I am developing an application that uses Entity Framework and WPF with MVVM design pattern. I have a number of entities, but to keep it simple, lets work with just a subset of them. Companies which can contain Contacts, Addresses, and PhoneNumbers; the aforementioned Contacts which contain Addresses, PhoneNumbers and Shipments.
Originally I was loading these entities from the database and leaving the DataContext open to use again later. This presented problems from a concurrency aspect as any of the objects that got updated may very well not be represented correctly in other parts of the program since those parts had not been updated.
I am trying something new. In my MainViewModel (which can easily be accessed from every other ViewModel) I have several ObservableCollections (my generic collection of choice when working with WPF Databound objects) which I intent to temporarily store my entities. Also there are several methods which will take care of adding entities to the context, attaching, detaching, saving changes etc. In my Record Collection displaying ViewModels (for instance the CompaniesViewModel) I have a backgroundworker load the entities, detach them, and store them in the Collections in MainViewModel. Unfortunately, I apparently have to create explicit queries such as:
Dim results = From Comp As Company In RVShipContext.Companies _
.Include("Contacts.PhoneNumbers") _
.Include("Addresses") _
.Include("PhoneNumbers") Select Comp
Mind you, that is not so much of a problem, except when it comes time to load up a Contact and now I have to go back (attach, query, detach) to get the Addresses, and Shipments. Now, I don't have a real problem with that, but some of these graphs are going to be several layers deep. A Shimpent contains one or more Packages contains one or more DiskSets contains several disks.
Is there a way to load large graphs easily when using detached entities?
Is there a way to use LazyLoading effectively when using detached entities?
Is keeping my entities in a central set of collections (accessible to other parts of the program) a good idea or should I abandon it before I spend an inordinate amount of time setting up infrastructure for it?
Any help would be greatly appreciated!
You should abandon your current approach. I don't understand WPF an VMMV but in case of MVP (model-view-presenter) you should use one ObjectContext / DbContext instance per Presenter and keep your entities attached. Check this article - it is about NHibernate but principle is the same. Similar approach should be used with VMMV.
The complexity of your query looks like you are trying to load everything to shared context which is very problematic solution. Especially detaching entities in stateful application like WPF app looks like big overhead and a lot of work to do when attaching changes back to context instead of using attached entities and let the context track changes for you. Also keeping entities attached solves the problem with lazy loading.
Related
I'm implementing a C++/CLI class library that does some low-level device-related stuff and exposes a few managed classes. This library is about to be utilized by a few C# WPF projects.
One of the classes (called CalibrationRecord) consists of a few public properties, and some of them are collections, currently implemented as generic Lists. One of the WPF project has to be able to edit those collections (i.e. implement CRUD operations).
I'm confused whether it would be better to:
A. Implement those collections as ObservableCollections and be able to use them directly from WPF bindings
B. Add another layer in the client app/another DLL and wrap CalibrationRecord in ObservableCalibrationRecord, where collections are ObservableCollections and properties implement INotifyPropertyChanged
I think that B is a "cleaner" solution because this way my class lib has no knowledge of WPF-related interfaces and classes, however, there would be plenty of additional work to implement this layer, and it would be just plain boring boilerplate code, so A seems tempting.
Which solution would you recommend? Or maybe I'm missing some simpler solution?
Personal anecdotes / opinion only here - but I would recommend Option B as well. ObservableCollections in your Model objects can be overkill - the ObservableCollection can raise a lot of notifications that you may not need (as the collection may not be viewed at that time) and seems to blur the business code with your UI code.
One issue I ran into personally while using a similar setup to your Option B, where the data is stored in both a List and in an ObservableCollection, is whether or not you want a copy of the List data in your ObservableCollection or the actual model data object itself. Obviously if you have the actual data in the ObservableCollection, than as the user updates the model object it will be refelected in your List; however, you can run into some design constraints where the Model object needs NotifyPropertyChanged handling, etc. - which can defeat some of the purpose of seperating the two. Otherwise, you have to take the objects in your ObservableCollection and synchronize them back to the List.
I ended up going with the synchronization approach, although that took a bit of extra work when the user was finished with their edits. In the end, the seperation between the two kept the UI editing code delineated from the business operational code / objects, which was worth it.
Well , In MVVM pattern it is good practice that two models should not know or talk with each other. That means you should not create a object of one model in other and then register for event or performing similar things.
But why, what is wrong if two models know each other. I am writing the code and what will happen if both my models know each other.
