颤振位置堆栈小部件在中心

您可以在 Stack 中将 Positioned.fill 与 Align 一起使用：

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

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可能是最优雅的方式。

您可以简单地使用 Stack 中的 alignment 选项

child: Stack(
  alignment: Alignment.center
)

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删除所有内容，但是：

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。我建议您通过添加 Padding 来包围它：

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

这对我来说似乎比 Positioned 更合理，而且我不太了解您的 Column 只有一个孩子。

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对于到达这里但无法解决问题的任何人，我曾经通过将左右都设置为 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,
      ),
    ),
  ]
)

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感谢以上所有答案，我想分享一些在某些情况下可能会派上用场的东西。因此，让我们看看使用 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：

https://i.stack.imgur.com/9r284.png

如您所见，即使您不想要它，它也会用容器填充整个宽度，而您只希望容器包裹其子项。所以为此你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(),
                  ],
                )
            ),
          ],
        ),
      ),

https://i.stack.imgur.com/2k888.png

---

您可以在堆栈内使用对齐更改定位：

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

有关堆栈的更多信息：Exploring Stack

---

问题是容器的尺寸尽可能小。

只需给 Container 一个 width:（红色）就可以了。

width: MediaQuery.of(context).size.width

https://i.stack.imgur.com/CYvmz.png

  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),
                ),
              )
            ],
          ),
        )
      ],
    ),
  ),
),

---

看看我想出的这个解决方案

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

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对我来说最好的方法是使用 Align。我需要在封面图片的底部中心制作用户的个人资料图片。

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

这就像一个魅力。

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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'));
  }
}

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你也可以试试这个：

Center(
  child: Stack(
    children: [],
  ),
)

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