Flutter中如何让叠加组件居中显示？

Question

我在一个 stack 中有小部件，因此我想将我的按钮栏放置在堆栈的中心底部，但是什么都不起作用。小部件只会粘在左侧。这是我的代码。

new Positioned(
            bottom: 40.0,
            child: new Container(
              margin: const EdgeInsets.all(16.0),
              child: new Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
                  new Align(
                    alignment: Alignment.bottomCenter,
                    child: new ButtonBar(
                      alignment: MainAxisAlignment.center,
                      children: <Widget>[
                        new OutlineButton(
                          onPressed: () {
                            Navigator.push(
                                context,
                                new MaterialPageRoute(
                                    builder: (context) => new LoginPage()));
                          },
                          child: new Text(
                            "Login",
                            style: new TextStyle(color: Colors.white),
                          ),
                        ),
                        new RaisedButton(
                          color: Colors.white,
                          onPressed: () {
                            Navigator.push(
                                context,
                                new MaterialPageRoute(
                                    builder: (context) =>
                                        new RegistrationPage()));
                          },
                          child: new Text(
                            "Register",
                            style: new TextStyle(color: Colors.black),
                          ),
                        )
                      ],
                    ),
                  )
                ],
              ),
            ),
          )

我已经尝试了每一种居中对齐方式，请帮忙。

- spongyboss

---

你是否尝试在Column上使用 mainAxisSize: MainAxisSize.min？并且移除 Align 组件。 - Rémi Rousselet

是的，我尝试过了@RémiRousselet，它仍然靠左对齐。 - spongyboss

Answer 1

你可以在 Stack 中使用 Align 和 Positioned.fill：

Stack(
  children: <Widget>[      
    Positioned.fill(
      child: Align(
        alignment: Alignment.centerRight,
        child: ....                
      ),
    ),
  ],
),

- Yousif khalid

---

调用 Positioned.fill() 方法时，您将获得完整的尺寸（它将填充视图），然后 Align 可以在其中进行任何所需的对齐方式。如果您将 Align 替换为一个带有红色背景的容器，则堆栈将全部变成红色。 - Bernardo Ferrari

我只使用了对齐功能，它完成了工作。 - Jakub S.

Answer 2

可能是最优雅的方法。

您可以直接使用Stack中存在的alignment选项。

child: Stack(
  alignment: Alignment.center
)

- Sunit Gautam

---

确实，这是最优雅的方式。 - Vikalp

完美运作，不会像Positioned.fill那样拉伸内容。 - kashlo

目前最佳答案！！或许是因为这个功能最近被添加到 Stack widget 中。 - Pablo Insua

是的，太棒了。 - Masahiro Aoki

但是当您在堆栈上有多个子项并且只想让其中一个位于中心时，该如何实现呢？ - parkorian

Answer 3

如果有人到达这里，无法解决他们的问题，我曾经通过将右侧和左侧都设置为0来使我的小部件水平居中，如下所示：

Stack(
  children: <Widget>[
    Positioned(
      top: 100,
      left: 0,
      right: 0,
      child: Text("Search",
        style: TextStyle(
          color: Color(0xff757575),
          fontWeight: FontWeight.w700,
          fontFamily: "Roboto",
          fontStyle: FontStyle.normal,
          fontSize: 56.0,
        ),
        textAlign: TextAlign.center,
      ),
    ),
  ]
)

- Amir Ghezelbash

---

你能解释一下为什么需要左和右值吗？ - Abdul Rahman Shamair

在网页中常用的一种做法。 - esengineer

现在这就是我所说的简单解决方案！谢谢伙计。 - Anoop Thiruonam

不，它只是使其占据整个宽度。 - Eight Rice

这只能工作是因为textAlign: center。小部件被拉伸到边缘。 - Danny Dulai

Answer 4

只保留这个，删除其他所有内容：

Align(
  alignment: Alignment.bottomCenter,
  child: new ButtonBar(
    alignment: MainAxisAlignment.center,
    children: <Widget>[
      new OutlineButton(
        onPressed: () {
          Navigator.push(
            context,
            new MaterialPageRoute(
              builder: (context) => new LoginPage()));
        },
        child: new Text(
          "Login",
          style: new TextStyle(color: Colors.white),
        ),
      ),
      new RaisedButton(
        color: Colors.white,
        onPressed: () {
          Navigator.push(
            context,
            new MaterialPageRoute(
              builder: (context) =>
                new RegistrationPage()));
        },
        child: new Text(
          "Register",
          style: new TextStyle(color: Colors.black),
        ),
      )
    ],
  ),
)

