What is Mocking

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What is Mocking

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If your mock involves a network request  another alternative is to have a real test server to hit  You can use a service to generate a request and response for your testing

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Mock is a method object that simulates the behavior of a real method object in controlled ways  Mock objects are used in unit testing    Often a method under a test calls other external services or methods within it  These are called dependencies  Once mocked  the dependencies behave the way we defined them    With the dependencies being controlled by mocks   we can easily test the behavior of the method that we coded  This is Unit testing   What is the purpose of mock objects   Mocks vs stubs  Unit tests vs Functional tests

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Mocking is generating pseudo-objects that simulate real objects behaviour for tests

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The purpose of mocking types is to sever dependencies in order to isolate the test to a specific unit  Stubs are simple surrogates  while mocks are surrogates that can verify usage  A mocking framework is a tool that will help you generate stubs and mocks    EDIT  Since the original wording mention  type mocking  I got the impression that this related to TypeMock  In my experience the general term is just  mocking   Please feel free to disregard the below info specifically on TypeMock   TypeMock Isolator differs from most other mocking framework in that it works my modifying IL on the fly  That allows it to mock types and instances that most other frameworks cannot mock  To mock these types instances with other frameworks you must provide your own abstractions and mock these    TypeMock offers great flexibility at the expense of a clean runtime environment  As a side effect of the way TypeMock achieves its results you will sometimes get very strange results when using TypeMock

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Prologue  If you look up the noun mock in the dictionary you will find that one of the definitions of the word is something made as an imitation   Mocking is primarily used in unit testing  An object under test may have dependencies on other  complex  objects  To isolate the behavior of the object you want to replace the other objects by mocks that simulate the behavior of the real objects  This is useful if the real objects are impractical to incorporate into the unit test  In short  mocking is creating objects that simulate the behavior of real objects   At times you may want to distinguish between mocking as opposed to stubbing  There may be some disagreement about this subject but my definition of a stub is a  quot minimal quot  simulated object  The stub implements just enough behavior to allow the object under test to execute the test  A mock is like a stub but the test will also verify that the object under test calls the mock as expected  Part of the test is verifying that the mock was used correctly  To give an example  You can stub a database by implementing a simple in-memory structure for storing records  The object under test can then read and write records to the database stub to allow it to execute the test  This could test some behavior of the object not related to the database and the database stub would be included just to let the test run  If you instead want to verify that the object under test writes some specific data to the database you will have to mock the database  Your test would then incorporate assertions about what was written to the database mock

