angular-cn/aio/content/guide/structural-directives.md

Structural Directives

结构型指令

This guide looks at how Angular manipulates the DOM with structural directives and how you can write your own structural directives to do the same thing.

在本章中，我们将看看Angular如何用结构型指令操纵DOM树，以及我们该如何写自己的结构型指令来完成同样的任务。

Try the .

试试。

{@a definition}

What are structural directives?

什么是结构型指令？

Structural directives are responsible for HTML layout. They shape or reshape the DOM's structure, typically by adding, removing, or manipulating elements.

结构型指令的职责是HTML布局。 它们塑造或重塑DOM的结构，比如添加、移除或维护这些元素。

As with other directives, you apply a structural directive to a host element. The directive then does whatever it's supposed to do with that host element and its descendants.

像其它指令一样，你可以把结构型指令应用到一个宿主元素上。 然后它就可以对宿主元素及其子元素做点什么。

Structural directives are easy to recognize. An asterisk (*) precedes the directive attribute name as in this example.

结构型指令非常容易识别。 在这个例子中，星号（*）被放在指令的属性名之前。

No brackets. No parentheses. Just *ngIf set to a string.

没有方括号，没有圆括号，只是把*ngIf设置为一个字符串。

You'll learn in this guide that the asterisk (*) is a convenience notation and the string is a microsyntax rather than the usual template expression. Angular desugars this notation into a marked-up <ng-template> that surrounds the host element and its descendents. Each structural directive does something different with that template.

在这个例子中，我们将学到星号(*)这个简写方法，而这个字符串是一个微语法，而不是通常的模板表达式。 Angular会解开这个语法糖，变成一个<ng-template>标记，包裹着宿主元素及其子元素。 每个结构型指令都可以用这个模板做点不同的事情。

Three of the common, built-in structural directives—NgIf, NgFor, and NgSwitch...—are described in the Template Syntax guide and seen in samples throughout the Angular documentation. Here's an example of them in a template:

三个常用的内置结构型指令 —— NgIf、NgFor和NgSwitch...。 我们在模板语法一章中讲过它，并且在Angular文档的例子中到处都在用它。下面是模板中的例子：

This guide won't repeat how to use them. But it does explain how they work and how to write your own structural directive.

本章不会重复讲如何使用它们，而是解释它们的工作原理以及如何写自己的结构型指令。

Directive spelling

指令的拼写形式

Throughout this guide, you'll see a directive spelled in both UpperCamelCase and lowerCamelCase. Already you've seen NgIf and ngIf. There's a reason. NgIf refers to the directive class; ngIf refers to the directive's attribute name.

在本章中，我们将看到指令同时具有两种拼写形式大驼峰UpperCamelCase和小驼峰lowerCamelCase，比如我们已经看过的NgIf和ngIf。 这里的原因在于，NgIf引用的是指令的类名*，而ngIf引用的是指令的属性名。

A directive class is spelled in UpperCamelCase (NgIf). A directive's attribute name is spelled in lowerCamelCase (ngIf). The guide refers to the directive class when talking about its properties and what the directive does. The guide refers to the attribute name when describing how you apply the directive to an element in the HTML template.

指令的类名拼写成大驼峰形式（NgIf），而它的属性名则拼写成小驼峰形式（ngIf）。 本章会在谈论指令的属性和工作原理时引用指令的类名，在描述如何在HTML模板中把该指令应用到元素时，引用指令的属性名。

There are two other kinds of Angular directives, described extensively elsewhere: (1) components and (2) attribute directives.

还有另外两种Angular指令，在本开发指南的其它地方有讲解：(1) 组件 (2) 属性型指令。

A component manages a region of HTML in the manner of a native HTML element. Technically it's a directive with a template.

组件可以在原生HTML元素中管理一小片区域的HTML。从技术角度说，它就是一个带模板的指令。

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.

属性型指令会改变某个元素、组件或其它指令的外观或行为。 比如，内置的NgStyle指令可以同时修改元素的多个样式。

You can apply many attribute directives to one host element. You can only apply one structural directive to a host element.

我们可以在一个宿主元素上应用多个属性型指令，但只能应用一个结构型指令。

{@a ngIf}

NgIf case study

NgIf案例分析

NgIf is the simplest structural directive and the easiest to understand. It takes a boolean expression and makes an entire chunk of the DOM appear or disappear.

