Container

Container（容器控件）在Flutter是经常使用的控件，它就相当于我们HTML里的<div>标签。
容器属性：
Alignment：这个属性针对的是Container内child的对齐方式，也就是容器子内容的对齐方式，并不是容器本身的对齐方式。
child ：容器内的子组件。
clipBehavior:裁剪行为。当Container.decaration中有clipPath时此值生效。默认值为：Clip.none
color:Container（容器）的背景色（color和dec0ration不能同时存在）
constraints：对child附加的约束。构造函数的width和height参数与constraints参数相结合来设置此属性。
dec0ration： Container 的(背景)修饰器，层级比child低。主要的功能是设置背景和边框。
foregroundDecoration: Container 的(前景)修饰器。层级比child高。
其他：margin,padding,height,width,transform等比较简单可以自行查阅官方文档
示例：

        Container(
          alignment: Alignment.center,
          constraints: BoxConstraints(minHeight: 200, minWidth: 200),
          decoration: BoxDecoration(
            color: Colors.red,//背景颜色
            border: Border.all(//边框设置
              color: Colors.red,
              width: 1,
            ),
          ),
          height: 100,
          width: 100,
          child: Text(
            '测试测试',
            style: TextStyle(color: Colors.blue),
          ),
        ),

效果如图：

屏幕快照 2020-07-10 下午4.55.14.png

这里我们宽高设置100，但是实际尺寸是200这是因为我们设置了 constraints属性去掉后效果如图：

屏幕快照 2020-07-10 下午7.27.16.png

BoxConstraints 类源码如下

class BoxConstraints extends Constraints {
  /// Creates box constraints with the given constraints.
  const BoxConstraints({
    this.minWidth = 0.0,
    this.maxWidth = double.infinity,
    this.minHeight = 0.0,
    this.maxHeight = double.infinity,
  }) : assert (minWidth != null),
       assert (maxWidth != null),
       assert (minHeight != null),
       assert (maxHeight != null);

  /// Creates box constraints that is respected only by the given size.
  BoxConstraints.tight(Size size)
    : minWidth = size.width,
      maxWidth = size.width,
      minHeight = size.height,
      maxHeight = size.height;

  /// Creates box constraints that require the given width or height.
  ///
  /// See also:
  ///
  ///  * [new BoxConstraints.tightForFinite], which is similar but instead of
  ///    being tight if the value is non-null, is tight if the value is not
  ///    infinite.
  const BoxConstraints.tightFor({
    double width,
    double height,
  }) : minWidth = width ?? 0.0,
       maxWidth = width ?? double.infinity,
       minHeight = height ?? 0.0,
       maxHeight = height ?? double.infinity;

  /// Creates box constraints that require the given width or height, except if
  /// they are infinite.
  ///
  /// See also:
  ///
  ///  * [new BoxConstraints.tightFor], which is similar but instead of being
  ///    tight if the value is not infinite, is tight if the value is non-null.
  const BoxConstraints.tightForFinite({
    double width = double.infinity,
    double height = double.infinity,
  }) : minWidth = width != double.infinity ? width : 0.0,
       maxWidth = width != double.infinity ? width : double.infinity,
       minHeight = height != double.infinity ? height : 0.0,
       maxHeight = height != double.infinity ? height : double.infinity;

  /// Creates box constraints that forbid sizes larger than the given size.
  BoxConstraints.loose(Size size)
    : minWidth = 0.0,
      maxWidth = size.width,
      minHeight = 0.0,
      maxHeight = size.height;

  /// Creates box constraints that expand to fill another box constraints.
  ///
  /// If width or height is given, the constraints will require exactly the
  /// given value in the given dimension.
  const BoxConstraints.expand({
    double width,
    double height,
  }) : minWidth = width ?? double.infinity,
       maxWidth = width ?? double.infinity,
       minHeight = height ?? double.infinity,
       maxHeight = height ?? double.infinity;

  /// The minimum width that satisfies the constraints.
  final double minWidth;

  /// The maximum width that satisfies the constraints.
  ///
  /// Might be [double.infinity].
  final double maxWidth;

  /// The minimum height that satisfies the constraints.
  final double minHeight;

  /// The maximum height that satisfies the constraints.
  ///
  /// Might be [double.infinity].
  final double maxHeight;

  /// Creates a copy of this box constraints but with the given fields replaced with the new values.
  BoxConstraints copyWith({
    double minWidth,
    double maxWidth,
    double minHeight,
    double maxHeight,
  }) {
    return BoxConstraints(
      minWidth: minWidth ?? this.minWidth,
      maxWidth: maxWidth ?? this.maxWidth,
      minHeight: minHeight ?? this.minHeight,
      maxHeight: maxHeight ?? this.maxHeight,
    );
  }
....
}

const BoxConstraints({
    this.minWidth = 0.0,
    this.maxWidth = double.infinity,
    this.minHeight = 0.0,
    this.maxHeight = double.infinity,
  }) : assert (minWidth != null),
       assert (maxWidth != null),
       assert (minHeight != null),
       assert (maxHeight != null);

what is the fuck ? 这个写法是神魔鬼???
这是一个dart的语法糖

  const BoxConstraints({
    this.minWidth = 0.0,
    this.maxWidth = double.infinity,
    this.minHeight = 0.0,
    this.maxHeight = double.infinity,
  })

相当于

const BoxConstraints({
    double minWidth = 0.0,
    double maxWidth = double.infinity,
    double minHeight = 0.0,
    double maxHeight = double.infinity,
  }){
    this.minWidth = minWidth;
    this.minWidth = maxWidth;
    this.minWidth = minHeight;
    this.minWidth = maxHeight;
}

冒号后面是dart： 初始化列表——Initializer list语法糖（子类构造器的方法体执行之前，初始化实例变量。不同的初始化变量之间用逗号分隔开）。
这里并没有初始化实例变量而是用assert校验输入的值