How to extend inherit components

Question

I would like to create extensions for some components already deployed in Angular 2  without having to rewrite them almost completely  as the base component could undergo changes and wish these changes were also reflected in its derived components   I created this simple example to try to explain better my questions   With the following base component app base-panel component ts   import  Component  Input  from  angular2 core     Component       selector   base-panel       template    lt div class  panel   style background-color   color   click   onClick  event   gt   content   lt  div gt        styles          panel      padding  50px              export class BasePanelComponent        Input   content  string     color  string    red      onClick event       console log  Click color      this color           Would you like to create another derivative component only alter  for example  a basic component behavior in the case of the example color  app my-panel component ts   import  Component  from  angular2 core   import  BasePanelComponent  from    base-panel component    Component       selector   my-panel       template    lt div class  panel   style background-color   color   click   onClick  event   gt   content   lt  div gt        styles          panel      padding  50px              export class MyPanelComponent extends BasePanelComponent     constructor         super        this color    blue           Complete working example in Plunker     Note  Obviously this example is simple and could be solved otherwise no need to use inheritance  but it is intended only to illustrate the real problem    As you can see in the implementation of the derivative component app my-panel component ts  much of the implementation was repeated  and the single part really inherited was the class BasePanelComponent  but the  Component had to basically be completely repeated  not just the changed portions  as the selector   my-panel    Is there some way to make a literally full inheritance of a component Angular2  inheriting the class definition of the markings annotations  as for example  Component   Edit 1 - Feature Request     Feature request angular2 added to the project on GitHub  Extend Inherit angular2 components annotations  7968   Edit 2 - Closed Request     The request was closed  for this reason  that briefly would not know how to merge the decorator will be made  Leaving us with no options  So my opinion is is quoted in the Issue

User · Answer

just use inheritance Extend parent class in child class and declare constructor with parent class parameter and this parameter use in super     1 parent class  Component       selector   teams-players-box       templateUrl    maxweb app app teams-players-box component html     export class TeamsPlayersBoxComponent       public  userProfile UserProfile      public  user img any      public  box class string  about-team teams-blockbox       public fullname string      public  index any      public  isView string      indexnumber number      constructor          public  userProfilesSvc  UserProfiles          public  router Router          2 child class  Component        selector    teams-players-eligibility        templateUrl    maxweb app app teams-players-eligibility component html     export class TeamsPlayersEligibilityComponent extends TeamsPlayersBoxComponent       constructor  public  userProfilesSvc  UserProfiles              public  router Router                super  userProfilesSvc  router

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Let us understand some key limitations  amp  features on Angular   s component inheritance system   The component only inherits the class logic    All meta-data in the  Component decorator is not inherited  Component  Input properties and  Output properties are inherited  Component lifecycle is not inherited    These features are very important to have in mind so let us examine each one independently   The Component only inherits the class logic  When you inherit a Component  all logic inside is equally inherited  It is worth noting that only public members are inherited as private members are only accessible in the class that implements them   All meta-data in the  Component decorator is not inherited  The fact that no meta-data is inherited might seem counter-intuitive at first but  if you think about this it actually makes perfect sense  If you inherit from a Component say  componentA   you would not want the selector of ComponentA  which you are inheriting from to replace the selector of ComponentB which is the class that is inheriting  The same can be said for the template templateUrl as well as the style styleUrls   Component  Input and  Output properties are inherited  This is another feature that I really love about component Inheritance in Angular  In a simple sentence  whenever you have a custom  Input and  Output property  these properties get inherited   Component lifecycle is not inherited  This part is the one that is not so obvious especially to people who have not extensively worked with OOP principles  For example  say you have ComponentA which implements one of Angular   s many lifecycle hooks like OnInit  If you create ComponentB and inherit ComponentA  the OnInit lifecycle from ComponentA won t fire until you explicitly call it even if you do have this OnInit lifecycle for ComponentB   Calling Super Base Component Methods  In order to have the ngOnInit   method from ComponentA fire  we need to use the super keyword and then call the method we need which in this case is ngOnInit  The super keyword refers to the instance of the component that is being inherited from which in this case will be ComponentA

