IoC



Inversion of Control


One Definition:

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()
    {
        checker = new SpellChecker();
    }
}

What we've done here is create a dependency between the TextEditor and the SpellChecker. In an IoC scenario we would instead do something like this:

public class TextEditor
{
    private ISpellChecker checker;
    public TextEditor(ISpellChecker checker)
    {
        this.checker = checker;
    }
}

Now, the client creating the TextEditor class has the control over which SpellChecker implementation to use. We're injecting the TextEditor with the dependency.

This is just a simple example, there's a good series of articles by Simone Busoli that explains it in greater detail.

<<

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 ...

Do not overuse this pattern

>>

Another One:

Inversion of Control is what you get when you 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.

Another One

What is Inversion of Control?

If you follow these simple two steps, you have done inversion of control:

  1. Separate what-to-do part from when-to-do part.
  2. 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)
  • Interfaces. Component client (when-to-do part) -- Component Interface implementation (what-to-do part)
  • xUnit fixure. 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)

The Last Word:


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 connection between your code is not in the code anymore. Instead it is in XML configuration files or annotations and the 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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 Drupal makes use of the inversion of control design pattern, in which modular functionality is called by the framework at the
appropriate time. These opportunities for modules to do their thing are called hooks.

































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