Lifecycle hookslink
\nA component instance has a lifecycle that starts when Angular instantiates the component class and renders the component view along with its child views.\nThe lifecycle continues with change detection, as Angular checks to see when data-bound properties change, and updates both the view and the component instance as needed.\nThe lifecycle ends when Angular destroys the component instance and removes its rendered template from the DOM.\nDirectives have a similar lifecycle, as Angular creates, updates, and destroys instances in the course of execution.
\nYour application can use lifecycle hook methods to tap into key events in the lifecycle of a component or directive in order to initialize new instances, initiate change detection when needed, respond to updates during change detection, and clean up before deletion of instances.
\nPrerequisiteslink
\nBefore working with lifecycle hooks, you should have a basic understanding of the following:
\n- \n
- TypeScript programming. \n
- Angular app-design fundamentals, as described in Angular Concepts. \n
Responding to lifecycle eventslink
\nYou can respond to events in the lifecycle of a component or directive by implementing one or more of the lifecycle hook interfaces in the Angular core library.\nThe hooks give you the opportunity to act on a component or directive instance at the appropriate moment, as Angular creates, updates, or destroys that instance.
Each interface defines the prototype for a single hook method, whose name is the interface name prefixed with ng.\nFor example, the OnInit interface has a hook method named ngOnInit(). If you implement this method in your component or directive class, Angular calls it shortly after checking the input properties for that component or directive for the first time.
You don't have to implement all (or any) of the lifecycle hooks, just the ones you need.
\n\nLifecycle event sequencelink
\nAfter your application instantiates a component or directive by calling its constructor, Angular calls the hook methods you have implemented at the appropriate point in the lifecycle of that instance.
\nAngular executes hook methods in the following sequence. You can use them to perform the following kinds of operations.
\n| Hook method | \nPurpose | \nTiming | \n
|---|---|---|
\n ngOnChanges()\n | \n \n Respond when Angular sets or resets data-bound input properties.\nThe method receives a Note that this happens very frequently, so any operation you perform here impacts performance significantly.\nSee details in Using change detection hooks in this document. \n | \n \n Called before Note that if your component has no inputs or you use it without providing any inputs, the framework will not call | \n
\n ngOnInit()\n | \n \n Initialize the directive or component after Angular first displays the data-bound properties\nand sets the directive or component's input properties.\nSee details in Initializing a component or directive in this document. \n | \n \n Called once, after the first | \n
\n ngDoCheck()\n | \n \n Detect and act upon changes that Angular can't or won't detect on its own.\nSee details and example in Defining custom change detection in this document. \n | \n \n Called immediately after | \n
\n ngAfterContentInit()\n | \n \n Respond after Angular projects external content into the component's view, or into the view that a directive is in. \nSee details and example in Responding to changes in content in this document. \n | \n \n Called once after the first | \n
\n ngAfterContentChecked()\n | \n \n Respond after Angular checks the content projected into the directive or component. \nSee details and example in Responding to projected content changes in this document. \n | \n \n Called after | \n
\n ngAfterViewInit()\n | \n \n Respond after Angular initializes the component's views and child views, or the view that contains the directive. \nSee details and example in Responding to view changes in this document. \n | \n \n Called once after the first | \n
\n ngAfterViewChecked()\n | \n \n Respond after Angular checks the component's views and child views, or the view that contains the directive. \n | \n \n Called after the | \n
\n ngOnDestroy()\n | \n \n Cleanup just before Angular destroys the directive or component.\nUnsubscribe Observables and detach event handlers to avoid memory leaks.\nSee details in Cleaning up on instance destruction in this document. \n | \n \n Called immediately before Angular destroys the directive or component. \n | \n
Lifecycle example setlink
\nThe AppComponent.\nIn each case a parent component serves as a test rig for\na child component that illustrates one or more of the lifecycle hook methods.
The following table lists the exercises with brief descriptions.\nThe sample code is also used to illustrate specific tasks in the following sections.
\n| Component | \nDescription | \n
|---|---|
| \n Peek-a-boo\n | \n\n Demonstrates every lifecycle hook.\nEach hook method writes to the on-screen log. \n | \n
| \n Spy\n | \n\n Shows how you can use lifecycle hooks with a custom directive.\nThe | \n
| \n OnChanges\n | \n\n Demonstrates how Angular calls the | \n
| \n DoCheck\n | \n\n Implements the | \n
| \n AfterView\n | \n\n Shows what Angular means by a view.\nDemonstrates the | \n
| \n AfterContent\n | \n\n Shows how to project external content into a component and\nhow to distinguish projected content from a component's view children.\nDemonstrates the | \n
| \n Counter\n | \n\n Demonstrates a combination of a component and a directive, each with its own hooks. \n | \n
Initializing a component or directivelink
\nUse the ngOnInit() method to perform the following initialization tasks.
