angular-cn/public/docs/ts/latest/guide/lifecycle-hooks.jade

block includes
  include ../_util-fns

- var top="vertical-align:top"

:marked
  # Component Lifecycle
  A Component has a lifecycle managed by Angular itself. Angular creates it, renders it, creates and renders its children,
  checks it when its data-bound properties change, and destroys it before removing it from the DOM.

  Angular offers **component lifecycle hooks**
  that give us visibility into these key moments and the ability to act when they occur.

  We cover these hooks in this chapter and demonstrate how they work in code.
  
  * [The lifecycle hooks](#hooks-overview)
  * [The hook-call sequence](#hook-sequence)
  * [Other Angular lifecycle hooks](#other-lifecycles)
  * [The lifecycle sample](#the-sample)
    * [All](#peek-a-boo)
    * [Spying OnInit and OnDestroy](#spy) 
    * [OnChanges](#onchanges)
    * [DoCheck](#docheck)
    * [AfterViewInit and AfterViewChecked](#afterview)
    * [AfterContentInit and AfterContentChecked](#aftercontent)

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

a#hooks-overview
.l-main-section
:marked
  ## Component lifecycle Hooks
  Directive and component instances have a lifecycle
  as Angular creates, updates, and destroys them.

  Developers can tap into key moments in that lifecycle by implementing
  one or more of the *Lifecycle Hook* interfaces in the Angular `core` library.
  
  Each interface has a single hook method whose name is the interface name prefixed with `ng`.
  For example, the `OnInit` interface has a hook method named `ngOnInit`.
  We might implement it in a component class like this:
+makeExample('lifecycle-hooks/ts/app/peek-a-boo.component.ts', 'ngOnInit', 'peek-a-boo.component.ts (excerpt)')(format='.')
:marked
  No directive or component will implement all of them and some of the hooks only make sense for components.
  Angular only calls a directive/component hook method *if it is defined*.
block optional-interfaces
  .l-sub-section
    :marked
      ### Interface optional?
      The interfaces are optional for JavaScript and Typescript developers from a purely technical perspective.
      The JavaScript language doesn't have interfaces. 
      Angular can't see TypeScript interfaces at runtime because they disappear from the transpiled JavaScript.

      Fortunately, they aren't necessary.
      We don't have to add the lifecycle hook interfaces to our directives and components to benefit from the hooks themselves.

      Angular instead inspects our directive and component classes and calls the hook methods *if they are defined*.
      Angular will find and call methods like `ngOnInit()`, with or without the interfaces.

      Nonetheless, we strongly recommend adding interfaces to TypeScript directive classes
      in order to benefit from strong typing and editor tooling.  
  
:marked
  Here are the component lifecycle hook methods:

  ### Directives and Components

table(width="100%")
  col(width="20%")
  col(width="80%") 
  tr
    th Hook
    th Purpose
  tr(style=top)
    td ngOnInit
    td
      :marked
        Initialize the directive/component after Angular initializes the data-bound input properties.
  tr(style=top)
    td ngOnChanges
    td
      :marked
        Respond after Angular sets a data-bound input property. 
        The method receives a `changes` object of current and previous values.
  tr(style=top)
    td ngDoCheck
    td
      :marked
        Detect and act upon changes that Angular can't or won't
        detect on its own. Called every change detection run.
  tr(style=top)
    td ngOnDestroy
    td
      :marked
        Cleanup just before Angular destroys the directive/component. 
        Unsubscribe observables and detach event handlers to avoid memory leaks.

:marked
  ### Components only

table(width="100%")
  col(width="20%")
  col(width="80%") 
  tr
    th Hook
    th Purpose
  tr(style=top)
    td ngAfterContentInit
    td
      :marked
        After Angular projects external content into its view.
  tr(style=top)
    td ngAfterContentChecked
    td
      :marked
        After Angular checks the bindings of the external content that it projected into its view.
  tr(style=top)
    td ngAfterViewInit
    td
      :marked
        After Angular creates the component's view(s).
  tr(style=top)
    td ngAfterViewChecked
    td
      :marked
        After Angular checks the bindings of the component's view(s).
:marked
  Angular does not call the hook methods in this order.

