i added map(), reduce() and where(qlint : string) to a Spring4D fork of mine.
While i was programming these functions, i found out that there is a differnce in the behaviour of the lists, when they are created in different ways.
If i create them with TList<TSomeClass>.create the objects in the enumerables are of the type TSomeClass.
If i create them with TCollections.CreateList<TSomeClass> the objects in the enumerables are of the type TObject.
So the question is:
Is there a downside by using TList<TSomeClass>.create ?
Or in other words: Why should i use TCollections.CreateList<TSomeClass> ?
btw: with TCollections.CreateList i got a TObjectList and not a TList. So it should be called TCollections.CreateObjectList... but that's another story.
Depending on the compiler version many of the Spring.Collections.TCollections.Create methods are applying what the compiler is unable to: folding the implementation into only a very slim generic class. Some methods are doing that from XE on, some only since XE7 ( GetTypeKind intrinsic function makes it possible to do the type resolution at compile time - see the parameterless TCollections.CreateList<T> for example).
This greatly reduces the binary size if you are creating many different types of IList<T> (where T are classes or interfaces) because it folds them into TFolded(Object|Interface)List<T>. However via the interface you are accessing the items as what you specified them and also the ElementType property returns the correct type and not only TObject or IInterface. On Berlin it adds less than 1K for every different object list while it would add around 80K if the folding is not applied due to all the internal classes involved for the different operations you can call on an IList<T>.
As for TCollections.CreateList<T> returning an IList<T> that is backed by a TFoldedObjectList<T> when T is a class that is completely as designed. Since the OwnsObject was passed as False it has the exact same behavior as a TList<T>.
The Spring4D collections are interface based so it does not matter what class is behind an interface as long as it behaves accordingly to the contract of the interface.
Make sure that you only carry the lists around as IList<T> and not TList<T> - you can create them both ways (with the benefits I mentioned before when using the TCollections methods). In our own application some places are still using the constructor of the classes while many other places are using the static methods from Spring.Collections.TCollections.
BTW:
I saw the activity in your fork and imo there is no need to implement Map/Reduce because that is already there. Since the Spring4D collections are modelled after .NET they are called Select and Aggregate (see Spring.Collections.TEnumerable). They are not available on IEnumerable<T> directly though because interfaces must not have generic parameterized methods.
Is there a way to set up bindings imperatively. An example use case:
var el2 = new MyElement();
el2.myProp = this.$.anotherElement.anotherProp
That won't setup a binding, it just assigns the value or object. I'd like to find a way to do something like:
el2.myProp.bindTo(this.$.anotherElement.anotherProp)
Possible?
Polymer 1.0 does not support this at the moment - as explained by #kevinpschaaf in Github https://github.com/Polymer/polymer/issues/1778.
(comment by #kevinpschaaf)
No, we don't currently support this, outside of dom-bind, which is the
only template implementation that late-binds instance children. You
can document.createElement('template', 'dom-bind'), then you can
dynamically append children with binding annotations to its content,
and the bindings will only be evaluated once the dom-bind is attached
to the document. See tests here that show this usage of it:
https://github.com/Polymer/polymer/blob/master/test/unit/dom-bind.html#L95
Note that dom-bind does not currently allow binding to outer scope, so
it has limited use in custom element templates (it's main use case is
for binding between elements in the main document), and that's not
likely to change short-term.
We are achieving a lot of performance optimization by baking the
binding connections into the prototype at registration time for an
element (rather than at instance time), and we haven't built up enough
of the machinery to easily allow runtime addition/removal of bindings.
When coding, I often come across the following pattern:
-A method calls another method (Fine), but the method being called/callee takes parameters, so in the wrapping method, I pass in parameters. Problem is, this dependency carrying can go on and on. How could I avoid this (any sample code appreciated)?
Thanks
Passing a parameter along just because a lower-layer component needs it is a sign of a Leaky Abstraction. It can often be more effective to refactor dependencies to aggregate services and hide each dependency behind an interface.
Cross-cutting concerns (which are often the most common reason to pass along parameters) are best addressed by Decorators.
If you use a DI Container with interception capabilities, you can take advantage of those to implement Decorators very efficiently (some people refer to this as a container's AOP capabilities).
You can use a dependency injection framework. One such is Guice: see http://code.google.com/p/google-guice/
Step 1: Instead of passing everything as separate arguments, group the arguments into a class, let's say X.
Step 2: Add getters to the class X to get the relevant information. The callee should use the getters to get the information instead of relying on parameters.
Step 3: Create an interface class of which class X inherits. Put all the getters in the interface (in C++ this is as pure virtual methods).
Step 4: Make the called methods only depend on the interface.