- \n
- \n
Perform complex initializations outside of the constructor.\nComponents should be cheap and safe to construct.\nYou should not, for example, fetch data in a component constructor.\nYou shouldn't worry that a new component will try to contact a remote server when\ncreated under test or before you decide to display it.
\nAn
\nngOnInit()is a good place for a component to fetch its initial data.\nFor an example, see the Tour of Heroes tutorial. \n
- \n
- \n
Set up the component after Angular sets the input properties.\nConstructors should do no more than set the initial local variables to simple values.
\nKeep in mind that a directive's data-bound input properties are not set until after construction.\nIf you need to initialize the directive based on those properties, set them when
\nngOnInit()runs.\n\nThe
\nngOnChanges()method is your first opportunity to access those properties.\nAngular callsngOnChanges()beforengOnInit(), but also many times after that.\nIt only callsngOnInit()once. \n
Cleaning up on instance destructionlink
\nPut cleanup logic in ngOnDestroy(), the logic that must run before Angular destroys the directive.
This is the place to free resources that won't be garbage-collected automatically.\nYou risk memory leaks if you neglect to do so.
\n- \n
- Unsubscribe from Observables and DOM events. \n
- Stop interval timers. \n
- Unregister all callbacks that the directive registered with global or application services. \n
The ngOnDestroy() method is also the time to notify another part of the application that the component is going away.
General exampleslink
\nThe following examples demonstrate the call sequence and relative frequency of the various lifecycle events, and how the hooks can be used separately or together for components and directives.
\n\nSequence and frequency of all lifecycle eventslink
\nTo show how Angular calls the hooks in the expected order, the PeekABooComponent demonstrates all of the hooks in one component.
In practice you would rarely, if ever, implement all of the interfaces the way this demo does.
\nThe following snapshot reflects the state of the log after the user clicked the Create... button and then the Destroy... button.
\n![\"Peek-a-boo\"](\"generated/images/guide/lifecycle-hooks/peek-a-boo.png\")
\nThe sequence of log messages follows the prescribed hook calling order:\nOnChanges, OnInit, DoCheck (3x), AfterContentInit, AfterContentChecked (3x),\nAfterViewInit, AfterViewChecked (3x), and OnDestroy.
Notice that the log confirms that input properties (the name property in this case) have no assigned values at construction.\nThe input properties are available to the onInit() method for further initialization.
Had the user clicked the Update Hero button, the log would show another OnChanges and two more triplets of DoCheck, AfterContentChecked and AfterViewChecked.\nNotice that these three hooks fire often, so it is important to keep their logic as lean as possible.
Use directives to watch the DOMlink
\nThe Spy example demonstrates how you can use hook method for directives as well as components.\nThe SpyDirective implements two hooks, ngOnInit() and ngOnDestroy(), in order to discover when a watched element is in the current view.
This template applies the SpyDirective to a <div> in the ngFor hero repeater managed by the parent SpyComponent.
The example does not perform any initialization or clean-up.\nIt just tracks the appearance and disappearance of an element in the view by recording when the directive itself is instantiated and destroyed.
\nA spy directive like this can provide insight into a DOM object that you cannot change directly.\nYou can't touch the implementation of a native <div>, or modify a third party component.\nYou can, however watch these elements with a directive.
The directive defines ngOnInit() and ngOnDestroy() hooks\nthat log messages to the parent via an injected LoggerService.
You can apply the spy to any native or component element, and see that it is initialized and destroyed\nat the same time as that element.\nHere it is attached to the repeated hero <div>:
Each spy's creation and destruction marks the appearance and disappearance of the attached hero <div>\nwith an entry in the Hook Log as seen here:
![\"Spy](\"generated/images/guide/lifecycle-hooks/spy-directive.gif\")
\nAdding a hero results in a new hero <div>. The spy's ngOnInit() logs that event.
The Reset button clears the heroes list.\nAngular removes all hero <div> elements from the DOM and destroys their spy directives at the same time.\nThe spy's ngOnDestroy() method reports its last moments.
Use component and directive hooks togetherlink
\nIn this example, a CounterComponent uses the ngOnChanges() method to log a change every time the parent component increments its input counter property.
This example applies the SpyDirective from the previous example to the CounterComponent log, in order to watch the creation and destruction of log entries.
Using change detection hookslink
\nAngular calls the ngOnChanges() method of a component or directive whenever it detects changes to the input properties.\nThe onChanges example demonstrates this by monitoring the OnChanges() hook.
The ngOnChanges() method takes an object that maps each changed property name to a\nSimpleChange object holding the current and previous property values.\nThis hook iterates over the changed properties and logs them.
The example component, OnChangesComponent, has two input properties: hero and power.
The host OnChangesParentComponent binds to them as follows.
Here's the sample in action as the user makes changes.