If you are going to circumvent MVVM in any way you want to have a good reason (otherwise you might as well just write it as traditional code-behind and save some typing) :)
There are several benefits of using MVVM, which are mainly to do with very loose coupling, unit testing and reuse.
The basic principal of loose coupling goes something like this:
A View knows how to display data of a certain shape, but has no idea where the data comes from.
A ViewModel provides a certain shape of data and settings, that views can use, but has no idea who is displaying it.
A Model knows how to hold generic data, but no idea who is consuming it
A Controller (missing from most MVVM explanations) decides what data to fetch, what data to display and where to display it. This is where the decision making belongs.
If you require your ViewModels to know about each other, then you are most likely missing a Controller object that manages your 2 ViewModels and associated Models/Views.
If you can provide more information about why your models are currently linked, I may be able to suggest an alternative.
It's often perfectly appropriate for view models to interoperate with each other. Parent/child relationships in the view involve the parent view model holding a collection of child view models, after all. There are any number of circumstances in which, for instance, the availability of commands in the parent view depend on the state of its children. It's sometimes appropriate to delegate this to a separate controller class, but it's also often not worth the effort to do this.
It really depends on the overall complexity of your application. Sometimes that kind of effort is essential; sometimes it's just gold-plating.
I'm trying to get to grips with MVVM and so I've read a bunch of articles - most focus on the View -> ViewModel relationship and there's general agreement about what's what. The ViewModel -> Model relationship and what constitutes the Model gets less focus and there's disagreement. I'm confused and would like some help. For example, this article describes the Model as a business object whereas this article describes a class which manages business objects. Are either of these right or is it something else?
I think you are on the right track. The "model" is vague in a lot of cases because it is different things to other people, and rightly so.
For me, my business objects that come back from my WCF service I consider my model. Because of this my projects don't have that pretty file structure with the holy trinity of namespaces: *.Models, *.ViewModels, and *.Views. I personally consider objects coming back from business logic or anything of that nature the "model".
Some people tend to lump both the business objects and the business logic together and call that the "Model", but I find that a little confusing because I picture a Model to be sort of more static than I do business logic, but it's semantics.
So when you look at examples of MVVM projects and don't see anything very clearly "Model", it's just because folks treat them differently. Unless an application is very standalone, I would actually be very suspicious of an application with an actual *.Model namespace, to be honest.
The other thing that is great here is that many times you already have an investment in these types of business objects and I think a lot of people see "MVVM" and immediately assume they need to start defining the "M", even though what they already have is perfectly fine.
The confusion between a Model and a ViewModel is pretty common, too. Essentially I know I need a ViewModel if I need a combination of data and behavior. For example, I wouldn't expect INotifyPropertyChanged to be implemented on a Model, but I would a ViewModel.
From the other answers it should be obvious that the relationship between ViewModel and Model is somewhat fuzzy. Be aware that there is nothing stopping you from having ViewModel and Model in the same class, and when your requirements in a particular area are simple enough maybe this is all that you need!
How you structure the separation between ViewModel and Model will very much depend on the needs of the project or software that requires it, how demanding your deadlines are and how much you care about having a well structured and maintainable code base.
Separating ViewModel and Model is simply a way of structuring your code. There are many different ways of structuring your code, even within this pattern! It should be no surprise then that you will hear different approaches preached by different programmers. The main thing is that the separation can help to simplify and make reusable the independent portions of code. When you have cleanly separated business data, business logic and presentation logic you can easily mix, match and reuse your views, logic and data to create new UIs. The separated and simplified code is also often easier to understand, test, debug and maintain.
Obviously not everyone will agree with this answer. I think that is part of the inherent fuzziness of the problem. In general you need to consider and trade-off the advantages versus the costs of having a separation between ViewModel and Model and know that it is not always a simple task to decide what goes in the ViewModel and what goes in the Model. It will probably help to lay down some ground rules that you or your organisation will follow and then evolve your rules as you understand which level of separation best suits your problem domain.
I think it is worth mentioning that I used to use a similar approach to MVVM when programming Windows Forms and the fact the WPF has more direct support for this (in the form of data binding and commands) has really made my day.
There are a lot of different implementations and interpretations.
In my mind, however, the value of the ViewModel comes from coordination.
The Model is representative of business data. It encapsulates scalar information, as opposed to process.
The View is obviously the presentation of the model.
The ViewModel is a coordinator. In my opinion, the job of the view model is to coordinate between the view and the model. It should NOT contain business logic, but in fact interface with business services.
