I use Caliburn.Micro with Spring.net instead of the default simple IoC. My custom Bootstrapper (derrived from Caliburn's BootstrapperBase) is working and I can define the ViewModels within Spring.net. But the the Views are still resolved by reflection (name convention) from the execution assembly. I used the following method of the Bootstrapper to add Assemblies for resolving the Views for the ViewModels.
protected override IEnumerable<Assembly> SelectAssemblies()
{
// hmm, want to change the way how the view is resolved... how to do this?
// ... use IoC or MEF for this task instead?
return new[]
{
// don't want to add every dll here
this.GetType().Assembly,
Assembly.Load("MyViewModels.Assembly")
};
}
How to change the behaviour of resolving views and using IoC or MEF for this task?
The Problem is that the Bootstrapper has no virtual method to override which resolves a requested view. What is the starting point to change this behaviour? I thought there must exist something like
protected virtual Control ResolveViewForModel(Type modelType) {...}
Thanks for any hints.
First of all, I don't know caliburn.micro so this might be wrong.
Looking at the ViewLocator method LocateTypeForModelType it seems that it asks the AssemblySource for available types which should be checked against the View-naming conventions.
Since all of the above are static classes I suspect there is no way to inherit and override that behaviour. Since they are static, one could just add assemblies to the public observable dictionary - which feels a little bit of a hack and SelectAssemblies seems like the proper way.
However, it seems to me that since there are conventions for resolving Views and ViewModels one could do the same for assemblies which brings us to the question: how do you decide which assemblies to scan for ViewModels/Views.
That strategy can be built into the SelectAssemblies method.
If you want to change how caliburn.micro finds the right views in those assemblies, effectively changing/adding to the exisiting conventions, there is an explanation in their wiki.
To finally answer your question: "Resolve Views through IoC or MEF instead of using SelectedAssemblies() method": Imo this kind of defeats the philosophy of Caliburn.Micro:
Caliburn.Micro uses conventions to resolve views from given assemblies - trying to use an IoC container instead of a name / namespace based convention contradicts that approach.
Related
I'm in the process of teaching myself the MVVM pattern by dividing the pattern into its core facets and learning those facets one by one.
My question is related to dependency injection. What is it, and why/when should I use it? I've looked at Jason Dolinger's excellent MVVM intro video and I see he uses Unity. This might be strange to ask but how would I implement dependency injection WITHOUT using Unity? I basically want to understand the concept of dependency injection and how to use it without having to implement other frameworks/tools (for now).
Thanks.
I think it's good that you want to understand DI without using a framework, the concept is not terribly difficult to wrap your head around.
Let's say you want to use some form of transportation.
interface ITransportation
{
Transport();
}
An initial implementation of a method that uses a form of transportation might look like this:
public void Move()
{
ITransportation car = new Car();
car.Transport();
}
The problem with that method is that it is now dependent on a Car class. We should pass our transportation object in for added flexibility. This is inversion of control and is closely related to DI.
public void Move(ITransportation tr)
{
tr.Transport();
}
As you can see, we don't need to know anything about a specific DI framework. You might also want to check out the ninject DI by hand tutorial.
Just to extend #Andy's answer
Dependency Injection is one of the forms of the Dependency Inversion Principle
To achieve the decoupling of dependencies (as typically found in layered architecture),
DI is commonly used for instantiation scenarios such as basic new() and patterns like Factory method. In addition to being able to inject a new dependency instance every time (e.g. like factory), containers can also be set up to inject named instances, singleton instances, etc - i.e. IoC containers usually also take on the responsibility of managing the lifespans of objects as well.
One potential 'mindset shift' is that dependencies now potentially become publicly visible on concrete classes, since DI typically injects via constructors or public Get / Set properties. This may seem strange if you are used to using OO encapsulation, where dependencies of a class are seen as implementation and should be hidden from the 'outside' i.e. class method signatures.
However, by implementing Interface / Concrete class separation (as you should, not only for decoupling but also for testing / mocking purposes), the injection constructors / property injection methods will not be on the interface, so encapsulation is again in place.