在我的理论中，额外的Container正在破坏它。我建议您只需添加Padding来包围它：

根据我的理论，额外的Container会破坏它。建议您通过添加Padding进行包围。

Padding(
  padding: EdgeInsets.only(bottom: 20.0),
  child: Align...
),

这对我来说比Positioned更合理，而且我不太理解你的只有一个子节点的Column。

- creativecreatorormaybenot

---

哇，谢谢你，老兄。说实话，我从来不知道Align可以在Stack中定位小部件。 - spongyboss

Answer 5

感谢以上所有回答，我想分享一些可能在某些情况下有用的东西。那么让我们看看当您使用Positioned:( right: 0.0, left:0.0, bottom:0.0)时会发生什么：

      Padding(
        padding: const EdgeInsets.all(4.0),
        child: Stack(
          children: <Widget>[
            Positioned(
                bottom: 0.0,
                right: 0.0,
                left: 0.0,
                child: Padding(
                  padding: const EdgeInsets.symmetric(horizontal: 8.0),
                  child: Container(
                      color: Colors.blue,
                      child: Center(
                        child: Text('Hello',
                          style: TextStyle(color: Color(0xffF6C37F),
                          fontSize: 46, fontWeight: FontWeight.bold),),
                      )
                  ),
                )
            ),
          ],
        ),
      ),

这将是上述代码的输出：

如您所见，它将使用容器的整个宽度填充，即使您不需要它，只想让容器包裹其子元素。因此，您可以尝试下面的技巧：

      Padding(
        padding: const EdgeInsets.all(4.0),
        child: Stack(
          children: <Widget>[
            Positioned(
                bottom: 0.0,
                right: 0.0,
                left: 0.0,
                child: Row(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: <Widget>[
                    Container(),
                    Padding(
                      padding: const EdgeInsets.symmetric(horizontal: 8.0),
                      child: Container(
                          color: Colors.blue,
                          child: Text('Hello',
                            style: TextStyle(color: Color(0xffF6C37F), 
                            fontSize: 46, fontWeight: FontWeight.bold),)
                      ),
                    ),
                    Container(),
                  ],
                )
            ),
          ],
        ),
      ),

- Taba

Answer 6

你可以使用Stack中的Align来改变已定位的位置：

Align(
  alignment: Alignment.bottomCenter,
  child: ... ,
),

了解有关 Stack 的更多信息：探索 Stack

- Marco

---

到目前为止，这是最好的解决方案，特别适用于定位“Text”元素。使用left:0和right:0的Positioned会水平扩展实际小部件。 - Osman

Answer 7

问题在于容器的尺寸最小化。只需为容器（红色）设置一个宽度，就可以解决问题。{{width: MediaQuery.of(context).size.width}}

  new Positioned(
  bottom: 0.0,
  child: new Container(
    width: MediaQuery.of(context).size.width,
    color: Colors.red,
    margin: const EdgeInsets.all(0.0),
    child: new Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        new Align(
          alignment: Alignment.bottomCenter,
          child: new ButtonBar(
            alignment: MainAxisAlignment.center,
            children: <Widget>[
              new OutlineButton(
                onPressed: null,
                child: new Text(
                  "Login",
                  style: new TextStyle(color: Colors.white),
                ),
              ),
              new RaisedButton(
                color: Colors.white,
                onPressed: null,
                child: new Text(
                  "Register",
                  style: new TextStyle(color: Colors.black),
                ),
              )
            ],
          ),
        )
      ],
    ),
  ),
),

- chemamolins

---

为什么你不在这里使用列/行呢？ - user1717140

有不同的解决方案可以实现预期的结果。我的回答是试图解释为什么OP提供的解决方案无法工作。 - chemamolins

Answer 8

看看我想出来的这个解决方案

Positioned(child: SizedBox(child: CircularProgressIndicator(), width: 50, height: 50,), left: MediaQuery.of(context).size.width / 2 - 25);

