Here's what I know so far:
DI lets me build reusable, unit-testable components
DI is verbose because it requires that I explicitly set the dependencies (via constructor or method. I still don't understand the interface injection though). This is why a container or a service locator is needed.
Container is better than service locator because classes won't need to be aware of the existence of it.
But I found these problems:
Some classes will now depend from Container? If I don't use the default config for every class, as described in my services file, some classes will need to call the container to re-configure the needed object.
On page 79 from this slide http://www.slideshare.net/fabpot/dependency-injection-with-php-53, Fabien Potencier said that a Container does not manage all the objects, only those with a single instance (yet not singletons). I'm even more confused now.
Any help is greatly appreciated. =)
Some classes will now depend from Container?
No. That's why you use dependency injection as opposed to service location.
On page 79 from this slide...
See page 82, it says "Unlike model objects". Honestly I'd never explain it like that ("Objects with only one instance (!= Singletons)" is either wrong or something very PHP specific, it doesn't apply to dependency injection or IoC+DI containers in general), but I bet what he was trying to explain is that the container usually manages service-like things, not model-like things.
Related
In building my current (first) Windows Phone app it requires me to create a Windows Runtime Component to achieve the functionality I require. In order for this setup to work and not duplicate a lot of code from my PCLs into the task class itself, I wanted to use the MMVMCross IOC that I am already using throughout the application.
Unfortunately, the Background Task (IBackgroundTask) is executed in an entirely different process. Trying to utilize the IOC via Mvx.Resolve throws a NullReferenceException. I cannot figure out how to initialize the IOC as the standard "setup.cs" method does not work in the Runtime Component.
I do not need the entire MVVMCross stack for this -- just the IOC.
Thank you.
I finally figured it out. I have to re-register on the background task, but to initialize you would call the basic initialize method on the simple IOC container:
Cirrious.CrossCore.IoC.MvxSimpleIoCContainer.Initialize();
Plugins were a problem, as the standard plugin mechanism is not available, but you can manually register the interfaces such as this:
Mvx.LazyConstructAndRegisterSingleton<IMvxFileStore>(() => new MvxWindowsCommonBlockingFileStore());
Of course, you can still register your other types and interfaces as you normally would.
I wonder if it's possible to get an instance of the JAX-RS Application a resource is attached on. Ideally a way that isn't dependent to a specific implementation. For example using dependency injection...
Thanks very much for your help,
Thierry
As stated in The Spec
5.2.1 Application
The instance of the application-supplied Application subclass can be injected into a class field or method parameter using the #Context annotation. Access to the Application subclass instance allows configuration information to be centralized in that class. Note that this cannot be injected into the Application subclass itself since this would create a circular dependency.
but from I've experienced, it will most likely not be the actual instance, but a proxy. Also if you're looking to alter anything on it, I'm not sure it's possible. It might be read-only.
I am using Ninject in substitution of MEF and I was wondering if it's possible to get lazy instances via standard kernel methods and not via [inject] .
I need this since when building up my application's menu I have to pass all particular view models and then if the user is enabled on that function to add it to the menu
Thanks
Sure thing, you can inject a Lazy<T> and the value will only be instanciated when you access Lazy<T>.Value.
You can also inject a Func<T> and use it to create T whenever you like (with the func, every call creates a new instance).
Of course you can also do IResolutionRoot.Get<Lazy<T>>() or IResolutionRoot.Get<Func<T>>(), but usually that's a sign of bad design (service locator), so use constructor injection when it's feasible.
EDIT: When is the "enabling of the user" happening? Is it a one time thing? What is being displayed before and after?
There might be other/better designs to achieve this but it's hard to say with that little information.
I have been playing around the last couple of days with different solutions for mapping DTO's to entities for a VS2013, EF6, WCF Service App project.
It is a fairly large project that is currently undergoing a major refactoring to bring the legacy code under test (as well as port the ORM from OpenAccess to EF6).
To be honest I had never used AutoMapper before but what I saw I really liked so I set out to test it out in a demo app and to be honest I am a bit ashamed that I have been unable to achieve a working solution after hours of tinkering and Googling. Here is a breakdown of the project:
WCF Service Application template based project (.svc file w/code behind).