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Other answers explain what mocking is  Let me walk you through it with different examples  And believe me  it s actually far more simpler than you think  tl dr It s an instance of the original class  It has other data injected into so you avoid testing the injected parts and solely focus on testing the implementation details of your class functions  Simple example  class Foo       func add  num1  Int  num2  Int  - gt  Int      Line A          return num1   num2    Line B          let unit   Foo      unit under test assertEqual unit add 1 5  6   As you can see  I m not testing LineA ie I m not validating the input parameters  I m not validating to see if num1  num2 are an Integer  I have no asserts against that  I m only testing to see if LineB  my implementation  given the mocked values 1 and 5 is doing as I expect  Obviously in the real word this can become much more complex  The parameters can be a custom object like a Person  Address  or the implementation details can be more than a single    But the logic of testing would be the same  Non-coding Example  Assume you re building a machine that identifies the type and brand name of electronic devices for an airport security  The machine does this by processing what it sees with its camera  Now your manager walks in the door and asks you to unit-test it  Then you as a developer you can either bring 1000 real objects  like a MacBook pro  Google Nexus  a banana  an iPad etc in front of it and test and see if it all works  But you can also use mocked objects  like an identical looking MacBook pro  with no real internal parts  or a plastic banana in front of it  You can save yourself from investing in 1000 real laptops and rotting bananas  The point is you re not trying to test if the banana is fake or not  Nor testing if the laptop is fake or not  All you re doing is testing if your machine once it sees a banana it would say not an electronic device and for a MacBook Pro it would say  Laptop  Apple  To the machine  the outcome of its detection should be the same for fake mocked electronics and real electronics  If your machine also factored in the internals of a laptop  x-ray scan  or banana then your mocks  internals need to look the same as well  But you could also use a gadget with a friend motherboard  Had your machine tested whether or not devices can power on then well you d need real devices  The logic mentioned above applies to unit-testing of actual code as well  That is a function should work the same with real values you get from real input  and interactions  or mocked values you inject during unit-testing  And just as how you save yourself from using a real banana or MacBook  with unit-tests  and mocking  you save yourself from having to do something that causes your server to return a status code of 500  403  200  etc  forcing your server to trigger 500 is only when server is down  while 200 is when server is up  It gets difficult to run 100 network focused tests if you have to constantly wait 10 seconds between switching over server up and down   So instead you inject mock a response with status code 500  200  403  etc and test your unit function with a injected mocked value  Be aware  Sometimes you don t correctly mock the actual object  Or you don t mock every possibility  E g  your fake laptops are dark  and your machine accurately works with them  but then it doesn t work accurately with white fake laptops  Later when you ship this machine to customers they complain that it doesn t work all the time  You get random reports that it s not working  It takes you 3 months of time to finally figure out that the color of fake laptops need to be more varied so you can test your modules appropriately  For a true coding example  your implementation may be different for status code 200 with image data returned vs 200 with image data not returned  For this reason it s good to use an IDE that provides code coverage e g  the image below shows that your unit-tests don t ever go through the lines marked with brown    image source  Real world coding Example  Let s say you are writing an iOS application and have network calls Your job is to test your application  To test identify whether or not the network calls work as expected is NOT YOUR RESPONSIBILITY   It s another party s  server team  responsibility to test it  You must remove this  network  dependency and yet continue to test all your code that works around it  A network call can return different status codes  404  500  200  303  etc with a JSON response  Your app is suppose to work for all of them  in case of errors  your app should throw its expected error   What you do with mocking is you create  imaginary   similar to real  network responses  like a 200 code with a JSON file  and test your code without  making the real network call and waiting for your network response   You manually hardcode return the network response for ALL kinds of network responses and see if your app is working as you expect   you never assume test a 200 with incorrect data  because that is not your responsibility  your responsibility is to test your app with a correct 200  or in case of a 400  500  you test if your app throws the right error  This creating imaginary   similar to real is known as mocking  In order to do this  you can t use your original code  your original code doesn t have the pre-inserted responses  right    You must add something to it  inject insert that dummy data which isn t normally needed  or a part of your class   So you create an instance the original class and add whatever  here being the network HTTPResponse  data OR in the case of failure  you pass the correct errorString  HTTPResponse  you need to it and then test the mocked class  Long story short  mocking is to simplify and limit what you are testing and also make you feed what a class depends on  In this example you avoid testing the network calls themselves  and instead test whether or not your app works as you expect with the injected outputs responses        by mocking classes Needless to say  you test each network response separately   Now a question that I always had in my mind was  The contracts end points and basically the JSON response of my APIs get updated constantly  How can I write unit tests which take this into consideration  To elaborate more on this  let   s say model requires a key field named username  You test this and your test passes  2 weeks later backend changes the key s name to id  Your tests still passes  right  or not  Is it the backend developer   s responsibility to update the mocks  Should it be part of our agreement that they provide updated mocks  The answer to the above issue is that  unit tests   your development process as a client-side developer should would catch outdated mocked response  If you ask me how  well the answer is  Our actual app would fail  or not fail yet not have the desired behavior  without using updated APIs   hence if that fails   we will make changes on our development code  Which again leads to our tests failing    which we   ll have to correct it   Actually if we are to do the TDD process correctly we are to not write any code about the field unless we write the test for it   and see it fail and then go and write the actual development code for it   This all means that backend doesn   t have to say     hey we updated the mocks      it eventually happens through your code development debugging    Because it   s all part of the development process  Though if backend provides the mocked response for you then it s easier  My whole point on this is that  if you can   t automate getting updated mocked API response then  some human interaction is required ie manual updates of JSONs and having short meetings to make sure their values are up to date will become part of your process  This section was written thanks to a slack discussion in our CocoaHead meetup group  For iOS devs only  A very good example of mocking is this Practical Protocol-Oriented talk by Natasha Muraschev  Just skip to minute 18 30  though the slides may become out of sync with the actual video     I really like this part from the transcript   Because this is testing   we do want to make sure that the get function from the Gettable is called  because it can return and the function could theoretically assign an array of food items from anywhere  We need to make sure that it is called

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There are plenty of answers on SO and good posts on the web about mocking  One place that you might want to start looking is the post by Martin Fowler Mocks Aren t Stubs where he discusses a lot of the ideas of mocking   In one paragraph - Mocking is one particlar technique to allow testing of a unit of code with out being reliant upon dependencies  In general  what differentiates mocking from other methods is that mock objects used to replace code dependencies will allow expectations to be set - a mock object will know how it is meant to be called by your code and how to respond     Your original question mentioned TypeMock  so I ve left my answer to that below   TypeMock is the name of a commercial mocking framework   It offers all the features of the free mocking frameworks like RhinoMocks and Moq  plus some more powerful options   Whether or not you need TypeMock is highly debatable - you can do most mocking you would ever want with free mocking libraries  and many argue that the abilities offered by TypeMock will often lead you away from well encapsulated design   As another answer stated  TypeMocking  is not actually a defined concept  but could be taken to mean the type of mocking that TypeMock offers  using the CLR profiler to intercept  Net calls at runtime  giving much greater ability to fake objects  not requirements such as needing interfaces or virtual methods

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I would think the use of the TypeMock isolator mocking framework would be TypeMocking   It is a tool that generates mocks for use in unit tests  without the need to write your code with IoC in mind

[unit-testing] What is Mocking?

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