Flutter 布局报错The following assert

编写 flutter 代码碰到一个错误

======== Exception caught by rendering library =====================================================
The following assertion was thrown during performLayout():
BoxConstraints forces an infinite width.

These invalid constraints were provided to _RenderColoredBox's layout() function by the following function, which probably computed the invalid constraints in question:
  RenderConstrainedBox.performLayout (package:flutter/src/rendering/proxy_box.dart:268:14)
The offending constraints were: BoxConstraints(w=Infinity, h=44.0)
The relevant error-causing widget was: 
  Container file:///Users/project_github/flutter/my_flutter_demo/lib/error_page.dart:20:20
When the exception was thrown, this was the stack: 
#0      BoxConstraints.debugAssertIsValid.<anonymous closure>.throwError (package:flutter/src/rendering/box.dart:517:9)
#1      BoxConstraints.debugAssertIsValid.<anonymous closure> (package:flutter/src/rendering/box.dart:559:21)
#2      BoxConstraints.debugAssertIsValid (package:flutter/src/rendering/box.dart:565:6)
#3      RenderObject.layout (package:flutter/src/rendering/object.dart:1677:24)
#4      RenderConstrainedBox.performLayout (package:flutter/src/rendering/proxy_box.dart:268:14)
...
The following RenderObject was being processed when the exception was fired: RenderConstrainedBox#e8eb9 relayoutBoundary=up2 NEEDS-LAYOUT NEEDS-PAINT NEEDS-COMPOSITING-BITS-UPDATE
...  parentData: bottom=0.0; offset=Offset(0.0, 515.0) (can use size)
...  constraints: BoxConstraints(unconstrained)
...  size: Size(360.0, 44.0)
...  additionalConstraints: BoxConstraints(w=Infinity, h=44.0)
RenderObject: RenderConstrainedBox#e8eb9 relayoutBoundary=up2 NEEDS-LAYOUT NEEDS-PAINT NEEDS-COMPOSITING-BITS-UPDATE
  parentData: bottom=0.0; offset=Offset(0.0, 515.0) (can use size)
  constraints: BoxConstraints(unconstrained)
  size: Size(360.0, 44.0)
  additionalConstraints: BoxConstraints(w=Infinity, h=44.0)
...  child: _RenderColoredBox#4f7c5 NEEDS-LAYOUT NEEDS-PAINT NEEDS-COMPOSITING-BITS-UPDATE
...    parentData: <none> (can use size)
...    constraints: BoxConstraints(w=360.0, h=44.0)
...    size: Size(360.0, 44.0)
...    behavior: opaque
...    child: RenderPositionedBox#c8a6c NEEDS-LAYOUT NEEDS-PAINT NEEDS-COMPOSITING-BITS-UPDATE
...      parentData: <none> (can use size)
...      constraints: BoxConstraints(w=360.0, h=44.0)
...      size: Size(360.0, 44.0)
...      alignment: center
...      textDirection: ltr
...      widthFactor: expand
...      heightFactor: expand
...      child: RenderParagraph#2f2e8 relayoutBoundary=up1 NEEDS-LAYOUT NEEDS-PAINT NEEDS-COMPOSITING-BITS-UPDATE
...        parentData: offset=Offset(152.0, 11.5) (can use size)
...        constraints: BoxConstraints(0.0<=w<=360.0, 0.0<=h<=44.0)
...        size: Size(56.0, 21.0)
...        textAlign: start
...        textDirection: ltr
...        softWrap: wrapping at box width
...        overflow: clip
...        locale: en_US
...        maxLines: unlimited
...        text: TextSpan
...          debugLabel: ((englishLike body1 2014).merge(blackMountainView bodyText2)).merge(unknown)
...          inherit: false
...          color: Color(0xff000000)
...          family: Roboto
...          size: 14.0
...          weight: 500
...          baseline: alphabetic
...          decoration: TextDecoration.none
...          "确认"
====================================================================================================

我的代码

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

class ErrorPage extends StatefulWidget {
  @override
  _MyErrorPageState createState() => _MyErrorPageState();

}

class _MyErrorPageState extends State<ErrorPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("测试页面"),),
      body: Stack(
        children: [
          Positioned(
            bottom: 0,
            child: Container(
            color: Colors.green,
            width: double.infinity,
            height: 44,
            alignment: Alignment.center,
            child: Text(
              "确认",
              style: TextStyle(
                  color: Colors.black,
                  fontSize: 14,
                  fontWeight: FontWeight.w500
              ),
            ),
          )
          )
        ],
      )
    );
  }

}

解决办法

给Positioned 设置一个宽度即可

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

class ErrorPage extends StatefulWidget {
  @override
  _MyErrorPageState createState() => _MyErrorPageState();
}

class _MyErrorPageState extends State<ErrorPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("测试页面"),
        ),
        body: Stack(
          children: [
            Positioned(
                bottom: 0,
                width: MediaQuery.of(context).size.width,//给 Positioned 设置一个宽度即可
                child: Container(
                  color: Colors.green,
                  width: double.infinity,
                  height: 44,
                  alignment: Alignment.center,
                  child: Text(
                    "确认",
                    style: TextStyle(
                        color: Colors.black,
                        fontSize: 14,
                        fontWeight: FontWeight.w500),
                  ),
                ))
          ],
        ));
  }
}