Almost every MVVM example I've come across has both the Model and ViewModel implementing INotifyPropertyChanged.
Other sources (ones which focus on domain modeling) seem to suggest that Models should be incredibly plain (something to do with separation of concerns?) with essentially no references to anything. Unfortunately, those sources don't use MVVM.
I'm trying to reconcile the two.
-I'm relatively new to programming and completely new to design patterns and the like so please try to go easy on me.
Edit: Let me rephrase my question. Given that the answer to the above seems to be "sometimes one and sometimes the other," WHEN should you do one and and when should you do the other. Also, how would each be implemented?
(That isn't answered in the other post. It just has them arguing with each other).
Models have to implement INotifyPropertyChanged to inform the view model that they have changed. If the model doesn't implement INotifyPropertyChanged (or another equivalent interface), then the view model is stuck using polling or other similarly inefficient methods to detect changes in the model state.
This MSDN page may be useful reading to further understand the roles of the various components that make up the MVVM pattern.
I have no idea if this is a best practice, but I have my ViewModel set up so that it is the only active entity. The Model is only directly changed when created by reading from a database (and then loaded into a ViewModel), or before saving to database (extracting from ViewModel, modifying Model properties that only matter to the database, like foreign keys).
If for some reason you desire being able to have multiple ViewModels connected to the same Model or have a need to change a Model from under a ViewModel, then you'd have a good reason to implement INotifyPropertyChanged on the Model.
I'm a relative amateur, so take what I say with a grain of salt. But this is what I've been gathering, and enforcing this separation has, I think, made my code cleaner and easier to understand and debug. So for my own projects, I'm going to try avoiding implementing INotifyPropertyChanged on my Models if I can avoid it.
Related
Recently, I'm learning MVVM design pattern!
In my way, I write the database function in the model, and let the viewmodel to call the database function in the model, then the viewmodel get database data and set to the viewmodel notfiypropertychanged. This is my way that I currently use!
There are some question confused me about model,
I read many article that told me model just a contain data and not more business logic, and here is my question, if model just a data container and I need to let my viewmodel to call the database then get data and set to the model, I think this way is very strange and the viewmodel code is make heavy in it. Is anybody have another method to do it? Thank you!
The Model:
"model just a contain data and not more business logic"
A model is a class which describe an entity within your domain logic. What is a domain? Starbuck's domain is coffee beverages and employees (among others), Ford's domain is cars, assembly lines, and employees. NYTimes's domain is articles, issues, supply routes, subscribers, etc, etc.
A model contains data AND logic. You can have multiple models to describe your domain.
You can place your data calls in your model, but it's more common to have helper classes, data access layers (DAL), keeping all your database calls in one place, and not spread them around.
The ViewModel:
The viewmodel sits between your domain model and your view. It's a class which expose properties of the model and represents the state of the view. The viewmodel may only exposes a subset of all the properties in the model that the UI needs to display, but it could also add properties of its own, for example; is the user in edit mode? have changes been made that needs saving? etc. The selling point with MVVM is that the UI binds to these properties on the viewmodel which is a mechanism to keep the UI up to date with changes, and this extra layer of abstraction conveniently decouples the view to any logic in your model making it more robust for code changes in either direction and testable. There's more to say on this topic, but I'll leave it to you to read on up it.
There are many good resources and info on the MVVM pattern; from blogs Model-View-ViewModel (MVVM) Explained, to Microsoft The MVVM Pattern and here on SO.
If you prefer videos, Pluralsight have good video tutorials on the MVVM pattern
Practical MVVM and WPF MVVM In Depths. They have a free 30-day trial.
"just a data container"
Such classes that only hold data to be passed around are often called Data Transfer Objects (DTO). It's common to keep them small and return collections of these from fetch database data method calls.
I've done some research in this and also found it quite confusing. First thing I like to point out is that code-patterns are abstract. Meaning that you have loads of different way to implement it / tweak it.
What most people have told me is that in "real-life" applications you usually have layers of services.
One of these services is where you get data from a database.
The Model job (in my opinion) is to give the developer knowledge about the database data and structure. One Model to one database-tabel.
It also helps in checking if the data is correct (Format checking, Type of data and so on) before sending it to the DB.
There isn't a clear answer on how to use model. I've seen loads of different implementations, and they all have been implemented for specific tasks.
