angular-docs-cn/public/docs/ts/latest/guide/structural-directives.jade

block includes
  include ../_util-fns

:marked
  One of the defining features of a single page application is its manipulation
  of the DOM tree. Instead of serving a whole new page every time a user
  navigates, whole sections of the DOM appear and disappear according
  to the application state. In this chapter we'll to look at how Angular
  manipulates the DOM and how we can do it ourselves in our own directives.

  In this chapter we will
  - [learn what structural directives are](#definition)
  - [study *ngIf*](#ngIf)
  - [discover the <template> element](#template)
  - [understand the asterisk (\*) in **ngFor*](#asterisk)
  - [write our own structural directive](#unless)

  Try the <live-example></live-example>.

<a id="definition"></a>
.l-main-section
:marked
  ## What are structural directives?

  There are three kinds of Angular directives:
  1. Components
  1. Attribute directives
  1. Structural directives

  The *Component* is really a directive with a template.
  It's the most common of the three directives and we write lots of them as we build our application.

  The [*Attribute* directive](attribute-directives.html) changes the appearance or behavior of an element.
  The built-in [NgStyle](template-syntax.html#ngStyle) directive, for example,
  can change several element styles at the same time.
  We can use it to render text bold, italic, and lime green by binding to a
  component property that requests such a sickening result.

  A *Structural* directive changes the DOM layout by adding and removing DOM elements.
  We've seen three of the built-in structural directives in other chapters: [ngIf](template-syntax.html#ngIf),
  [ngSwitch](template-syntax.html#ngSwitch) and [ngFor](template-syntax.html#ngFor).

+makeExample('structural-directives/ts/app/structural-directives.component.html', 'structural-directives')(format=".")


<a id="ngIf"></a>
.l-main-section
:marked
  ## NgIf Case Study

  Let’s focus on `ngIf`. It's a great example of a structural
  directive: it takes a boolean and makes an entire chunk of DOM appear
  or disappear.

+makeExample('structural-directives/ts/app/structural-directives.component.html', 'ngIf')(format=".")

:marked
  The `ngIf` directive does not hide the element.
  Using browser developer tools we can see that, when the condition is true, the top
  paragraph is in the DOM and the bottom disused paragraph is completely
  absent from the DOM! In its place are empty `<script>` tags.

figure.image-display
  img(src='/resources/images/devguide/structural-directives/element-not-in-dom.png' alt="element not in dom")

:marked
  ### Why *remove* rather than *hide*?
  We could hide the unwanted paragraph by setting its css `display` style to `none`.
  The element would remain in the DOM while invisible. Instead we removed it with `ngIf`.

  The difference matters. When we hide an element,
  the component's behavior continues.
  It remains attached to its DOM element. It continues to listen to events.
  Angular keeps checking for changes that could affect data bindings.
  Whatever the component was doing it keeps doing.

  Although invisible, the component &mdash; and all of its descendent components &mdash;
  tie up resources that might be more useful elsewhere.
  The performance and memory burden can be substantial and the user may not benefit at all.

  On the positive side, showing the element again is very quick.
  The component's previous state is preserved and ready to display.
  The component doesn't re-initialize &mdash; an operation that could be expensive.

  `ngIf` is different.
  Setting `ngIf` to false **does** affect the component's resource consumption.
  Angular removes the element from DOM, stops change detection for the associated component,
  detaches it from DOM events (the attachments that it made) and destroys the component.
  The component can be garbage-collected (we hope) and free up memory.

  Components often have child components which themselves have children.
  All of them are destroyed when `ngIf` destroys the common ancestor.
  This cleanup effort is usually a good thing.

  Of course it isn't *always* a good thing.
  It might be a bad thing if we need that particular component again soon.

  The component's state might be expensive to re-construct.
  When `ngIf` becomes `true` again, Angular recreates the component and its subtree.
  Angular runs every component's initialization logic again. That could be expensive ... as when
  a component re-fetches data that had been in memory just moments ago.
.l-sub-section
  :marked
    *Design thought*: minimize initialization effort and consider caching state in a
    companion service.
:marked
  Although there are pros and cons to each approach,
  in general it is best to use `ngIf` to remove unwanted components rather than
  hide them.

  **These same considerations apply to every structural directive, whether built-in or custom.**
  We should ask ourselves &mdash; and the users of our directives &mdash; to think carefully
  about the consequences of adding and removing elements and of creating and destroying components.

