Typescript How to extend two classes

Question

I want to save my time and reuse common code across classes that extend PIXI classes  a 2d webGl renderer library   Object Interfaces  module Game Core       export interface IObject         export interface IManagedObject extends IObject          getKeyInManager key  string   string          setKeyInManager key  string   IObject           My issue is that the code inside getKeyInManager and setKeyInManager will not change and I want to reuse it  not to duplicate it  here is the implementation  export class ObjectThatShouldAlsoBeExtended      private  keyInManager  string       public getKeyInManager key  string   string          return this  keyInManager             public setKeyInManager key  string   DisplayObject          this  keyInManager   key          return this           What I want to do is to automatically add  through a Manager add    the key used in the manager to reference the object inside the object itself in its property  keyInManager  So  let s take an example with a Texture  Here goes the TextureManager module Game Managers       export class TextureManager extends Game Managers Manager            public createFromLocalImage name  string  relativePath  string   Game Core Texture              return this add name  Game Core Texture fromImage  quot   quot    relativePath   get name                      When I do this add    I want the Game Managers Manager add   method to call a method which would exist on the object returned by Game Core Texture fromImage  quot   quot    relativePath   This object  in this case would be a Texture  module Game Core          I must extend PIXI Texture  but I need to inject the methods in IManagedObject      export class Texture extends PIXI Texture             I know that IManagedObject is an interface and cannot contain implementation  but I don t know what to write to inject the class ObjectThatShouldAlsoBeExtended inside my Texture class  Knowing that the same process would be required for Sprite  TilingSprite  Layer and more  I need experienced TypeScript feedback advice here  it must be possible to do it  but not by multiple extends since only one is possible at the time  I didn t find any other solution

User · Answer

TypeScript supports decorators  and using that feature plus a little library called typescript-mix you can use mixins to have multiple inheritance with just a couple of lines     The following line is only for intellisense to work interface Shopperholic extends Buyer  Transportable     class Shopperholic        The following line is where we  extend  from other 2 classes    use  Buyer  Transportable   this    price   2000

User · Answer

I would suggest using the new mixins approach described there  https   blogs msdn microsoft com typescript 2017 02 22 announcing-typescript-2-2   This approach is better  than the  applyMixins  approach described by Fenton  because the autocompiler would help you and show all the methods   properties from the both base and 2nd inheritance classes   This approach might be checked on the TS Playground site   Here is the implementation   class MainClass       testMainClass             alert  testMainClass             const addSecondInheritance    BaseClass    new    args       gt        return class extends BaseClass           testSecondInheritance                 alert  testSecondInheritance                          Prepare the new class  which  inherits  2 classes  MainClass and the cass declared in the addSecondInheritance method  const SecondInheritanceClass   addSecondInheritance MainClass      Create object from the new prepared class const secondInheritanceObj   new SecondInheritanceClass    secondInheritanceObj testMainClass    secondInheritanceObj testSecondInheritance

User · Answer

If you don t like using multi-inheritance  use extends and implements together to stay safe  class C extends B implements A        implements A here

User · Answer

A very hacky solution would be to loop through the class you want to inherit from adding the functions one by one to the new parent class  class ChildA       public static x   5    class ChildB       public static y   6    class Parent     for  const property in ChildA        Parent property    ChildA property    for  const property in ChildB        Parent property    ChildB property      Parent x    5 Parent y    6   All properties of ChildA and ChildB can now be accessed from the Parent class  however they will not be recognised meaning that you will receive warnings such as Property  x  does not exist on  typeof Parent

User · Answer

I found an up-to-date  amp  unparalleled solution  https   www npmjs com package ts-mixer You are welcome

