1 Card
Flutter Card 组件，卡片组件具有圆角和阴影，看起来有立体感。
2 构造函数
Card({ Key key, this.color, this.shadowColor, this.elevation, this.shape, this.borderOnForeground = true, this.margin, this.clipBehavior, this.child, this.semanticContainer = true, })
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3 常用属性
3.1 color：颜色
color:Colors.blue500,
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3.2 elevation：阴影大小
elevation: elevation: 5.0,
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3.3 shape：Card 的阴影效果
shape:const RoundedRectangleBorder(borderRadius: BorderRadius.all(Radius.circular(14.0))),
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3.4 margin：外边距
margin: EdgeInsets.only(top: 10.0,bottom: 10.0,left: 10.0,right: 10.0)
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3.5 child
child: Text("内容"),
