I have a WPF application where I want to apply MVVM for the presentation layer and DDD for the whole application. I am very confused with how I should apply the architecture. Can you give me some advises as it feels like I have completely messed up at the moment with the following design try:
I have 4 layers:
Presentation Layer : This is where my WPF client application lies in.
Application Layer : This is where I have my services that's supposed communicate with domain services for business rules and does CRUD. It works simply as an anti-corruption layer between Presentation and Domain layers.
Domain Layer : This is where I have my aggregates, domain objects, and some services that reveals business rules, such as IsTooOld(Person person)
Infrastructure Layer : This is the lowest layer, infrastructure lies in here, IRepository, IEntity etc..
Let's achieve a simple scenario with those DDD based layers: Have a Person object in a database, map it, CRUD the database, check persons' birthday and show it to the user.
Presentation Layer
I'll start with the WPF part. I create the following classes :
PersonView : XAML view of the person
PersonViewModel : ViewModel that provides functionality for the PersonView. PersonView binds to this and this ViewModel provides values from PersonModel
PersonModel : This is the MVVM model that my PersonViewModel is tightly coupled to.
Domain Layer
This is good enough for the presentation layer. I now want to connect to the database to retrieve a person object to present it.
I must create:
PersonEntity in Domain Layer : An aggregate for the database entity, used for mappings with the database. It lies in Domain layer.
Person in Domain Layer : This is the domain model of DDD. I'll put some logic here and I don't want to send around entity objects as DDD suggests.
Application Layer
Ok, I already have 3 person models that are pretty similar to each other. How about some more for data access and the services?
PersonService in Application Layer : When my presentation layer wants to communicate with this layer it needs to convert its PersonModel (MVVM model) to Person (domain model). This service in application layer then converts Person (domain model) it into PersonEntity (entity object) and does CRUD with the database. This service uses also another PersonService (see below) in Domain layer to check/apply some business rules.
PersonService in Domain Layer : This layer works only with Person domain object. It has business-related rules like bool IsTooOld(Person person).
To summarize, I've ended up with 7 classes for a simple scenario :
Presentation.WpfClient.View.PersonView
Presentation.WpfClient.ViewModel.PersonViewModel
Presentation.WpfClient.Model.PersonModel
Application.ApplicationServices.PersonService
Domain.Application.Services.PersonService
Domain.Application.Models.Person
Domain.Application.DbEntities.PersonEntity (the reason I've created this is that I can't use mapping for complex domain objects so I just put some data annotations here instead of mapping domain objects)
This feels very very awkward. I'm not sure how I should re-structure it and benefit both domain driven design and MVVM patterns. I'm really stuck and I'm really looking forward for any advises or real life examples for applying both MVVM and domain driven design. I'm also open for any feedback for naming conventions or strategies for minimizing this much of work for simple operations.
I still have two concrete questions though :
Should I remove models from presentation layer (MVVM models) and use only models from domain layer (DDD models) ? Isn't it violation of MVVM at this point?
Should I merge my entity (database) models with domain models? Isn't it violation of DDD?
Update
Decisions I've taken:
Use domain model for MVVM's model (removed PersonModel)
Use external mappings for the same model to the database (removed PersonEntity added PersonMappings). Using persistence model is far more costly than simply mapping it. See : http://enterprisecraftsmanship.com/2016/04/05/having-the-domain-model-separate-from-the-persistence-model/ from Vladimir's answer.
Finally it looks like this:
Presentation.WpfClient.View.PersonView
Presentation.WpfClient.ViewModel.PersonViewModel
Application.ApplicationServices.PersonService (crud with some application related logic)
Application.ApplicationServices.Mappings (I have the repository abstractions and mappings here)
Domain.Application.People.Person (person object in it's bounded context, object is smart enough to handle domain logic)
This is way too many classes for a single concept.
Should I remove models from presentation layer (MVVM models) and use only models from domain layer (DDD models) ? Isn't it violation of MVVM at this point?
Yes, this is a preferable solution is many cases, especially if you don't use a communication mechanism like WCF. No violation here as MVVM doesn't impose a particular implementation of the model part.
