颤振布局问题：堆栈 z 顺序不起作用

Question

栈子布局的 z-order 与我的预期不同。

我在堆栈中有 4 个带有键的小部件：0、1、2、3。我尝试更改它们的 z 顺序并重建堆栈小部件。这是我的代码：

class TestBringToFrontPage extends StatefulWidget {
  TestBringToFrontPage({Key key}) : super(key: key);

  @override
  State<TestBringToFrontPage> createState() => TestBringToFrontState();
}

class TestBringToFrontState extends State<TestBringToFrontPage> {
  List<Widget> _widgets = [];

  @override
  Widget build(BuildContext context) {
    return Stack(children: <Widget>[
      ..._widgets,
      Positioned(
          bottom: 0,
          right: 0,
          child: RaisedButton(
            child: Text(
              "click",
              style: TextStyle(color: Colors.white),
            ),
            onPressed: () {
              setState(() {
                _swap(0, 2);
                _swap(1, 3);
                print("$_widgets");
              });
            },
          )),
    ]);
  }

  @override
  void initState() {
    super.initState();

    const double start = 100;
    const double size = 100;
    _widgets = <Widget>[
      _buildWidget(key: const Key("0"), color: Colors.blue, offset: const Offset(start, start), size: const Size(size, size)),
      _buildWidget(key: const Key("1"), color: Colors.green, offset: const Offset(start + size / 2, start), size: const Size(size, size)),
      _buildWidget(key: const Key("2"), color: Colors.yellow, offset: const Offset(start, start + size / 2), size: const Size(size, size)),
      _buildWidget(key: const Key("3"), color: Colors.red, offset: const Offset(start + size / 2, start + size / 2), size: const Size(size, size)),
    ];
  }

  Widget _buildWidget({Key key, Color color, Offset offset, Size size}) {
    final label = (key as ValueKey<String>)?.value;
    return Positioned(
        key: key,
        left: 0,
        top: 0,
        child: Container(
          transform: Matrix4.identity()..translate(offset.dx, offset.dy, 0),
          width: size.width,
          height: size.height,
          decoration: BoxDecoration(border: Border.all(color: Colors.black, width: 1.0), color: color),
          child: Text(
            label,
            textAlign: TextAlign.left,
            style: TextStyle(
              color: Colors.black,
              fontWeight: FontWeight.w700,
              decoration: TextDecoration.none,
              fontSize: 15.0,
            ),
          ),
        ));
  }

  void _swap(int x, int y) {
    final w = _widgets[x];
    _widgets[x] = _widgets[y];
    _widgets[y] = w;
  }
}

在开始时，堆栈子级按 z 顺序排列（从下到上）：0,1,2,3。屏幕显示： Starting screen state image

通过单击按钮，我希望堆栈子级以新的 z 顺序排列：2,3,0,1。屏幕应显示： Expected screen state image

但与预期不同，屏幕显示： Actual screen state image.

点击按钮时的控制台日志：“[Positioned-[](left: 0.0, top: 0.0), Positioned-[](left: 0.0, top: 0.0), Positioned-[](left: 0.0, top: 0.0), Positioned-[](left: 0.0, top: 0.0)]"

在 Flutter 检查器窗口中，堆栈子项的顺序正确：2,3,0,1。但是堆栈呈现错误。

你知道我错在哪里还是颤振布局问题？ 提前谢谢你，

Answer 1

在玩了一会儿你的代码之后，我发现每个块上的 Key 都处于某种状态，这会在 setState 期间弄乱它们的顺序。移除钥匙会给我你所期望的行为。我不知道为什么会发生这种情况，但至少现在你有一个解决方案，如果你的目标不需要密钥。

不管他们是不是，如果我是你，我会提交一个错误报告，因为我很确定这种行为不是故意的。

这是我使用的有效代码（我清理了一点，所以它不会覆盖initState）：

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: TestBringToFrontPage(),
    );
  }
}

class TestBringToFrontPage extends StatefulWidget {
  TestBringToFrontPage({Key key}) : super(key: key);

  @override
  State<TestBringToFrontPage> createState() => TestBringToFrontState();
}

class TestBringToFrontState extends State<TestBringToFrontPage> {
  static const start = 50.0;
  static const size = 250.0;

  final _widgets = <Widget>[
      _buildWidget(label: "0", color: Colors.blue, offset: const Offset(start, start), size: const Size(size, size)),
      _buildWidget(label: "1", color: Colors.green, offset: const Offset(start + size / 2, start), size: const Size(size, size)),
      _buildWidget(label: "2", color: Colors.yellow, offset: const Offset(start, start + size / 2), size: const Size(size, size)),
      _buildWidget(label: "3", color: Colors.red, offset: const Offset(start + size / 2, start + size / 2), size: const Size(size, size)),
    ];

  @override
  Widget build(BuildContext context) {
    return Stack(children: <Widget>[
      ..._widgets,
      Positioned(
          bottom: 0,
          right: 0,
          child: RaisedButton(
            child: Text(
              "click",
              style: TextStyle(color: Colors.white),
            ),
            onPressed: () {
              setState(() {
                _swap(0, 2);
                _swap(1, 3);
                print("$_widgets");
              });
            },
          )),
    ]);
  }

  static Widget _buildWidget({String label, Color color, Offset offset, Size size}) {
    return Positioned(
        left: 0,
        top: 0,
        child: Container(
          transform: Matrix4.identity()..translate(offset.dx, offset.dy, 0),
          width: size.width,
          height: size.height,
          decoration: BoxDecoration(border: Border.all(color: Colors.black, width: 1.0), color: color),
          child: Text(
            label,
            textAlign: TextAlign.left,
            style: TextStyle(
              color: Colors.black,
              fontWeight: FontWeight.w700,
              decoration: TextDecoration.none,
              fontSize: 30.0,
            ),
          ),
        ));
  }

  void _swap(int x, int y) {
    final w = _widgets[x];
    _widgets[x] = _widgets[y];
    _widgets[y] = w;
  }
}