When I'm designing multiple views under the MVVM pattern, does each view get its own ViewModel or do they all share the same one? I understand that this is ultimately a flexible decision, but what is the best practice?
My gut tells me to have a ViewModel for each view (i.e. each separate UI window). All of the blog examples of MVVM show a single view but not much beyond that.
Yes, basically the idea is that your viewModel should only be used by one view. If you use a viewModel to populate an area (like in ASP.NET MVC) then that viewModel is "reused" each time that view is presented in difference places.
This article is a discussion by Josh Smith of the MVVM pattern. Then, Ward Bell discusses in this article what he thinks Josh left out, while maintaining that Josh's work is still very strong.
Ward does an excellent job of laying out the complexities of this pattern and showing the tension that exists. Here is his take on the tension:
In my experience there is a “dialog” between View and ViewModel design. The VM exists to serve a view even as it strives for independence from any particular concrete view. A VM is useless if there is no view that will work with it; clearly the VM developer must
heed the imprecations of the View developer.
On the other hand, in business applications the application’s imperatives – what the view must do to satisfy business requirements – are the province of the programmer and are > best articulated through the capabilities of the ViewModel.
Therein lies the necessary tension between View and ViewModel design. As a developer my allegiance is with the ViewModel (“the application should do something worthwhile”) but it would be silly to defend that allegiance at the expense of the View (“a good UX is essential to making an application easy to learn and to use”).
To me thats a subjective statement- text book, I would say definately a 1-1 pairing - and there is certainly nothing wrong w/ being proactive and setting the paradigm by having a 1-1. However, if you have multiple views each representing a slice of the same data you dont necessarily have to have 1 per- you could reuse the same one across multiple views until you had a deviation. The purpose of the view model being a bridge between the ui and the business layer...if the business functions are the same you are either going to have a common view model interface or base and cast down or you are going to be replicating the same logic multiple times. If the view is the only thing that changes I see no issue in reusing the same view model until there is deviation.
In wpf, you should be binding to values on the model so unless your view model has a reference to your concrete view that shouldnt be an issue. Even it does, you could abstract out to a view contract (interface) and change the property to be that type instead.
Yup, each view should have its on ViewModel.
I don't know WPF inside out, but generally the ViewModel mediates between a UI component and a business logic component. In other words: It's specific to a view/model pair - this is the only reason for this component to exist...
HTH.
Related
While adding extra functionality to the main view I have in my application, I've realized that the amount of code will soon become a problem (currently around 600 lines of code in my viewmodel and I still have plenty to add).
I've been looking around for articles on how to split/design your view into smaller components, but I haven't found a satisfying solution. One person suggested using child viewmodels but that presented with other problems (dependency between the viewmodels).
I've thought of using user controls, but there is nothing on the View that I use on other Views so it kind of defeats the purpose of user controls.
What is a correct approach in this situation?
Thanks,
Adrian
If you want to split a view into component parts, then you need to do view composition. If you are building an MVVM app, then you should really be using an MVVM framework. Something like Caliburn.Micro makes view composition incredibly easy.
There doesn't necessarily have to be dependencies between the view models, they should only know about what they need in order to produce their view. This could be a subset of the business object that the parent view model contains. As the parent view model will have references to all of the child view models, it can pass the relevant parts of the business object to them at the point of their construction.
I would agree with using Caliburn Micro.
However, to play devil's advocate you could split out your ViewModel File into separate files (same class name) and use the partial keyword before the class keyword. Its generally tidier and one step-away (non-breaking precursor) from breaking-up into separate classes.
I also agree Caliburn.Micro is a good solution for splitting your application in smaller components.
In Caliburn.Micro the communication between viewmodels is based on the Event aggregator pattern.
This makes a loose coupling between viewmodels
Splitting up is not ideal.
It looks as if the Caliburn toolkit focuses on events, whereas my application largely relies on ICommand implementations.
To me, the first encounter with Caliburn.Micro has been unsatisfactory.
The setup appeared to be tailored to VS2010 - which sounded promissing to me - because I do have VS2010 pro. But I got lost in the setup's of Silverlight.
Compared to toolkits like Prism it lacks the ease of an start.
It just takes to much time to switch now.
I use my own MVVM paradigm, it is less abstract than the Caliburn, it integrates multilanguage support everywhere, and it just faces one acceptable problem of some sources getting too big because of the nature of the Binding/DataContext paradigm.
For this problem I accept that "partial class" is a solution - even though I know there is a more elegant solution available.
In the heat of my work, I cannot change to another toolkit.
So I gently wait for Microsoft to allow for more flexibility around that Binding/DataContext paradigm.
It may be the case that Caliburn shows more intelligence allocating a viewmodel to some item. Does it ? (I think it does.)
