class Rectangle { num left, top, width, height; Rectangle(this.left, this.top, this.width, this.height); // 定义两个计算属性： right 和 bottom。 num get right => left + width; set right(num value)=>left = value - width; num get bottom => top + height; set bottom(num value) => top = value - height;}void main() { var rect = Rectangle(3, 4, 20, 15); assert(rect.left == 3); rect.right = 12; assert(rect.left == -8);}
