小菜在学习过程中，为了调整或适配 Widget 大小时，会设置 Widget 或嵌套使用一些约束 Widget；小菜针对性学习一下这一系列的约束 Box；
SizedBox
源码分析
class SizedBox extends SingleChildRenderObjectWidget { // 创建固定大小的约束Box const SizedBox({ Key key, this.width, this.height, Widget child }) : super(key: key, child: child);
// 创建父类允许最大尺寸的约束Box const SizedBox.expand({ Key key, Widget child }) : width = double.infinity, height = double.infinity, super(key: key, child: child);
// 创建父类允许最小尺寸的约束Box const SizedBox.shrink({ Key key, Widget child }) : width = 0.0, height = 0.0, super(key: key, child: child);
// 创建指定大小的约束Box SizedBox.fromSize({ Key key, Widget child, Size size }) : width = size?.width, height = size?.height, super(key: key, child: child);}
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分析源码可知，SizeBox 继承自 SingleChildRenderObjectWidget 仅提供子 Child 的存储并不提供更新逻辑；且 SizedBox 提供了多种使用方法，小菜逐一尝试；
案例尝试
1. SizedBox({ Key key, this.width, this.height, Widget child })
SizedBox 可自定义 width 和 height，当限制宽高时，子 Widget 无论宽高如何，均默认填充；通过设置 double.infinity 填充父类 Widget 宽高，注意此时父类要有限制，不可是无限宽高；当 width 和 height 未设置时，根据子 Widget 大小展示；
return SizedBox(width: 100.0, height: 100.0, child: Container(color: Colors.teal.withOpacity(0.4)));return SizedBox(width: 100.0, height: 100.0, child: Container(color: Colors.red.withOpacity(0.4), width: 200.0, height: 200.0));return SizedBox(width: 100.0, height: 100.0, child: Container(color: Colors.pink.withOpacity(0.4), width: 50.0, height: 50.0));return SizedBox(width: double.infinity, height: 100.0, child: Container(color: Colors.blueAccent.withOpacity(0.4)));
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2. SizedBox.expand({ Key key, Widget child })
.expand 是便捷方法，默认 width 和 height 均为 double.infinity，填充整个父类 Widget；
return SizedBox.expand(child: Container(color: Colors.blueAccent.withOpacity(0.4)));
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3. SizedBox.shrink({ Key key, Widget child })
.shrink 同样是便捷方法，但 width 和 height 均为 0.0，无论子 Widget 设置多少均不展示；小菜暂时未想出此类方法的应用场景；
return SizedBox.shrink(child: Container(color: Colors.pink.withOpacity(0.4)));
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4. SizedBox.fromSize({ Key key, Widget child, Size size })
.fromSize 便捷方式与基本的构建方式基本相同，只是 width 和 height 由 Size 封装，小菜用此方法实现与基本方式相同的效果；
return SizedBox.fromSize(size: Size(100.0, 100.0), child: Container(color: Colors.teal.withOpacity(0.4)));return SizedBox.fromSize(size: Size(100.0, 100.0), child: Container(color: Colors.red.withOpacity(0.4), width: 200.0, height: 200.0));return SizedBox.fromSize(size: Size(100.0, 100.0), child: Container(color: Colors.pink.withOpacity(0.4), width: 50.0, height: 50.0));return SizedBox.fromSize(size: Size(double.infinity, 100.0), child: Container(color: Colors.blueAccent.withOpacity(0.4)));
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ConstrainedBox
源码分析
class ConstrainedBox extends SingleChildRenderObjectWidget { ConstrainedBox({ Key key, @required this.constraints, Widget child, })}
class BoxConstraints extends Constraints { // 用指定的约束大小创建框架大小 const BoxConstraints({ this.minWidth = 0.0, this.maxWidth = double.infinity, this.minHeight = 0.0, this.maxHeight = double.infinity, });
// 仅用指定大小创建框架大小 BoxConstraints.tight(Size size) : minWidth = size.width, maxWidth = size.width, minHeight = size.height, maxHeight = size.height;
// 用指定的约束大小创建框架大小 const BoxConstraints.tightFor({ double width, double height, })
// 创建需要给定宽度或高度的框约束，除非它们是无限的 const BoxConstraints.tightForFinite({ double width = double.infinity, double height = double.infinity, })
// 创建禁止大小大于给定大小的框约束 BoxConstraints.loose(Size size) : minWidth = 0.0, maxWidth = size.width, minHeight = 0.0, maxHeight = size.height;
// 创建扩展为填充另一个框约束的框约束 const BoxConstraints.expand({ double width, double height, })}
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分析源码可知，ConstrainedBox 同样是继承自 SingleChildRenderObjectWidget；约束方式主要是对 constraints 的操作；相对于 SizedBox 约束更为灵活；
案例尝试
1. BoxConstraints()
BoxConstraints 的构造方法可以设置最大最小宽度和高度四个属性，在实际场景中可以灵活运用；其约束的大小按照 minWidth <= child.width <= maxWidth && minHeight <= child.Height <= maxHeight，当我们不设置宽高尺寸时，minWidth = 0.0 && minHeight = 0.0 && maxWidth = double.infinity && maxHeight = double.infinity;