What might be another option is to define a custom (xaml useable) object that triggers a custom allocator which control is to be allocated to which viewmodel. How about that ?
I'm trying to get up to speed with using WPF and the Prism framework which is heavily associated with the MVVM pattern. I've ready many different descriptions, examples and discussions on MVVM and each one is slightly different and has left me a little confused.
My understand is as follows:
The MVVM pattern has 3 parts to it :-
Model - the classes that hold the application data/information.
View - The visual elements of the application.
ViewModel - The logic, state and other behaviour associated with the visual elements. It takes data from the model and exposes it (possibly with some data conversion/formatting) in such a way that the View can use it directly.
What I'm not sure about is:
Do these 3 parts cover every part of the application? Or can there be parts of the application that are outside these 3 parts?
Is it the ViewModel or some other part that is responsible for populating the Model?
Thanks in advance
Absolutely not. Unless they do. If your application is simple, then everything can be handled in the View, ViewModel or Model(s). If your application is complex, and best practices dictate you break out logic into their own types (communication logic, repository logic, etc), then there is no stopping you. MVVM is only concerned with view-centric logic within the View, application logic within the ViewModel, and the means of storing information for transmission between the two.
The ViewModel is solely tasked with interpreting user actions and readying the results of logic within Models so that the View may display this information to the user. In some cases, it makes sense that a Model itself hold some logic so that it may respond to user actions. However, it has been my experience that this mini-ViewModel-Model design is in reaction to design decisions by inexperienced developers. Once you get the real hang of MVVM, you usually don't have to (or want to) put any code in your Models apart from validation logic.
Think of Model, ViewModel and View as logical layers that handle Business, application flow and presentation respectively. For example, a ViewModel class may delegate complex or reusable UI interactions to a separate service that doesn't correspond to any particular view but still belongs to ViewModel layer.
Yes, ViewModel stands between UI and Model.
The question is stated in the subject: are the advantages of Model-View-ViewModel (MVVM) pattern worth the overhead?
In many cases, implementing the view model involves quite significant overhead of duplicating the Model properties and sometimes synchronization between Model and ViewModel data members. For example, currently in Silverlight 4 & WCF RIA, View Models are not generated (if the developer follows the MVVM pattern, it is up to him to create the view models, often duplicating the corresponding Model's properties at ViewModel, that do nothing significant but refer to Model as the storage).
Why not extending the Model class, providing additional properties to make it easy to be consumed by the View instead?
Why not extending the Model class, providing additional properties to make it easy to be consumed by the View instead?
Effectively that is what the PresentationModel is for. Which MVVM is strongly based on. The difference is that the ViewModel is the model for the view and not the model for the data. So you are concerned around more how the view behaves with the data.
If you have a simple UI that all it does is present the model then I would suggest expose the Model on a property of the ViewModel and bind to that. Make sure though the model does implement INotifyPropertyChanged etc.
The power of the ViewModel is when you have things to do in response to a user action. The ViewModel can then support Commands, calling out to services and validation and thus leaving the Model as a data container
Why not extending the Model class, providing additional properties to make it easy to be consumed by the View instead?
In the simple cases, this is all the ViewModel is doing - wrapping up the Model so that its extended in a way that's consumable by the View. If your Model can be bound directly, you're welcome to do so.
That being said, there is more to the ViewModel layer than just wrapping the model - this is also where the application specific logic - ie: the application's plumbing, will occur. Something has to make the requests from the Model classes correctly and compose together the logic.
If you are concerned about extra work, you can always create a ViewModelBase (INotifyPropertyChanged , Errors/Validation, generic stuff) to be inherited by your ViewModel, it will minimize things that you think may cost you time to replicate. And also, Silverlight/Wpf provides us with binding which greatly reduces our coding, besides the fact that XAML also does that by providing functionalities through markup. Besides that you can further the design by using screens, controllers, etc.
For me, I do not see any "overhead" with regards to using MVVM; if there were, it'd be worth it. It properly deals with the Separation of Concerns. It provides a good platform for development especially in teams where people may take care of different aspects of the application without affecting other team members codes (especially between developers and designers).
Hope this helps
Benefits of MVVM
Reduced complexity.
Isolation of Designing and Development.
Dependency injection.
Major advantage is when you have a Well MVVM structured Windows Phone application and want to develop same for Windows Metro Desktop, Only thing u want to concentarte on design as the same view model can be used as it is.
Hope it helps.
i know some Mvvm Frameworks that introduced in this thread
please describe or give me link for that what are them useful for?
not information about MVVM about MVVM Framework.
thanks :)
i want to know :
What Is MVVM Framework?
I think your question is not really precise. As far as I understand, you ask for the features of each framework?!