我们重点看下ngIf。它是一个很好的结构型指令案例：它接受一个布尔值，并据此让一整块DOM树出现或消失。

The ngIf directive doesn't hide elements with CSS. It adds and removes them physically from the DOM. Confirm that fact using browser developer tools to inspect the DOM.

ngIf指令并不是使用CSS来隐藏元素的。它会把这些元素从DOM中物理删除。 使用浏览器的开发者工具就可以确认这一点。

The top paragraph is in the DOM. The bottom, disused paragraph is not; in its place is a comment about "bindings" (more about that later).

可以看到第一段文字出现在了DOM中，而第二段则没有，在第二段的位置上是一个关于“绑定”的注释（稍后有更多讲解）。

When the condition is false, NgIf removes its host element from the DOM, detaches it from DOM events (the attachments that it made), detaches the component from Angular change detection, and destroys it. The component and DOM nodes can be garbage-collected and free up memory.

当条件为假时，NgIf会从DOM中移除它的宿主元素，取消它监听过的那些DOM事件，从Angular变更检测中移除该组件，并销毁它。 这些组件和DOM节点可以被当做垃圾收集起来，并且释放它们占用的内存。

Why remove rather than hide?

为什么移除而不是隐藏？

A directive could hide the unwanted paragraph instead by setting its display style to none.

指令也可以通过把它的display风格设置为none而隐藏不需要的段落。

While invisible, the element remains in the DOM.

当不可见时，这个元素仍然留在DOM中。

The difference between hiding and removing doesn't matter for a simple paragraph. It does matter when the host element is attached to a resource intensive component. Such a component's behavior continues even when hidden. The component stays attached to its DOM element. It keeps listening to events. Angular keeps checking for changes that could affect data bindings. Whatever the component was doing, it keeps doing.

对于简单的段落，隐藏和移除之间的差异影响不大，但对于资源占用较多的组件是不一样的。当我们隐藏掉一个元素时，组件的行为还在继续 —— 它仍然附加在它所属的DOM元素上， 它也仍在监听事件。Angular会继续检查哪些能影响数据绑定的变更。 组件原本要做的那些事情仍在继续。

Although invisible, the component—and all of its descendant components—tie up resources. The performance and memory burden can be substantial, responsiveness can degrade, and the user sees nothing.

虽然不可见，组件及其各级子组件仍然占用着资源，而这些资源如果分配给别人可能会更有用。 在性能和内存方面的负担相当可观，响应度会降低，而用户却可能无法从中受益。

On the positive side, showing the element again is quick. The component's previous state is preserved and ready to display. The component doesn't re-initialize—an operation that could be expensive. So hiding and showing is sometimes the right thing to do.

当然，从积极的一面看，重新显示这个元素会非常快。 组件以前的状态被保留着，并随时可以显示。 组件不用重新初始化 —— 该操作可能会比较昂贵。 这时候隐藏和显示就成了正确的选择。

But in the absence of a compelling reason to keep them around, your preference should be to remove DOM elements that the user can't see and recover the unused resources with a structural directive like NgIf .

但是，除非有非常强烈的理由来保留它们，否则我们更倾向于移除用户看不见的那些DOM元素，并且使用NgIf这样的结构型指令来收回用不到的资源。

These same considerations apply to every structural directive, whether built-in or custom. Before applying a structural directive, you might want to pause for a moment to consider the consequences of adding and removing elements and of creating and destroying components.

同样的考量也适用于每一个结构型指令，无论是内置的还是自定义的。 我们应该提醒自己以及我们指令的使用者，来仔细考虑添加元素、移除元素以及创建和销毁组件的后果。

{@a asterisk}

The asterisk (*) prefix

星号（*）前缀

Surely you noticed the asterisk (*) prefix to the directive name and wondered why it is necessary and what it does.

你可能注意到了指令名的星号（*）前缀，并且困惑于为什么需要它以及它是做什么的。

Here is *ngIf displaying the hero's name if hero exists.

这里的*ngIf会在hero存在时显示英雄的名字。

The asterisk is "syntactic sugar" for something a bit more complicated. Internally, Angular translates the *ngIf attribute into a <ng-template> element, wrapped around the host element, like this.

