I'm new to MVVM. Currently I'm developing a WPF project in C# that will have a SQl Server backend and I'll be using a standard WCF service to communicate with it. All the tutorials I've seen on MVVM thus far always seem to use some static data repository such as an xml file for their backend. I haven't yet seen implemenations using a database and a data access layer, so I'm confused as to where my WCF service fits in. The service has all data objects defined in it, so does the service itself then become the model? In addition, how do I go about including the service in the ViewModel so that the designer doesn't throw an error stating that it cannot create an instance f the service class? Any help here would be greatly appreciated as I find it strange that so many tutorials on this subject omit the most prctical implementation for a line-of-business application.
PS I would like to steer clear of WCF RIA services and Silverlight as the Silverlight's lack of support for commands makes the book I'm following (Pro WPF and Silverlight MVVM Effective Application Development with Model-View-ViewModel) difficult to understand.
OK, I'll try to get you up to speed ...
First, I do recognize the question about the model and the object model exposed with WCF. Are they the same? Well, I would like to make that assumption currently, for sake of simplicity. So then we do not need the model part of MVVM on the client side? Depends ...
The ViewModel is in the driving seat. We let it create the client proxy to your WCF service. The objects used in the request and returned as result makes your model. Anything you want to cache on the client side or is not directly bindable with the UI will be put in properties in your model container class. Generate bindable properties from these model properties for consumption in your UI. Every else will just be direct properties in your view model.
About WCF and the data access layer, there are a few important thing to recognize. First of all, you will need to have a seperation between you logical (information) model and your physical (database) model. One reason is to abstract your database technology away from your application. Another to allow small deviations between your application / domain logic and your physical implementation. Make sure your (entity) model classes are generic enough to support changes in your UI without having to modify the complete application stack for every UI change.
It is hard to talk about this matter without a clear example, so to wrap up I would like to invite you to look at http://aviadezra.blogspot.com/2010/10/silverlight-mvvm-odata-wcf-data.html. I know, it IS using WCF data services and SilverLight. Don't be mad at me directly for directing to this sample and give me the thumb down. It is just such a damn good example of what you want to achieve and what to introduce and what to think about setting up such an application. Just replace Silverlight by WPF and Data Services by regular typed data contracts and the rest of the story will help to get your thoughts clear.
Hope it helps you in your quest!
Related
I'm building a WPF application using MVVM pattern, and Caliburn.Micro is the framework choice to boost up the development.
Different from a conventional MVVM-based application, I add a business layer (BL) below the ViewModel (VM) layer to handle logic for specific business cases. VM is left with data binding and simple conversion/presentation logic. Below BL is an extra Data Access layer (DAL) that encapsulates the Data model (DM) underneath built with Entity Framework.
I'm pretty new to both WPF, MVVM and, of course, know almost nothing about Caliburn. I have read plenty of questions and answers about the Caliburn usage and now trying to use what I've learnt so far in my application.
My questions are:
Does it sound okay with the above layered architecture?
In the application bootstrapper, is it correct that we can register all services that will later be used (like EventAgreggator (EA), WindowManager or extra security and validation services), and also all the concerned VMs? These should be injected into VM instances via constructors or so (supposed I'll be using SimpleContainer). So from any VM that are properly designed and instantiated, we can have these services ready to be used. If I understand correctly, Caliburn and its IoC maintain a kind of global state so that different VMs can use and share it.
Navigation: I know this topic has been discussed so many times. But just to be sure I'm doing the right way: There'd be a ShellViewModel acting as the main window for the whole application with different VMs (or screens) loaded dynamically. Each VM can inherit either Screen or ViewModelBase or NotifyChangedBase. When I'm in, let's say, VM A and want to switch to VM B. I'd from inside VM A send a message (using EA) to the ShellViewModel, saying that I want to change to B. ShellViewModel receives the message and reloads its CurrentViewModel property. What should be a proper data structure to maintain the list of VMs to be loaded? How can stuffs like Conductor or WindowManger come into the place?
Can/Should Caliburn in one way or another support the access to the database (via EF). Or this access should be exposed to VM and/or BL only?
Thanks a lot!
Different from a conventional MVVM-based application, I add a business layer (BL) below the ViewModel (VM) layer
That's the standard case. ViewModels can't/shouldn't contain business logic which is considered to be part of the Model (Model in MVVM is considered a layer, not an object or data structure) in the MVVM. ViewModel is for presentation logic only.
Yes, as long as your Business (Domain) Layer has no dependency on the DAL (no reference to it's assembly). Repository interfaces should be defined in the Business Layer, their implementations in the Data Access Layer.
Yes, Bootstrapper is where you build your object graph (configuration the IoC container).
Registering ViewModels: Depends on IoC framework. Some frameworks let you resolve unregistered types, as long as they are not abstract or interfaces (i.e. Unity). Not sure about Caliburn, haven't used it. If the IoC supports it, you don't need to register them.
One possible way to do it. I prefer navigation services though, works better for passing around parameters to views and windows that are not yet instantiated and you always know there is exactly one objects handling it.