For example, if you have a view that is a list of widgets, and the widgets are grabbed from a service, then I'd posit:
The Model is a List<Widget>
The View is a ListBox bound to the ViewModel property Widgets
The ViewModel exposes the Widgets property. It also has a IWidgetService reference it can call to in order to get those Widgets.
In this case, the view is coordinating with a business object so the view doesn't have to know anything about it. The model should be ignorant of view models, views, and every thing else ... they should exist independent of how they are used. The IWidgetService would get bound to the view model using some source of dependency injection container, either constructor injection with Unity or an import using MEF, etc.
Hope that makes sense ... don't overload your viewmodel. Think of it as a coordinator that understands business objects and the model, but has no knowledge of the view or how business process is performed.
The value added by the model is its decoupling from the ViewModel and the View. Think if you had to construct and maintain business logic in the ViewModel you would have lots of duplicate code.
For instance - if you had a car game with a GearBoxView (a control in the CockpitView), CarViewModel and CarModel - the advantage of abstracting what is in the CarModel from the CarViewModel is that the CarModel can be used in the WorldViewModel and any other ViewModel. The CarModel can have relationships with other Models (GearsModel, WheelModel, etc).
Your question specifically asked about using a Model as a business object or to manage business objects: my answer is it can do both - and should be responsible for both.
Heres an example
public class WorldModel //Is a business object (aka game object)
{
private List<CarModel> _cars;
public List<CarModel> Cars
{
get //Here's some management of other business objects
{
//hits NetworkIO in a multiplayer game
if(_cars == null)
{
_cars = myExternalDataSource.GetCarsInMyGame();
}
return _cars;
}
}
public Level CurrentRaceCourse { get; set; }
public CourseTime TimeElapsed { get; set; }
}
I think of the model as something that contains the smallest units of business entities. The entities in the model are used not only across the view models in my application but even across applications. So one model provides for many applications and, for those applications using MVVM, many view models.
The view model is an arbitrary collection of entities from the model that are brought together to serve whatever the view needs. If a view requires 2 of these and 1 of those, then its view model provisions them from the model. Generally, I have 1 view model per view.
So a model is like a grocery store. A view model is like a shopping cart. And a view is like a household.
Each household has unique requirements. And each household has its own shopping cart that cherry picks what the household needs from the grocery store.
My thoughts
(The "Model")
Have one model. Just data no methods (except if apt for the platform some -simple- getters/setters).
(The "View Model")
To my mind the rationale for a view model is:
(1) to provide a lower-RAM-requirement backup copy of fields so views hidden behind other views can be unloaded and reloaded (to conserve RAM until they reappear from behind views covering them). Obviously this is a general concept that may not be useful or worthwhile to your app.
(2) in apps with more complex data models, it is less work to lay out all application fields in a view model than to create one reduced model corresponding to the fields of each possible data change, and easier to maintain, and often not significantly slower performance wise.
If neither of these apply, use of a view model is inapt in my view.
If view models are apt, have a one to one relationship of view models to views.
It may be important to note/remind/point out that with a lot of UI toolkits if the exact same "String" object is referenced twice (both in the model and the view model) then the memory used by the String object itself is not double, it is only a little more (enough to store an extra reference to the String).
(The "View")
The only code in the view should be (the required) to show/hide/rearrange/populate controls for initial view load and (when user is scrolling or clicking show/hide detail buttons etc) showing/hiding parts of the view, and to pass any more significant events to the "rest" of the code. If any text formatting or drawing or similar is required, the view should call the "rest" of the code to do that dirty work.
(The "View Model" revisited)
If the (...facts of which views are showing and...) the values of view fields are to be persistent ie survive app shutdown/restart, the view model is part of the model :--: otherwise it is not.
(The "View" revisited)
The view ensures that the view model is as synch'ed with the view in terms of field changes as is appropriate, which may be very synched (on each character change in a text field) or for example only upon initial form population or when user clicks some "Go" button or requests app shutdown.
(The "Rest")
App start event: Populate the model from SQL/network/files/whatever. If view model persistent, construct views attached to view models, otherwise create initial view model(s) and create initial views attached to them.
On commit after user transaction or on app shutdown event: Send model to SQL/networkl/files/whatever.
Allow the user to ("effectively") edit the view model through the view (whether you should update the view model on the minutest change of a character in a text field or only when the user clicks some "Go" button depends on the particular app you are writing and what is easiest in the UI toolkit you are using).
On some application event: the event handlers look at the data in the view model (new data from the user), update the model as required, create/delete views and view models as required, flush the model / view models as required (to conserve RAM).