星号是一个用来简化更复杂语法的“语法糖”。 从内部实现来说，Angular把*ngIf 属性 翻译成一个<ng-template> 元素 并用它来包裹宿主元素，代码如下：

• The *ngIf directive moved to the <ng-template> element where it became a property binding,[ngIf].

  *ngIf指令被移到了<ng-template>元素上。在那里它变成了一个属性绑定[ngIf]。

• The rest of the <div>, including its class attribute, moved inside the <ng-template> element.

  <div>上的其余部分，包括它的class属性在内，移到了内部的<ng-template>元素上。

The first form is not actually rendered, only the finished product ends up in the DOM.

第一种形态永远不会真的渲染出来。 只有最终产出的结果才会出现在DOM中。

Angular consumed the <ng-template> content during its actual rendering and replaced the <ng-template> with a diagnostic comment.

Angular会在真正渲染的时候填充<ng-template>的内容，并且把<ng-template>替换为一个供诊断用的注释。

The NgFor and NgSwitch... directives follow the same pattern.

NgFor和NgSwitch...指令也都遵循同样的模式。

{@a ngFor}

Inside *ngFor

*ngFor内幕

Angular transforms the *ngFor in similar fashion from asterisk (*) syntax to <ng-template> element.

Angular会把*ngFor用同样的方式把星号（*）语法的template属性转换成<ng-template>元素。

Here's a full-featured application of NgFor, written both ways:

这里有一个NgFor的全特性应用，同时用了这三种写法：

This is manifestly more complicated than ngIf and rightly so. The NgFor directive has more features, both required and optional, than the NgIf shown in this guide. At minimum NgFor needs a looping variable (let hero) and a list (heroes).

它明显比ngIf复杂得多，确实如此。 NgFor指令比本章展示过的NgIf具有更多的必选特性和可选特性。 至少NgFor会需要一个循环变量（let hero）和一个列表（heroes）。

You enable these features in the string assigned to ngFor, which you write in Angular's microsyntax.

我们可以通过把一个字符串赋值给ngFor来启用这些特性，这个字符串使用Angular的微语法。

Everything outside the ngFor string stays with the host element (the <div>) as it moves inside the <ng-template>. In this example, the [ngClass]="odd" stays on the <div>.

ngFor字符串之外的每一样东西都会留在宿主元素（<div>）上，也就是说它移到了<ng-template>内部。 在这个例子中，[ngClass]="odd"留在了<div>上。

{@a microsyntax}

Microsyntax

微语法

The Angular microsyntax lets you configure a directive in a compact, friendly string. The microsyntax parser translates that string into attributes on the <ng-template>:

Angular微语法能让我们通过简短的、友好的字符串来配置一个指令。 微语法解析器把这个字符串翻译成<ng-template>上的属性：

• The let keyword declares a template input variable that you reference within the template. The input variables in this example are hero, i, and odd. The parser translates let hero, let i, and let odd into variables named, let-hero, let-i, and let-odd.

  let关键字声明一个模板输入变量，我们会在模板中引用它。本例子中，这个输入变量就是hero、i和odd。 解析器会把let hero、let i和let odd翻译成命名变量let-hero、let-i和let-odd。

• The microsyntax parser takes of and trackBy, title-cases them (of -> Of, trackBy -> TrackBy), and prefixes them with the directive's attribute name (ngFor), yielding the names ngForOf and ngForTrackBy. Those are the names of two NgFor input properties . That's how the directive learns that the list is heroes and the track-by function is trackById.

  微语法解析器接收of和trackby，把它们首字母大写（of -> Of, trackBy -> TrackBy）， 并且给它们加上指令的属性名（ngFor）前缀，最终生成的名字是ngForOf和ngForTrackBy。 还有两个NgFor的输入属性，指令据此了解到列表是heroes，而track-by函数是trackById。

• As the NgFor directive loops through the list, it sets and resets properties of its own context object. These properties include index and odd and a special property named $implicit.

  NgFor指令在列表上循环，每个循环中都会设置和重置它自己的上下文对象上的属性。 这些属性包括index和odd以及一个特殊的属性名$implicit（隐式变量）。

• The let-i and let-odd variables were defined as let i=index and let odd=odd. Angular sets them to the current value of the context's index and odd properties.

  let-i和let-odd变量是通过let i=index和let odd=odd来定义的。 Angular把它们设置为上下文对象中的index和odd属性的当前值。

• The context property for let-hero wasn't specified. Its intended source is implicit. Angular sets let-hero to the value of the context's $implicit property which NgFor has initialized with the hero for the current iteration.