With messages, there could be 0, 1 or many objects listening to it. Up to you
What do you mean with support access to the database? You can use it to inject your repositories and/or services into your ViewModels, other than that there isn't much DB related stuff to it.
I have recently created a pretty robust API built around Entity Framework's DbContext. I am using a lot of metadata programming and taking advantage of the fact that I can get my data with a call like DbContext.Set(typeof(Customer)). Only, in my API I do not know at compile time what type I will be passing to the Set method. This is working very well with EntityFramework and I would like to add another layer abstraction and have it work with both EntityFramework or DataServiceContext. So, I really have two questions.
Firstly, and more specifically, is there a DataServiceContext (i.e. odata/wcf) equivalent to the DbContext.Set(type) method?
Secondly, and more generally, is there a good resource that compares the APIs provided by DbContext with DataServiceContext?
EntityFramework and DataServices client API should not be mixed. Even though they look similar they are not. DbSet represents entity set. I don't think there is a strong contract around entity sets in DataServiceContext. Instead the name of the entity set is passed to methods that need to know this (e.g. look at DataServiceContext.AddObject() or DataServiceContext.CreateQuery() methods) as strings. In some sense it makes it much easier to program the DataServiceContext dynamically. On the other hand you still need to know what is on the other side of the pipe (i.e. the server). As said above WCF Data Services and EntityFramework are different technologies (even though they can work together) and their APIs, though similar, serve different purposes. Therefore comparing them would be like comparing apples to oranges.
The DbContext API in the client side is not the same from DbContext on server side. The main goal is to expose the data and model, which can be done pretty well. I think you may be overengeneering your app, since WCF Data Services can provide enough funcionalities.
Here is a link from Ladislav Mrnka, who is very good at entity framework, he shows how you could expose your robust api with WCF Data Services.
Implement WCF Data Service using the Repository Pattern
I have posted this same question in the msdn forums, but nothing yet ..
http://social.msdn.microsoft.com/Forums/en-US/wcf/thread/60cf36d1-c11a-4d8a-9446-f1d299db1222
I'm working on a project that is an MVC app that will be sourced data via a WCF service that may or may not be getting data via EF, but will definitely be using Stored Procedures..
The MVC app will maintain state in the session, and the entity-tracking portion of this state would preferably function much like the RIA Services DomainContext. Whether or not this context encapsulates saves and changesets is not really all that important, but how entities are loaded into the context and relate to one another (navigation properties) are.
Question 1: Is there such a pattern/solution in existence?
Question 2: Should the MVC and WCF layers share the same DTOs/Entities via a class library? (thereby maintaining state-awareness, navigation properties, etc on both ends of the pipe?)
Question 3: Does using WCF Data Services help solve these problems?
Question 4: Is this all misguided and is there a better approach?
Pretty basic stuff here..
The solution is use a WCF Data Service, and in the client add a Service Reference pointing to it. The client-side proxy will include a proxy and the context classes I was looking for, similar to RIA. If you're accustomed to RIA, there will be some differences and caveats, but by and large it's easy to work through and provides a client-side proxy to your server-side ObjectContext (or whatever repository you expose through the DataService)
I have an architectural question about EF and WCF.
We are developing a three-tier application using Entity Framework (with an Oracle database), and a GUI based on WPF. The GUI communicates with the server through WCF.
Our data model is quite complex (more than a hundred tables), with lots of relations. We are currently using the default EF code generation template, and we are having a lot of trouble with tracking the state of our entities.
The user interfaces on the client are also fairly complex, sometimes an object graph with more than 50 objects are sent down to a single user interface, with several layers of aggregation between the entities. It is an important goal to be able to easily decide in the BLL layer, which of the objects have been modified on the client, and which objects have been newly created.
What would be the clearest approach to manage entities and entity states between the two layers? Self tracking entities? What are the most common pitfalls in this scenario?
Could those who have used STEs in a real production environment tell their experiences?
STEs are supposed to solve this scenario but they are not silver bullet. I have never used them in real project (I don't like them) but I spent some time playing with them. The main pitfalls I found are:
Coupling your data layer with your client application - you must share entity assembly between projects (it also means it is .NET only solution but it should not be a problem in your case)
Large data transfers - you pass 50 entities to clients, client change single entity and you will pass 50 entities back. It will require some fighting with STEs to avoid passing unnecessary data
Unnecessary updates to database - normally when EF works with attached entities it track changes on property level but with STEs it track changes on entity level. So if user modify single property in entity with 100 properties it will generate update with setting all of them. It will require modifying template and adding property level change tracking to avoid this.
Client application should use STEs directly (binding STEs to UI) to get most of its self tracking ability. Otherwise you will have to implement code which will move data from UI back to self tracking entity and modify its state.
They are not proxied = they don't support lazy loading (in case of WCF service it is good behavior)
I described today the way to solve this without STEs. There is also related question about tracking over web services (check #Richard's answer and provided links).