Re : "Doing DI by hand" without Unity etc
What you would need to do is to code your own IoC container, which then is responsible for 'building up' instances of classes - during each 'build up', you would scan the class for dependencies (which are configured in the container, e.g. by config, by attributes, or simply just by convention, e.g. all public settable properties, or any class parameters on a constructor will be assumed to be dependencies). You would then create (if necessary) and inject this 'dependency' instance onto the object (e.g. by using reflection). And then recursively, all dependencies of these dependencies need to be built up etc. You would then also need to provide lifespan management for each of the objects, e.g. Singletons etc.
I think my question is a long shot.
Lets say I have an attribute:
public sealed class MyCustomAttribute: ActionFilterAttribute
Used on a class method
[MyCustomAttribute]
public virtual ActionResult Options(FormCollection collection)
Now, I need to add a contructor's parameter
public MyCustomAttribute(IMyDependentObject dependentObject)
{
(...)
}
(You propably notice that it's some Asp.NET MCV code)
I would like to use DI to create this attribute. Asp.NET MVC code automatically create it and I don't know how/where I could write code to use Castle istead.
Any ideas?
As far a I konw castle does not support injection of existing objects, which makes it impossible to inject attributes as their construction is not under your control. Other IoC containers such as Ninject support injection of existing objects. They inject properties of your attribut filter. See http://github.com/ninject/ninject.web.mvc for an extension that exactly does what you need.
What you can do if you want to stay on castle is to inject your own ControllerActionInvoker derived from ControllerActionInvoker (AsyncControllerActionInvoker in case of async controller) into all controllers. In your own invoker you override GetFilters. Additionally to the Filters returned by the base you add FilterInfos that are created by castle.
The decision which filters infos are created and added can be achieved with various strategies e.g.:
Add an own custom attribute that contains the information e.g. name of a binding
A configuration file/database
May you consider switching to MVC3 this makes all a bit easier. As you can register your own FilterProvider which makes all much easier. In this FilterProvider you have to decide which filter info you want to add. See again the two strategies above. See http://bradwilson.typepad.com/blog/2010/07/service-location-pt4-filters.html for information about MVC3 and filters.
I am working in a project that has two main parts: a class library assembly and the main application. Both are using Castle Windsor for IoC and both manually setup their list of of components in code (to aid refactoring and prevent the need for a config file). Currently the main application has code like this:
public static void Main()
{
// Perform library IoC setup
LibraryComponent.Init();
// Perform application IoC setup
IoC.Register<IXyz, Abc>("abc");
// etc, etc, ...
// Start the application code ...
}
However the call to initialise the library doesn't seem like a good solution. What is the best way to setup a class library that uses an IoC container to decouple its internal components?
Edit:
Lusid proposed using a static method on each public component in the library that would in turn make the call to initialise. One possible way to make this a bit nicer would be to use something like PostSharp to do this in an aspect-oriented way. However I was hoping for something a bit more elegant ;-)
Lusid also proposed using the AppDomain.AssemblyLoad event to perform custom steps at load time, however I am really after a way to avoid the client assembly from requiring any setup code.
Thanks!
I'm not sure if I'm understanding exactly the problem you are trying to solve, but my first guess is that you are looking for a way to decouple the need to call the Init method from your main application.
One method I've used in the past is a static constructor on a static class in the class library:
static public class LibraryComponent {
static LibraryComponent() {
Init();
}
}
If you have multiple class libraries, and would like a quick and dirty way of evaluating all of them as they are loaded, here's a (kinda hairy) way:
[STAThread]
static void Main()
{
AppDomain.CurrentDomain.AssemblyLoad += new AssemblyLoadEventHandler(CurrentDomain_AssemblyLoad);
}
static void CurrentDomain_AssemblyLoad(object sender, AssemblyLoadEventArgs args)
{
IEnumerable<Type> types = args.LoadedAssembly.GetTypes()
.Where(t => typeof(IMyModuleInterface).IsAssignableFrom(t));
foreach (Type t in types)
{
doSomethingWithType(t);
}
}
The Where clause could be anything you want, of course. The code above would find any class deriving from IMyModuleInterface in each assembly that gets loaded into the current AppDomain, and then I can do something with it, whether it be registering dependencies, maintaining an internal list, whatever.
Might not be exactly what you are looking for, but hopefully it helps in some way.
