What is Inversion of Control

Question

Inversion of Control  IoC  can be quite confusing when it is first encountered    What is it  Which problem does it solve  When is it appropriate to use and when not

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Inversion of Control is a generic principle, while Dependency Injection realises this principle as a design pattern for object graph construction (i.e. configuration controls how the objects are referencing each other, rather than the object itself controlling how to get the reference to another object).

Looking at Inversion of Control as a design pattern, we need to look at what we are inverting. Dependency Injection inverts control of constructing a graph of objects. If told in layman's term, inversion of control implies change in flow of control in the program. Eg. In traditional standalone app, we have main method, from where the control gets passed to other third party libraries(in case, we have used third party library's function), but through inversion of control control gets transferred from third party library code to our code, as we are taking the service of third party library. But there are other aspects that need to be inverted within a program - e.g. invocation of methods and threads to execute the code.

For those interested in more depth on Inversion of Control a paper has been published outlining a more complete picture of Inversion of Control as a design pattern (OfficeFloor: using office patterns to improve software design http://doi.acm.org/10.1145/2739011.2739013 with a free copy available to download from http://www.officefloor.net/about.html).

What is identified is the following relationship:

Inversion of Control (for methods) = Dependency (state) Injection + Continuation Injection + Thread Injection

Summary of above relationship for Inversion of Control available - http://dzone.com/articles/inversion-of-coupling-control

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Let to say that we make some meeting in some hotel   Many people  many carafes of water  many plastic cups   When somebody want to drink  she fill cup  drink and throw cup on the floor   After hour or something we have a floor covered of plastic cups and water   Let invert control   The same meeting in the same place  but instead of plastic cups we have a waiter with one glass cup  Singleton   and she all of time offers to guests drinking   When somebody want to drink  she get from waiter glass  drink and return it back to waiter   Leaving aside the question of the hygienic  last form of drinking process control is much more effective and economic   And this is exactly what the Spring  another IoC container  for example  Guice  does  Instead of let to application create what it need using new keyword  taking plastic cup   Spring IoC container all of time offer to application the same instance  singleton  of needed object glass of water    Think about yourself as organizer of such meeting  You need the way to message to hotel administration that  meeting members will need glass of water but not piece of cake   Example -  public class MeetingMember        private GlassOfWater glassOfWater                public void setGlassOfWater GlassOfWater glassOfWater           this glassOfWater   glassOfWater              your glassOfWater object initialized and ready to use          spring IoC  called setGlassOfWater method itself in order to       offer to meetingMember glassOfWater instance      Useful links -   http   adfjsf blogspot in 2008 05 inversion-of-control html http   martinfowler com articles injection html http   www shawn-barrett com blog post Tip-of-the-day-e28093-Inversion-Of-Control aspx

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So number 1 above  What is Inversion of Control  Maintenance is the number one thing it solves for me   It guarantees I am using interfaces so that two classes are not intimate with each other     In using a container like Castle Windsor  it solves maintenance issues even better   Being able to swap out a component that goes to a database for one that uses file based persistence without changing a line of code is awesome  configuration change  you re done    And once you get into generics  it gets even better   Imagine having a message publisher that receives records and publishes messages   It doesn t care what it publishes  but it needs a mapper to take something from a record to a message   public class MessagePublisher lt RECORD MESSAGE gt        public MessagePublisher IMapper lt RECORD MESSAGE gt  mapper IRemoteEndpoint endPointToSendTo                setup           I wrote it once  but now I can inject many types into this set of code if I publish different types of messages   I can also write mappers that take a record of the same type and map them to different messages  Using DI with Generics has given me the ability to write very little code to accomplish many tasks   Oh yeah  there are testability concerns  but they are secondary to the benefits of IoC DI    I am definitely loving IoC DI   3   It becomes more appropriate the minute you have a medium sized project of somewhat more complexity   I would say it becomes appropriate the minute you start feeling pain