• The API guide describes additional NgFor directive properties and context properties.

  API参考手册中描述了NgFor指令的其它属性和上下文属性。

• NgFor is implemented by the NgForOf directive. Read more about additional NgForOf directive properties and context properties NgForOf API reference.

  NgFor是由NgForOf指令来实现的。请参阅NgForOf API reference来了解NgForOf指令的更多属性及其上下文属性。

These microsyntax mechanisms are available to you when you write your own structural directives. Studying the source code for NgIf and NgForOf is a great way to learn more.

这些微语法机制在你写自己的结构型指令时也同样有效，参考NgIf的源码 和NgFor的源码 可以学到更多。

{@a template-input-variable}

{@a template-input-variables}

Template input variable

模板输入变量

A template input variable is a variable whose value you can reference within a single instance of the template. There are several such variables in this example: hero, i, and odd. All are preceded by the keyword let.

模板输入变量是这样一种变量，你可以在单个实例的模板中引用它的值。 这个例子中有好几个模板输入变量：hero、i和odd。 它们都是用let作为前导关键字。

A template input variable is not the same as a template reference variable, neither semantically nor syntactically.

模板输入变量和模板引用变量是不同的，无论是在语义上还是语法上。

You declare a template input variable using the let keyword (let hero). The variable's scope is limited to a single instance of the repeated template. You can use the same variable name again in the definition of other structural directives.

我们使用let关键字（如let hero）在模板中声明一个模板输入变量。 这个变量的范围被限制在所重复模板的单一实例上。 事实上，我们可以在其它结构型指令中使用同样的变量名。

You declare a template reference variable by prefixing the variable name with # (#var). A reference variable refers to its attached element, component or directive. It can be accessed anywhere in the entire template.

而声明模板引用变量使用的是给变量名加#前缀的方式（#var）。 一个引用变量引用的是它所附着到的元素、组件或指令。它可以在整个模板的任意位置访问。

Template input and reference variable names have their own namespaces. The hero in let hero is never the same variable as the hero declared as #hero.

模板输入变量和引用变量具有各自独立的命名空间。let hero中的hero和#hero中的hero并不是同一个变量。

{@a one-per-element}

One structural directive per host element

每个宿主元素上只能有一个结构型指令

Someday you'll want to repeat a block of HTML but only when a particular condition is true. You'll try to put both an *ngFor and an *ngIf on the same host element. Angular won't let you. You may apply only one structural directive to an element.

有时我们会希望只有当特定的条件为真时才重复渲染一个 HTML 块。 你可能试过把*ngFor和*ngIf放在同一个宿主元素上，但Angular 不允许。这是因为我们在一个元素上只能放一个结构型指令。

The reason is simplicity. Structural directives can do complex things with the host element and its descendents. When two directives lay claim to the same host element, which one takes precedence? Which should go first, the NgIf or the NgFor? Can the NgIf cancel the effect of the NgFor? If so (and it seems like it should be so), how should Angular generalize the ability to cancel for other structural directives?

原因很简单。结构型指令可能会对宿主元素及其子元素做很复杂的事。当两个指令放在同一个元素上时，谁先谁后？NgIf优先还是NgFor优先？NgIf可以取消NgFor的效果吗？ 如果要这样做，Angular 应该如何把这种能力泛化，以取消其它结构型指令的效果呢？

There are no easy answers to these questions. Prohibiting multiple structural directives makes them moot. There's an easy solution for this use case: put the *ngIf on a container element that wraps the *ngFor element. One or both elements can be an ng-container so you don't have to introduce extra levels of HTML.

对这些问题，没有办法简单回答。而禁止多个结构型指令则可以简单地解决这个问题。 这种情况下有一个简单的解决方案：把*ngIf放在一个"容器"元素上，再包装进 *ngFor 元素。 这个元素可以使用ng-container，以免引入一个新的HTML层级。

{@a ngSwitch}

Inside NgSwitch directives

NgSwitch 内幕

The Angular NgSwitch is actually a set of cooperating directives: NgSwitch, NgSwitchCase, and NgSwitchDefault.

Angular 的 NgSwitch 实际上是一组相互合作的指令：NgSwitch、NgSwitchCase 和 NgSwitchDefault。

Here's an example.