a(id="hook-sequence")
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:marked
  ## Lifecycle sequence
  *After* Angular creates a component/directive by `new`-ing its constructor, 
  it calls the lifecycle hook methods in the following sequence at specific moments:  
table(width="100%")
  col(width="20%")
  col(width="80%") 
  tr
    th Hook
    th Timing
  tr(style=top)
    td ngOnChanges
    td
      :marked
        before `ngOnInit` and when a data-bound input property value changes.
  tr(style=top)
    td ngOnInit
    td
      :marked
        after the first `ngOnChanges`.
  tr(style=top)
    td ngDoCheck
    td
      :marked
        during every Angular change detection cycle.
  tr(style=top)
    td ngAfterContentInit
    td
      :marked
        after projecting content into the component.
  tr(style=top)
    td ngAfterContentChecked
    td
      :marked
         after every check of projected component content.
  tr(style=top)
    td ngAfterViewInit
    td
      :marked
        after initializing the component's views and child views.
  tr(style=top)
    td ngAfterViewChecked
    td
      :marked
         after every check of the component's views and child views.
  tr(style=top)
    td ngOnDestroy
    td
      :marked
         just before Angular destroys the directive/component.

a(id="other-lifecycles")
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:marked
  ## Other lifecycle hooks
  
  Other Angular sub-systems may have their own lifecycle hooks apart from the component hooks we've listed. 
  
  A parallel can be drawn between `ngOnInit` and `routerOnActivate`. 
  Both are prefixed so as to avoid collision, and both run right when a component is 'booting' up.
  
  3rd party libraries might implement their hooks as well in order to give us, the developers, more
  control over how these libraries are used.

.l-main-section#the-sample
:marked
  ## Lifecycle exercises

  The <live-example></live-example>
  demonstrates the lifecycle hooks in action through a series of exercises
  presented as components under the control of the root `AppComponent`.

  They follow a common pattern:  a *parent* component serves as a test rig for 
  a *child* component that illustrates one or more of the lifecycle hook methods.
  
  Here's a brief description of each exercise: 

table(width="100%")
  col(width="20%")
  col(width="80%") 
  tr
    th Component
    th Description
  tr(style=top)
    td <a href="#peek-a-boo">Peek-a-boo</a>
    td
      :marked
        Demonstrates every lifecycle hook.
        Each hook method writes to the on-screen log.
  tr(style=top)
    td <a href="#spy">Spy</a>
    td
      :marked
        Directives have lifecycle hooks too.
        We create a `SpyDirective` that logs when the element it spies upon is 
        created or destroyed using the `ngOnInit` and `ngOnDestroy` hooks.
        
        We apply the `SpyDirective` to a `<div>` in an `ngFor` *hero* repeater
        managed by the parent `SpyComponent`.
  tr(style=top)
    td <a href="#onchanges">OnChanges</a>
    td
      :marked
        See how Angular calls the `ngOnChanges` hook with a `changes` object
        every time one of the component input properties changes.
        Shows how to interpret the `changes` object.
  tr(style=top)
    td <a href="#docheck">DoCheck</a> 
    td
      :marked
        Implements an `ngDoCheck` method with custom change detection.
        See how often Angular calls this hook and watch it post changes to a log.
  tr(style=top)
    td <a href="#afterview">AfterView</a>
    td
      :marked
        Shows what Angular means by a *view*.
        Demonstrates the `ngAfterViewInit` and `ngAfterViewChecked` hooks.
  tr(style=top)
    td <a href="#aftercontent">AfterContent</a>
    td
      :marked
        Shows how to project external content into a component and
        how to distinguish projected content from a component's view children.
        Demonstrates the `ngAfterContentInit` and `ngAfterContentChecked` hooks.
  tr(style=top)
    td Counter
    td
      :marked
        Demonstrates a combination of a component and a directive 
        each with its own hooks.
        
        In this example, a `CounterComponent` logs a change (via `ngOnChanges`)
        every time the parent component increments its input counter property.
        Meanwhile, we apply the `SpyDirective` from the previous example
        to the `CounterComponent` log and watch log entries be created and destroyed.
    
:marked
  We discuss the exercises in further detail over this chapter as we learn more about the lifecycle hooks.

a(id="peek-a-boo")
.l-main-section
:marked
  ## Peek-a-boo: all hooks
  The `PeekABooComponent` demonstrates all of the hooks in one component.