Should I merge my entity (database) models with domain models? Isn't it violation of DDD?
Also, yes. Separating an entity into two (domain entity and "persistence" entity) usually leads to over-complication and anemic domain model. Here's more on this: Having the domain model separated from the persistence model.
Overall, I recommend you to look at this example. This looks like exactly what you need: a fully-fledged application written in WPF with the use of MVVM and DDD.
DDD is most beneficial in handling comlex business problem. In real project, it should be used in more flexible way deponding on how complex the domain is. I would like to use a multiple-strategies approach to adopt it.
For most of case which mostly is about CRUD data with few change in data structure. I would only use:
Presentation Layer : ViewModel
Service Layer: ViewModel<->Domin object
Domain Layer: Domain object same as Entity
For more complex Domain, where there are many important and reusable business logic on Domain object, then I would add core service (like your Domain.Application.Services.PersonService).
If there are also complex business logic needed for process data for easier mapping data between Presentation Layer and Domin Layer. I would add another model in Service Layer, similar as your Presentation.WpfClient.Model.PersonModel.
So basically the architecture you have righ now is ready for handling the case that your Person domain is very complex in your project. But I can't see it so far from your description.
To answer your questions:
You can remove the Presentation.WpfClient.Model.PersonModel in your MVVM models. It doesn't violate MVVM because in this case, your Domain Object is the Model, And you have Presentation.WpfClient.ViewModel.PersonViewModel as ViewModel.
You can merge Entity with Domain object if your Person domain doesn't have complex business logic.
For your first question, using the domain layer models as MVVM models is not a violation of MVVM (See definition of Model here)
For more elaboration on the subject and an answer to your second question see this: Entities VS Domain Models VS View Models
Related
WPF Application using MVVM design
I'm using EF as my ORM exposed via a service layer to my front end.
Reading around everything I've read says I should implement automapper in the service layer, but I don't see how that makes sense here.
The ViewModel is the one I wish to differ from the data entities surely so I should do the mapping in the ViewModel?
Or should I be doing it twice (seems overkill) ie map the EF entities to DTOs for the Service to pass to the Front End which then converts them into the ViewModel objects.
I wouldn't bother with DTOs or Automapper in an MVVM client application in the first place. Just use EF types as Model types, and build ViewModels that compose Model types together.
But I wouldn't be concerned at all about coupling between the ViewModel and and Model. The ViewModel is already a later of indirection between the View and the Model/Service layer, and you're almost always deploying all the parts together. It's generally acceptable for a ViewModel to include Model types directly.
But if you do want to put Automapper somewhere, it belongs between the EF types and the Model types, just like it would be if your Services layer was a REST service and you wanted to use DTOs instead of EF Entity types. In that case you may want to change the REST service without redeploying the application, and so having DTOs allow you to make database/EF changes without changing the data that the client receives.
I have been trying to learn more about Uncle Bob's Clean Architecture from blogs, article, and videos.
If I were to use a database in this architecture, then what should the UI (as a framework such as web or form) know about the database? Or more generally, how should data flow between two or more pieces/parts that are in the same layer?
For example, The UI would talk to my adapter(s)/gateway(s) to interact with the business entities. To Read/Write, I can see that the UI could call whatever class/classes that can access the database and pass in the adapter(s)/gateway(s), so that it can interact with the business entities.
public class SomeUI
{
public static void Main(string[] args)
{
SomeAdapter adapter = new SomeAdapter();
SomeDataAccess db = new SomeDataAccess();
db.Save(adapter);
}
}
public class SomeDataAccess
{
public void Save(SomeAdapter adapter)
{
//Interact with database
}
}
public class SomeAdapter
{
//properties
}
Many of the articles barely vary off of this one (https://subvisual.co/blog/posts/20-clean-architecture). I have not found a good article that covers how pieces that are in the same layer should work with each other. So, articles referring to that would be an acceptable answer.
This does not appear to violate the Dependency Rule, but it feels like I am not doing something right, since I am making a dependency between my UI and database. I believe that I may be over-thinking the concept, and I believe it may be from hammering away at learning the 3-tier architecture (UI -> BLL -> DAL).