Yes it might come out that some ViewModels become heavy-loaded with code and functions to perform, but then it might not be because of the code-pattern. It can be because of poor structure in code. At my work right now I've found a ViewModel that contained over 3000 lines of code (Which is WAY to much). This could easily be seperated into atleast 3 different ViewModels and Views, but as I said, poor code structure causes problem.
One thing I recommend for you to read up on is
IoC - Inversion of Control
DoP - Dependency inversion principle
DI - Dependency Injection
Hope this helps in someway explaining your question.
I read many article that told me model just a contain data and not more business logic
A model may be represented by a service, a database access layer or a business layer so there is nothing wrong with your current approach. The business logic does belong to the model.
But sometimes people tend to refer to a "child" type as a model. For example, if your view model exposes a List<Item> property, the Item class may considered to be a model. Perhaps your confusion comes from this.
This kind of classes (Item) typically implement the INotifyPropertyChanged interface to provide change notifications and are effectively rather just "nested" view models that don't contain any business logic though.
Hope that makes sense.
I am trying to understand the MVVM pattern and there is some issue that is not clear. My Xamarin Forms application will have several Views (which is quite obvious). Must everyone of them be binded to different ModelView class? Or maybe there should be only one MV?
Yes and No
Yes -> Basically the idea is that your ViewModel should only be used by one view. If you use a ViewModel to populate an area or whatever then that ViewModel is "reused" each time that view is presented in difference places.
No -> You can use multiple models in a view model. The purpose of the view model is to abstract away the business / data layer (i.e. the model).
However, using more than one model usually indicates that the view is too large. You might want to split it into user controls (which have their own view models).
References:
With MVVM, does each UI window have its own ViewModel?
In MVVM, is every ViewModel coupled to just one Model?
I've always thought of "each View has its own VM" - the model as being a sub-set of a comprehensive design.
I will say from painful experience: do not design models in isolation based solely on the view/UI they support. Without a comprehensive model back end you will have a hard time integrating all of the pieces into a coherent, complete business model that works. Anemic classes, redundant bits, incomplete and wrong validations, same properties-different names, incompatibilities w/ existing code, gobs of hacky glue code, Programmers breaking each other's code, no re-usability, no reuse of existing classes. For us it all came to a head in a 3 month test-fail-fix tilt-a-whirl of embarrassment.
My question is fairly simple and already asked in the title.
Here's the context: I've got a domain with entities and repositories. The result of a query is mapped into DTO and sent to the GUI.
The GUI is implemented with WPF and for the mapping, I need classes that implement INotifyPropertyChanged.
My first idea is to have DTO that implement this interface because I foresee a lot of work to map again my DTO into items that implement INotifyPropertyChanged.
Is it a good practice? Has it pitfalls I haven't seen? What is the "official" good practice for this situation?
DTOs are supposed to be very simple, lightweight, data transfer objects. Because of this, I wouldn't implement anything on them other than their data. Also, I believe if serializing the class to/from a WCF server, the properties need to all be public, so you can't make things like the Id read-only
I would create Model classes that implement INotifyPropertyChanged and IDataErrorInfo for property changed notification and validation purposes, and have them accept a DTO in the Constructor. Using something like AutoMapper will make mapping a DTO to a Model pretty simple
Even though it is a DTO, there isn't much reason to not implement INPC.
INPC is in every .net impl that I can think of, so you aren't taking extra dependencies that you might want to avoid at both ends of a connection (usually why you would use a DTO)
Using NotifyPropertyWeaver you can do it with very little code.
Just because your DTO implements that interface, I don't think it makes it any less of a DTO.
The wikipedia definition of DTO says that there is no behavior in a DTO. You have now added behavior in the form of the PropertyChanged event, but given that the whole reason to use a DTO is for remote objects (http://msdn.microsoft.com/en-us/library/ms978717.aspx) I am still convinced it is OK.
Fowler states that the point of a DTO is to reduce the number of parameters in a remote call. (http://martinfowler.com/eaaCatalog/dataTransferObject.html) This doesn't even say that you can't add behavior.
INPC away!
The best practice with WPF would be to use the MVVM pattern (Model-View-ViewModel).
In this instance, your DTO is the Model. You should not pass the Model directly to the View but instead wrap it into a ViewModel that can in turn implement the notification mechanisms.