  In real life, we'd rarely if ever implement all of the interfaces like this.
  We do so in peek-a-boo in order to watch Angular call the hooks in the expected order.
  
  In this snapshot, we clicked the *Create...* button and then the *Destroy...* button.
figure.image-display
  img(src="/resources/images/devguide/lifecycle-hooks/peek-a-boo.png" alt="Peek-a-boo")
:marked
  The sequence of log messages follows the prescribed hook calling order: 
  `OnChanges`, `OnInit`, `DoCheck`&nbsp;(3x), `AfterContentInit`, `AfterContentChecked`&nbsp;(3x), 
  `AfterViewInit`, `AfterViewChecked`&nbsp;(3x), and `OnDestroy`.
  
.l-sub-section
  :marked
    The constructor isn't an Angular hook *per se*.
    We log in it to confirm that input properties (the `name` property in this case) have no assigned values at construction.
:marked
  Had we clicked the *Update Hero* button, we'd have seen another `OnChanges` and two more triplets of 
  `DoCheck`, `AfterContentChecked` and `AfterViewChecked`.
  Clearly these three hooks fire a *lot* and we must keep the logic we put in these hooks 
  as lean as possible!

  Our next examples focus on hook details.

.a(id="spy")
.l-main-section
:marked
  ## Spying *OnInit* and *OnDestroy*
  
  We're going undercover for these two hooks. We want to know when an element is initialized or destroyed,
  but we don't want *it* to know we're watching. 
  
  This is the perfect infiltration job for a directive. 
  Our heroes will never know it's there.
  
.l-sub-section
  :marked
    Kidding aside, we're emphasizing two key points: 
    
    1. Angular calls hook methods for *directives* as well as components.
    
    2. A spy directive can gives us insight into a DOM object that we cannot change directly.
    Obviously we can't change the implementation of a native `div`. 
    We can't modify a third party component either.
    But we can watch both with a directive.

  
:marked
  Our sneaky spy directive is simple,  consisting almost entirely of `ngOnInit` and `ngOnDestroy` hooks 
  that log messages to the parent via an injected `LoggerService`.
  
+makeExample('lifecycle-hooks/ts/app/spy.directive.ts', 'spy-directive')(format=".")

:marked
  We can apply the spy to any native or component element and it'll be initialized and destroyed
  at the same time as that element. 
  Here we attach it to the repeated hero `<div>`
+makeExample('lifecycle-hooks/ts/app/spy.component.html', 'template')(format=".")

:marked
  Each spy's birth and death marks the birth and death of the attached hero `<div>` 
  with an entry in the *Hook Log* as we see here:
  
figure.image-display
  img(src='/resources/images/devguide/lifecycle-hooks/spy-directive.gif' alt="Spy Directive")

:marked
  Adding a hero results in a new hero `<div>`. The spy's `ngOnInit` logs that event.
  We see a new entry for each hero.
  
  The *Reset* button clears the `heroes` list. 
  Angular removes all hero divs from the DOM and destroys their spy directives at the same time.
  The spy's `ngOnDestroy` method reports its last moments.
  
  The `ngOnInit` and `ngOnDestroy` methods have more vital roles to play in real applications.
  Let's see why we need them.
    
  ### OnInit
  
  We turn to `ngOnInit` for two main reasons:
  1. To perform complex initializations shortly after construction
  1. To set up the component after Angular sets the input properties
  
  An `ngOnInit` often fetches data for the component as shown in the 
  [Tutorial](../tutorial/toh-pt4.html#oninit) and [HTTP](server-communication.html#oninit) chapters.
  
  We don't fetch data in a component constructor. Why?
  Because experienced developers agree that components should be cheap and safe to construct. 
  We shouldn't worry that a new component will try to contact a remote server when
  created under test or before we decide to display it.
  Constructors should do no more than set the initial local variables to simple values.
  
  When a component must start working _soon_ after creation,
  we can count on Angular to call the `ngOnInit` method to jumpstart it.
  That's where the heavy initialization logic belongs.
  
  Remember also that a directive's data-bound input properties are not set until _after construction_.
  That's a problem if we need to initialize the directive based on those properties.
  They'll have been set when our `ngOninit` runs.
.l-sub-section
  :marked
    Our first opportunity to access those properties is the `ngOnChanges` method which
    Angular calls before `ngOnit`. But Angular calls `ngOnChanges` many times after that.
    It only calls `ngOnit` once.
:marked
  ### OnDestroy
  
  Put cleanup logic in `ngOnDestroy`, the logic that *must* run before Angular destroys the directive.
  