例子如下：

The switch value assigned to NgSwitch (hero.emotion) determines which (if any) of the switch cases are displayed.

一个值(hero.emotion)被被赋值给了NgSwitch，以决定要显示哪一个分支。

NgSwitch itself is not a structural directive. It's an attribute directive that controls the behavior of the other two switch directives. That's why you write [ngSwitch], never *ngSwitch.

NgSwitch本身不是结构型指令，而是一个属性型指令，它控制其它两个switch指令的行为。 这也就是为什么我们要写成[ngSwitch]而不是*ngSwitch的原因。

NgSwitchCase and NgSwitchDefault are structural directives. You attach them to elements using the asterisk (*) prefix notation. An NgSwitchCase displays its host element when its value matches the switch value. The NgSwitchDefault displays its host element when no sibling NgSwitchCase matches the switch value.

NgSwitchCase 和 NgSwitchDefault 都是结构型指令。 因此我们要使用星号（*）前缀来把它们附着到元素上。 NgSwitchCase会在它的值匹配上选项值的时候显示它的宿主元素。 NgSwitchDefault则会当没有兄弟NgSwitchCase匹配上时显示它的宿主元素。

The element to which you apply a directive is its host element. The <happy-hero> is the host element for the happy *ngSwitchCase. The <unknown-hero> is the host element for the *ngSwitchDefault.

指令所在的元素就是它的宿主元素。 <happy-hero> 是 *ngSwitchCase 的宿主元素。 <unknown-hero> 是 *ngSwitchDefault 的宿主元素。

As with other structural directives, the NgSwitchCase and NgSwitchDefault can be desugared into the <ng-template> element form.

像其它的结构型指令一样，NgSwitchCase 和 NgSwitchDefault 也可以解开语法糖，变成 <ng-template> 的形式。

{@a prefer-asterisk}

Prefer the asterisk (*) syntax.

优先使用星号（*）语法

The asterisk (*) syntax is more clear than the desugared form. Use <ng-container> when there's no single element to host the directive.

星号（*）语法比不带语法糖的形式更加清晰。 如果找不到单一的元素来应用该指令，可以使用<ng-container>作为该指令的容器。

While there's rarely a good reason to apply a structural directive in template attribute or element form, it's still important to know that Angular creates a <ng-template> and to understand how it works. You'll refer to the <ng-template> when you write your own structural directive.

虽然很少有理由在模板中使用结构型指令的属性形式和元素形式，但这些幕后知识仍然是很重要的，即：Angular会创建<ng-template>，还要了解它的工作原理。 当需要写自己的结构型指令时，我们就要使用<ng-template>。

{@a template}

The <ng-template>

*<ng-template>*指令

The <ng-template> is an Angular element for rendering HTML. It is never displayed directly. In fact, before rendering the view, Angular replaces the <ng-template> and its contents with a comment.

<ng-template>是一个 Angular 元素，用来渲染HTML。 它永远不会直接显示出来。 事实上，在渲染视图之前，Angular 会把<ng-template>及其内容替换为一个注释。

If there is no structural directive and you merely wrap some elements in a <ng-template>, those elements disappear. That's the fate of the middle "Hip!" in the phrase "Hip! Hip! Hooray!".

如果没有使用结构型指令，而仅仅把一些别的元素包装进<ng-template>中，那些元素就是不可见的。 在下面的这个短语"Hip! Hip! Hooray!"中，中间的这个 "Hip!"（欢呼声） 就是如此。

Angular erases the middle "Hip!", leaving the cheer a bit less enthusiastic.

Angular 抹掉了中间的那个 "Hip!" ，让欢呼声显得不再那么热烈了。

A structural directive puts a <ng-template> to work as you'll see when you write your own structural directive.

结构型指令会让<ng-template>正常工作，在我们写自己的结构型指令时就会看到这一点。

{@a ngcontainer}

{@a ng-container}

Group sibling elements with <ng-container>

使用<ng-container>把一些兄弟元素归为一组

There's often a root element that can and should host the structural directive. The list element (<li>) is a typical host element of an NgFor repeater.

通常都要有一个根元素作为结构型指令的数组。 列表元素（<li>）就是一个典型的供NgFor使用的宿主元素。

When there isn't a host element, you can usually wrap the content in a native HTML container element, such as a <div>, and attach the directive to that wrapper.

