前言

做视频监控项目时需要需要展示多分屏，比如2x2、3x3、414等等，如果每一种分屏都单独实现会很麻烦，而且不能支持用户定制。最好的方式还是实现一个通用的分屏容器，而且采样比例计算位置大小，可以适配任意尺寸。

一、如何实现？

最直观的实现方式是获取控件宽高然后按比例计算，但是flutter在build的时候无法获取位置宽高信息，只有绘制之后才能获取，所以这种方式并不容易实现，比较简单的方式应该是使用Row、Column结合Flexible。

1、数值转成分数

需要转换的数值

 final Rect rect; //子控件位置大小,比例值范围0-1

定义一个分数对象

//分数
class Rational {
  int den = 1; //分母
  int num = 0; //分子
  Rational(this.num, this.den);
  //通过double构造，accuracy小数点后精度
  factory Rational.fromDouble(double d, {int accuracy = 5}) {
    int den = 1;
    while (d > d.toInt() && accuracy-- > 0) {
      d *= 10;
      den *= 10;
    }
    return Rational(d.toInt(), den);
  }
}

转成分数并对齐分母

    //将位置大小转成分数
    final width = Rational.fromDouble(rect.width);
    final x = Rational.fromDouble(rect.left);
    final height = Rational.fromDouble(rect.height);
    final y = Rational.fromDouble(rect.top);
    //对齐分母
    if (width.den != x.den) {
      final den = width.den;
      width.den *= x.den;
      width.num *= x.den;
      x.den *= den;
      x.num *= den;
    }
    //对齐分母
    if (height.den != y.den) {
      final den = height.den;
      height.den *= y.den;
      height.num *= y.den;
      y.den *= den;
      y.num *= den;
    }

2、Row+Flexible布局横向

我们利用Row的自动布局，以及Flexible的比例布局的特性，根据上面的分数计算出控件比例的位置大小对应的flex值即可。

 Row(
      children: [
        Flexible(
          flex: x.num,
          child: Container(),
        ),
        Flexible(
          flex: width.num,
          child: child/*子控件，加上纵向布局则是Column*/
        ),
        Flexible(flex: width.den - width.num - x.num, child: Container()),
      ],
    );
  }

3、Column+Flexible布局纵向

我们利用Column的自动布局，以及Flexible的比例布局的特性，根据上面的分数计算出控件比例的位置大小对应的flex值即可。

Column(
            children: [
              Flexible(
                flex: y.num,
                child: Container(),
              ),
              Flexible(flex: height.num, child: child/*子控件*/),
              Flexible(
                flex: height.den - height.num - y.num,
                child: Container(),
              ),
            ],
          )

二、完整代码

proportion.dart

import 'package:flutter/material.dart';

//比例布局控件,
class Proportion extends StatelessWidget {
  final Rect rect; //位置大小,比例值范围0-1
  final Widget child;
  const Proportion({
    super.key,
    this.rect = const Rect.fromLTWH(0, 0, 1, 1),
    required this.child,
  });

  @override
  Widget build(BuildContext context) {
    //实现按比例显示布局
    final width = Rational.fromDouble(rect.width);
    final x = Rational.fromDouble(rect.left);
    final height = Rational.fromDouble(rect.height);
    final y = Rational.fromDouble(rect.top);
    if (width.den != x.den) {
      final den = width.den;
      width.den *= x.den;
      width.num *= x.den;
      x.den *= den;
      x.num *= den;
    }
    if (height.den != y.den) {
      final den = height.den;
      height.den *= y.den;
      height.num *= y.den;
      y.den *= den;
      y.num *= den;
    }
    return Row(
      children: [
        Flexible(
          flex: x.num,
          child: Container(),
        ),
        Flexible(
          flex: width.num,
          child: Column(
            children: [
              Flexible(
                flex: y.num,
                child: Container(),
              ),
              Flexible(flex: height.num, child: child),
              Flexible(
                flex: height.den - height.num - y.num,
                child: Container(),
              ),
            ],
          ),
        ),
        Flexible(flex: width.den - width.num - x.num, child: Container()),
      ],
    );
  }
}

//分数
class Rational {
  int den = 1; //分母
  int num = 0; //分子
  Rational(this.num, this.den);
  //通过double构造，accuracy小数点后精度
  factory Rational.fromDouble(double d, {int accuracy = 5}) {
    int den = 1;
    while (d > d.toInt() && accuracy-- > 0) {
      d *= 10;
      den *= 10;
    }
    return Rational(d.toInt(), den);
  }
}