  This is the time to notify another part of the application that this component is going away.
  
  This is the place to free resources that won't be garbage collected automatically.
  Unsubscribe from observables and DOM events. Stop interval timers.
  Unregister all callbacks that this directive registered with global or application services.
  We risk memory leaks if we neglect to do so.

.l-main-section
:marked
  ## OnChanges
  
  We monitor the `OnChanges` hook in this example. 
  Angular calls its `ngOnChanges` method whenever it detects changes to ***input properties*** of the component (or directive).
  
  Here is our implementation of the hook.
+makeExample('lifecycle-hooks/ts/app/on-changes.component.ts', 'ng-on-changes', 'OnChangesComponent (ngOnChanges)')(format=".")
:marked
  The `ngOnChanges` method takes an object that maps each changed property name to a 
  [SimpleChange](../api/core/index/SimpleChange-class.html) object with the current and previous property values.
  We iterate over the changed properties and log them.
  
  The input properties for our example `OnChangesComponent` are `hero` and `power`.
+makeExample('lifecycle-hooks/ts/app/on-changes.component.ts', 'inputs')(format=".")
:marked
  The parent binds to them like this:

+makeExample('lifecycle-hooks/ts/app/on-changes-parent.component.html', 'on-changes')
:marked
  Here's the sample in action as we make changes.

figure.image-display
  img(src='/resources/images/devguide/lifecycle-hooks/on-changes-anim.gif' alt="OnChanges")
  
:marked
  We see log entries as the string value of the *power* property changes. But the `ngOnChanges` did not catch changes to `hero.name`
  That's surprising at first. 
  
  Angular only calls the hook when the value of the input property changes. 
  The value of the `hero` property is the *reference to the hero object*.
  Angular doesn't care that the hero's own `name` property changed. 
  The hero object *reference* didn't change so, from Angular's perspective, there is no change to report!

.l-main-section
:marked
  ## DoCheck
  We can use the `DoCheck` hook to detect and act upon changes that Angular doesn't catch on its own.
.l-sub-section
  :marked
    With this method we can detect a change that Angular overlooked. 
    What we do with that information to refresh the display is a separate matter.
:marked
  The *DoCheck* sample extends the *OnChanges* sample with this implementation of `DoCheck`:
+makeExample('lifecycle-hooks/ts/app/do-check.component.ts', 'ng-do-check', 'DoCheckComponent (ngDoCheck)')(format=".")
:marked
  We manually check everything that we care about, capturing and comparing against previous values.
  We write a special message to the log when there are no substantive changes 
  to the hero or the power so we can keep an eye on the method's performance characteristics.
  
  The results are illuminating:

figure.image-display
  img(src='/resources/images/devguide/lifecycle-hooks/do-check-anim.gif' alt="DoCheck")
:marked
  We now are able to detect when the hero's `name` has changed. But we must be careful.
  
  The `ngDoCheck` hook is called with enormous frequency &mdash; 
  after _every_ change detection cycle no matter where the change occurred.
  It's called over twenty times in this example before the user can do anything. 
  
  Most of these initial checks are triggered by Angular's first rendering of *unrelated data elsewhere on the page*.
  Mere mousing into another input box triggers a call.
  Relatively few calls reveal actual changes to pertinent data.
  Clearly our implementation must be very lightweight or the user experience may suffer.

.l-sub-section
  :marked
    We also see that the `ngOnChanges` method is called in contradiction of the 
    [incorrect API documentation](../api/core/index/DoCheck-class.html).

.l-main-section
:marked
  ## AfterView
  The *AfterView* sample explores the `AfterViewInit` and `AfterViewChecked` hooks that Angular calls
  *after* it creates a component's child views.
  
  Here's a child view that displays a hero's name in an input box:
+makeExample('lifecycle-hooks/ts/app/after-view.component.ts', 'child-view', 'ChildComponent')(format=".")
:marked
  The `AfterViewComponent` displays this child view *within its template*:
+makeExample('lifecycle-hooks/ts/app/after-view.component.ts', 'template', 'AfterViewComponent (template)')(format=".")
:marked
  The following hooks take action based on changing values *within the child view* 
  which we can only reach by querying for the child view via the property decorated with 
  [@ViewChild](../api/core/index/ViewChild-var.html).

