NgIf是结构化指令的实现class，ngif是指令的属性名，应用在HTML代码里。
除了structural指令外，Angular常用的还有Component指令和属性指令两种。
A component manages a region of HTML in the manner of a native HTML element. Technically it’s a directive with a template.
An attribute directive changes the appearance or behavior of an element, component, or another directive. For example, the built-in NgStyle directive changes several element styles at the same time.
structural指令和host元素是1：1关系，attribute指令和host元素可以是N:1关系。
NgIf takes a boolean expression and makes an entire chunk of the DOM appear or disappear.
使用微语法(一个字符串)配置结构性指令。微语法解析器把这个字符串里的内容解析成的属性：
The parser translates let hero, let i, and let odd into variables named let-hero, let-i, and let-odd.
As the NgFor directive loops through the list, it sets and resets properties of its own context object. These properties can include, but aren’t limited to, index, odd, and a special property named $implicit.
NgFor遍历list，在每次循环过程体内部设置它自己上下文对象的属性，比如index，odd和$implicit.
Template input variable
A template input variable is a variable whose value you can reference within a single instance of the template. 可以在模板的某一个具体实例内被使用。
里面包含的元素不会出现在最后的HTML页面里：
还有一种情况，某些html元素要求其子元素必须是一种特殊的类型，比如的子元素必须是, 二者中间不能再引入或者等中间层级。像下图这种设计，最后是看不到下拉菜单的：是一种grouping element，不会干预HTML正常的布局和样式，因为Angular不会将其渲染到最终的HTML中去。