Lifecycle Hooks 4.0
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 lifecycle hooks that provide visibility into these key life moments and the ability to act when they occur.
A directive has the same set of lifecycle hooks, minus the hooks that are specific to component content and views.
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
Each interface has a single hook method whose name is the interface name prefixed with
For example, the
OnInit interface has a hook method named
that Angular calls shortly after creating the component:
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.
After creating a component/directive by calling its constructor, Angular calls the lifecycle hook methods in the following sequence at specific moments:
|Hook||Purpose and Timing|
Respond when Angular (re)sets data-bound input properties.
The method receives a
Initialize the directive/component after Angular first displays the data-bound properties and sets the directive/component’s input properties.
Called once, after the first
Detect and act upon changes that Angular can’t or won’t detect on its own.
Called during every change detection run, immediately after
Respond after Angular projects external content into the component’s view.
Called once after the first
A component-only hook.
Respond after Angular checks the content projected into the component.
Called after the
A component-only hook.
Respond after Angular initializes the component’s views and child views.
Called once after the first
A component-only hook.
Respond after Angular checks the component’s views and child views.
Called after the
A component-only hook.
Cleanup just before Angular destroys the directive/component. Unsubscribe observables and detach event handlers to avoid memory leaks.
Called just before Angular destroys the directive/component.
Other lifecycle hooks
Other Angular sub-systems may have their own lifecycle hooks apart from these component hooks.
The router, for instance, also has its own router lifecycle
hooks that allow us to tap into
specific moments in route navigation.
A parallel can be drawn between
routerOnActivate. Both are
prefixed so as to avoid collision, and both run right when a component is
3rd party libraries might implement their hooks as well in order to give developers more control over how these libraries are used.
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:
Demonstrates every lifecycle hook. Each hook method writes to the on-screen log.
Directives have lifecycle hooks too.
This example applies the
See how Angular calls the
Shows what Angular means by a view.
Shows how to project external content into a component and
how to distinguish projected content from a component’s view children.
Demonstrates a combination of a component and a directive each with its own hooks.
In this example, a
The remainder of this chapter discusses selected exercises in further detail.
Peek-a-boo: all hooks
PeekABooComponent demonstrates all of the hooks in one component.
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.
This snapshot reflects the state of the log after the user clicked the Create… button and then the Destroy… button.
The sequence of log messages follows the prescribed hook calling order:
AfterViewChecked (3x), and
The constructor isn’t an Angular hook per se.
The log confirms that input properties (the
name property in this case) have no assigned values at construction.
Had the user clicked the Update Hero button, the log would show another
OnChanges and two more triplets of
Clearly these three hooks fire a often. Keep the logic in these hooks as lean as possible!
The next examples focus on hook details.
Spying OnInit and OnDestroy
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. The heroes will never know they’re being watched.
Kidding aside, pay attention to two key points:
Angular calls hook methods for directives as well as components.
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.
The sneaky spy directive is simple, consisting almost entirely of
that log messages to the parent via an injected
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 it is attached to the repeated hero
Each spy’s birth and death marks the birth and death of the attached hero
with an entry in the Hook Log as seen here:
Adding a hero results in a new hero
<div>. The spy’s
ngOnInit logs that event.
The Reset button clears the
Angular removes all hero
<div> elements from the DOM and destroys their spy directives at the same time.
ngOnDestroy method reports its last moments.
ngOnDestroy methods have more vital roles to play in real applications.
ngOnInit for two main reasons:
- to perform complex initializations shortly after construction
- to set up the component after Angular sets the input properties
Experienced developers agree that components should be cheap and safe to construct.
Misko Hevery, Angular team lead, explains why you should avoid complex constructor logic.
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.
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
ngOnChanges method is your first opportunity to access those properties.
ngOnInit … and many times after that.
It only calls
You can count on Angular to call the
ngOnInit method soon after creating the component.
That’s where the heavy initialization logic belongs.
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 app 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 app services. You risk memory leaks if you neglect to do so.
Angular calls its
ngOnChanges method whenever it detects changes to input properties of the component (or directive).
This example monitors the
ngOnChanges method takes an object that maps each changed property name to a
SimpleChange object holding the current and previous property values.
This hook iterates over the changed properties and logs them.
The example component,
OnChangesComponent, has two input properties:
OnChangesParentComponent binds to them like this:
Here’s the sample in action as the user makes changes.
The log entries appear as the string value of the power property changes.
ngOnChanges does not catch changes to
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!
DoCheck hook to detect and act upon changes that Angular doesn’t catch on its own.
Use this method to detect a change that Angular overlooked.
The DoCheck sample extends the OnChanges sample with the following
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
so you can see how often
DoCheck is called. The results are illuminating:
ngDoCheck hook can detect when the hero’s
name has changed, it has a frightful cost.
This 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.
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 will suffer.
The AfterView sample explores the
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:
AfterViewComponent displays this child view within its template:
The following hooks take action based on changing values within the child view which can only be reached by querying for the child view via the property decorated with @ViewChild.
Abide by the unidirectional data flow rule
doSomething method updates the screen when the hero name exceeds 10 characters.
Why does the
doSomething method wait a tick before updating
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 the hook updates the component’s data-bound
comment property immediately (try it!).
LoggerService.tick() postpones the log update
for one turn of the browser’s update cycle … and that’s just long enough.
Here’s AfterView in action
Notice that Angular frequently calls
AfterViewChecked, often when there are no changes of interest.
Write lean hook methods to avoid performance problems.
The AfterContent sample explores the
AfterContentChecked hooks that Angular calls
after Angular projects external content into the component.
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.
Angular 1 developers know this technique as transclusion.
Consider this variation on the previous AfterView example.
This time, instead of including the child view within the template, it imports the content from
AfterContentComponent’s parent. Here’s the parent’s template.
Notice that the
<my-child> tag is tucked between the
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:
<ng-content> tag is a placeholder for the external content.
It tells Angular where to insert that content.
In this case, the projected content is the
<my-child> from the parent.
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.
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.
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 can only be reached by querying for it via the property decorated with @ContentChild.
No unidirectional flow worries with AfterContent…
doSomething method update’s the component’s data-bound
comment property immediately.
There’s no need to wait.
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.
There is a small window between the
AfterView... hooks to modify the host view.