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If anyone is looking for an updated solution  Fernando s answer is pretty much perfect  Except that ComponentMetadata has been deprecated  Using Component instead worked for me    The full Custom Decorator CustomDecorator ts file looks like this   import  zone js   import  reflect-metadata   import   Component   from   angular core   import   isPresent   from   angular platform-browser src facade lang    export function CustomComponent annotation  any      return function  target  Function        var parentTarget   Object getPrototypeOf target prototype  constructor      var parentAnnotations   Reflect getMetadata  annotations   parentTarget        var parentAnnotation   parentAnnotations 0       Object keys parentAnnotation  forEach key   gt          if  isPresent parentAnnotation key                 verify is annotation typeof function         if typeof annotation key       function              annotation key    annotation key  call this  parentAnnotation key             else if               force override in annotation base            isPresent annotation key                        annotation key    parentAnnotation key                                  var metadata   new Component annotation        Reflect defineMetadata  annotations     metadata    target           Then import it in to your new component sub-component component ts file and use  CustomComponent instead of  Component like this   import   CustomComponent   from    CustomDecorator   import   AbstractComponent   from  path to file          CustomComponent     selector   subcomponent     export class SubComponent extends AbstractComponent      constructor         super              Add new logic here

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if you read through the CDK libraries and the material libraries  they re using inheritance but not so much for components themselves  content projection is king IMO  see this link https   blog angular-university io angular-ng-content  where it says  quot the key problem with this design quot  I know this doesn t answer your question but I really think inheriting   extending components should be avoided  Here s my reasoning  If the abstract class extended by two or more components contains shared logic  use a service or even create a new typescript class that can be shared between the two components  If the abstract class    contains shared variables or onClicketc functions  Then there will be duplication between the html of the two extending components views  This is bad practice  amp  that shared html needs to be broken into Component s   These Component s   parts  can be shared between the two components  Am I missing other reasons for having an abstract class for components  An example I saw recently was components extending AutoUnsubscribe  import   Subscription   from  rxjs   import   OnDestroy   from   angular core   export abstract class AutoUnsubscribeComponent implements OnDestroy     protected infiniteSubscriptions  Array lt Subscription gt      constructor         this infiniteSubscriptions              ngOnDestroy         this infiniteSubscriptions forEach  subscription    gt          subscription unsubscribe                   this was bas because throughout a large codebase  infiniteSubscriptions push   was only used 10 times  Also importing  amp  extending AutoUnsubscribe actually takes more code than just adding mySubscription unsubscribe   in the ngOnDestroy   method of the component itself  which required additional logic anyway

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Angular 2 version 2 3 was just released  and it includes native component inheritance  It looks like you can inherit and override whatever you want  except for templates and styles  Some references    New features in Angular 2 3 Component Inheritance in Angular 2 A Plunkr demonstrating component inheritance

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You can inherit  Input   Output   ViewChild  etc  Look at the sample    Component       template        export class BaseComponent        Input   someInput  any    something         Output   someOutput  EventEmitter lt void gt    new EventEmitter lt void gt          Component       selector   app-derived       template    lt div  click   someOutput emit    gt   someInput   lt  div gt        providers              provide  BaseComponent  useExisting  DerivedComponent            export class DerivedComponent

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Now that TypeScript 2 2 supports Mixins through Class expressions we have a much better way to express Mixins on Components  Mind you that you can also use Component inheritance since angular 2 3  discussion  or a custom decorator as discussed in other answers here  However  I think Mixins have some properties that make them preferable for reusing behavior across components     Mixins compose more flexibly  i e  you can mix and match Mixins on existing components or combine Mixins to form new Components Mixin composition remains easy to understand thanks to its obvious linearization to a class inheritance hierarchy  You can more easily avoid issues with decorators and annotations that plague component inheritance  discussion    I strongly suggest you read the TypeScript 2 2 announcement above to understand how Mixins work  The linked discussions in angular GitHub issues provide additional detail    You ll need these types    export type Constructor lt T gt    new     args  any      gt  T   export class MixinRoot       And then you can declare a Mixin like this Destroyable mixin that helps components keep track of subscriptions that need to be disposed in ngOnDestroy    export function Destroyable lt T extends Constructor lt    gt  gt  Base  T      return class Mixin extends Base implements OnDestroy       private readonly subscriptions  Subscription              protected registerSubscription sub  Subscription          this subscriptions push sub              public ngOnDestroy           this subscriptions forEach x   gt  x unsubscribe           this subscriptions length   0     release memory                To mixin Destroyable into a Component  you declare your component like this    export class DashboardComponent extends Destroyable MixinRoot       implements OnInit  OnDestroy           Note that MixinRoot is only necessary when you want to extend a Mixin composition  You can easily extend multiple mixins e g  A extends B C D    This is the obvious linearization of mixins I was talking about above  e g  you re effectively composing an inheritnace hierarchy A - gt  B - gt  C - gt  D   In other cases  e g  when you want to compose Mixins on an existing class  you can apply the Mixin like so    const MyClassWithMixin   MyMixin MyClass     However  I found the first way works best for Components and Directives  as these also need to be decorated with  Component or  Directive anyway

