget>[ 
            ...
          ]),
          Positioned(
            top: 20,
            height: 100,
            width: screenSize.width - 5,
            left: 0,
            child: Header(),
          ),
        ]),
      ),
    );
  }
}