- RIX

---

在使用相对较小的小部件时，这实际上是有效的，您可以设置固定宽度。 - Levi Roelofsma

Answer 9

我最有效的方法是使用Align。我需要将用户的个人资料图片放在封面图片的底部中心位置。

return Container(
  height: 220,
  color: Colors.red,
  child: Stack(
    children: [
      Container(
        height: 160,
        color: Colors.yellow,
      ),
      Align(
        alignment: Alignment.bottomCenter,
        child: UserProfileImage(),
      ),
    ],
  ),
);

这个工作得像魔法一样。

- theCaptainXgod

Answer 10

它可以像Positioned.center一样工作，您确定小部件的中心并设置小部件和点的相同中心。

import 'dart:developer';

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
import 'dart:math' as math;

class AlignPositioned extends SingleChildRenderObjectWidget {
  const AlignPositioned({
    Key? key,
    this.alignment = Alignment.center,
    required this.centerPoint,
    this.widthFactor,
    this.heightFactor,
    Widget? child,
  })  : assert(widthFactor == null || widthFactor >= 0.0),
        assert(heightFactor == null || heightFactor >= 0.0),
        super(key: key, child: child);

  final AlignmentGeometry alignment;
  final Offset centerPoint;

  final double? widthFactor;

  final double? heightFactor;

  @override
  _RenderPositionedBox createRenderObject(BuildContext context) {
    return _RenderPositionedBox(
      alignment: alignment,
      widthFactor: widthFactor,
      heightFactor: heightFactor,
      textDirection: Directionality.maybeOf(context),
      centerPoint: this.centerPoint,
    );
  }

  @override
  void updateRenderObject(
      BuildContext context, _RenderPositionedBox renderObject) {
    renderObject
      ..alignment = alignment
      ..widthFactor = widthFactor
      ..heightFactor = heightFactor
      ..textDirection = Directionality.maybeOf(context)
      ..centerPoint = centerPoint;
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties
        .add(DiagnosticsProperty<AlignmentGeometry>('alignment', alignment));
    properties
        .add(DoubleProperty('widthFactor', widthFactor, defaultValue: null));
    properties
        .add(DoubleProperty('heightFactor', heightFactor, defaultValue: null));
  }
}

class _RenderPositionedBox extends RenderAligningShiftedBox {
  Offset centerPoint;

  _RenderPositionedBox({
    RenderBox? child,
    double? widthFactor,
    double? heightFactor,
    AlignmentGeometry alignment = Alignment.center,
    TextDirection? textDirection,
    required this.centerPoint,
  })  : assert(widthFactor == null || widthFactor >= 0.0),
        assert(heightFactor == null || heightFactor >= 0.0),
        _widthFactor = widthFactor,
        _heightFactor = heightFactor,
        super(child: child, alignment: alignment, textDirection: textDirection);

  double? get widthFactor => _widthFactor;
  double? _widthFactor;

  set widthFactor(double? value) {
    assert(value == null || value >= 0.0);
    if (_widthFactor == value) return;
    _widthFactor = value;
    markNeedsLayout();
  }

  set alignment(AlignmentGeometry value) {
    super.alignment = value;
    _resolvedAlignment = alignment.resolve(textDirection);
  }

  double? get heightFactor => _heightFactor;
  double? _heightFactor;
  late Alignment _resolvedAlignment = alignment.resolve(textDirection);

  set heightFactor(double? value) {
    assert(value == null || value >= 0.0);
    if (_heightFactor == value) return;
    _heightFactor = value;
    markNeedsLayout();
  }

  @override
  Size computeDryLayout(BoxConstraints constraints) {
    final bool shrinkWrapWidth =
        _widthFactor != null || constraints.maxWidth == double.infinity;
    final bool shrinkWrapHeight =
        _heightFactor != null || constraints.maxHeight == double.infinity;
    if (child != null) {
      final Size childSize = child!.getDryLayout(constraints.loosen());
      return constraints.constrain(Size(
        shrinkWrapWidth
            ? childSize.width * (_widthFactor ?? 1.0)
            : double.infinity,
        shrinkWrapHeight
            ? childSize.height * (_heightFactor ?? 1.0)
            : double.infinity,
      ));
    }
    return constraints.constrain(Size(
      shrinkWrapWidth ? 0.0 : double.infinity,
      shrinkWrapHeight ? 0.0 : double.infinity,
    ));
  }

