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docs(lifecycle-hooks): make it easier to get to call sequence (#2465) 

Per Ben Lesh suggestion. Also converts away from "we"
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Ward Bell 2016-09-24 12:37:22 -07:00 committed by GitHub
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2
public/docs/_examples/lifecycle-hooks/ts/app/peek-a-boo.component.ts
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@ -21,7 +21,7 @@ export class PeekABoo implements OnInit {
// implement OnInit's `ngOnInit` method
ngOnInit() { this.logIt(`OnInit`); }
protected logIt(msg: string) {
logIt(msg: string) {
this.logger.log(`#${nextId++} ${msg}`);
}
}

260
public/docs/ts/latest/guide/lifecycle-hooks.jade
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@ -9,13 +9,18 @@ block includes
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.
that provide visibility into these key moments and the ability to act when they occur.
## Table of Contents
* [The lifecycle hooks](#hooks-overview)
* [The hook-call sequence](#hook-sequence)
* [Other Angular lifecycle hooks](#other-lifecycles)
* [_What_ they are](#hook-descriptions)
* [_When_ they are called](#hook-sequence)
+ifDocsFor('ts|js')
:marked
* [Interfaces are optional (technically)](#interface-optional)
:marked
* [Other Angular lifecycle hooks](#other-lifecycle-hooks)
<br><br>
* [The lifecycle sample](#the-sample)
* [All](#peek-a-boo)
* [Spying OnInit and OnDestroy](#spy)
@ -32,38 +37,20 @@ a#hooks-overview
## 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:
For example, the `OnInit` interface has a hook method named `ngOnInit`
that Angular calls shortly after creating the component:
+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.
No directive or component will implement all of the lifecycle hooks and some of the hooks only make sense for components.
Angular only calls a directive/component hook method *if it is defined*.
+ifDocsFor('ts|js')
.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.
a#hook-descriptions
:marked
Here are the component lifecycle hook methods:
Here are descriptions of the component lifecycle hook methods followed by a chart describing [when they are called](#hook-sequence):
### Directives and Components
@ -129,7 +116,7 @@ table(width="100%")
:marked
Angular does not call the hook methods in this order.
a(id="hook-sequence")
a#hook-sequence
.l-main-section
:marked
## Lifecycle sequence
@ -182,18 +169,37 @@ table(width="100%")
:marked
just before Angular destroys the directive/component.
a(id="other-lifecycles")
+ifDocsFor('ts|js')
a#interface-optional
.l-main-section
:marked
## Interface are optional (technically)
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.
You don't have to add the lifecycle hook interfaces to directives and components to benefit from the hooks themselves.
Angular instead inspects directive and component classes and calls the hook methods *if they are defined*.
Angular finds and calls methods like `ngOnInit()`, with or without the interfaces.
Nonetheless, it's good practice to add interfaces to TypeScript directive classes
in order to benefit from strong typing and editor tooling.
a#other-lifecycle-hooks
.l-main-section
:marked
## Other lifecycle hooks
Other Angular sub-systems may have their own lifecycle hooks apart from the component hooks we've listed.
Other Angular sub-systems may have their own lifecycle hooks apart from these component hooks.
block other-angular-subsystems
//- N/A for TS.
:marked
3rd party libraries might implement their hooks as well in order to give us, the developers, more
3rd party libraries might implement their hooks as well in order to give developers more
control over how these libraries are used.
.l-main-section#the-sample
@ -226,10 +232,10 @@ table(width="100%")
td
:marked
Directives have lifecycle hooks too.
We create a `SpyDirective` that logs when the element it spies upon is
A `SpyDirective` can log 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
This example applies the `SpyDirective` to a `<div>` in an `ngFor` *hero* repeater
managed by the parent `SpyComponent`.
tr(style=top)
td <a href="#onchanges">OnChanges</a>
@ -266,22 +272,22 @@ table(width="100%")
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.
Meanwhile, the `SpyDirective` from the previous example is applied
to the `CounterComponent` log where it watches log entries being created and destroyed.
:marked
We discuss the exercises in further detail over this chapter as we learn more about the lifecycle hooks.
The remainder of this chapter discusses selected exercises in further detail.
a(id="peek-a-boo")
a#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.
You would rarely, if ever, implement all of the interfaces like this.
The peek-a-boo exists to show how Angular calls the hooks in the expected order.
In this snapshot, we clicked the *Create...* button and then the *Destroy...* button.
This snapshot reflects the state of the log after the user 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
@ -292,135 +298,138 @@ figure.image-display
.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.
The log confirms 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
Had the user clicked the *Update Hero* button, the log would show 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!
Clearly these three hooks fire a *often*. Keep the logic in these hooks as lean as possible!
Our next examples focus on hook details.
The next examples focus on hook details.
.a(id="spy")
a#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.
Go undercover with these two spy hooks to discover when an element is initialized or destroyed.
This is the perfect infiltration job for a directive.
Our heroes will never know it's there.
The heroes will never know they're being watched.