常用布局（可选）
proportions.dart

import 'package:flutter/material.dart';

import 'proportion.dart';

//常用布局,需配合stack作为父容器使用
class Proportions {
  Proportions._();
  //全屏
  static List<Proportion> fullScreen({
    required Widget child,
  }) =>
      [
        Proportion(
          rect: const Rect.fromLTWH(0, 0, 1, 1),
          child: child,
        )
      ];

  //二分屏
  static List<Proportion> halfScreen({
    required Widget left,
    required Widget right,
  }) =>
      [
        Proportion(
          rect: const Rect.fromLTWH(0, 0, 0.5, 1),
          child: left,
        ),
        Proportion(
          rect: const Rect.fromLTWH(0.5, 0, 0.5, 1),
          child: right,
        ),
      ];

  //四分屏
  static List<Proportion> quadScreen({
    required List<Widget> children,
  }) {
    return [
      Proportion(
        rect: const Rect.fromLTWH(0, 0, 0.5, 0.5),
        child: children[0],
      ), //左上
      Proportion(
        rect: const Rect.fromLTWH(0.5, 0, 0.5, 0.5),
        child: children[1],
      ), //右上
      Proportion(
        rect: const Rect.fromLTWH(0, 0.5, 0.5, 0.5),
        child: children[2],
      ), //左下
      Proportion(
        rect: const Rect.fromLTWH(0.5, 0.5, 0.5, 0.5),
        child: children[3],
      ), //右下
    ];
  }

  //6  分屏
  static List<Proportion> sixScreen({
    required List<Widget> children,
  }) {
    return [
      Proportion(
        rect: const Rect.fromLTWH(0, 0, 0.666, 0.666),
        child: children[0],
      ), //左上
      Proportion(
        rect: const Rect.fromLTWH(0.666, 0, 0.333, 0.333),
        child: children[1],
      ), //右上
      Proportion(
        rect: const Rect.fromLTWH(0.666, 0.333, 0.333, 0.333),
        child: children[2],
      ), //右中
      Proportion(
        rect: const Rect.fromLTWH(0.666, 0.666, 0.333, 0.333),
        child: children[3],
      ), //右下
      Proportion(
        rect: const Rect.fromLTWH(0.333, 0.666, 0.333, 0.333),
        child: children[4],
      ), //中下
      Proportion(
        rect: const Rect.fromLTWH(0, 0.666, 0.333, 0.333),
        child: children[5],
      ), //左下
    ];
  }

  //8  分屏
  static List<Proportion> eightScreen({
    required List<Widget> children,
  }) {
    return [
      Proportion(
        rect: const Rect.fromLTWH(0, 0, 0.75, 0.75),
        child: children[0],
      ), //左上
      Proportion(
        rect: const Rect.fromLTWH(0.75, 0, 0.25, 0.25),
        child: children[1],
      ), //右上
      Proportion(
        rect: const Rect.fromLTWH(0.75, 0.25, 0.25, 0.25),
        child: children[2],
      ), //右中1
      Proportion(
        rect: const Rect.fromLTWH(0.75, 0.5, 0.25, 0.25),
        child: children[3],
      ), //右中2
      Proportion(
        rect: const Rect.fromLTWH(0.75, 0.75, 0.25, 0.25),
        child: children[4],
      ), //右下
      Proportion(
        rect: const Rect.fromLTWH(0.5, 0.75, 0.25, 0.25),
        child: children[5],
      ), //中下2
      Proportion(
        rect: const Rect.fromLTWH(0.25, 0.75, 0.25, 0.25),
        child: children[6],
      ), //中下1
      Proportion(
        rect: const Rect.fromLTWH(0, 0.75, 0.25, 0.25),
        child: children[7],
      ), //左下
    ];
  }

  //9  分屏
  static List<Proportion> nightScreen({
    required List<Widget> children,
  }) {
    int n = 0;
    return [
      ...children.getRange(0, 9).map(
        (element) {
          final i = n++;
          return Proportion(
            rect: Rect.fromLTWH(
              (i % 3) * 0.333,
              (i ~/ 3) * 0.333,
              0.333,
              0.333,
            ),
            child: element,
          );
        },
      )
    ];
  }

