不支持背景图片，仅支持linear-gradient方法
background-image: linear-gradient({{direction}}, red, yellow)
style 动态切换 background
{{ testStyle}}
class 动态切换 background
{{testClass}}
scroll-view 组件
background-image: linear-gradient(to right, cyan, yellow);
background-image: linear-gradient(to bottom, blue, green);
to right
to bottom
style.setProperty 动态设置 background-image
background-image: {{backgroundImage}}
当前 background-image: {{backgroundImage}}
第一个枚举值，'' (空字符串) - 空值情况