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Using IoC you are not new ing up your objects  Your IoC container will do that and manage the lifetime of them   It solves the problem of having to manually change every instantiation of one type of object to another   It is appropriate when you have functionality that may change in the future or that may be different depending on the environment or configuration used in

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Creating an object within class is called tight coupling  Spring removes this dependency by following a design pattern DI IOC   In which object of class in passed in constructor rather than creating in class  More over we give super class reference variable in constructor to define more general structure

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Inversion of control is a pattern used for decoupling components and layers in the system  The pattern is implemented through injecting dependencies into a component when it is constructed  These dependences are usually provided as interfaces for further decoupling and to support testability  IoC   DI containers such as Castle Windsor  Unity are tools  libraries  which can be used for providing IoC  These tools provide extended features above and beyond simple dependency management  including lifetime  AOP   Interception  policy  etc  a  Alleviates a component from being responsible for managing it s dependencies  b  Provides the ability to swap dependency implementations in different environments  c  Allows a component be tested through mocking of dependencies  d  Provides a mechanism for sharing resources throughout an application  a  Critical when doing test-driven development  Without IoC it can be difficult to test  because the components under test are highly coupled to the rest of the system  b  Critical when developing modular systems  A modular system is a system whose components can be replaced without requiring recompilation  c  Critical if there are many cross-cutting concerns which need to addressed  partilarly in an enterprise application

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I shall write down my simple understanding of this two terms   For quick understanding just read examples    Dependency Injection DI   Dependency injection generally means passing an object on which method depends  as a parameter to a method  rather than having the method create the dependent object  What it means in practice is that the method does not depends directly on a particular implementation  any implementation that meets the requirements can be passed as a parameter  With this objects tell thier dependencies  And spring makes it available  This leads to loosely coupled application development   Quick Example EMPLOYEE OBJECT WHEN CREATED                IT WILL AUTOMATICALLY CREATE ADDRESS OBJECT     if address is defines as dependency by Employee object    Inversion of Control IoC  Container  This is common characteristic of frameworks  IOC manages java objects     from instantiation to destruction through its BeanFactory  -Java components that are instantiated by the IoC container are called beans  and the IoC container manages a bean s scope  lifecycle events  and any AOP features for which it has been configured and coded   QUICK EXAMPLE Inversion of Control is about getting freedom  more flexibility  and less dependency  When you are using a desktop computer  you are slaved  or say  controlled   You have to sit before a screen and look at it  Using keyboard to type and using mouse to navigate  And a bad written software can slave you even more  If you replaced your desktop with a laptop  then you somewhat inverted control  You can easily take it and move around  So now you can control where you are with your computer  instead of computer controlling it   By implementing Inversion of Control  a software object consumer get more controls options over the software objects  instead of being controlled or having less options   Inversion of control as a design guideline serves the following purposes   There is a decoupling of the execution of a certain task from implementation  Every module can focus on what it is designed for  Modules make no assumptions about what other systems do but rely on their contracts  Replacing modules has no side effect on other modules I will keep things abstract here  You can visit following links for detail understanding of the topic  A good read with example  Detailed explanation

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Suppose you are an object  And you go to a restaurant   Without IoC  you ask for  apple   and you are always served apple when you ask more    With IoC  You can ask for  fruit   You can get different fruits each time you get served  for example  apple  orange  or water melon    So  obviously  IoC is preferred when you like the varieties

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The IoC principle helps in designing loosely coupled classes which make them testable  maintainable and extensible