You ask:
If I were to use a database in this architecture, then what should the UI (as a framework such as web or form) know about the database? Or more generally, how should data flow between two or more pieces/parts that are in the same layer?
There is no such term as UI component in Clean Architecture. In Clean Architecture terms, the UI would be the presentation layer or the delivery mechanism, decomposed in the following components:
The view model generator (or the presenter to use Uncle Bob's terms), which is in charge of encapsulating the business rules for the UI. This should access the business model in order to generate the view model from it. The business model is passed to the presenter's method inside the interactor response object by its caller, the interactor.
The view model which holds data for the view and passed to the view indirectly via for example an event.
The dumb view which is now decoupled from the domain model and all layers, displays data of the view model.
Decomposing that this way insures better testability, better SRP and more decoupling with application, domain and infrastructure layers.
So your presentation layer should know absolutely nothing about the infrastructure layer.
Maybe you got confused by examples using some kind of Web Form component/library? This kind of component propose interconnected functionalities each in relation with several architecture layers: domain, application and presentation… So Web Form components are particularly delicate to adapt satisfactory in a Clean Architecture. Due to this inflexibility, I'm still struggling figuring out what is the best way to integrate a Web Form component in my Clean Architecture implementations...
Finally, to make it clear, you said:
For example, The UI would talk to my adapter(s)/gateway(s) to interact with the business entities. To Read/Write, I can see that the UI could call whatever class/classes that can access the database and pass in the adapter(s)/gateway(s), so that it can interact with the business entities.
It's not UI's responsibility to interact with your entities but as its name suggest it's interactor's responsibility (interactor = use case). Interactor are meant to encapsulate application business rules, they represent the application layer. They can CRUD your entities via the Entity Gateway which is your adapter to the infrastructure layer which could be an ORM, a REST API or whatever...
Edit #1:
Since a picture worth a thousand words here is Uncle Bob's UML class diagram representing the structure (and the data flow between the concerned components) of the Clean Architecture:
Edit #2:
It seems to me that your representation of the flow of control in a Clean Architecture is kind of reversed. Taking as reference the above diagram and Uncle Bob's analogy:
If you don't want your code to be dependent of a thing, make this thing a plugin.
(Said otherwise, make that thing the client of your code that you want independent from it.)
In a Clean Architecture, you want the presentation layer, or more contextually, the delivery mechanism (Controller + Presenter + ViewModel + View) to be a plugin of your business layer (which is composed of components on the right side of the communication channel boundary).
I have been doing more research into other examples of clean architecture.
(source).
From the diagram above, it looks like App (business entities and use cases) talks back and forth with Delivery (Externals: UI). The Delivery is used to talk to External (Externals: DAL).
Delivery is where you implement the delivery mechanism of your application itself. Delivery is where your app is integrated with external data sources and shown to the user. This means in simplest terms the UI, but it also means creating concrete versions of the external objects, such as data jacks, and also calling the actions of the app itself.
-Retro Mocha
So, that leads me to believe that yes the code example at the top is valid, but I am still open to hear if anyone else has more to provide on the answer.
In my WPF application I'm using the MVVM pattern and the repository pattern. I have a repository that provides a list of aircraft objects. From a user perspective, I want to display these aircrafts in several different ways (e.g. on a map, in lists, in textual form, etc..), and also allow the user to filter the objects. To achieve this I have built views and viewmodels for each of the different ways of representing the data.
My problem now is that I'm not sure what the best practice is for making the list of aircraft objects available for all the different viewmodels. Here's some of the alternatives I've considered:
1.Inject the repository into each viewmodel, and then get all the objects from the repo.
2.Inject the repository into a MainViewModel that retrieves all the objects from the repo, and then inject the object collection into all other viewmodels that needs it.
So in sum: I have a set of viewmodels that all make use of the same collection of model objects. What is the best practice for sharing this collection between the viewmodels when using the repository pattern?