You could have a registration module. Basically LibraryComponent.Init() function takes an IRegistrar to wire everything up.
The IRegistrar could basically have a function Register(Type interface, Type implementation). The implimentor would map that function back to their IOC container.
The downside is that you can't rely on anything specific to the container your using.
Castle Windsor actually has a concept called facilities that are basically just ways of wrapping standardised pieces of configuration. In this model, you would simply add the two facilities to the container and they would do the work.
Of course, this wouldn't really be better than calling a library routine to do the work unless you configured the facilities in a configuration file (consider binsor). If you are really allergic to configuration files, your current solution is probably the best.
When Instantiating a class, Windsor by default treats all public properties of the class as optional dependencies and tries to satisfy them. In my case, this creates a rather complicated circular dependency which causes my application to hang.
How can I explicitly tell Castle Windsor that it should not be trying to satisfy a public property? I assume there must be an attribute to that extent. I can't find it however so please let me know the appropriate namespace/assembly.
If there is any way to do this without attributes (such as Xml Configuration or configuration via code) that would be preferable since the specific library where this is happening has to date not needed a dependency on castle.
You can use the Castle.Core.DoNotWireAttribute attribute to stop a property from being wired up by the IoC container (this is in the Castle.Core assembly, which means your library only needs to take a dependency on the lightweight Castle.Core assembly - if for example you want to use the code without an inversion of control container altogether, or in a different IoC container).
I don't believe there's any way to prevent wiring from occurring in the Xml configuration, but it would be reasonably easy to add support for this - if I had to do this I would probably:
Introduce some kind of attribute on the property declaration in the xml: <myprop wire="false" />
Inherit from PropertiesDependenciesModelInspector, overriding the InspectProperties method to apply some additional logic to identifying which properties should be added as dependencies to the components model (inspecting the model.Configuration for the wire="false" attribute/value pair).
Inherit from DefaultComponentModelBuilder and override the InitializeContributors to include your replacement PropertiesDependenciesModelInspector - or just remove the existing properties contributor and add your own at run time via the AddContributor/RemoveContributor methods.
Replace the ComponentModelBuilder service instance assigned to the kernel of your container.
Another approach which could work for you is to just manually remove the dependencies from the model before any instances of the service are requested ie.
kernel.GetHandler(typeof(MyComponent)).ComponentModel.Dependencies.RemoveAll(d => d.DependencyKey == "PropertyThatShouldNotBeWired");
YMMV with that approach though - especially if you have startable services or other facilities which may be eagerly instantiating your component after it's registered.
I created a facility to help with this:
Castle.Facilities.OptionalPropertyInjection
I do not know which version of Castle you guys were using at that time, but none of the solution mentioned were working. Plus, there is a lot of dead links.
With castle 3.1, here the solution I came up with (thanks to some castle source code digging):
container.Register(Component.For(type)
.LifestyleTransient()
.Properties( propertyInfo => propertyInfo.PropertyType != typeof(MyOtherType)));
The 'Properties' function adds a property filter used by castle when constructing the ComponentModel. In my case, all properties dependency will be satisfied except the property type 'MyOtherType'.
Maybe it will be helpful for someone. In Windsor 4.1 there is PropertiesIgnore method during registration.
Component.For<Role>().LifestyleTransient().PropertiesIgnore((model, propertyInfo) => true)
DoNotWireAttribute
Class: http://svn.castleproject.org:8080/svn/castle/trunk/Core/Castle.Core/Attributes/DoNotWireAttribute.cs
Test: http://svn.castleproject.org:8080/svn/castle/trunk/InversionOfControl/Castle.Windsor.Tests/IgnoreWireTestCase.cs
This can be achieved by the following code:
var container = new WindsorContainer();
// We don't want to inject properties, only ctors
var propInjector = container.Kernel.ComponentModelBuilder
.Contributors
.OfType<PropertiesDependenciesModelInspector>()
.Single();
container.Kernel.ComponentModelBuilder.RemoveContributor(propInjector);
Source Castle Windsor Documentation
Posted this on the google groups forum too here: http://groups.google.com/group/castle-project-devel/browse_thread/thread/43aa513817bd057a
Suppose that the ApplicationSettings class is a general repository of settings that apply to my application such as TimeoutPeriod, DefaultUnitOfMeasure, HistoryWindowSize, etc... And let's say MyClass makes use of one of those settings - DefaultUnitOfMeasure.