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I understand that the answer has already been given here  But I still think  some basics about the inversion of control have to be discussed here in length for future readers   Inversion of Control  IoC  has been built on a very simple principle called Hollywood Principle  And it says that      Don t call us  we ll call you   What it means is that don t go to the Hollywood to fulfill your dream rather if you are worthy then Hollywood will find you and make your dream comes true  Pretty much inverted  huh   Now when we discuss about the principle of IoC  we use to forget about the Hollywood  For IoC  there has to be three element  a Hollywood  you and a task like to fulfill your dream   In our programming world  Hollywood represent a generic framework  may be written by you or someone else   you represent the user code you wrote and the task represent the thing you want to accomplish with your code  Now you don t ever go to trigger your task by yourself  not in IoC  Rather you have designed everything in such that your framework will trigger your task for you  Thus you have built a reusable framework which can make someone a hero or another one a villain  But that framework is always in charge  it knows when to pick someone and that someone only knows what it wants to be   A real life example would be given here  Suppose  you want to develop a web application  So  you create a framework which will handle all the common things a web application should handle like handling http request  creating application menu  serving pages  managing cookies  triggering events etc   And then you leave some hooks in your framework where you can put further codes to generate custom menu  pages  cookies or logging some user events etc  On every browser request  your framework will run and executes your custom codes if hooked then serve it back to the browser   So  the idea is pretty much simple  Rather than creating a user application which will control everything  first you create a reusable framework which will control everything then write your custom codes and hook it to the framework to execute those in time   Laravel and EJB are examples of such a frameworks   Reference   https   martinfowler com bliki InversionOfControl html  https   en wikipedia org wiki Inversion of control

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Inversion of control is when you go to the grocery store and your wife gives you the list of products to buy   In programming terms  she passed a callback function getProductList   to the function you are executing - doShopping     It allows user of the function to define some parts of it  making it more flexible

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Since already there are many answers for the question but none of them shows the breakdown of Inversion Control term I see an opportunity to give a more concise and useful answer   Inversion of Control is a pattern that implements the Dependency Inversion Principle  DIP   DIP states the following  1  High-level modules should not depend on low-level modules  Both should depend on abstractions  e g  interfaces   2  Abstractions should not depend on details  Details  concrete implementations  should depend on abstractions    There are three types of Inversion of Control   Interface Inversion  Providers shouldn   t define an interface  Instead  the consumer should define the interface and providers must implement it  Interface Inversion allows eliminating the necessity to modify the consumer each time when a new provider added    Flow Inversion  Changes control of the flow  For example  you have a console application where you asked to enter many parameters and after each entered parameter you are forced to press Enter  You can apply Flow Inversion here and implement a desktop application where the user can choose the sequence of parameters    entering  the user can edit parameters  and at the final step  the user needs to press Enter only once   Creation Inversion  It can be implemented by the following patterns  Factory Pattern  Service Locator  and Dependency Injection  Creation Inversion helps to eliminate dependencies between types moving the process of dependency objects creation outside of the type that uses these dependency objects  Why dependencies are bad  Here are a couple of examples  direct creation of a new object in your code makes testing harder  it is impossible to change references in assemblies without recompilation  OCP principle violation   you can   t easily replace a desktop-UI by a web-UI

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Inversion of control is about transferring control from library to the client  It makes more sense when we talk about a client that injects  passes  a function value  lambda expression  into a higher order function  library function  that controls  changes  the behavior of the library function  So  a simple implementation  with huge implications  of this pattern is a higher order library function  which accepts another function as an argument   For example  library functions like  quot map quot    quot flatMap quot  are IoC implementations  A client or framework that injects library dependencies  which carry behavior  into libraries may also be considered IoC

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IoC   DI to me is pushing out dependencies to the calling objects  Super simple   The non-techy answer is being able to swap out an engine in a car right before you turn it on  If everything hooks up right  the interface   you are good

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Wikipedia Article  To me  inversion of control is turning your sequentially written code and turning it into an delegation structure  Instead of your program explicitly controlling everything  your program sets up a class or library with certain functions to be called when certain things happen   It solves code duplication  For example  in the old days you would manually write your own event loop  polling the system libraries for new events  Nowadays  most modern APIs you simply tell the system libraries what events you re interested in  and it will let you know when they happen   Inversion of control is a practical way to reduce code duplication  and if you find yourself copying an entire method and only changing a small piece of the code  you can consider tackling it with inversion of control  Inversion of control is made easy in many languages through the concept of delegates  interfaces  or even raw function pointers  It is not appropriate to use in all cases  because the flow of a program can be harder to follow when written this way  It s a useful way to design methods when writing a library that will be reused  but it should be used sparingly in the core of your own program unless it really solves a code duplication problem