In a WPF application I'll typically create service objects which encapsulate repositories which in turn contain a session object each, but I use dependency injection to inject the actual implementations. One of the major benefits of doing this is that everything can be easily mocked for unit testing, but another one is that you now have total control over the scope of these objects. If you're pulling your data from a database then you'll have different strategies for session management depending on whether you're developing a windows app vs a web site (say) and in enterprise applications this requirement will change even within the same code base. There's a good article on Germán Schuager's blog discussing the pros and cons of various session management strategies but for WPF applications using one session per form seems to be a good one. If you're using something like Ninject then you simply scope your ISession object to the view model for your top-level "page" and then all the objects within the logical hierarchy for that page can create their own repositories without needing to know about each other. Since they're all sharing the same session object they're also sharing the same entities in the repository cache and thus the model is now being shared by multiple view models. The only thing that remains is to add property notification to the entity classes in your domain layer so that changes made by one view model propagate through to the others, but that's a topic for another post (hint: your DB layer should provide some mechanism for injecting your own wrapper proxies to add property change notification automatically using things like Castle DynamicProxy etc).
The general rule of thumb is that EACH view should have it's own ViewModel, you can reuse ViewModels via inheritance or adding properties but again a view "DESERVES" it's own ViewModel.
You can have your views leverage interfaces, your viewmodels can implement interfaces.
We have a complex composite financial application where we needed to share various model objects across viewmodels, So we took an approach: we created a high level Model Object which holds collections of other model entities. This high level model is returned/populated in servicelayer. We share this high level model object in many viewmodels and with many PRISM modules using event aggregator .
So in your case you may have a AircarftsData Model, which can maintain collection of aircrafts. you can populate this model using your repository. Then You can pass this AircarftsData in various viewmodels.
We have shipped our application in production with this design and faced no issues as such. One thing you might want to be careful of Memory leakage of this Model object. If somehow any child object of this Model is leaked in memory then whole high level model may remain in memory.
Your data should be located in some place... (Service, Database, File, Etc...).
In that case, you should have a class which manage yours client requests:
GetAllAircrafts, Create, Update, Etc...
This class somtimes called XXXService.
I think that this is where you should expose your collection of models.
This service can be injected to the view models and share the collection of models through a get property (Maybe as a read only collection...?)
Good luck !
I am Java programmer who tries to investigate CakePHP - currently I have problem with application structure/design. I could not understand where to put core logic of application.
When I am developing in JavaEE, common approach looks like following:
Model classes are simple beans which represent data entities (products, people etc) - mostly like data structures with getters/setters;
Controller classes are simple enough classes which aggregate necessary data and inject them into dedicated View template which is then sent to user;
DAO (DataAccessObject) or Repository classes are ones which can load and store entities into database;
Service classes are usually singletons which contains certain business-logic methods - these are called by controllers, by other services or by scheduled actions, on the other hand they themselves call DAO / Repository methods to fetch or modify data.
For example if I have entities Person, Product and Order, when user selects some product and clicks "put it into my cart/basket" new Order for this Person should be created and this Product should be added to this Order (we may check that Person is not bad debtor and that Product is present at store etc.) - all this work is performed in methods of OrderService called by some controller.
Usually some kind of IOC (Inversion of Control) is used so that all services and controllers have links to necessary services etc.
Now I am slightly bewildered about how this all is done in CakePHP. Where should I put this business-logic etc. ?
In CakePHP the model layer is made up from collection of active record instances, called AppModel. They combine the storage-related logic (that you would usually put in DAOs and/or Repositories) with business logic (what usually went into your "models").
Any other domain related logic (from your Service) becomes part of controller.
If you want to know, how you are supposed to implement domain business logic in CakePHP, just look up articles which praise active record pattern.
Personal opinion
CakePHP and CodeIgniter are two of the worst frameworks in PHP. They are filled with bad practices.
Actually, if you were doing correct-ish MVC, then model layer would contain all of the business logic and everything that is related to it. Model layer is made of DAOs, Repositories, Domain Objects (what you call "models") and Services.
While your descriptions of Java-based code indicates, that you are kinda moving in that direction, CakePHP is not even remotely close to it.