当没有这样一个单一的宿主元素时，我们可以把这些内容包裹在一个原生的HTML容器元素中，比如<div>，并且把结构型指令附加到这个"包裹"上。

Introducing another container element—typically a <span> or <div>—to group the elements under a single root is usually harmless. Usually ... but not always.

但引入另一个容器元素（通常是<span>或<div>）来把一些元素归到一个单一的根元素下，通常也会带来问题。注意，是"通常"而不是"总会"。

The grouping element may break the template appearance because CSS styles neither expect nor accommodate the new layout. For example, suppose you have the following paragraph layout.

这种用于分组的元素可能会破坏模板的外观表现，因为CSS的样式既不曾期待也不会接受这种新的元素布局。 比如，假设你有下列分段布局。

You also have a CSS style rule that happens to apply to a <span> within a <p>aragraph.

而我们的CSS样式规则是应用于<p>元素下的<span>的。

The constructed paragraph renders strangely.

这样渲染出来的段落就会非常奇怪。

The p span style, intended for use elsewhere, was inadvertently applied here.

本来为其它地方准备的p span样式，被意外的应用到了这里。

Another problem: some HTML elements require all immediate children to be of a specific type. For example, the <select> element requires <option> children. You can't wrap the options in a conditional <div> or a <span>.

另一个问题是：有些HTML元素需要所有的直属下级都具有特定的类型。 比如，<select>元素要求直属下级必须为<option>，那么我们就没办法把这些选项包装进<div>或<span>中。

When you try this,

如果这样做：

the drop down is empty.

下拉列表就是空的。

The browser won't display an <option> within a <span>.

浏览器不会显示<span>中的<option>。

<ng-container> to the rescue

<ng-container> 的救赎

The Angular <ng-container> is a grouping element that doesn't interfere with styles or layout because Angular doesn't put it in the DOM.

Angular的<ng-container>是一个分组元素，但它不会污染样式或元素布局，因为 Angular 压根不会把它放进 DOM 中。

Here's the conditional paragraph again, this time using <ng-container>.

下面是重新实现的条件化段落，这次我们使用<ng-container>。

It renders properly.

这次就渲染对了。

Now conditionally exclude a select <option> with <ng-container>.

我们再用<ng-container>来根据条件排除选择框中的某个<option>。

The drop down works properly.

下拉框也工作正常。

The <ng-container> is a syntax element recognized by the Angular parser. It's not a directive, component, class, or interface. It's more like the curly braces in a JavaScript if-block:

<ng-container>是一个由 Angular 解析器负责识别处理的语法元素。 它不是一个指令、组件、类或接口，更像是 JavaScript 中 if 块中的花括号。

if (someCondition) { statement1; statement2; statement3; }

Without those braces, JavaScript would only execute the first statement when you intend to conditionally execute all of them as a single block. The <ng-container> satisfies a similar need in Angular templates.

没有这些花括号，JavaScript 只会执行第一句，而你原本的意图是把其中的所有语句都视为一体来根据条件执行。 而<ng-container>满足了 Angular 模板中类似的需求。

{@a unless}

Write a structural directive

写一个结构型指令

In this section, you write an UnlessDirective structural directive that does the opposite of NgIf. NgIf displays the template content when the condition is true. UnlessDirective displays the content when the condition is false.

在本节中，我们会写一个名叫UnlessDirective的结构型指令，它是NgIf的反义词。 NgIf在条件为true的时候显示模板内容，而UnlessDirective则会在条件为false时显示模板内容。

Creating a directive is similar to creating a component.

创建指令很像创建组件。

• Import the Directive decorator (instead of the Component decorator).

  导入Directive装饰器（而不再是Component）。

• Import the Input, TemplateRef, and ViewContainerRef symbols; you'll need them for any structural directive.

  导入符号Input、TemplateRef 和 ViewContainerRef，我们在任何结构型指令中都会需要它们。

• Apply the decorator to the directive class.

  给指令类添加装饰器。

• Set the CSS attribute selector that identifies the directive when applied to an element in a template.

  设置 CSS 属性选择器 ，以便在模板中标识出这个指令该应用于哪个元素。

Here's how you might begin:

这里是起点：

The directive's selector is typically the directive's attribute name in square brackets, [appUnless]. The brackets define a CSS attribute selector.