  //16  分屏
  static List<Proportion> sixteenScreen({
    required List<Widget> children,
  }) {
    int n = 0;
    return [
      ...children.getRange(0, 16).map(
        (element) {
          final i = n++;
          return Proportion(
            rect: Rect.fromLTWH((i % 4) * 0.25, (i ~/ 4) * 0.25, 0.25, 0.25),
            child: element,
          );
        },
      )
    ];
  }

  //414分屏
  static List<Proportion> fourOneFourScreen({
    required List<Widget> children,
  }) {
    int n = 0;
    return [
      //左4
      ...children.getRange(0, 4).map(
        (element) {
          final i = n++;
          return Proportion(
            rect: Rect.fromLTWH((i ~/ 4) * 0.25, (i % 4) * 0.25, 0.25, 0.25),
            child: element,
          );
        },
      ),
      //中间
      Proportion(
        rect: const Rect.fromLTWH(0.25, 0, 0.5, 1),
        child: children[4],
      ),
      //右边4
      ...children.getRange(5, 9).map(
        (element) {
          final i = n++ + 8;
          return Proportion(
            rect: Rect.fromLTWH((i ~/ 4) * 0.25, (i % 4) * 0.25, 0.25, 0.25),
            child: element,
          );
        },
      )
    ];
  }
}

三、使用示例

1、基本用法

设置子控件位置大小。一般配合stack作为父容器使用

    Proportion(
      rect: Rect.fromLTRB(0, 0, 0.5, 0.5), //子控件位置大小，(0, 0, 0.5, 0.5)表示左上1/4的区域
      child: ColoredBox(color: Colors.red), //子控件
    );

2、四分屏

final List<int> _nums = [1, 2, 3, 4, 5, 6, 7, 8, 9];

 Stack(
            children: Proportions.quadScreen(children: [
          ..._nums.map((e) => Container(
                constraints: const BoxConstraints.expand(),
                decoration: BoxDecoration(
                    border: Border.all(color: Colors.deepPurple.shade300)),
                child: Center(child: Text("video $e")),
              ))

在这里插入图片描述

3、六分屏

final List<int> _nums = [1, 2, 3, 4, 5, 6, 7, 8, 9];

 Stack(
            children: Proportions.sixScreen(children: [
          ..._nums.map((e) => Container(
                constraints: const BoxConstraints.expand(),
                decoration: BoxDecoration(
                    border: Border.all(color: Colors.deepPurple.shade300)),
                child: Center(child: Text("video $e")),
              ))

在这里插入图片描述

4、八分屏

final List<int> _nums = [1, 2, 3, 4, 5, 6, 7, 8, 9];

 Stack(
            children: Proportions.eightScreen(children: [
          ..._nums.map((e) => Container(
                constraints: const BoxConstraints.expand(),
                decoration: BoxDecoration(
                    border: Border.all(color: Colors.deepPurple.shade300)),
                child: Center(child: Text("video $e")),
              ))

在这里插入图片描述

5、九分屏

final List<int> _nums = [1, 2, 3, 4, 5, 6, 7, 8, 9];

 Stack(
            children: Proportions.nightScreen(children: [
          ..._nums.map((e) => Container(
                constraints: const BoxConstraints.expand(),
                decoration: BoxDecoration(
                    border: Border.all(color: Colors.deepPurple.shade300)),
                child: Center(child: Text("video $e")),
              ))

在这里插入图片描述

6、414分屏

final List<int> _nums = [1, 2, 3, 4, 5, 6, 7, 8, 9];

 Stack(
            children: Proportions.fourOneFourScreen(children: [
          ..._nums.map((e) => Container(
                constraints: const BoxConstraints.expand(),
                decoration: BoxDecoration(
                    border: Border.all(color: Colors.deepPurple.shade300)),
                child: Center(child: Text("video $e")),
              ))

在这里插入图片描述
始终保持比例

总结

以上就是今天要讲的内容，本文用的是比较简单的方式实现了比例布局控件，其主要特点是可以灵活使用，尤其是方便视频分屏预览的实现。本质上也是对一类布局规则的总结得出的一个通用的控件，因为考虑到2x2、3x3还是可以写死的，但是到了4x4、5x5写死则需要16、25个参数，那就必须改用数组，也就意味着需要根据规则计算位置，那和本文一样了。所以本文的控件是有实际使用意义的。