Then again, it might be that my understanding of MVC is just wrong.
The controllers should only contain logic relevant for the whole thing being a web application. Your business logic belongs into the models. I think it is one of the basic mistakes that you find in many cakePHP applications, that to much logic is put into the controllers, which actually belongs into the models.
In CakePHP. the "M" is just a bunch of Data Models instead of Domain Models.
In my opinion. CakePHP is made for RAD development. It is not a good fit for enterprise applications.
My opinion though.
I've been wondering what lives in the community for quite some time now;
I'm talking about big, business-oriented WPF/Silverlight enterprise applications.
Theoretically, there are different models at play:
Data Model (typically, linked to your db tables, Edmx/NHibarnate/.. mapped entities)
Business Model (classes containing actual business logic)
Transfer Model (classes (dto's) exposed to the outside world/client)
View Model (classes to which your actual views bind)
It's crystal clear that this separation has it's obvious advantages;
But does it work in real life? Is it a maintenance nightmare?
So what do you do?
In practice, do you use different class models for all of these models?
I've seen a lot of variations on this, for instance:
Data Model = Business Model: Data Model implemented code first (as POCO's), and used as business model with business logic on it as well
Business Model = Transfer Model = View Model: the business model is exposed as such to the client; No mapping to DTO's, .. takes place. The view binds directly to this Business Model
Silverlight RIA Services, out of the box, with Data Model exposed: Data Model = Business Model = Transfer Model. And sometimes even Transfer Model = View Model.
..
I know that the "it depends" answer is in place here;
But on what does it depend then;
Which approaches have you used, and how do you look back on it?
Thanks for sharing,
Regards,
Koen
Good question. I've never coded anything truly enterprisey so my experience is limited but I'll kick off.
My current (WPF/WCF) project uses Data Model = Business Model = Transfer Model = View Model!
There is no DB backend, so the "data model" is effectively business objects serialised to XML.
I played with DTO's but rapidly found the housekeeping arduous in the extreme, and the ever present premature optimiser in me disliked the unnecessary copying involved. This may yet come back to bite me (for instance comms serialisation sometimes has different needs than persistence serialisation), but so far it's not been much of a problem.
Both my business objects and view objects required push notification of value changes, so it made sense to implement them using the same method (INotifyPropertyChanged). This has the nice side effect that my business objects can be directly bound to within WPF views, although using MVVM means the ViewModel can easily provide wrappers if needs be.
So far I haven't hit any major snags, and having one set of objects to maintain keeps things nice and simple. I dread to think how big this project would be if I split out all four "models".
I can certainly see the benefits of having separate objects, but to me until it actually becomes a problem it seems wasted effort and complication.
As I said though, this is all fairly small scale, designed to run on a few 10's of PCs. I'd be interested to read other responses from genuine enterprise developers.
The seperation isnt a nightmare at all, infact since using MVVM as a design pattern I hugely support its use. I recently was part of a team where I wrote the UI component of a rather large product using MVVM which interacted with a server application that handled all the database calls etc and can honestly say it was one of the best projects I have worked on.
This project had a
Data Model (Basic Classes without support for InotifyPropertyChanged),
View Model (Supports INPC, All business logic for associated view),
View (XAML only),
Transfer Model(Methods to call database only)
I have classed the Transfer model as one thing but in reality it was built as several layers.
I also had a series of ViewModel classes that were wrappers around Model classes to either add additional functionality or to change the way the data was presented. These all supported INPC and were the ones that my UI bound to.
I found the MVVM approach very helpfull and in all honesty it kept the code simple, each view had a corresponding view model which handled the business logic for that view, then there were various underlying classes which would be considered the Model.
I think by seperating out the code like this it keeps things easier to understand, each View Model doesnt have the risk of being cluttered because it only contains things related to its view, anything we had that was common between the viewmodels was handled by inheritance to cut down on repeated code.
The benefit of this of course is that the code becomes instantly more maintainable, because the calls to the database was handled in my application by a call to a service it meant that the workings of the service method could be changed, as long as the data returned and the parameters required stay the same the UI never needs to know about this. The same goes for the UI, having the UI with no codebehind means the UI can be adjusted quite easily.