  @override
  void performLayout() {
    final BoxConstraints constraints = this.constraints;
    final bool shrinkWrapWidth =
        _widthFactor != null || constraints.maxWidth == double.infinity;
    final bool shrinkWrapHeight =
        _heightFactor != null || constraints.maxHeight == double.infinity;

    if (child != null) {
      child!.layout(constraints.loosen(), parentUsesSize: true);
      size = constraints.constrain(Size(
        shrinkWrapWidth
            ? child!.size.width * (_widthFactor ?? 1.0)
            : double.infinity,
        shrinkWrapHeight
            ? child!.size.height * (_heightFactor ?? 1.0)
            : double.infinity,
      ));

      final BoxParentData childParentData = child!.parentData! as BoxParentData;
      final moveX = _resolvedAlignment.x - 1;
      final moveY = _resolvedAlignment.y - 1;
      log(_resolvedAlignment.y.toString());
      childParentData.offset = this.centerPoint +
          Offset(
            child!.size.width / 2 * moveX,
            child!.size.height / 2 * moveY,
          );
      // log(_resolvedAlignment.x.toString());
      // log(_resolvedAlignment.y.toString());

      // _resolvedAlignment.alongOffset(size - child!.size as Offset);
    } else {
      size = constraints.constrain(Size(
        shrinkWrapWidth ? 0.0 : double.infinity,
        shrinkWrapHeight ? 0.0 : double.infinity,
      ));
    }
  }

  @override
  void debugPaintSize(PaintingContext context, Offset offset) {
    super.debugPaintSize(context, offset);
    assert(() {
      final Paint paint;
      if (child != null && !child!.size.isEmpty) {
        final Path path;
        paint = Paint()
          ..style = PaintingStyle.stroke
          ..strokeWidth = 1.0
          ..color = const Color(0xFFFFFF00);
        path = Path();
        final BoxParentData childParentData =
            child!.parentData! as BoxParentData;
        if (childParentData.offset.dy > 0.0) {
          // vertical alignment arrows
          final double headSize =
              math.min(childParentData.offset.dy * 0.2, 10.0);
          path
            ..moveTo(offset.dx + size.width / 2.0, offset.dy)
            ..relativeLineTo(0.0, childParentData.offset.dy - headSize)
            ..relativeLineTo(headSize, 0.0)
            ..relativeLineTo(-headSize, headSize)
            ..relativeLineTo(-headSize, -headSize)
            ..relativeLineTo(headSize, 0.0)
            ..moveTo(offset.dx + size.width / 2.0, offset.dy + size.height)
            ..relativeLineTo(0.0, -childParentData.offset.dy + headSize)
            ..relativeLineTo(headSize, 0.0)
            ..relativeLineTo(-headSize, -headSize)
            ..relativeLineTo(-headSize, headSize)
            ..relativeLineTo(headSize, 0.0);
          context.canvas.drawPath(path, paint);
        }
        if (childParentData.offset.dx > 0.0) {
          // horizontal alignment arrows
          final double headSize =
              math.min(childParentData.offset.dx * 0.2, 10.0);
          path
            ..moveTo(offset.dx, offset.dy + size.height / 2.0)
            ..relativeLineTo(childParentData.offset.dx - headSize, 0.0)
            ..relativeLineTo(0.0, headSize)
            ..relativeLineTo(headSize, -headSize)
            ..relativeLineTo(-headSize, -headSize)
            ..relativeLineTo(0.0, headSize)
            ..moveTo(offset.dx + size.width, offset.dy + size.height / 2.0)
            ..relativeLineTo(-childParentData.offset.dx + headSize, 0.0)
            ..relativeLineTo(0.0, headSize)
            ..relativeLineTo(-headSize, -headSize)
            ..relativeLineTo(headSize, -headSize)
            ..relativeLineTo(0.0, headSize);
          context.canvas.drawPath(path, paint);
        }
      } else {
        paint = Paint()..color = const Color(0x90909090);
        context.canvas.drawRect(offset & size, paint);
      }
      return true;
    }());
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties
        .add(DoubleProperty('widthFactor', _widthFactor, ifNull: 'expand'));
    properties
        .add(DoubleProperty('heightFactor', _heightFactor, ifNull: 'expand'));
  }
}

- Hellomik U