`
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 `
`. The spy's `ngOnInit` logs that event.
We see a new entry for each hero.
-
- The *Reset* button clears the `heroes` list.
+
+ 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
+
+ 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.
+ 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.
@@ -380,11 +383,11 @@ figure.image-display
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.
@@ -393,17 +396,17 @@ figure.image-display
.l-main-section
:marked
## OnChanges
-
- We monitor the `OnChanges` hook in this example.
+
+ 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
+ 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
@@ -415,14 +418,14 @@ figure.image-display
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.
+ 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.
+ 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
@@ -431,27 +434,27 @@ figure.image-display
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.
+ 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
+ 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 —
+
+ The `ngDoCheck` hook is called with enormous frequency —
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.
-
+ 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.
@@ -459,7 +462,7 @@ figure.image-display
.l-sub-section
:marked
- We also see that the `ngOnChanges` method is called in contradiction of the
+ 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
@@ -467,15 +470,15 @@ figure.image-display
## 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
+ 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=".")
@@ -483,15 +486,15 @@ figure.image-display
: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
@@ -510,19 +513,19 @@ figure.image-display
## 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
+ 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=".")
@@ -530,11 +533,11 @@ figure.image-display
Notice that the `
` tag is tucked between the `` 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 `` tag is a *placeholder* for the external content.
+ The `` tag is a *placeholder* for the external content.
They tell Angular where to insert that content.
In this case, the projected content is the `` from the parent.
figure.image-display
@@ -547,26 +550,26 @@ figure.image-display
: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.
-
+ 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
+ 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).
-
+ 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.