+makeExample('lifecycle-hooks/ts/app/after-view.component.ts', 'hooks', 'AfterViewComponent (class excerpts)')(format=".")
.a(id="wait-a-tick")
:marked
  ### Abide by the unidirectional data flow rule
  The `doSomething` method updates the screen when the hero name exceeds 10 characters.
  
+makeExample('lifecycle-hooks/ts/app/after-view.component.ts', 'do-something', 'AfterViewComponent (doSomething)')(format=".")
:marked
  Why does the `doSomething` method wait a tick before updating `comment`?
  
  Because we must adhere to Angular's unidirectional data flow rule which says that
  we may not update the view *after* it has been composed.
  Both hooks fire after the component's view has been composed.
  
  Angular throws an error if we update component's data-bound `comment` property immediately (try it!).
block tick-methods
  :marked
    The `LoggerService.tick` methods, which are implemented by a call to `setTimeout`, postpone the update one turn of the of the browser's JavaScript cycle ... and that's long enough.

:marked
  Here's *AfterView* in action
figure.image-display
  img(src='/resources/images/devguide/lifecycle-hooks/after-view-anim.gif' alt="AfterView")
:marked
  Notice that Angular frequently calls `AfterViewChecked`, often when there are no changes of interest.
  Write lean hook methods to avoid performance problems.

.l-main-section
:marked
  ## AfterContent
  The *AfterContent* sample explores the `AfterContentInit` and `AfterContentChecked` hooks that Angular calls
  *after* Angular projects external content into the component.
  
  ### Content projection
  *Content projection* is a way to import HTML content from outside the component and insert that content
  into the component's template in a designated spot.
 
.l-sub-section
  :marked
    Angular 1 developers know this technique as *transclusion*.
  
:marked
  We'll illustrate with a variation on the [previous](#afterview) example 
  whose behavior and output is almost the same.
  
  This time, instead of including the child view within the template, we'll import it from
  the `AfterContentComponent`'s parent. Here's the parent's template.
+makeExample('lifecycle-hooks/ts/app/after-content.component.ts', 'parent-template', 'AfterContentParentComponent (template excerpt)')(format=".")
:marked
  Notice that the `<my-child>` tag is tucked between the `<after-content>` tags.
  We never put content between a component's element tags *unless we intend to project that content
  into the component*.
  
  Now look at the component's template:
+makeExample('lifecycle-hooks/ts/app/after-content.component.ts', 'template', 'AfterContentComponent (template)')(format=".")
:marked
  The `<ng-content>` tag is a *placeholder* for the external content. 
  They tell Angular where to insert that content.
  In this case, the projected content is the `<my-child>` from the parent.
figure.image-display
  img(src='/resources/images/devguide/lifecycle-hooks/projected-child-view.png' width="230" alt="Projected Content")
:marked
.l-sub-section
  :marked
    The tell-tale signs of *content projection* are (a) HTML between component element tags
    and (b) the presence of `<ng-content>` tags in the component's template.
:marked
  ### AfterContent hooks
  *AfterContent* hooks are similar to the *AfterView* hooks. The key difference is the kind of child component
  that we're looking for. 
  
  * The *AfterView* hooks concern `ViewChildren`, the child components whose element tags
  appear *within* the component's template.
  
  * The *AfterContent* hooks concern `ContentChildren`, the child components that Angular
  projected into the component.
  
  The following *AfterContent* hooks take action based on changing values in a  *content child*
  which we can only reach by querying for it via the property decorated with 
  [@ContentChild](../api/core/index/ContentChild-var.html).

+makeExample('lifecycle-hooks/ts/app/after-content.component.ts', 'hooks', 'AfterContentComponent (class excerpts)')(format=".")

:marked
   ### No unidirectional flow worries
   
   This component's `doSomething` method update's the component's data-bound `comment` property immediately.
   There's no [need to wait](#wait-a-tick). 
   
   Recall that Angular calls both *AfterContent* hooks before calling either of the *AfterView* hooks.
   Angular completes composition of the projected content *before* finishing the composition of this component's view.
   We still have a window of opportunity to modify that view.