My reading of proper use of Inversion of Control Containers - and please correct me if I'm wrong on this - is that you define the dependencies of a class in its constructor:
public class MyClass {
public MyClass(IDataSource ds, UnitOfMeasure default_uom) {...}
}
and then call instantiate your class with something like
var mc = IoC.Container.Resolve<MyClass>();
Where IDataSource has been assigned a concrete implementation and default_uom has been wired up to instantiate from the ApplicationSettings.DefaultUnitOfMeasure property. I've got to wonder however, if all these hoops are really that necessary to jump through. What trouble am I setting myself up for should I do
public class MyClass {
public MyClass(IDataSource ds) {
UnitOfMeasure duom = IoC.Container.Resolve<UnitOfMeasure>("default_uom");
}
}
Yes, many of my classes end up with a dependency on IoC.Container but that is a dependency that most of my classes will have anyways. It seems like I maybe should make full use of it as long as the classes are coupled. Please Agile gurus, tell me where I'm wrong.
IoC.Container.Resolve("default_uom");
I see this as a classic anti-pattern, where you are using the IoC container as a service locater - the key issues that result are:
Your application no longer fails-fast if your container is misconfigured (you'll only know about it the first time it tries to resolve that particular service in code, which might not occur except for a specific set of logic/circumstances).
Harder to test - not impossible of course, but you either have to create a real (and semi-configured) instance of the windsor container for your tests or inject the singleton with a mock of IWindsorContainer - this adds a lot of friction to testing, compared to just being able to pass the mock/stub services directly into your class under test via constructors/properties.
Harder to maintain this kind of application (configuration isn't centralized in one location)
Violates a number of other software development principles (DRY, SOC etc.)
The concerning part of your original statement is the implication that most of your classes will have a dependency on your IoC singleton - if they're getting all the services injected in via constructors/dependencies then having some tight coupling to IoC should be the exception to the rule - In general the only time I take a dependency on the container is when I'm doing something tricky i.e. trying to avoid a circular dependency problems, or wish to create components at run-time for some reason, and even then I can often avoid taking a dependency on anything more then a generic IServiceProvider interface, allowing me to swap in a home-bake IoC or service locater implementation if I need to reuse the components in an environment outside of the original project.
I usually don't have many classes depending on my IoC container. I usually try to wrap the IoC stuff in a facade object that I inject into other classes, usually most of my IoC injection is done only in the higher layers of my application though.
If you do things your way you can't test MyClass without creating a IoC configuration for your tests. This will make your tests harder to maintain.
Another problem is that you're going to have powerusers of your software who want to change the configuration editing your IoC config files. This is something I'd want to avoid. You could split up your IoC config into a normal config file and the IoC specific stuff. But then you could just as well use the normal .Net config functionality to read the configuration.
Yes, many of my classes end up with a dependency on IoC.Container but that is a dependency that most of my classes will have anyways.
I think this is the crux of the issue. If in fact most of your classes are coupled to the IoC container itself chances are you need to rethink your design.
Generally speaking your app should only refer to the container class directly once during the bootstrapping. After you have that first hook into the container the rest of the object graph should be entirely managed by the container and all of those objects should be oblivious to the fact that they were created by an IoC container.
To comment on your specific example:
public class MyClass {
public MyClass(IDataSource ds) {
UnitOfMeasure duom = IoC.Container.Resolve<UnitOfMeasure>("default_uom");
}
}
This makes it harder to re-use your class. More specifically it makes it harder to instantiate your class outside of the narrow usage pattern you are confining it to. One of the most common places this will manifest itself is when trying to test your class. It's much easier to test that class if the UnitOfMeasure can be passed to the constructor directly.
Also, your choice of name for the UOM instance ("default_uom") implies that the value could be overridden, depending on the usage of the class. In that case, you would not want to "hard-code" the value in the constructor like that.
Using the constructor injection pattern does not make your class dependent on the IoC, just the opposite it gives clients the option to use the IoC or not.