User · Answer

I think there is a much better approach  that allows for solid type-safety and scalability   First declare interfaces that you want to implement on your target class   interface IBar     doBarThings    void     interface IBazz     doBazzThings    void     class Foo implements IBar  IBazz      Now we have to add the implementation to the Foo class  We can use class mixins that also implements these interfaces   class Base     type Constructor lt I   Base gt    new     args  any      gt  I   function Bar lt T extends Constructor gt  constructor  T   Base as any      return class extends constructor implements IBar       public doBarThings           console log  Do bar                   function Bazz lt T extends Constructor gt  constructor  T   Base as any      return class extends constructor implements IBazz       public doBazzThings           console log  Do bazz                    Extend the Foo class with the class mixins   class Foo extends Bar Bazz    implements IBar  IBazz     public doBarThings         super doBarThings        console log  Override mixin           const foo   new Foo    foo doBazzThings       Do bazz  foo doBarThings       Do bar     Override mixin

User · Answer

There are so many good answers here already  but i just want to show with an example that you can add additional functionality to the class being extended   function applyMixins derivedCtor  any  baseCtors  any          baseCtors forEach baseCtor   gt            Object getOwnPropertyNames baseCtor prototype  forEach name   gt                if  name      constructor                     derivedCtor prototype name    baseCtor prototype name                                        class Class1       doWork             console log  Working             class Class2       sleep             console log  Sleeping             class FatClass implements Class1  Class2       doWork       gt  void        gt           sleep       gt  void        gt             x  number   23      private  z  number   80       get z    number           return this  z             set z newZ            this  z   newZ             saySomething y  string            console log  Just saying   y                applyMixins FatClass   Class1  Class2      let fatClass   new FatClass     fatClass doWork    fatClass saySomething  nothing    console log fatClass x

User · Answer

In design patterns there is a principle called  quot favouring composition over inheritance quot   It says instead of inheriting Class B from Class A  put an instance of class A inside class B as a property and then you can use functionalities of class A inside class B  You can see some examples of that here and here

User · Answer

Unfortunately typescript does not support multiple inheritance  Therefore there is no completely trivial answer  you will probably have to restructure your program  Here are a few suggestions    If this additional class contains behaviour that many of your subclasses share  it makes sense to insert it into the class hierarchy  somewhere at the top  Maybe you could derive the common superclass of Sprite  Texture  Layer      from this class   This would be a good choice  if you can find a good spot in the type hirarchy  But I would not recommend to just insert this class at a random point  Inheritance expresses an  Is a - relationship  e g  a dog is an animal  a texture is an instance of this class  You would have to ask yourself  if this really models the relationship between the objects in your code  A logical inheritance tree is very valuable If the additional class does not fit logically into the type hierarchy  you could use aggregation  That means that you add an instance variable of the type of this class to a common superclass of Sprite  Texture  Layer      Then you can access the variable with its getter setter in all subclasses  This models a  Has a - relationship    You could also convert your class into an interface  Then you could extend the interface with all your classes but would have to implement the methods correctly in each class  This means some code redundancy but in this case not much    You have to decide for yourself which approach you like best  Personally I would recommend to convert the class to an interface   One tip  Typescript offers properties  which are syntactic sugar for getters and setters  You might want to take a look at this  http   blogs microsoft co il gilf 2013 01 22 creating-properties-in-typescript

User · Answer

There is a little known feature in TypeScript that allows you to use Mixins to create re-usable small objects  You can compose these into larger objects using multiple inheritance  multiple inheritance is not allowed for classes  but it is allowed for mixins - which are like interfaces with an associated implenentation    More information on TypeScript Mixins  I think you could use this technique to share common components between many classes in your game and to re-use many of these components from a single class in your game   Here is a quick Mixins demo    first  the flavours that you want to mix   class CanEat       public eat             alert  Munch Munch              class CanSleep       sleep             alert  Zzzzzzz               Then the magic method for Mixin creation  you only need this once somewhere in your program      function applyMixins derivedCtor  any  baseCtors  any          baseCtors forEach baseCtor   gt            Object getOwnPropertyNames baseCtor prototype  forEach name   gt                 if  name      constructor                     derivedCtor prototype name    baseCtor prototype name                                          And then you can create classes with multiple inheritance from mixin flavours   class Being implements CanEat  CanSleep           eat       gt  void          sleep       gt  void    applyMixins  Being   CanEat  CanSleep      Note that there is no actual implementation in this class - just enough to make it pass the requirements of the  interfaces   But when we use this class - it all works   var being   new Being        Zzzzzzz    being sleep

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