blood_magic

Dependency Injection in Objective-C with Blood and Magic

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MVC is not sufficient

Every day iOS apps become more cumbersome and bulky, which makes MVC approach not sufficient.

We are seeing more and more classes for different purposes: app logic extracts into services, models extends with decorators, view separates into partials and so on. And – what is more important – there are now a lot of dependencies that need to be managed somehow.

Singleton is often used to resolve this problem; this is a kind of global variable that everyone can access. How many times have you seen such a code:

A lot of projects use this approach, but it entails a few flaws:

  • it’s hard to mock or stub Singletons while testing classes that use it
  • Singleton is a global variable by the nature
  • from the SRP point of view, object should not control his Singleton behaviour

The first problem is easy to solve – use object property:

But this approach entails another issue – someone should ‘fill’ this property.

Dependency Injection

These issues are not unique for Objective-C. If we take a look at more ‘industrial’ languages, e.g. Java or C++, we can find a solution. Widely used approach in Java – Dependency Injection(DI).

DI allows injecting requestManager as a singleton in the application, but uses mock object while testing. No one is aware of singleton however – neither RequestManager, nor ViewController – because all this stuff is managed by DI framework.

You can find a lot of Objective-C implementations of the Dependency Injection pattern on GitHub, but they have some disadvantages:

  • description of dependencies via macros or string constants
  • injection occurs only if object is created in a special way (this would not work at least with UIViewController from Storyboard and with UIView from Nib)
  • injected class should implement some protocol (this would not work with third party libraries)
  • initialization can’t be placed into a separate module
  • uses XML to describe dependencies

BloodMagic

So, let’s look into another framework (also with its set of disadvantages) – BloodMagic.

BloodMagic provides some sort of custom attributes. It’s designed to be extensible, so more features are coming soon. Currently, only one attribute is implemented – Lazy.

This attribute allows to initialize objects on demand with a minimum of boilerplate code. Therefore, instead of the following sheets:

we can simply write:

And that’s it. Both @dynamic properties are created by a call to self.progressViewService and self.resourceLoader. They are released in the same way as a standard properties – after ViewController is deallocated.

BloodMagic and Dependency Injection

+new method is used for object creation here by default. But it also has a capability of writing custom initializers which is a key feature of BMLazy as a DI framework.

Creating custom initializer is a bit cumbersome, but works well:

propertyClass – initializer registers for property of this class.

initializer – block, which is called to initialize property. If this block is nil, or initializer is not found for concrete property, then object is created via +new class method.

sender – container class instance.

Also, initializer has a containerClass property, which allows us to describe creation of the same property based on class container. For example:

Thus, for UsersViewController and ProjectsViewController different objects are created. By default containerClass equals to NSObject class.

Initializers help to get rid of various shared* methods and hardcoding, described in the beginning:

Organize initializers

We may have a lot of initializers, so it makes sense to move them into a separate place.

Nice solution is to store them in different files and use attributes of a compiler (it’s a normal practice). BloodMagic has also a simple macros that hides this attribute – lazy_initializer. We should simply create a source file without header and add it to the compilation phase.

Here is an example:

lazy_initializer will be replaced with __attribute__((constructor)) static void. Attribute constructor means that this method will be called before main function (here are details: GCC. Function Attributes).

Plans for the near future

  • implement support of @protocol, e.g.: @property (nonatomic, strong) id<ResourceLoader> loader;
  • explain how it works
  • describe how to add new attribute

Stay tuned!

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