指令的选择器通常是把指令的属性名括在方括号中，如[myUnless]。 这个方括号定义出了一个 CSS 属性选择器。

The directive attribute name should be spelled in lowerCamelCase and begin with a prefix. Don't use ng. That prefix belongs to Angular. Pick something short that fits you or your company. In this example, the prefix is app.

该指令的属性名应该拼写成小驼峰形式，并且带有一个前缀。 但是，这个前缀不能用ng，因为它只属于 Angular 本身。 请选择一些简短的，适合你自己或公司的前缀。 在这个例子中，前缀是my。

The directive class name ends in Directive per the style guide. Angular's own directives do not.

指令的类名用Directive结尾，参见风格指南。 但 Angular 自己的指令例外。

TemplateRef and ViewContainerRef

TemplateRef 和 ViewContainerRef

A simple structural directive like this one creates an embedded view from the Angular-generated <ng-template> and inserts that view in a view container adjacent to the directive's original <p> host element.

像这个例子一样的简单结构型指令会从 Angular 生成的<ng-template>元素中创建一个内嵌的视图，并把这个视图插入到一个视图容器中，紧挨着本指令原来的宿主元素<p>（译注：注意不是子节点，而是兄弟节点）。

You'll acquire the <ng-template> contents with a TemplateRef and access the view container through a ViewContainerRef.

我们可以使用TemplateRef取得<ng-template>的内容，并通过ViewContainerRef来访问这个视图容器。

You inject both in the directive constructor as private variables of the class.

我们可以把它们都注入到指令的构造函数中，作为该类的私有属性。

The appUnless property

myUnless 属性

The directive consumer expects to bind a true/false condition to [appUnless]. That means the directive needs an appUnless property, decorated with @Input

该指令的使用者会把一个true/false条件绑定到[myUnless]属性上。 也就是说，该指令需要一个带有@Input的myUnless属性。

Read about @Input in the Template Syntax guide.

要了解关于@Input的更多知识，参见模板语法一章。

Angular sets the appUnless property whenever the value of the condition changes. Because the appUnless property does work, it needs a setter.

一旦该值的条件发生了变化，Angular 就会去设置 myUnless 属性，这时候，我们就需要为它定义一个设置器（setter）。

• If the condition is falsy and the view hasn't been created previously, tell the view container to create the embedded view from the template.

  如果条件为假，并且以前尚未创建过该视图，就告诉视图容器（ViewContainer）根据模板创建一个内嵌视图。

• If the condition is truthy and the view is currently displayed, clear the container which also destroys the view.

  如果条件为真，并且视图已经显示出来了，就会清除该容器，并销毁该视图。

Nobody reads the appUnless property so it doesn't need a getter.

没有人会读取myUnless属性，因此它不需要定义设置器（getter）。

The completed directive code looks like this:

完整的指令代码如下：

Add this directive to the declarations array of the AppModule.

把这个指令添加到AppModule的declarations数组中。

Then create some HTML to try it.

然后创建一些 HTML 来试用一下。

When the condition is falsy, the top (A) paragraph appears and the bottom (B) paragraph disappears. When the condition is truthy, the top (A) paragraph is removed and the bottom (B) paragraph appears.

当condition为false时，顶部的段落就会显示出来，而底部的段落消失了。 当condition为true时，顶部的段落被移除了，而底部的段落显示了出来。

{@a summary}

Summary

小结

You can both try and download the source code for this guide in the .

你可以去中下载本章的源码。

Here is the source from the src/app/ folder.

本章相关的代码如下：

You learned

我们学到了

• that structural directives manipulate HTML layout.

  结构型指令可以操纵 HTML 的元素布局。

• to use <ng-container> as a grouping element when there is no suitable host element.

  当没有合适的容器元素时，可以使用<ng-container>对元素进行分组。

• that the Angular desugars asterisk (*) syntax into a <ng-template>.

  Angular 会把星号（*）语法解开成<ng-template>。

• how that works for the NgIf, NgFor and NgSwitch built-in directives.

  内置指令NgIf、NgFor和NgSwitch的工作原理。

• about the microsyntax that expands into a <ng-template>.

  微语法如何展开成<ng-template>。

• to write a custom structural directive, UnlessDirective.

  写了一个自定义结构型指令 —— UnlessDirective。