You can find detailed information here and here. However, at least one of these links has already been given in the thread you mentioned...
EDIT:
Basically, an MVVM framework is a collection of classes which are commonly used in applications utilising the MVVM (Model-View-ViewModel) pattern. This may include messaging systems to communicate between independent parts of a software, dependency injection techniques, base classes for ViewModels, project/class templates, validation mechanisms, commonly used commands, techniques for displaying dialog boxes, and so on...
To completely understand such a framework, you will have to understand the MVVM pattern first. Because only then (or even only after you did your first MVVM project) you will have an understanding of the problems and/or challenges of this pattern.
To use Mvvm framework just simply follow below steps:
You have a model and a view-model with the same name.
View-models are not supposed to be wrappers around models. The job of a view-model is to broker requests for external services such as the loading and saving of data. The data itself, as well as validation and most of the business logic, should be in the models.
I can’t emphasize this enough. Whenever you create a view-model that wraps a model by delegation you introduce a huge hole in your API. Specially, anything with a direct reference to the model can change a property in such a way that the view-model and thus the UI are never notified. Likewise, any changes to calculated fields in the model won’t be propagated back to the view-model.
You have a view and a view-model with the same name.
Ideally view-models are agnostic to the screens they are used by. This is especially true in a WPF application where multiple windows may be sharing the same instance of a view-model.
For smaller applications such you may only need a single view-model for the whole application. For larger applications you may need one for the main functionality and one for each secondary aspect such as configuration management.
You have no code behind.
In absolute terms code behind is neither a good nor a bad thing. It is merely a place to put logic that is specific to a single view or control. So when I see a view with no code-behind at all I immediately check for the following mistakes:
Does the view-model touch specific controls by name?
Is the view-model being given access to controls via a command parameter?
Is EventToCommand or another leaky behavior being used in place of simple event handler?
EventToCommand from MVVM Light is especially bad because it will prevent controls from being garbage collected after they are removed from the screen.
View-models are listening to property changed notifications
If a model has a longer life-span then the view-model that listens to its events then you probably have a memory leak. Unlike views which have an unloaded event, view-models don’t have a good story for life-cycle management. So if they attach an event to a model that may out-last them then the view-model will be leaked.
Well this question relates to MVVM pattern and i could good and fast answers on this forum so I thought to ask and clear the confusions i had about the pattern.
I am quite new to MVVM approach. I appreciate the pattern and understand the principals behind it. Maybe I have not worked that much with the pattern that’s why there are a few confusions.
If there is a scenario in which I want to load few parts of my WPF page dynamically with XAML and still want to be compliant with MVVM approach.
The confusion is:
Where the logic of loading a view dynamically with XAML reside.
Whether I should have a single ViewModel for my WPF page or each seperate part have its own viewmodel with interactions with other viewmodel classes.
What if I had to build control tree displayed on the GUI using C# code in the codebehind itself.
For the controls created using code should I do the commandbindings in the codebehind of the view itself.
Where the logic for loading goes is something not really addressed by the pattern itself. There's an interesting blog post about this by Ward Bell. There's any number of ways to skin this cat, and they're all compatible with MVVM. Not really the answer you're looking for, I know, but it's honest :). Check out Ward's blog post... you'll get a much more in depth discussion of this topic.
As for whether or not to have a single VM for the page, or one for each control, that just depends. Generally, I have one for the page. If there's some portion that reusable elsewhere, I break it out into a user control with it's own VM, which means we have a VM within a VM. I don't agree with rockeye on this one. There isn't a one-to-one relationship between V-VM-M. You're Models are designed according to business needs, with NO regard to presentation at all. You're ViewModels are designed according to your presentation needs, and may encapsulate more than one Model. In fact, it's very common for them to encapsulate many models.
Like rockeye, I don't understand your last question.
I am also quite new to mvvm, but i will try to answer :
Where the logic of loading a view dynamically with XAML reside
If you mean "how can i show the view associated with my business object?", IMHO, you don't have to care about this. Usually, your VMs have corresponding views. With dataTemplate, you use only VM in the code but Views are displayed automatically.
2 Whether i should have a single ViewModel for my WPF page or each seperate part have its own viewmodel with interactions with other viewmodel classes
It seems you have a top-bottom approach. I see the mvvm more as bottom-up : models (business objects) -> ViewModels -> Views. Every model should have its own ViewModel and view. So you can't have a whole WPF page in a viewModel unless you model represents a page.
3 What if i had to build control tree displayed on the GUI using C# code in the codebehind itself. For the controls created using code should i do the commandbindings in the codebehind of the view itself.
Don't understand. I think you may take a look at dataTemplate, it might be helpfull.