When new view must be shown: Populate each viewmodel from the model just after the viewmodel is created. Then create view attached to view model.
(Related issue: what if any data set primarily for display (not editing) should not be entirely loaded into RAM?)
For sets of objects that should not be entirely held in RAM cause of RAM use considerations, create an abstract interface to access information on the overall count of objects and also to access the objects one at a time.
The interface and its "interface consumer" may have to deal with the number of objects being unknown/estimated and/or changing depending on the API source providing the objects. This interface can be the same for the model and the view model.
(Related issue: what if any data set primarily for editing should not be entirely loaded into RAM?)
Use a custom paging system through a slightly different interface. Support modified bits for fields and deleted nits for objects - these kept in the object. Page unused data out to disk. Use encryption if necessary. When editing of set done, iterate it (loading in pages at a time - one page can be say 100 objects) and write all data or only changes in transaction or batch as appropriate.
(Conceptual significance of MVVM?)
Clean platform-agnostic way to allow and lose data changes in views without corrupting model; and to allow only validated data through to the model which remains as the "master" sanitised version of data.
Crucial to understanding the why is that flows from view model to model are conditional on data validation of user input whereas flows in the opposite direction from model to view model are not.
The separation is achieved by placing code that knows about all three (M/V/VM) into a core object responsible for handling application events including startup and shutdown at a high level. This code necessarily references individual fields as well as objects. If it didn't I don't think easy separation of the other objects can be achieved.
In response to your original question, it is a question of degree of interrelationships of validation rules on update of fields in the model.
Models are flat where they can be but with references to submodels, directly for one-to-one relationships or through arrays or other container objects for one-to-many relationships.
If the complexity of validation rules is such that merely validating a successful user form fill or incoming message against a list of field regular expressions and numeric ranges (and checking any objects against a cached or specially fetched copy of relevant reference objects and/or keys) is enough to guarantee that updates to business objects will be 'with integrity', and the rules are tested by the application as part of the event handler, then the model can just have simple getters and setters.
The application may perhaps (best) do this directly in-line in event handlers where the number of rules is so simple.
In some cases it may be better even to put these simple rules in the setters on the model but this validation overhead is then incurred on database load unless you have extra functions for setting without validate. So for simpler data models I tend to keep the validation in application event handlers to avoid writing these extra setters.
But if the rules are more complex either:
(a) a single method taking a special object that is really a composite of the usual business objects containing data for myriad field changes is written for each complex model change, and the method can succeed or fail depending on validation of the rules - facade pattern;
or
(b) a "dry run" / hypothesis / "trial" copy of the model or a subset of the model is created first, setting one property at a time, and then a validation routine run on the copy. If validation is successful, the changes are assimilated into the main model otherwise the data is discarded and an error raised.
The simple getter/setter approach is in my view preferred when analysing each individual transaction unless the vast majority of updates for your app are complex, in which case one can use the facade pattern throughout.
The other way the model ends up getting more complex than a bunch of fields with (possibly) simple getters/setters is if you start "enhancing" classes' getters/setters (with an O2R mapper tool), or adding extra aspects like calls to transaction monitoring APIs, security APIs (to check permissions, for logging etc), accounting APIs, methods that pre-fetch any related data needed for a get or set, or whatever upon get or set. See "aspect-oriented programming" for an exposition on this area.
I am creating a WPF/MVVM framework which generates the code for the model classes.
I'm planning to have for each database-table/web-service (e.g. "Customers") two model classes:
a singular model class (e.g. "Customer")
and a plural model class (e.g. "Customers")
The singular model class has all of its properties (FirstName, LastName, etc.) plus all of it methods which make sense for a singular instance, e.g. Save(), Delete(), CalculateSalary(), etc.
The plural model class has a collection of singular model objects, plus the same methods since you would want to also perform on a group of singular objects, e.g. Save(), Delete(), CalculateSalary(), but also particular methods such as Sort(), and methods that made it very easy to certain groups, e.g. LoadAllGoldCustomers(), or even LoadWithSql(string sql), etc.
I've done a framework like this before (PHP) and it made for very easy to write and understand code like this:
Customers customers = new Customers("all");
customers.CalculateSalary();
A couple inherited classes (Item and Items) took most of the code out of the individual singular and plural classes for each database table, which made a very clean environment to program in.
However, I have rarely seen other applications do this singular/plural model class split. Instead, there is almost always just one class for each database table, e.g. Customer and this class has any plural methods necessary, e.g. GetCustomers(string sql), etc.