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I found a very clear example here which explains how the  control is inverted    Classic code  without Dependency injection   Here is how a code not using DI will roughly work    Application needs Foo  e g  a controller   so  Application creates Foo Application calls Foo    Foo needs Bar  e g  a service   so   Foo creates Bar  Foo calls Bar    Bar needs Bim  a service  a repository        so  Bar creates Bim  Bar does something     Using dependency injection   Here is how a code using DI will roughly work    Application needs Foo  which needs Bar  which needs Bim  so  Application creates Bim Application creates Bar and gives it Bim Application creates Foo and gives it Bar Application calls Foo   Foo calls Bar   Bar does something     The control of the dependencies is inverted from one being called to the one calling   What problems does it solve   Dependency injection makes it easy to swap with the different implementation of the injected classes  While unit testing you can inject a dummy implementation  which makes the testing a lot easier   Ex  Suppose your application stores the user uploaded file in the Google Drive  with DI your controller code may look like this   class SomeController       private  storage       function   construct StorageServiceInterface  storage                 this- gt storage    storage             public function myFunction                   return  this- gt storage- gt getFile  fileName            class GoogleDriveService implements StorageServiceInterface       public function authenticate  user         public function putFile  file         public function getFile  file         When your requirements change say  instead of GoogleDrive you are asked to use the Dropbox  You only need to write a dropbox implementation for the StorageServiceInterface  You don t have make any changes in the controller as long as Dropbox implementation adheres to the StorageServiceInterface   While testing you can create the mock for the StorageServiceInterface with the dummy implementation where all the methods return null or any predefined value as per your testing requirement    Instead if you had the controller class to construct the storage object with the new keyword like this   class SomeController       private  storage       function   construct                  this- gt storage   new GoogleDriveService               public function myFunction                   return  this- gt storage- gt getFile  fileName             When you want to change with the Dropbox implementation you have to replace all the lines where new GoogleDriveService object is constructed and use the DropboxService  Besides when testing the SomeController class the constructor always expects the GoogleDriveService class and the actual methods of this class are triggered   When is it appropriate and when not  In my opinion you use DI when you think there are  or there can be  alternative implementations of a class

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What is Inversion of Control  If you follow these simple two steps  you have done inversion of control   Separate what-to-do part from when-to-do part  Ensure that when part knows as little as possible about what part  and vice versa   There are several techniques possible for each of these steps based on the technology language you are using for your implementation  -- The inversion part of the Inversion of Control  IoC  is the confusing thing  because inversion is the relative term  The best way to understand IoC is to forget about that word  -- Examples  Event Handling  Event Handlers  what-to-do part  -- Raising Events  when-to-do part  Dependency Injection  Code that constructs a dependency  what-to-do part  -- instantiating and injecting that dependency for the clients when needed  which is usually taken care of by the DI tools such as Dagger  when-to-do-part   Interfaces  Component client  when-to-do part  -- Component Interface implementation  what-to-do part  xUnit fixture  Setup and TearDown  what-to-do part  -- xUnit frameworks calls to Setup at the beginning and TearDown at the end  when-to-do part  Template method design pattern  template method when-to-do part -- primitive subclass implementation what-to-do part DLL container methods in COM  DllMain  DllCanUnload  etc  what-to-do part  -- COM OS  when-to-do part