Using Unity 3.x for my IoC container and thus creating my own ServiceHostFactory inheriting from UnityServiceHostFactory.
Using current AutoMapper nuget package.
DTO's and DAL are in two separate libraries as expected, both of which are referenced by the service app project.
My goal is simple (I think): Wire up and create all of my maps in my composition root and inject the necessary objects (using my DI container) into the class that has domain knowledge of the DTO's and a reference to my DAL library. Anyone that needs a transformation would therefore only need to reference the transformation library.
The problem: Well, there are a couple of them...
1) I cannot find a working example of AutoMapper in Unity anywhere. The code snippet that is referenced many times across the web for registering AutoMapper in Unity (see below) references a Configuration class that doesn't seem to exist anymore and I cannot find any documentation on its deprecation:
container.RegisterType<AutoMapper.Configuration, AutoMapper.Configuration>(new PerThreadLifetimeManager(), new InjectionConstructor(typeof(ITypeMapFactory),
AutoMapper.Mappers.MapperRegistry.AllMappers())).RegisterType<ITypeMapFactory,
TypeMapFactoy>().RegisterType<IConfiguration, AutoMapper.Configuration>().RegisterType<IConfigurationProvider,
AutoMapper.Configuration>().RegisterType<IMappingEngine, MappingEngine>();
2) Where to create the maps themselves... I would assuming that I could perform this operation right in my ServiceHostFactory but is that the correct place? There is a Bootstrapper project out there but I have not gone down that road (yet) and would like to avoid it if possible.
3) Other than the obviously necessary reference to AutoMapper in the DTO lib, what would I be injecting into the instantition, the configuration object (assuming IConfiguration or IConfigurationProvider) and which class I am injecting into the constructor of the WCF service to gain access to the necessary object.
I know #3 is a little vague but since I cannot get AutoMapper bound in my Unity container, I cannot test/trial/error to figure out the other issues.
Any pointers would be greatly appreciated.
UPDATE
So I now have a working solution that is testing correctly but would still like to get confirmation that I am following any established best practices.
First off, the Unity container registration for AutoMapper (as of 11/13/2013) v3.x looks like this:
container
.RegisterType<ConfigurationStore, ConfigurationStore>
(
new ContainerControlledLifetimeManager()
, new InjectionConstructor(typeof(ITypeMapFactory)
, MapperRegistry.AllMappers())
)
.RegisterType<IConfigurationProvider, ConfigurationStore>()
.RegisterType<IConfiguration, ConfigurationStore>()
.RegisterType<IMappingEngine, MappingEngine>()
.RegisterType<ITypeMapFactory, TypeMapFactory>();
Right after all of my container registrations, I created and am calling a RegisterMaps() method inside of ConfigureContainer(). I created a test mapping that does both an auto mapping for like named properties as well as a custom mapping. I did this in my demo app for two reasons primarily:
I don't yet know AutoMapper in a WCF app hosted in IIS and injected with Unity well enough to fully understand its behavior. I do not seem to have to inject any kind of configuration object into my library that does the transformations and I am still reading through the source to understand its implementation.
As I understand it, there is a caching mechanism at play here and that if a mapping is not found in cache that it will create it on the fly. While this is great in theory, the only way I could then test my mappings that were occurring in my composition root was to do some sort of custom mapping and then call Mapper.Map in the library that performs mapping and returns the DTO.
All of that blathering aside, here is what I was able to accomplish.
WCF Service App (composition root) injects all of the necessary objects including my DtoConversionMapper instance.
The project is made up of the WCF Service App (comp root), DtoLib, DalLib, ContractsLib (interfaces).
In my ServiceFactoryHost I am able to create mappings, including custom mappings (i.e. map unlike named properties between my DTO and EF 6 entity).
The DtoConversionMapper class lives in the DtoLib library and looks like this: IExampleDto GetExampleDto(ExampleEntity entity);
Any library with a reference to the DtoLib can convert back and forth, including the Service App where the vast majority of these calls will take place.
Any guiding advice would be greatly appreciated but I do have a working demo now that I can test things out with while I work through this large refactoring.