We have developed a layered application with STE's. A user interface layer with ASP.NET and ModelViewPresenter, a business layer, a WCF service layer and the data layer with Entity Framework.
When I first read about STE's the documentation said that they are easier then using custom DTO's. They should be the 'quick and easy way' and that only on really big projects you should use hand written DTO's.
But we've run in a lot of problems using STE's. One of the main problems is that if your entities come from multiple service calls (for example in a master detail view) and so from different contexts you will run into problems when composing the graphs on the server and trying to save them. So our server function still have to check manually which data has changed and then recompose the object graph on the server. A lot has been written about this topic but it's still not easy to fix.
Another problem we ran into was that the STE's wouldn't work without WCF. The change tracking is activated when the entities are serialized. We've originally designed an architecture where WCF could be disabled and the service calls would just be in process (this was a requirement for our unit tests, which would run a lot faster without wcf and be easier to setup). It turned out that STE's are not the right choice for this.
I've also noticed that developers sometimes included a lot of data in their query and just send it to the client instead of really thinking about which data they needed.
After this project we've decided to use custom DTO's with automapper from server to client and use the POCO template in our data layer in a new project.
So since you already state that your project is big I would opt for custom DTO's and service functions that are a specifically created for one goal instead of 'Update(Person person)' functions that send a lot of data
Hope this helps :)
I really like OData (WCF Data Services). In past projects I have coded up so many Web-Services just to allow different ways to read my data.
OData gives great flexibility for the clients to have the data as they need it.
However, in a discussion today, a co-worker pointed out that how we are doing OData is little more than giving the client application a connection to the database.
Here is how we are setting up our WCF Data Service (Note: this is the traditional way)
Create an Entity Framework (E)F Data Model of our database
Publish that model with WCF Data Services
Add Security to the OData feed
(This is where it is better than a direct connection to the SQL Server)
My co-worker (correctly) pointed out that all our clients will be coupled to the database now. (If a table or column is refactored then the clients will have to change too)
EF offers a bit of flexibility on how your data is presented and could be used to hide some minor database changes that don't affect the client apps. But I have found it to be quite limited. (See this post for an example) I have found that the POCO templates (while nice for allowing separation of the model and the entities) also does not offer very much flexibility.
So, the question: What do I tell my co-worker? How do I setup my WCF Data Services so they are using business oriented contracts (like they would be if every read operation used a standard WCF Soap based service)?
Just to be clear, let me ask this a different way. How can I decouple EF from WCF Data Services. I am fine to make up my own contracts and use AutoMapper to convert between them. But I would like to not go directly from EF to OData.
NOTE: I still want to use EF as my ORM. Rolling my own ORM is not really a solution...
If you use your custom classes instead of using classes generated directly by EF you will also change a provide for WCF Data Services. It means you will no more pass EF context as generic parameter to DataService base class. This will be OK if you have read only services but once you expect any data modifications from clients you will have a lot of work to do.
Data services based on EF context supports data modifications. All other data services use reflection provider which is read only by default until you implement IUpdatable on your custom "service context class".
Data services are technology for creating quickly services exposing your data. They are coupled with their context and it is responsibility of the context to provide abstraction. If you want to make quick and easy services you are dependent on features supported by EF mapping. You can make some abstractions in EDMX, you can make projections (DefiningQuery, QueryView) etc. but all these features have some limitations (for example projections are readonly unless you use stored procedures for modifications).
Data services are not the same as providing connection to database. There is one very big difference - connection to database will ensure only access and execution permissions but it will not ensure data security. WCF Data Services offer data security because you can create interceptors which will add filters to queries to retrieve only data the user is allowed to see or check if he is allowed to modify the data. That is the difference you can tell your colleague.
In case of abstraction - do you want a quick easy solution or not? You can inject abstraction layer between service and ORM but you need to implement mentioned method and you have to test it.
Most simple approach:
DO NOT PUBLISH YOUR TABLES ;)
Make a separate schema
Add views to this
Put those views to EF and publish them.
The views are decoupled from the tables and thus can be simplified and refactored separately.
Standard approach, also for reporting.
Apart from achieving more granular data authorisation (based of certain field values etc) OData also allows your data to be accessible via open standards like JSON/Xml over Http using OAuth. This is very useful for the web/mobile applications. Now you could create a web service to expose your data but that will warrant a change every time your client needs change in the data requirements (e.g. extra fields needed) whereas OData allows this via OData queries. In a big enterprise this is also useful for designing security at infrastructure level as it will only allow the text based (http) calls which can be inspected/verified for security threats via network firewalls'.
You have some other options for your OData client. Have a look at Simple.OData.Client, described in this article: http://www.codeproject.com/Articles/686240/reasons-to-consume-OData-feeds-using-Simple-ODa
And in case you are familiar with Simple.Data microORM, there is an OData adapter for it:
https://github.com/simplefx/Simple.OData/wiki
UPDATE. My recommendations go for client choice while your question is about setting up your server side. Then of course they are not what you are asking. I will leave however my answer so you aware of client alternatives.