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Before using Inversion of Control you should be well aware of the fact that it has its pros and cons and you should know why you use it if you do so   Pros    Your code gets decoupled so you can easily exchange implementations of an interface with alternative implementations It is a strong motivator for coding against interfaces instead of implementations It s very easy to write unit tests for your code because it depends on nothing else than the objects it accepts in its constructor setters and you can easily initialize them with the right objects in isolation    Cons    IoC not only inverts the control flow in your program  it also clouds it considerably  This means you can no longer just read your code and jump from one place to another because the connections that would normally be in your code are not in the code anymore  Instead it is in XML configuration files or annotations and in the code of your IoC container that interprets these metadata  There arises a new class of bugs where you get your XML config or your annotations wrong and you can spend a lot of time finding out why your IoC container injects a null reference into one of your objects under certain conditions    Personally I see the strong points of IoC and I really like them but I tend to avoid IoC whenever possible because it turns your software into a collection of classes that no longer constitute a  real  program but just something that needs to be put together by XML configuration or annotation metadata and would fall  and falls  apart without it

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Answering only the first part  What is it   Inversion of Control  IoC  means to create instances of dependencies first and latter instance of a class  optionally injecting them through constructor   instead of creating an instance of the class first and then the class instance creating instances of dependencies  Thus  inversion of control inverts the flow of control of the program  Instead of the callee controlling the flow of control  while creating dependencies   the caller controls the flow of control of the program

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To understanding the concept  Inversion of Control  IoC  or Dependency Inversion Principle  DIP  involves two activities  abstraction  and inversion  Dependency Injection  DI  is just one of the few of the inversion methods   To read more about this you can read my blog  Here   What is it    It is a practice where you let the actual behavior come from outside of the boundary  Class in Object Oriented Programming   The boundary entity only knows the abstraction  e g interface  abstract class  delegate in Object Oriented Programming  of it     What problems does it solve    In term of programming  IoC try to solve monolithic code by making it modular  decoupling various parts of it  and make it unit-testable     When is it appropriate and when not    It is appropriate most of the time  unless you have situation where you just want monolithic code  e g very simple program

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Inversion of control means you control how components  classes  behave  Why its called  inversion  because before this pattern the classes were hard wired and were definitive about what they will do e g   you import a library that has a TextEditor and SpellChecker classes  Now naturally this SpellChecker would only check spellings for English language  Suppose if you want the TextEditor to handle German language and be able to spell check you have any control over it     with IoC this control is inverted i e  its given to you  how  the library would implement something like this   It will have a TextEditor class and then it will have a ISpeallChecker  which is an interface instead of a concret SpellChecker class  and when you configure things in IoC container e g  Spring you can provide your own implementation of  ISpellChecker  which will check spelling for German language  so the control of how spell checking will work is ineverted is taken from that Library and given to you  Thats IoC

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It seems that the most confusing thing about  IoC  the acronym and the name for which it stands is that it s too glamorous of a name - almost a noise name   Do we really need a name by which to describe the difference between procedural and event driven programming  OK  if we need to  but do we need to pick a brand new  bigger than life  name that confuses more than it solves

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For example  task 1 is to create object  Without IOC concept  task 1 is supposed to be done by Programmer But With IOC concept  task 1 would be done by container   In short Control gets inverted from Programmer to container  So  it is called as inversion of control   I found one good example here

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Really not understanding why there are lots of wrong answers and even the accepted is not quite accurate making things hard to understand  The truth is always simple and clean    As  Schneider commented in  Mark Harrison s answer  please just read Martin Fowler s post discussing IoC    https   martinfowler com bliki InversionOfControl html   One of the most I love is      This phenomenon is Inversion of Control  also known as the Hollywood Principle -  Don t call us  we ll call you      Why    Wiki for IoC  I might quote a snippet       Inversion of control is used to increase modularity of the program and make it extensible     then further popularized in 2004 by Robert C  Martin and Martin Fowler    Robert C  Martin  the author of  lt  lt Clean Code  A Handbook of Agile Software Craftsmanship gt  gt     Martin Fowler  the author of  lt  lt Refactoring  Improving the Design of Existing Code gt  gt