I just noticed in the WPF Model-View-ViewModel Toolkit 0.1 walkthrough, they have you make two models their "Models" directory two classes:
Customer.cs (fields only)
CustomersDataSource.cs (one List Load() method)
Which seems to be a similar concept, just that the "plural" class is called a DataSource.
So now I am about to make another framework based in WPF/MVVM and can decide how I want to structure the model classes. I want the framework to be:
clear and easy to program against from the ViewModel, hence the clear separation of singular and plural model classes, you should just have to instantiate a singular or plural class and call a method on it and you have your data.
fit in well with the MVVM pattern (which I understand means to keep as simple as possible, just have properties and methods that the ViewModel can call, but implement no WPF-specific features such as INotifyProperityChanged)
want my datalayer to sit above any datasource, so if I use LINQ-to-SQL, I still call my own model classes, and if I want to switch to saving in Oracle, I write a lower data adapter layer for my classes to interact with that.
take advantage of LINQ in the best way possible
I would appreciate feedback from those who have developed datalayers for frameworks especially using WPF/MVVM/Composite Application Library and what characteristics you found worked best, or if you have worked with other frameworks such as CSLA, Subsonic, etc. Also, any experience or ideas on how LINQ changes/simplifies building a datalayer structure. Thanks.
Wow. That's a hard question to answer without having a day to sit down and speak with you. But here goes a shortened version anyway.
First, attempting to port a framework or any characteristics of that framework from one language to another seems like it maybe be trying to shove a square peg in a round hole. While I don't doubt that features (e.g. customer and customers) can be ported, I could certainly argue that they shouldn't be ported. Sticking with the customer.CalculateSalary exmaple, you could use .NET and create an extension method for IEnumerable that did the same thing, eliminating the need for that Customers class. I realize you may have other features as well, but that's just an example. Another example is using LINQ to sort IEnumerable.
Second, I have personally found that having the persistence methods (e.g. Save, Delete, etc.) inside of the object doesn't work well in a large system, particularly when dealing with WCF. It seems to work better in these scenarios to use some type of repository later, which seems like it would also play well with your point of switching to Oracle in the middle of development.
I also want to totally disagree with you on the bullet about fitting well into MVVM. To me, the view model is the glue between the model and the view. It is not only likely that the view model would need to know about the view (hence, WPF specific features), but desired that it know about it. ICommand is a critical interface for the view model to know about, and is one of the WPF-y assemblies (WindowsBase, PresentationCore, PresentationFramework, can't remember which). Also, INotifyPropertyChanged is also critical to data binding and should be implemented in all view models, and has nothing to do with WPF (comes from System.ComponentModel i think?).
That's my two cents. Again, it's really difficult to explain this in a short response to your question. I would recommend using the pattern for a while before making a framework for it. Good luck!
Background
WinForms application using NHibernate. Application is in MDI style and each MDI child form opens a new NHibernate session at Load which remains open for the life of the form.
Question
My application is basically an "order management" or "purchasing" system. One particular form uses a lot of "lookup" lists. Like a list of products, a list of vendors, a list of locations, a list of UnitsOfMeasurement, a list of PriceQuotes, etc.
Lots of lists, that all get loaded when the form is constructed.
Problem: I need the lookup lists, but I need the form to be a bit faster to load. The form is taking too long to perform all the lookups. How can I get better performance and keep my lookup lists?
My Thoughts
Can I load the lookup lists once and hold on to them for the life of the application, and periodically check to see if the lists are stale?
Can I load just the text description for the lists, and instead of holding a bunch of IList, IList, etc, I could hold a bunch of IList, and then when I save, perform the Gets against NHibernate to get the real object.
Is there some other approach that I just haven't thought of?
You should definitely cache slowly changing data to improve performance. How often you need to check for stale data depends on the type of data and your business, e.g. units of measure probably doesn't change as frequently as a list of products. You should also provide a method for manually refreshing lists so that the user can refresh them if something appears to be missing.
If you need the business objects in the list in order to perform a database operation, you can call ISession.Lock(obj) to lock the object into the current ISession. One thing to be aware of is that the lock doesn't automatically cascade to child objects: I think there's a mapping setting to do that or you can do it manually.
Are you sending lists of full objects to your UI? I recently worked on an app using DTO's between the data layer and the UI so I'm not sending the full object, just a description and an identifier. That could help you trim out some unneeded data. So basically when the screen loads a service call is made, nhibernate gets all of the objects I want for my list box, then the UI binds to the list. I bound my listbox display member to the description and the value member to the identifier.