  Let's see these dynamics at work. For fun, we'll stack the deck *against*
  our recommendation and consider a component called `heavy-loader` that
  ***pretends*** to load a ton of data when initialized.

  We'll display two instances of the component.  We toggle the visibility of the first one with CSS.
  We toggle the second into and out of the DOM with `ngIf`.

+makeTabs(
    `structural-directives/ts/app/structural-directives.component.html,
    structural-directives/ts/app/heavy-loader.component.ts`,
    'message-log,',
    'template (excerpt), heavy-loader.component.ts')

:marked
  We also log when a component is created or destroyed
  using the built-in `ngOnInit` and `ngOnDestroy` [lifecycle hooks](lifecycle-hooks.html).
  Here it is in action:

figure.image-display
  img(src='/resources/images/devguide/structural-directives/heavy-loader-toggle.gif' alt="heavy loader toggle")

:marked
  Both components are in the DOM at the start.
  First we toggle the component's visibility repeatedly. The component never leaves the DOM.
  When visible it's always the same instance and the log is quiet.

  Then we toggle the second component with `ngIf`.
  We create a new instance every time and the log shows that we're paying
  a heavy price to create and destroy it.

  If we really expected to "wink" the component like this, toggling visibility would be the better choice.
  In most UIs, when we "close" a component we're unlikely see it again for a long time, if ever.
  The `ngIf` would be preferred in that case.

<a id="template"></a>
.l-main-section
:marked
  ## The *&lt;template>* tag

  Structural directives, like `ngIf`, do their magic by using the
  [HTML 5 template tag](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/template).

  Outside of an Angular app, the `<template>` tag's default CSS `display` property is `none`.
  It's contents are ***invisible*** within
  a hidden [document fragment](https://developer.mozilla.org/en/docs/Web/API/DocumentFragment).

  Inside of an app, Angular ***removes*** the`<template>` tags and their children.
  The contents are gone &mdash; but not forgotten as we'll see soon.

  We can confirm these effects by wrapping the middle "hip" of the phrase "Hip! Hip! Hooray!" within a `<template>` tag.
+makeExample('structural-directives/ts/app/structural-directives.component.html', 'template-tag')(format=".")
:marked
  The display is a 'Hip! Hooray!', short of perfect enthusiasm. The DOM effects are different when Angular is in control.
figure.image-display
  img(src='/resources/images/devguide/structural-directives/template-in-out-of-a2.png' alt="template outside angular")

:marked
  Evidently Angular replaces the `<template>` tag and its contents with empty `<script>` tags.
  That's just its default behavior.
  It can do something different as we saw when applying a variety of `ngSwitch` directives to `<template>` tags:

+makeExample('structural-directives/ts/app/structural-directives.component.html', 'ngSwitch')(format=".")
:marked
  When one of those `ngSwitch` conditions is true, Angular inserts the template's content into the DOM.

  What does this have to do with `ngIf` and `ngFor`?  We didn't use a `<template>` tag with those directives.

<a id="asterisk"></a>
.l-main-section
:marked
  ## The asterisk (\*) effect
  Here are those directives again. See the difference?

+makeExample('structural-directives/ts/app/structural-directives.component.html', 'asterisk')(format=".")
:marked
  We're prefixing these directive names with an asterisk (\*).

  The asterisk is "syntactic sugar". It simplifies `ngIf` and `ngFor` for both the writer and the reader.
  Under the hood, Angular replaces the asterisk version with a more verbose `<template>` form.

  The next two `ngIf` examples are effectively the same and we may write in either style:

+makeExample('structural-directives/ts/app/structural-directives.component.html', 'ngIf-template')(format=".")

:marked
  Most of us would rather write in style (A).

  It's worth knowing that Angular expands style (A) into style (B).
  It moves the paragraph and its contents inside a `<template>` tag.
  It moves the directive up to the `<template>` tag where it becomes a property binding,
  surrounded in square brackets. The boolean value of the host component's `condition` property
  determines whether the templated content is displayed or not.

  Angular transforms `*ngFor` in a similar manner:

+makeExample('structural-directives/ts/app/structural-directives.component.html', 'ngFor-template')(format=".")
:marked
  The basic pattern is the same:&nbsp; create a `<template>`, relocate the content,
  and move the directive onto the `<template>`.