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Programming speaking   IoC in easy terms  It s the use of Interface as a way of specific something  such a field or a parameter  as a wildcard that can be used by some classes  It allows the re-usability of the code   For example  let s say that we have two classes   Dog and Cat    Both shares the same qualities states  age  size  weight  So instead of creating a class of service called DogService and CatService  I can create a single one called AnimalService that allows to use Dog and Cat only if they use the interface IAnimal   However  pragmatically speaking  it has some backwards   a  Most of the developers don t know how to use it   For example  I can create a class called Customer and I can create automatically  using the tools of the IDE  an interface called ICustomer  So  it s not rare to find a folder filled with classes and interfaces  no matter if the interfaces will be reused or not  It s called BLOATED  Some people could argue that  may be in the future we could use it     -   b  It has some limitings  For example  let s talk about the case of Dog and Cat and I want to add a new service  functionality  only for dogs   Let s say that I want to calculate the number of days that I need to train a dog  trainDays     for cat it s useless  cats can t be trained  I m joking    b 1  If I add trainDays   to the Service AnimalService then it also works with cats and it s not valid at all   b 2  I can add a condition in trainDays   where it evaluates which class is used  But it will break completely the IoC   b 3  I can create a new class of service called DogService just for the new functionality  But  it will increase the maintainability of the code because we will have two classes of service  with similar functionality  for Dog and it s bad

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To understand IoC  we should talk about Dependency Inversion   Dependency inversion  Depend on abstractions  not on concretions   Inversion of control  Main vs Abstraction  and how the Main is the glue of the systems     I wrote about this with some good examples  you can check them here   https   coderstower com 2019 03 26 dependency-inversion-why-you-shouldnt-avoid-it   https   coderstower com 2019 04 02 main-and-abstraction-the-decoupled-peers   https   coderstower com 2019 04 09 inversion-of-control-putting-all-together

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Inversion of Controls is about separating concerns    Without IoC  You have a laptop computer and you accidentally break the screen  And darn  you find the same model laptop screen is nowhere in the market  So you re stuck   With IoC  You have a desktop computer and you accidentally break the screen  You find you can just grab almost any desktop monitor from the market  and it works well with your desktop   Your desktop successfully implements IoC in this case  It accepts a variety type of monitors  while the laptop does not  it needs a specific screen to get fixed

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I ve read a lot of answers for this but if someone is still confused and needs a plus ultra  laymans term  to explain IoC here is my take   Imagine a parent and child talking to each other   Without IoC    Parent  You can only speak when I ask you questions and you can only act when I give you permission   Parent  This means  you can t ask me if you can eat  play  go to the bathroom or even sleep if I don t ask you   Parent  Do you want to eat   Child  No   Parent  Okay  I ll be back  Wait for me   Child   Wants to play but since there s no question from the parent  the child can t do anything    After 1 hour     Parent  I m back  Do you want to play   Child  Yes   Parent  Permission granted   Child   finally is able to play    This simple scenario explains the control is centered to the parent  The child s freedom is restricted and highly depends on the parent s question  The child can ONLY speak when asked to speak  and can ONLY act when granted permission   With IoC   The child has now the ability to ask questions and the parent can respond with answers and permissions  Simply means the control is inverted  The child is now free to ask questions anytime and though there is still dependency with the parent regarding permissions  he is not dependent in the means of speaking asking questions   In a technological way of explaining  this is very similar to console shell cmd vs GUI interaction   Which is answer of Mark Harrison above no 2 top answer   In console  you are dependent on the what is being asked displayed to you and you can t jump to other menus and features without answering it s question first  following a strict sequential flow   programmatically this is like a method function loop   However with GUI  the menus and features are laid out and the user can select whatever it needs thus having more control and being less restricted   programmatically  menus have callback when selected and an action takes place

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But I think you have to be very careful with it  If you will overuse this pattern  you will make very complicated design and even more complicated code    Like in this example with TextEditor  if you have only one SpellChecker maybe it is not really necessary to use IoC   Unless you need to write unit tests or something      Anyway  be reasonable  Design pattern are good practices but not Bible to be preached  Do not stick it everywhere