Final Update
I changed the demo project just a little by adding another library (MappingLib) and moved all of my DTO conversions and mappings to it in a static method. While I still call the static method in my composition root after the Unity container is initialized, this gives me the added flexibility of being able to call that same map creation method in my NUnit unit test libraries, effectively eliminating any duplication of code surrounding auto mapper and makes it very testable.
This is a problem which has been bugging me for a while now. I'm still pretty new with some of these patterns so you'll have to forgive me (and correct me) if I use any of the terms incorrectly.
My Methodology
I've created a game engine. All of the objects in my game engine use inversion of control to get dependencies. These dependencies all implement protocols and are never accessed directly in the project, other than during the bootstrapping phase. In order to get these objects, I have the concept of a service locator. The service locator's job is to locate an object which conforms to a specific protocol and return it. It's a lot like a factory, but it should handle the dependencies as well.
In order to provide the services to the service locator, I have what I call service specifiers. The service locator knows about all of the service specifiers in the project, and when an object is requested, attempts to get an instance of an object conforming to the provided protocol from each of them. This object is then returned to the caller. What's cool about this set up is the service specifier also knows about a service locator, so if it has any dependencies, it just asks the service locator for those specific dependencies.
To give an example, I have an object called HighScoreManager. HighScoreManager implements the PHighScoreManager protocol. At any time if an instance of PHighScoreManager is required, it can be retrieved by calling:
id<PHighScoreManager> highScoreManager = [ServiceLocator resolve: #protocol(PHighScoreManager)];
Thus, inversion of control. However, most of the time it isn't even necessary to do this, because most classes are located in a service specifier, if one required PHighScoreManager as a dependency, then it is retrieved through the service locator. Thus, I have a nice flat approach to inversion of control.
My Problem
Because I want the code from my game engine to be shared, I have it compiled as a static library. This works awesome for everything else, but seems to get a little tricky with the service locator. The problem is some services change on a game to game basis. In my above example, a score in one game might be a time and in another it might be points. Thus, HighScoreManager depends on an instance of PHighScoreCreator, which tells it how to create a PScore objecct.
In order to provide PHighScoreCreator to HighScoreManager, I need to have a service specifier for my game. The only way I could think of to accomplish this was to use the Cocoa version of reflections. After digging around, I found out classes were discoverable through NSBundle, but it seems there's no way to get the current bundle. Thus, if I want to be able to search out my service specifiers, I would have to compile my game logic into its own bundle, and then have the engine search out this bundle and load it. In order to do this I'd have to create a third project to house both the engine code and the game logic bundle, when in reality I'd like to just have a game project which used the engine static library.
My Real Question
So after all of that, my question is
Is there a better way to do what I'm trying to accomplish in Cocoa Touch, or
Is there a way to discover classes which conform to my service specifier protocol from the main bundle?
Thanks for the help and taking the time to read the question.
-helixed
Have a look at:
+[NSBundle mainBundle];
+[NSBundle bundleForClass:];
+[NSBundle bundleWithIdentifier:];
+[NSBundle allBundles];
+[NSBundle allFrameworks];
These allow you to interact programmatically with the various bundles at runtime. Once you have a bundle to work with there are a number of strategies you could employ to find the specific class(es) you are looking for. For example:
Retrieve the bundle identifier — this will be an NSString like #"com.example.GameEngineClient".
Transform it into a legal Objective-C class name by stripping everything before the last dot, or replacing all the dots with underscores, or whatever, and then appending a predefined protocol name. Your protocol from above, for instance, might result in a string like #"GameEngineClient_PHighScoreManager".
Get the bundle's designated class for your protocol using NSClassFromString().
Now you can create an instance of the class provided by the bundle author, that implements whatever protocol you have specified.
The Objective-C runtime is a beautiful thing!
Sounds like you need to use the functions of the Objective-C runtime. First you can get a list of all available classes via objc_getClassList. Then you can iterate over all the classes and check if they conform to your protocol with class_conformsToProtocol. You shouldn’t use +conformsToProtocol: messages here, since there are classes in the runtime that don’t support this selector.