Refactoring: Introduce Parameter Object
You have a group of parameters that naturally go together?
Replace them with an object.
http://www.refactoring.com/catalog/introduceParameterObject.html
The advantage of the parameter object is that the calls passing them around don't need to change if you add/remove parameters.
(given the context of your answers, I don't think that an IoC library or dependency injection patterns are really what you're after)
Since they cannot be (easily) unit tested, most developers choose to inject objects into Views. Since the views are not (normally) used to construct other views, that is where your DI chain ends. You may have the issue (which I have run into every once in ahwile) where you need to construct objects in the correct order especially when using a DI framework like Unity where an attemt to resolve the object will deadlock. The main thing you need to worry about is circular dependency. In order to do this, read the following article:
Can dependency injection prevent a circular dependency?
I don't really see the need for element to element binding in Silverlight 3 if using MVVM. Won't having one property directly affect another proper cause that property to be untestable?
To me, it makes more sense to do a two way binding to a explicit property defined in the ViewModel.
I agree that the use of MVVM severely deflates the usefulness of element to element binding.
Still, if all you are doing is binding two elements using a ViewModel property... what can you test? You can test that setting a property in the ViewModel sends a PropertyChanged event... but thats about it. Only when something else cares about that value is it useful to test a property like that.
In the simple cases, I can see element2element binding being more efficient and less code.
I'm refactoring a class and adding a new dependency to it. The class is currently taking its existing dependencies in the constructor. So for consistency, I add the parameter to the constructor.
Of course, there are a few subclasses plus even more for unit tests, so now I am playing the game of going around altering all the constructors to match, and it's taking ages.
It makes me think that using properties with setters is a better way of getting dependencies. I don't think injected dependencies should be part of the interface to constructing an instance of a class. You add a dependency and now all your users (subclasses and anyone instantiating you directly) suddenly know about it. That feels like a break of encapsulation.
This doesn't seem to be the pattern with the existing code here, so I am looking to find out what the general consensus is, pros and cons of constructors versus properties. Is using property setters better?
Well, it depends :-).
If the class cannot do its job without the dependency, then add it to the constructor. The class needs the new dependency, so you want your change to break things. Also, creating a class that is not fully initialized ("two-step construction") is an anti-pattern (IMHO).
If the class can work without the dependency, a setter is fine.
The users of a class are supposed to know about the dependencies of a given class. If I had a class that, for example, connected to a database, and didn't provide a means to inject a persistence layer dependency, a user would never know that a connection to the database would have to be available. However, if I alter the constructor I let the users know that there is a dependency on the persistence layer.
Also, to prevent yourself from having to alter every use of the old constructor, simply apply constructor chaining as a temporary bridge between the old and new constructor.
public class ClassExample
{
public ClassExample(IDependencyOne dependencyOne, IDependencyTwo dependencyTwo)
: this (dependnecyOne, dependencyTwo, new DependnecyThreeConcreteImpl())
{ }
public ClassExample(IDependencyOne dependencyOne, IDependencyTwo dependencyTwo, IDependencyThree dependencyThree)
{
// Set the properties here.
}
}
One of the points of dependency injection is to reveal what dependencies the class has. If the class has too many dependencies, then it may be time for some refactoring to take place: Does every method of the class use all the dependencies? If not, then that's a good starting point to see where the class could be split up.
Of course, putting on the constructor means that you can validate all at once. If you assign things into read-only fields then you have some guarantees about your object's dependencies right from construction time.
It is a real pain adding new dependencies, but at least this way the compiler keeps complaining until it's correct. Which is a good thing, I think.
If you have large number of optional dependencies (which is already a smell) then probably setter injection is the way to go. Constructor injection better reveals your dependencies though.
The general preferred approach is to use constructor injection as much as possible.
Constructor injection exactly states what are the required dependencies for the object to function properly - nothing is more annoying than newing up an object and having it crashing when calling a method on it because some dependency is not set. The object returned by a constructor should be in a working state.
Try to have only one constructor, it keeps the design simple and avoids ambiguity (if not for humans, for the DI container).
You can use property injection when you have what Mark Seemann calls a local default in his book "Dependency Injection in .NET": the dependency is optional because you can provide a fine working implementation but want to allow the caller to specify a different one if needed.
(Former answer below)
I think that constructor injection are better if the injection is mandatory. If this adds too many constructors, consider using factories instead of constructors.
The setter injection is nice if the injection is optional, or if you want to change it halfway trough. I generally don't like setters, but it's a matter of taste.
It's largely a matter of personal taste.
Personally I tend to prefer the setter injection, because I believe it gives you more flexibility in the way that you can substitute implementations at runtime.
Furthermore, constructors with a lot of arguments are not clean in my opinion, and the arguments provided in a constructor should be limited to non-optional arguments.