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I like this explanation  http   joelabrahamsson com inversion-of-control-an-introduction-with-examples-in-net   It start simple and shows code examples as well     The consumer  X  needs the consumed class  Y  to accomplish something  That   s all good and natural  but does X really need to know that it uses Y   Isn   t it enough that X knows that it uses something that has the behavior  the methods  properties etc  of Y without knowing who actually implements the behavior   By extracting an abstract definition of the behavior used by X in Y  illustrated as I below  and letting the consumer X use an instance of that instead of Y it can continue to do what it does without having to know the specifics about Y     In the illustration above Y implements I and X uses an instance of I  While it   s quite possible that X still uses Y what   s interesting is that X doesn   t know that  It just knows that it uses something that implements I   Read article for further info and description of benefits such as    X is not dependent on Y anymore More flexible  implementation can be decided in runtime Isolation of code unit  easier testing

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What is it   Inversion of  Coupling  Control  changes the direction of coupling for the method signature   With inverted control  the definition of the method signature is dictated by the method implementation  rather than the caller of the method   Full explanation here  Which problem does it solve   Top down coupling on methods   This subsequently removes need for refactoring   When is it appropriate to use and when not   For small well defined applications that are not subject to much change  it is likely an overhead   However  for less defined applications that will evolve  it reduces the inherent coupling of the method signature   This gives the developers more freedom to evolve the application  avoiding the need to do expensive refactoring of code   Basically  allows the application to evolve with little rework

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I agree with NilObject  but I d like to add to this      if you find yourself copying an entire method and only changing a small piece of the code  you can consider tackling it with inversion of control   If you find yourself copying and pasting code around  you re almost always doing something wrong   Codified as the design principle Once and Only Once

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IoC is about inverting the relationship between your code and third-party code  library framework     In normal s w development  you write the main   method and call  library  methods   You are in control    In IoC the  framework  controls main   and calls your methods  The Framework is in control      DI  Dependency Injection  is about how the control flows in the application  Traditional desktop application had control flow from your application main   method  to other library method calls  but with DI control flow is inverted that s framework takes care of starting your app  initializing it and invoking your methods whenever required   In the end you always win

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Inversion of Control is what you get when your program callbacks  e g  like a gui program   For example  in an old school menu  you might have   print  enter your name  read name print  enter your address  read address etc    store in database   thereby controlling the flow of user interaction   In a GUI program or somesuch  instead we say   when the user types in field a  store it in NAME when the user types in field b  store it in ADDRESS when the user clicks the save button  call StoreInDatabase   So now control is inverted    instead of the computer accepting user input in a fixed order  the user controls the order in which the data is entered  and when the data is saved in the database   Basically  anything with an event loop  callbacks  or execute triggers falls into this category

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The Inversion of Control  IoC  and Dependency Injection  DI  patterns are all about removing dependencies from your code  For example  say your application has a text editor component and you want to provide spell checking  Your standard code would look something like this  public class TextEditor        private SpellChecker checker       public TextEditor             this checker   new SpellChecker             What we ve done here creates a dependency between the TextEditor and the SpellChecker  In an IoC scenario we would instead do something like this  public class TextEditor        private IocSpellChecker checker       public TextEditor IocSpellChecker checker            this checker   checker           In the first code example we are instantiating SpellChecker  this checker   new SpellChecker      which means the TextEditor class directly depends on the SpellChecker class  In the second code example we are creating an abstraction by having the SpellChecker dependency class in TextEditor s constructor signature  not initializing dependency in class   This allows us to call the dependency then pass it to the TextEditor class like so  SpellChecker sc   new SpellChecker       dependency TextEditor textEditor   new TextEditor sc    Now the client creating the TextEditor class has control over which SpellChecker implementation to use because we re injecting the dependency into the TextEditor signature