The disadvantage is that sadly some things you just have to do in code behind for whatever reason, unless you really want to stick to MVVM and figure some overcomplicated solution so in some situations it can be hard or impossible to stick to a true MVVM implementation(in our company we considered this to be no code behind).
In conclusion I think that if you make use of inheritance properly, stick to the design pattern and enforce coding standards this approach works very well, if you start to deviate however things start to get messy.
Several layers doesn't lead to maintenance nightmare, moreover the less layers you have - the easier to maintain them. And I'll try to explain why.
1) Transfer Model shouldn't be the same as Data Model
For example, you have the following entity in your ADO.Net Entity Data Model:
Customer
{
int Id
Region Region
EntitySet<Order> Orders
}
And you want to return it from a WCF service, so you write the code like this:
dc.Customers.Include("Region").Include("Orders").FirstOrDefault();
And there is the problem: how consumers of the service will be assured that the Region and Orders properties are not null or empty? And if the Order entity has a collection of OrderDetail entities, will they be serialized too? And sometimes you can forget to switch off lazy loading and the entire object graph will be serialized.
And some other situations:
you need to combine two entities and return them as a single object.
you want to return only a part of an entity, for example all information from a File table except the column FileContent of binary array type.
you want to add or remove some columns from a table but you don't want to expose the new data to existing applications.
So I think you are convinced that auto generated entities are not suitable for web services.
That's why we should create a transfer model like this:
class CustomerModel
{
public int Id { get; set; }
public string Country { get; set; }
public List<OrderModel> Orders { get; set; }
}
And you can freely change tables in the database without affecting existing consumers of the web service, as well as you can change service models without making changes in the database.
To make the process of models transformation easier, you can use the AutoMapper library.
2) It is recommended that View Model shouldn't be the same as Transfer Model
Although you can bind a transfer model object directly to a view, it will be only a "OneTime" relation: changes of a model will not be reflected in a view and vice versa.
A view model class in most cases adds the following features to a model class:
Notification about property changes using the INotifyPropertyChanged interface
Notification about collection changes using the ObservableCollection class
Validation of properties
Reaction to events of the view (using commands or the combination of data binding and property setters)
Conversion of properties, similar to {Binding Converter...}, but on the side of view model
And again, sometimes you will need to combine several models to display them in a single view. And it would be better not to be dependent on service objects but rather define own properties so that if the structure of the model is changed the view model will be the same.
I allways use the above described 3 layers for building applications and it works fine, so I recommend everyone to use the same approach.
We use an approach similar to what Purplegoldfish posted with a few extra layers. Our application communicates primarily with web services so our data objects are not bound to any specific database. This means that database schema changes do not necessarily have to affect the UI.
We have a user interface layer comprising of the following sub-layers:
Data Models: This includes plain data objects that support change notification. These are data models used exclusively on the UI so we have the flexibility of designing these to suit the needs of the UI. Or course some of these objects are not plain as they contain logic that manipulate their state. Also, because we use a lot of data grids, each data model is responsible for providing its list of properties that can be bound to a grid.
Views: Our XAML definitions of the views. To accommodate for some complex requirements we had to resort to code behind in certain cases as sticking to a XAML only approach was too tedious.
ViewModels: This is where we define business logic for our views. These guys also have access to interfaces that are implemented by entities in our data access layer described below.
Module Presenter: This is typically a class that is responsible for initializing a module. Its task also includes registering the views and other entities associated with this module.
Then we have a Data Access layer which contains the following:
Transfer Objects: These are usually data entities exposed by the webservices. Most of these are autogenerated.
Data Adapters such as WCF client proxies and proxies to any other remote data source: These proxies typically implement one or more interfaces exposed to the ViewModels and are responsible for making all calls to the remote data source asynchronously, translating all responses to UI equivalent data models as required. In some cases we use AutoMapper for translation but all of this is done exclusively in this layer.
Our layering approach is a little complex so is the application. It has to deal with different types of data sources including webservices, direct data base access and other types of data sources such as OGC webservices.