This way the Model is pure data and doesn't need to worry about what is using it.
Also, there are several frameworks you can use to simplify the mapping work (e.g. Automapper).
I'm trying to create an silverlight application using the MVVM design pattern. It's a kind of bank application.
I've watched a lot of tutorials on MVVM but something makes me real confused.
I have about fiwe usercontrols representing my views "TransactionsView", "AccountView" etc and a bunch of models "UserProfile" - containing user password, username and a list of UserAccounts, "UserAccounts" - containing name, balance and a list of AccountTransactions, "AccountTransactions" - containing a name, and ammount.
Should i create one modelview which contains my userprofile or should i create a viewmodel for every view i have? I'm a doing right so far? Or have i got it completley wrong?
Thanks
In MVVM, ViewModels are usually 1-to-1 with Views. There isn't a parity between number of ViewModel and Models, though.
View: UI
ViewModel: Handles changes to view state, forwarding them to the model if/when appropriate. Sends notifications from the underlying program back to the user. It may also do initial UI validation.
Model: Actual "guts" of the application. Algorithms, data storage, system calls etc go here. I put program flow here. I've seen other people put it in the ViewModel. That part is up to you to figure out.
A View always needs a ViewModel, hence 1-to-1 (it could have sub-models, but I'll leave that up to you to decide on/deal with. I'd start off with 1-to-1).
A ViewModel usually needs Models to actually "do work", but how many classes/instances is up to each app/problem you're trying to solve.
From what you explain you are going in the right direction. What viewmodel you create is a bit up to you, MVVM is not set in stone - its just a method. What I found through trial and ERROR was that it was smart to understand it well before digging myself in too deep.
I read many articles that didn't explain MVVM in a way I could understand. Finally I found a couple of articles by Jeremiah Morrill that were straight to the point and easy to understand: Article 1 and article 2.
One ViewModel per view is recommended for MVVM.
There's no real hard and fast rules but essentially there's normally one ViewModel per View. You can get into a situation where you want to share a view model across multiple views but it's rare.
Imagine what you want to see on the screen and each state the screen / controls on the screen might be in, everything that is needed on that particular screen (view) should have a corresponding property in your ViewModel that you can bind the View to. So, this translate to a single ViewModel for a particular View. The ViewModel itself can be tied into one or more model(s) in the back. At least that's how I understand it.
After fighting for a while with maintaining model-viewmodel relationships (eg. creating vm instances for each instance of model) I've got some ideas that might be quite controversial, but I'm curious of opinions.
What if VM class was made to maintain a static list of containers for model instances.
Those could(or even should) be weak references so whenever model class instance is out of scope its viewmodel is automatically disposed. Another option would be to reuse vm instances.
Another idea that would work well with the first one might be creating an implicit cast operator from model to viewmodel class. I would always get the same instance of vm whenever casting from model instance.
What do you think about it ? Is this a hard violation of rules and MVVM pattern?
//edit
I should probably provide also what was the motive behind this: in my app I have multiple places where I use one of my model classes and need corresponding vm references. In every such place I need to observe a collection and react to changes -creating or removing vm instances. This is basically the same code that is repeated in many places => I thought of creating only one place to do that (implicit cast is just a candy it's not required to solve the real problem). Or maybe instead of static lists I should create a manager that would handle view model instance creation for all my classes?
First of all, I am not to sure whether your ideas violate the MVVM pattern. In my opinion it is not that important to fullfill patterns in every case. A pattern in my eyes suggests strategies to solve problems. In most cases it is not worth following a pattern 100% by all means. If there is a pragmatic solution you should rather use this one. Of course this should be solutions that lead you to your aims, e.g. unit testable, separation of UI and applicatiopn logic and so on.
Anyway, when I was reading your article the first time I thought implementing a cast operator is good idea. But if I am not wrong, you need to reference the view model in your model. I always try to avoid that to maximize re-use opportunities. But I think having such a reference does not violate the pattern. Maybe someone else can tell more about that.
For me your manager idea is the best way. I use a similar way to create view models. It depends on how many view model you need to create, but you should rather create new view models than re-using existing ones. Somewhere I read that view models should be some kind of state machine to the view. Following this idea, you never know in what state the view model is when you re-use it. So the preferred way is to create a new view model.
Just some thoughts! Maybe there are some other ideas...