.l-sub-section
:marked
Kidding aside, we're emphasizing two key points:
Kidding aside, pay attention to two key points:
1. Angular calls hook methods for *directives* as well as components.
1. Angular calls hook methods for *directives* as well as components.<br><br>
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.
2. A spy directive can provide insight into a DOM object that you cannot change directly.
Obviously you can't touch the implementation of a native `div`.
You can't modify a third party component either.
But you can watch both with a directive.
:marked
Our sneaky spy directive is simple, consisting almost entirely of `ngOnInit` and `ngOnDestroy` hooks
The 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
You 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>`
Here it is attached 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:
with an entry in the *Hook Log* as seen 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.
Angular removes all hero `<div>` elements 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
Use `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.
Experienced developers agree that components should be cheap and safe to construct.
.l-sub-section
:marked
Our first opportunity to access those properties is the `ngOnChanges` method which
Angular calls before `ngOnInit`. But Angular calls `ngOnChanges` many times after that.
Misko Hevery, Angular team lead,
[explains why](http://misko.hevery.com/code-reviewers-guide/flaw-constructor-does-real-work/)
you should avoid complex constructor logic.
:marked
Don't fetch data in a component constructor.
You shouldn't worry that a new component will try to contact a remote server when
created under test or before you decide to display it.
Constructors should do no more than set the initial local variables to simple values.
An `ngOnInit` is a good place for a component to fetch its initial data. The
[Tutorial](../tutorial/toh-pt4.html#oninit) and [HTTP](server-communication.html#oninit) chapter
show how.
Remember also that a directive's data-bound input properties are not set until _after construction_.
That's a problem if you need to initialize the directive based on those properties.
They'll have been set when `ngOninit` runs.
.l-sub-section
:marked
The `ngOnChanges` method is your first opportunity to access those properties.
Angular calls `ngOnChanges` before `ngOnInit` ... and many times after that.
It only calls `ngOnInit` once.
:marked
You can count on Angular to call the `ngOnInit` method _soon_ after creating the component.
That's where the heavy initialization logic belongs.
### 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 time to notify another part of the application that the 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.
You risk memory leaks if you 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.
This example monitors the `OnChanges` 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.
[SimpleChange](../api/core/index/SimpleChange-class.html) object holding the current and previous property values.
This hook iterates over the changed properties and logs them.
The input properties for our example `OnChangesComponent` are `hero` and `power`.
The example component, `OnChangesComponent`, has two input properties: `hero` and `power`.
+makeExample('lifecycle-hooks/ts/app/on-changes.component.ts', 'inputs')(format=".")
:marked
The parent binds to them like this:
The host `OnChangesParentComponent` 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.
Here's the sample in action as the user makes 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`
The log entries appear as the string value of the *power* property changes.
But the `ngOnChanges` does not catch changes to `hero.name`
That's surprising at first.
Angular only calls the hook when the value of the input property changes.
@ -431,39 +440,30 @@ figure.image-display
.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.
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.
Use this method to detect a change that Angular overlooked.
:marked
The *DoCheck* sample extends the *OnChanges* sample with this implementation of `DoCheck`:
The *DoCheck* sample extends the *OnChanges* sample with the following `ngDoCheck` hook:
+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:
This code inspects certain _values-of-interest_, capturing and comparing their current state against previous values.
It writes a special message to the log when there are no substantive changes to the `hero` or the `power`
so you can see how often `DoCheck` is called. 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;
While the `ngDoCheck` hook can detect when the hero's `name` has changed, it has a frightful cost.
This 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).
Clearly our implementation must be very lightweight or the user experience will suffer.
.l-main-section
:marked
@ -478,11 +478,11 @@ figure.image-display
+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
which can only be reached 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")
#wait-a-tick
:marked
### Abide by the unidirectional data flow rule
The `doSomething` method updates the screen when the hero name exceeds 10 characters.
@ -491,14 +491,14 @@ figure.image-display
: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's unidirectional data flow rule forbids updates to the view *after* it has been composed.
Both of these 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!).
Angular throws an error if the hook updates the 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.
The `LoggerService.tick_then()` postpones the log update
for one turn of the browser's JavaScript cycle ... and that's just long enough.
:marked
Here's *AfterView* in action
@ -523,22 +523,20 @@ figure.image-display
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
Consider this variation on the [previous _AfterView_](#afterview) example.
This time, instead of including the child view within the template, it imports the content 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
Never put content between a component's element tags *unless you 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.
It tells 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")
@ -549,8 +547,8 @@ figure.image-display
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.
*AfterContent* hooks are similar to the *AfterView* hooks.
The key difference is in the child component
* The *AfterView* hooks concern `ViewChildren`, the child components whose element tags
appear *within* the component's template.
@ -559,17 +557,17 @@ figure.image-display
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
which can only be reached 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
### No unidirectional flow worries with _AfterContent..._
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.
There is a small window between the `AfterContent...` and `AfterView...` hooks to modify the host view.