如何向可拖动组件添加 anchor (handle)，以便通过拖动调整其大小。

最佳答案

我认为此 SO post 中提供的示例原型(prototype)将是您想要实现的目标的理想选择。

1. Draw size handlers and a container;
2. Use GestureDetector to detect dragging;
3. Refresh the main container size and coordinates.

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Text Overflow Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        body: Demo(),
      ),
    );
  }
}

class Demo extends StatefulWidget {
  @override
  _DemoState createState() => _DemoState();
}

class _DemoState extends State<Demo> {
  @override
  Widget build(BuildContext context) {
    return Container(
      padding: EdgeInsets.all(60),
      child: ResizebleWidget(
        child: Text(
          '''I've just did simple prototype to show main idea.
  1. Draw size handlers with container;
  2. Use GestureDetector to get new variables of sizes
  3. Refresh the main container size.''',
        ),
      ),
    );
  }
}

class ResizebleWidget extends StatefulWidget {
  ResizebleWidget({required this.child});

  final Widget child;
  @override
  _ResizebleWidgetState createState() => _ResizebleWidgetState();
}

const ballDiameter = 30.0;

class _ResizebleWidgetState extends State<ResizebleWidget> {
  double height = 400;
  double width = 200;

  double top = 0;
  double left = 0;

  void onDrag(double dx, double dy) {
    var newHeight = height + dy;
    var newWidth = width + dx;

    setState(() {
      height = newHeight > 0 ? newHeight : 0;
      width = newWidth > 0 ? newWidth : 0;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Stack(
      children: <Widget>[
        Positioned(
          top: top,
          left: left,
          child: Container(
            height: height,
            width: width,
            color: Colors.red[100],
            child: widget.child,
          ),
        ),
        // top left
        Positioned(
          top: top - ballDiameter / 2,
          left: left - ballDiameter / 2,
          child: ManipulatingBall(
            onDrag: (dx, dy) {
              var mid = (dx + dy) / 2;
              var newHeight = height - 2 * mid;
              var newWidth = width - 2 * mid;

              setState(() {
                height = newHeight > 0 ? newHeight : 0;
                width = newWidth > 0 ? newWidth : 0;
                top = top + mid;
                left = left + mid;
              });
            },
          ),
        ),
        // top middle
        Positioned(
          top: top - ballDiameter / 2,
          left: left + width / 2 - ballDiameter / 2,
          child: ManipulatingBall(
            onDrag: (dx, dy) {
              var newHeight = height - dy;

              setState(() {
                height = newHeight > 0 ? newHeight : 0;
                top = top + dy;
              });
            },
          ),
        ),
        // top right
        Positioned(
          top: top - ballDiameter / 2,
          left: left + width - ballDiameter / 2,
          child: ManipulatingBall(
            onDrag: (dx, dy) {
              var mid = (dx + (dy * -1)) / 2;

              var newHeight = height + 2 * mid;
              var newWidth = width + 2 * mid;

              setState(() {
                height = newHeight > 0 ? newHeight : 0;
                width = newWidth > 0 ? newWidth : 0;
                top = top - mid;
                left = left - mid;
              });
            },
          ),
        ),
        // center right
        Positioned(
          top: top + height / 2 - ballDiameter / 2,
          left: left + width - ballDiameter / 2,
          child: ManipulatingBall(
            onDrag: (dx, dy) {
              var newWidth = width + dx;

              setState(() {
                width = newWidth > 0 ? newWidth : 0;
              });
            },
          ),
        ),
        // bottom right
        Positioned(
          top: top + height - ballDiameter / 2,
          left: left + width - ballDiameter / 2,
          child: ManipulatingBall(
            onDrag: (dx, dy) {
              var mid = (dx + dy) / 2;

              var newHeight = height + 2 * mid;
              var newWidth = width + 2 * mid;

              setState(() {
                height = newHeight > 0 ? newHeight : 0;
                width = newWidth > 0 ? newWidth : 0;
                top = top - mid;
                left = left - mid;
              });
            },
          ),
        ),
        // bottom center
        Positioned(
          top: top + height - ballDiameter / 2,
          left: left + width / 2 - ballDiameter / 2,
          child: ManipulatingBall(
            onDrag: (dx, dy) {
              var newHeight = height + dy;

              setState(() {
                height = newHeight > 0 ? newHeight : 0;
              });
            },
          ),
        ),
        // bottom left
        Positioned(
          top: top + height - ballDiameter / 2,
          left: left - ballDiameter / 2,
          child: ManipulatingBall(
            onDrag: (dx, dy) {
              var mid = ((dx * -1) + dy) / 2;

              var newHeight = height + 2 * mid;
              var newWidth = width + 2 * mid;

              setState(() {
                height = newHeight > 0 ? newHeight : 0;
                width = newWidth > 0 ? newWidth : 0;
                top = top - mid;
                left = left - mid;
              });
            },
          ),
        ),
        //left center
        Positioned(
          top: top + height / 2 - ballDiameter / 2,
          left: left - ballDiameter / 2,
          child: ManipulatingBall(
            onDrag: (dx, dy) {
              var newWidth = width - dx;

              setState(() {
                width = newWidth > 0 ? newWidth : 0;
                left = left + dx;
              });
            },
          ),
        ),
        // center center
        Positioned(
          top: top + height / 2 - ballDiameter / 2,
          left: left + width / 2 - ballDiameter / 2,
          child: ManipulatingBall(
            onDrag: (dx, dy) {
              setState(() {
                top = top + dy;
                left = left + dx;
              });
            },
          ),
        ),
      ],
    );
  }
}

class ManipulatingBall extends StatefulWidget {
  const ManipulatingBall({Key? key, required this.onDrag});

  final Function onDrag;

  @override
  _ManipulatingBallState createState() => _ManipulatingBallState();
}

class _ManipulatingBallState extends State<ManipulatingBall> {
  late double initX;
  late double initY;

  _handleDrag(details) {
    setState(() {
      initX = details.globalPosition.dx;
      initY = details.globalPosition.dy;
    });
  }

  _handleUpdate(details) {
    var dx = details.globalPosition.dx - initX;
    var dy = details.globalPosition.dy - initY;
    initX = details.globalPosition.dx;
    initY = details.globalPosition.dy;
    widget.onDrag(dx, dy);
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onPanStart: _handleDrag,
      onPanUpdate: _handleUpdate,
      child: Container(
        width: ballDiameter,
        height: ballDiameter,
        decoration: BoxDecoration(
          color: Colors.blue.withOpacity(0.5),
          shape: BoxShape.circle,
        ),
      ),
    );
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DoubleProperty('initX', initX));
  }
}

输出:

关于flutter - 如何在 flutter 中调整或调整 Draggable 组件的大小？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/55739713/