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update  Component inheritance is supported since 2 3 0-rc 0  original  So far  the most convenient for me is to keep template  amp  styles into separate  html  amp    css files and specify those through templateUrl and styleUrls  so it s easy reusable    Component       selector   my-panel       templateUrl   app components panel html        styleUrls    app components panel css     export class MyPanelComponent extends BasePanelComponent

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As far as I know component inheritance has not been implemented yet in Angular 2 and I m not sure if they have plans to  however since Angular 2 is using typescript  if you ve decided to go that route  you can use class inheritance by doing class MyClass extends OtherClass          For component inheritance I d suggest getting involved with the Angular 2 project by going to https   github com angular angular issues and submitting a feature request

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Components can be extended as same as a typescript class inheritance  just that you have to override the selector with a new name  All Input    and Output   Properties  from the Parent Component works as normal  Update   Component is a decorator   Decorators are applied during the declaration of class not on objects   Basically  decorators add some metadata to the class object and that cannot be accessed via inheritance   If you want to achieve the Decorator Inheritance I would Suggest writing a custom decorator  Something like below example   export function CustomComponent annotation  any        return function  target  Function        var parentTarget   Object getPrototypeOf target prototype  constructor       var parentAnnotations   Reflect getMetadata  annotations   parentTarget       var parentParamTypes   Reflect getMetadata  design paramtypes   parentTarget       var parentPropMetadata   Reflect getMetadata  propMetadata   parentTarget       var parentParameters   Reflect getMetadata  parameters   parentTarget        var parentAnnotation   parentAnnotations 0        Object keys parentAnnotation  forEach key   gt        if  isPresent parentAnnotation key              if   isPresent annotation key              annotation key    parentAnnotation key                                  Same for the other metadata     var metadata   new ComponentMetadata annotation        Reflect defineMetadata  annotations     metadata    target               Refer   https   medium com  ttemplier angular2-decorators-and-class-inheritance-905921dbd1b7

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Alternative Solution   This answer of Thierry Templier is an alternative way to get around the problem   After some questions with Thierry Templier  I came to the following working example that meets my expectations as an alternative to inheritance limitation mentioned in this question   1 - Create custom decorator   export function CustomComponent annotation  any      return function  target  Function        var parentTarget   Object getPrototypeOf target prototype  constructor      var parentAnnotations   Reflect getMetadata  annotations   parentTarget        var parentAnnotation   parentAnnotations 0       Object keys parentAnnotation  forEach key   gt          if  isPresent parentAnnotation key                 verify is annotation typeof function         if typeof annotation key       function              annotation key    annotation key  call this  parentAnnotation key             else if             force override in annotation base          isPresent annotation key                        annotation key    parentAnnotation key                                  var metadata   new Component annotation        Reflect defineMetadata  annotations     metadata    target           2 - Base Component with  Component decorator    Component        create seletor base for test override property   selector   master     template         lt div gt Test lt  div gt         export class AbstractComponent        3 - Sub component with  CustomComponent decorator    CustomComponent        override property annotation     selector   sub     selector   parentSelector    gt    return parentSelector    sub      export class SubComponent extends AbstractComponent     constructor             Plunkr with complete example

[angular] How to extend / inherit components?

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