  There are extra nuances stemming from
  Angular's [ngFor micro-syntax](template-syntax.html#ngForMicrosyntax) which expands
  into an additional `ngForOf` property binding (the iterable) and
  the `hero` template input variable (the current item in each iteration).

<a id="unless"></a>
.l-main-section
:marked
  ## Make a structural directive
  Let's write our own structural directive, an `Unless` directive, the not-so-evil twin of `ngIf`.

  Unlike `ngIf` which displays the template content when `true`,
  our directive displays the content when the condition is ***false***.

block unless-intro
  :marked
    Creating a directive is similar to creating a component.
    * import the `Directive` decorator.

    * add a CSS **attribute selector** (in brackets) that identifies our directive.

    * specify the name of the public `input` property for binding
    (typically the name of the directive itself).

    * apply the decorator to our implementation class.

    Here is how we begin:

+makeExample('structural-directives/ts/app/unless.directive.ts', 'unless-declaration', 'unless.directive.ts (excerpt)')(format=".")
.l-sub-section
  :marked
    ### Selector brackets [&nbsp;]
    The CSS syntax for selecting an attribute is a name in square brackets.
    We surround our directive name in square brackets. See *Directive configuration* on the
    [cheatsheet](cheatsheet.html).

    ### Selector name prefixes

    We recommend picking a selector name with a prefix to ensure
    that it cannot conflict with any standard HTML attribute, now or in the future.

    We do **not** prefix our `unless` directive name with **`ng`**.
    That prefix belongs to Angular and
    we don't want to confuse our directives with their directives.

    Our prefix is `my`.
:marked
  We'll need access to the template *and* something that can render its contents.
  We access the template with a `TemplateRef`.  The renderer is a `ViewContainerRef`.
  We inject both into our constructor as private variables.

+makeExample('structural-directives/ts/app/unless.directive.ts', 'unless-constructor')(format=".")

:marked
  The consumer of our directive will bind a boolean value to our directive's `myUnless` input property.
  The directive adds or removes the template based on that value.

  Let's add the `myUnless` property now as a setter-only property.

+makeExample('structural-directives/ts/app/unless.directive.ts', 'unless-set')(format=".")
.l-sub-section
  :marked
    The `@Input()` annotation marks this property as an input for the directive.
:marked
  Nothing fancy here: if the condition is false,
  we render the template, otherwise we clear the element content.

  The end result should look like this:

+makeExample('structural-directives/ts/app/unless.directive.ts', null, 'unless.directive.ts')

:marked
  Now we add it to the `directives`array of the host component and try it.
  First we add some test HTML to the template:

+makeExample('structural-directives/ts/app/structural-directives.component.html', 'myUnless')(format=".")
:marked
  We run it and it behaves as expected, doing the opposite of `ngIf`.
  When `condition` is `true`, the top paragraph is removed (replaced by `<script>` tags) and the bottom paragraph appears.
figure.image-display
  img(src='/resources/images/devguide/structural-directives/myUnless-is-true.png' alt="myUnless is true" )

:marked
  Our `myUnless` directive is dead simple. Surely we left something out.
  Surely `ngIf` is more complex?

  [Look at the source code](https://github.com/angular/angular/blob/master/modules/%40angular/common/src/directives/ng_if.ts).
  It's well documented and we shouldn't be shy
  about consulting the source when we want to know how something works.

  `ngIf` isn't much different! There are a few
  additional checks to improve performance (don't clear or recreate the
  view unless necessary) but otherwise it's much the same.

.l-main-section
:marked
  ## Wrap up
  Here is the pertinent source for this chapter.

+makeTabs(`
  structural-directives/ts/app/unless.directive.ts,
  structural-directives/ts/app/heavy-loader.component.ts,
  structural-directives/ts/app/structural-directives.component.ts,
  structural-directives/ts/app/structural-directives.component.html
  `,
  null,
  `unless.directive.ts,
   heavy-loader.component.ts,
   structural-directives.component.ts,
   structural-directives.component.html
   `)
:marked
  We learned that we can manipulate our HTML layout with
  structural directives like `ngFor` and `ngIf` and we
  wrote our own structural directive, `myUnless`, to do something similar.

  Angular offers more sophisticated techniques for managing layout
  such as *structural components* that can take external content
  and incorporate that content within their own templates.
  Tab and tab pane controls are good examples.

  We'll learn about structural components in a future chapter.