As long as the classes interface (API) is clear in what it needs to perform its task,
you're good.
I prefer constructor injection because it helps "enforce" a class's dependency requirements. If it's in the c'tor, a consumer has to set the objects to get the app to compile. If you use setter injection they may not know they have a problem until run time - and depending on the object, it might be late in run time.
I still use setter injection from time to time when the injected object maybe needs a bunch of work itself, like initialization.
I personally prefer the Extract and Override "pattern" over injecting dependencies in the constructor, largely for the reason outlined in your question. You can set the properties as virtual and then override the implementation in a derived testable class.
I perfer constructor injection, because this seems most logical. Its like saying my class requires these dependencies to do its job. If its an optional dependency then properties seem reasonable.
I also use property injection for setting things that the container does not have a references to such as an ASP.NET View on a presenter created using the container.
I dont think it breaks encapsulation. The inner workings should remain internal and the dependencies deal with a different concern.
One option that might be worth considering is composing complex multiple-dependencies out of simple single dependencies. That is, define extra classes for compound dependencies. This makes things a little easier WRT constructor injection - fewer parameters per call - while still maintaining the must-supply-all-dependencies-to-instantiate thing.
Of course it makes most sense if there's some kind of logical grouping of dependencies, so the compound is more than an arbitrary aggregate, and it makes most sense if there are multiple dependents for a single compound dependency - but the parameter block "pattern" has been around for a long time, and most of those that I've seen have been pretty arbitrary.
Personally, though, I'm more a fan of using methods/property-setters to specify dependencies, options etc. The call names help describe what is going on. It's a good idea to provide example this-is-how-to-set-it-up snippets, though, and make sure the dependent class does enough error checks. You might want to use a finite state model for the setup.
I recently ran into a situation where I had multiple dependencies in a class, but only one of the dependencies was necessarily going to change in each implementation. Since the data access and error logging dependencies would likely only be changed for testing purposes, I added optional parameters for those dependencies and provided default implementations of those dependencies in my constructor code. In this way, the class maintains its default behavior unless overridden by the consumer of the class.
Using optional parameters can only be accomplished in frameworks that support them, such as .NET 4 (for both C# and VB.NET, though VB.NET has always had them). Of course, you can accomplish similar functionality by simply using a property that can be reassigned by the consumer of your class, but you don't get the advantage of immutability provided by having a private interface object assigned to a parameter of the constructor.
All of this being said, if you are introducing a new dependency that must be provided by every consumer, you're going to have to refactor your constructor and all code that consumers your class. My suggestions above really only apply if you have the luxury of being able to provide a default implementation for all of your current code but still provide the ability to override the default implementation if necessary.
Constructor injection does explicitly reveal the dependencies, making code more readable and less prone to unhandled run-time errors if arguments are checked in the constructor, but it really does come down to personal opinion, and the more you use DI the more you'll tend to sway back and forth one way or the other depending on the project. I personally have issues with code smells like constructors with a long list of arguments, and I feel that the consumer of an object should know the dependencies in order to use the object anyway, so this makes a case for using property injection. I don't like the implicit nature of property injection, but I find it more elegant, resulting in cleaner-looking code. But on the other hand, constructor injection does offer a higher degree of encapsulation, and in my experience I try to avoid default constructors, as they can have an ill effect on the integrity of the encapsulated data if one is not careful.
Choose injection by constructor or by property wisely based on your specific scenario. And don't feel that you have to use DI just because it seems necessary and it will prevent bad design and code smells. Sometimes it's not worth the effort to use a pattern if the effort and complexity outweighs the benefit. Keep it simple.
This is an old post, but if it is needed in future maybe this is of any use:
https://github.com/omegamit6zeichen/prinject
I had a similar idea and came up with this framework. It is probably far from complete, but it is an idea of a framework focusing on property injection
It depends on how you want to implement.
I prefer constructor injection wherever I feel the values that go in to the implementation doesnt change often. Eg: If the compnay stragtegy is go with oracle server, I will configure my datsource values for a bean achiveing connections via constructor injection.
Else, if my app is a product and chances it can connect to any db of the customer , I would implement such db configuration and multi brand implementation through setter injection. I have just taken an example but there are better ways of implementing the scenarios I mentioned above.
When to use Constructor injection?
When we want to make sure that the Object is created with all of its dependencies and to ensure that required dependencies are not null.
When to use Setter injection?
When we are working with optional dependencies that can be assigned reasonable default values within the class. Otherwise, not-null checks must be performed everywhere the code uses the dependency.
Additionally, setter methods make objects of that class open to reconfiguration or re-injection at a later time.
Sources:
Spring documentation ,
Java Revisited