return ConstrainedBox(constraints: BoxConstraints(), child: Container(color: Colors.pink.withOpacity(0.4), child: _textWid('ConstrainedBox')));return ConstrainedBox(constraints: BoxConstraints(), child: Container(color: Colors.blueAccent.withOpacity(0.4), child: _textWid('ConstrainedBox: A widget that imposes additional constraints on its child.')));return ConstrainedBox(constraints: BoxConstraints(minWidth: 100.0, minHeight: 100.0), child: Container(color: Colors.orange.withOpacity(0.4), child: _textWid('ConstrainedBox')));return ConstrainedBox(constraints: BoxConstraints(maxHeight: 100.0, maxWidth: 100.0), child: Container(color: Colors.teal.withOpacity(0.4), child: _textWid('ConstrainedBox: A widget that imposes additional constraints on its child.')));
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2. BoxConstraints.tight(Size size)
.tight 构造方法中需要传一个固定尺寸的 Size 其中 minWidth == maxWidth == size.width && maxWidth == maxHeight == size.height；无论子 Widget 设置多大宽高均按照 Size 宽高约束；
return ConstrainedBox(constraints: BoxConstraints.tight(Size(110.0, 120.0)), child: Container(color: Colors.purple.withOpacity(0.4), child: _textWid('ConstrainedBox: A widget that imposes additional constraints on its child.')));
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3. BoxConstraints.tightFor()
小菜理解 .tightFor 方式是 .tight 方式的细化，可以自定义 width 和 height；若宽高未设置，则最大宽高为无限，最小宽高为 0.0；若设置宽高则以设置尺寸进行约束；
return ConstrainedBox(constraints: BoxConstraints.tightFor(height: 110.0), child: Container(color: Colors.teal.withOpacity(0.4), child: _textWid('ConstrainedBox: A widget that imposes additional constraints on its child.')));return ConstrainedBox(constraints: BoxConstraints.tightFor(width: 110.0, height: 120.0), child: Container(color: Colors.purple.withOpacity(0.4), child: _textWid('ConstrainedBox: A widget that imposes additional constraints on its child.')));
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4. BoxConstraints.tightForFinite()
.tightForFinite 与 .tightFor 类似，默认最大宽高为无限，最小宽高为 0.0，其判断的是设置宽高是否为无限，不是 double.infinity 则 minWidth == maxWidth == width && maxWidth == maxHeight == height 进行约束；
return ConstrainedBox(constraints: BoxConstraints.tightForFinite(), child: Container(color: Colors.teal.withOpacity(0.4), child: _textWid('ConstrainedBox: A widget that imposes additional constraints on its child.')));return ConstrainedBox(constraints: BoxConstraints.tightForFinite(width: 110.0, height: 120.0), child: Container(color: Colors.purple.withOpacity(0.4), child: _textWid('ConstrainedBox: A widget that imposes additional constraints on its child.')));
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5. BoxConstraints.loose()
.loose 方式限制给定 Size 大小，minWidth / minHeight 均为 0.0，最大宽高为 Size 大小；其中整体约束由 Size 限制，实际展示与子 Widget 大小相关；
return ConstrainedBox(constraints: BoxConstraints.loose(Size(110.0, 150.0)), child: Container(color: Colors.purple.withOpacity(0.4), child: _textWid('ConstrainedBox: A widget that imposes additional constraints on its child.')));return ConstrainedBox(constraints: BoxConstraints.loose(Size(110.0, 150.0)), child: Container(width: 200.0, height: 200.0, color: Colors.grey.withOpacity(0.6), child: _textWid('ConstrainedBox: A widget that imposes additional constraints on its child.')));return ConstrainedBox(constraints: BoxConstraints.loose(Size(110.0, 120.0)), child: Container(width: 100.0, height: 100.0, color: Colors.green.withOpacity(0.6), child: _textWid('ConstrainedBox: A widget that imposes additional constraints on its child.')));
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6. BoxConstraints.expand()
大部分 .expand 方式是填充方式，默认填满父 Widget，若不设置宽高，默认 minWidth == maxWidth == double.infinity && minHeight == maxHeight == double.infinity，使用时注意父类约束；
return ConstrainedBox(constraints: BoxConstraints.expand(), child: Container(color: Colors.teal.withOpacity(0.4), child: _textWid('ConstrainedBox: A widget that imposes additional constraints on its child.')));return ConstrainedBox(constraints: BoxConstraints.expand(width: 110.0, height: 150.0), child: Container(color: Colors.purple.withOpacity(0.4), child: _textWid('ConstrainedBox: A widget that imposes additional constraints on its child.')));
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Flutter 提供了多种约束 Box，所谓条条大路通罗马，同一种效果可以有多种实现方式，但小菜认为还是应该多多了解，尽量尝试最优方案；小菜对 Box 系列的研究还很浅显，有错误的地方请多多指导！
来源： 阿策小和尚
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