\n![\"OnChanges\"](\"generated/images/guide/lifecycle-hooks/on-changes-anim.gif\")
\nThe log entries appear as the string value of the power property changes.\nNotice, however, that the ngOnChanges() method does not catch changes to hero.name.\nThis is because Angular calls the hook only when the value of the input property changes.\nIn this case, hero is the input property, and the value of the hero property is the reference to the hero object.\nThe object reference did not change when the value of its own name property changed.
Responding to view changeslink
\nAs Angular traverses the view hierarchy during change detection, it needs to be sure that a change in a child does not attempt to cause a change in its own parent. Such a change would not be rendered properly, because of how unidirectional data flow works.
\nIf you need to make a change that inverts the expected data flow, you must trigger a new change detection cycle to allow that change to be rendered.\nThe examples illustrate how to make such changes safely.
\nThe AfterView sample explores the AfterViewInit() and AfterViewChecked() hooks that Angular calls\nafter it creates a component's child views.
Here's a child view that displays a hero's name in an <input>:
The AfterViewComponent displays this child view within its template:
The following hooks take action based on changing values within the child view,\nwhich can only be reached by querying for the child view via the property decorated with\n@ViewChild.
\nWait before updating the viewlink
\nIn this example, the doSomething() method updates the screen when the hero name exceeds 10 characters, but waits a tick before updating comment.
Both the AfterViewInit() and AfterViewChecked() hooks fire after the component's view has been composed.\nIf you modify the code so that the hook updates the component's data-bound comment property immediately, you can see that Angular throws an error.
The LoggerService.tick_then() statement postpones the log update\nfor one turn of the browser's JavaScript cycle, which triggers a new change-detection cycle.
Write lean hook methods to avoid performance problemslink
\nWhen you run the AfterView sample, notice how frequently Angular calls AfterViewChecked()-often when there are no changes of interest.\nBe very careful about how much logic or computation you put into one of these methods.
![\"AfterView\"](\"generated/images/guide/lifecycle-hooks/after-view-anim.gif\")
\nResponding to projected content changeslink
\nContent projection is a way to import HTML content from outside the component and insert that content\ninto the component's template in a designated spot.\nYou can identify content projection in a template by looking for the following constructs.
\n- \n
- HTML between component element tags. \n
- The presence of
<ng-content>tags in the component's template. \n
AngularJS developers know this technique as transclusion.
\nThe AfterContent sample explores the AfterContentInit() and AfterContentChecked() hooks that Angular calls after Angular projects external content into the component.
Consider this variation on the previous AfterView example.\nThis time, instead of including the child view within the template, it imports the content from\nthe AfterContentComponent's parent.\nThe following is the parent's template.
Notice that the <app-child> tag is tucked between the <after-content> tags.\nNever put content between a component's element tags unless you intend to project that content\ninto the component.
Now look at the component's template.
\nThe <ng-content> tag is a placeholder for the external content.\nIt tells Angular where to insert that content.\nIn this case, the projected content is the <app-child> from the parent.
![\"Projected](\"generated/images/guide/lifecycle-hooks/projected-child-view.png\")
\nUsing AfterContent hookslink
\nAfterContent hooks are similar to the AfterView hooks.\nThe key difference is in the child component.
\n- \n
- \n
The AfterView hooks concern
\nViewChildren, the child components whose element tags\nappear within the component's template. \n - \n
The AfterContent hooks concern
\nContentChildren, the child components that Angular\nprojected into the component. \n
The following AfterContent hooks take action based on changing values in a content child,\nwhich can only be reached by querying for them via the property decorated with\n@ContentChild.
\nThis component's doSomething() method updates the component's data-bound comment property immediately.\nThere's no need to delay the update to ensure proper rendering.
Angular calls both AfterContent hooks before calling either of the AfterView hooks.\nAngular completes composition of the projected content before finishing the composition of this component's view.\nThere is a small window between the AfterContent... and AfterView... hooks that allows you to modify the host view.
Defining custom change detectionlink
\nTo monitor changes that occur where ngOnChanges() won't catch them, you can implement your own change check, as shown in the DoCheck example.\nThis example shows how you can use the ngDoCheck() hook to detect and act upon changes that Angular doesn't catch on its own.
The DoCheck sample extends the OnChanges sample with the following ngDoCheck() hook:
This code inspects certain values of interest, capturing and comparing their current state against previous values.\nIt 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.\nThe results are illuminating.
![\"DoCheck\"](\"generated/images/guide/lifecycle-hooks/do-check-anim.gif\")
\nWhile the ngDoCheck() hook can detect when the hero's name has changed, it is very expensive.\nThis hook is called with enormous frequency—after every\nchange detection cycle no matter where the change occurred.\nIt'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.\nJust moving the cursor into another <input> triggers a call.\nRelatively few calls reveal actual changes to pertinent data.\nIf you use this hook, your implementation must be extremely lightweight or the user experience suffers.