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Inversion of control is an indicator for a shift of responsibility in the program  There is an inversion of control every time when a dependency is granted ability to directly act on the caller s space  The smallest IoC is passing a variable by reference  lets look at non-IoC code first  function isVarHello  var        return   var      quot Hello quot          Responsibility is within the caller  word    quot Hello quot   if  isVarHello  word          word    quot World quot      Let s now invert the control by shifting the responsibility of a result from the caller to the dependency  function changeHelloToWorld  amp  var           Responsibility has been shifted to the dependency     if   var      quot Hello quot              var    quot World quot             word    quot Hello quot   changeHelloToWorld  word     Here is another example using OOP   lt  php  class Human       private  hp   0 5       function consume Eatable  chunk                this- gt chew  chunk            chunk- gt unfoldEffectOn  this              function incrementHealth              this- gt hp              function isHealthy          function getHungry                    interface Eatable       public function unfoldEffectOn  body      class Medicine implements Eatable       function unfoldEffectOn  human               The dependency is now in charge of the human           human- gt incrementHealth             this- gt depleted   true            human   new Human     medicine   new Medicine    if    human- gt isHealthy           human- gt consume  medicine         var dump  medicine   var dump  human       Disclaimer  The real world human uses a message queue

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Inversion of Control   or IoC   is about getting freedom  You get married  you lost freedom and you are being controlled  You divorced  you have just implemented Inversion of Control  That s what we called   decoupled   Good computer system discourages some very close relationship   more flexibility  The kitchen in your office only serves clean tap water  that is your only choice when you want to drink  Your boss implemented Inversion of Control by setting up a new coffee machine  Now you get the flexibility of choosing either tap water or coffee   and less dependency  Your partner has a job  you don t have a job  you financially depend on your partner  so you are controlled  You find a job  you have implemented Inversion of Control  Good computer system encourages in-dependency    When you use a desktop computer  you have slaved  or say  controlled   You have to sit before a screen and look at it  Using the keyboard to type and using the mouse to navigate  And a badly written software can slave you even more  If you replace your desktop with a laptop  then you somewhat inverted control  You can easily take it and move around  So now you can control where you are with your computer  instead of your computer controlling it   By implementing Inversion of Control  a software object consumer gets more controls options over the software objects  instead of being controlled or having fewer options   With the above ideas in mind  We still miss a key part of IoC  In the scenario of IoC  the software object consumer is a sophisticated framework  That means the code you created is not called by yourself  Now let s explain why this way works better for a web application   Suppose your code is a group of workers  They need to build a car  These workers need a place and tools  a software framework  to build the car  A traditional software framework will be like a garage with many tools  So the workers need to make a plan themselves and use the tools to build the car  Building a car is not an easy business  it will be really hard for the workers to plan and cooperate properly  A modern software framework will be like a modern car factory with all the facilities and managers in place  The workers do not have to make any plan  the managers  part of the framework  they are the smartest people and made the most sophisticated plan  will help coordinate so that the workers know when to do their job  framework calls your code   The workers just need to be flexible enough to use any tools the managers give to them  by using Dependency Injection    Although the workers give the control of managing the project on the top level to the managers  the framework   But it is good to have some professionals help out  This is the concept of IoC truly come from   Modern Web applications with an MVC architecture depends on the framework to do URL Routing and put Controllers in place for the framework to call    Dependency Injection and Inversion of Control are related  Dependency Injection is at the micro level and Inversion of Control is at the macro level  You have to eat every bite  implement DI  in order to finish a meal  implement IoC

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A very simple written explanation can be found here  http   binstock blogspot in 2008 01 excellent-explanation-of-dependency html  It says -      Any nontrivial application is made up of two or more classes that   collaborate with each other to perform some business logic    Traditionally  each object is responsible for obtaining its own   references to the objects it collaborates with  its dependencies     When applying DI  the objects are given their dependencies at creation   time by some external entity that coordinates each object in the   system  In other words  dependencies are injected into objects

[oop] What is Inversion of Control?

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