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.
Related
So I stumbled upon a problem while learning MVVM. I had a TreeView that contained TextBlocks which I wanted to perform an action on when I double clicked any of the TextBlocks in the TreeView. I started to learn about Behaviors, and I have a great example of how a behavior is implemented but the example does not connect the Behavior to a ViewModel at all. So in other words, if I double click on the TextBlock, I have the Behavior class that catches it but I don't have any ViewModel to perform any actions.
Could someone take a moment and explain how these tie in? I was reviewing this article:
http://msdn.microsoft.com/en-us/library/gg430869(v=pandp.40).aspx
But I didn't seem to grasp what I was looking for.
MVVM concept provide us a decoupling mechanism in WPF application which means no more code in xaml.cs file. Attached behavior is different thing. It has not relation with MVVM.
But because if we have scenarios where I cant use MVVM e.g. Select the text of TextBox on double click. Which is a behavior you want to add on textbox.
You will prefer implement the double click functionality in xaml.cs file as it not reusable and also tightly coupled.
This is where behavior come into picture. We will create the behavior for TextBox and will attach it. Now you can attach this behavior to as many controls you want.
EDIT:
if you are using WPF 4.5. you can look Markup Extensions for events
If you want to do it with attached behavior. Create an attached behavior of double click event which has Command dependency property. Your double click behavior just raise the command attached and in xaml bind the command with viewmodel which I expect you know how.
Hope, I am able to answer you comment.
D J has a good answer, though I'd like to insert some additional thoughts to this discussion.
In my experience, view behaviors (whether code-behind, attached behaviors, blend behaviors, or custom control logic) are often useful when the view does not depend on a view-model to function properly. If a view (UserControl, Window, Page, etc.) does not logically make sense in the absence of a view-model, it might make more sense to remove behavior from the view and move it to the view-model.
While we can do pretty much anything under the sun with all types of behaviors, it is often not wise to. MVVM, for good reason, limits what we should do so that we can observe separation of concerns to improve our application's cohesiveness and to decouple our classes. These two things are what software maintainability is all about, and these concepts become increasingly important as the application grows into enterprise software.
It is important to think about the concerns that the behavior has. Understanding this will help us find a properly suited place for it. Here are some questions to help know where it belongs:
Does the behavior interact with a business domain model (this is the first 'M' in MVVM)?
Behavior that has a dependency (even a loosely coupled one) on a domain model should likely belong in the view-model (or service), and not in the view. For instance, if the behavior needs to save to or read from an external device (e.g. a database), it has a dependency that the view should never have. Wrap this logic in a service function.
Or if not using a heavily layered architecture, put this inside of a view-model.
Does the behavior interact with application services, domain services, infrastructure services, etc.?
For the same reasons as the prior answer, the behavior likely belongs in the view-model or a service class. The view should have no explicit knowledge of services or domain model objects as it would muddy up its responsibilities (or concerns) that the view has. A view should only be concerned the visual/physical aspect of the user UI. Many views should define a contract (i.e. a view-model interface) that it binds to in order to operate correctly.
Will the behavior need to be reused across different views of the same kind?
This is a bit of a tricky question. Many times we foresee being able to have a different presentation for the same content. In effect, the view in these cases is a thin wrapper around some structure. For instance, suppose for a e-mail application we have a summarized view for received e-mails as well as a detailed view. Both views might need to support the same behavior (e.g. delete, reply, forward). Because we are reusing the behavior across different views of the same kind, then the behavior should belong in a common, reusable place. View-model logic is a good place for this.
Will the behavior need to be reused across views of different kinds?
When the behavior needs to be reused across different kinds of views (e.g. TextBox, ComboBox), we likely need an attached behavior. Usually, we can know this because the views are so diverse that it is not possible for them to share a view-model interface. Given that the behavior is concerned with view-related responsibilities, then custom control logic, code-behind, attached behaviors, or blend behaviors are all suitable places.
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.
This question already has answers here:
Why use MVVM? [closed]
(13 answers)
Closed 2 years ago.
Why we go for MVVM over MVC or MVP while dealing with WPF?
What extra benefit we get by using this?
Edit:
To be honest , today I had an interview and I have been asked this question. I answered like INotifyPropertyChanged , ICommand,IValue Convertor.. but he was not satisfied. Henceforth I have put up this question
Thanks in advance
I'll point you to a particularly useful video by Jason Dolinger.
Coming from a WinForms world, implementing any MVX style pattern seemed like more hassle than it was worth but after working with WPF for a couple of years now, I can honestly say that I wouldn't consider anything less. The whole paradigm is supported out-of-the-box.
First off, the key benefit is enabling true separation between the view and model. What that means in real terms is that if/when your model needs to change, it can without the view needing to and vice-versa.
Secondly, while your model may contain all the data you might need in your view, you may want to abstract that data in such a way that your model doesn't support. For example, say your model contains a date property. In the model it can exist solely as a DateTime object but your view might want to present it in a completely different way. Without the viewmodel you'd either have to duplicate the property in the model to support the view or modify the property which could seriously obfuscate the 'model'.
You can also use a viewmodel to aggregate parts of your model that exist in separate classes/libraries to facilitate a more fluent interface for the view to deal with. It's very unlikely that you'll want to work with data in your code in the same way that a user will want to or will want that data presented to them.
On top of that, you get support for automatic two-way data binding between the view and viewmodel.
There really is a whole bunch of extra stuff that I could bang on about but Jason say's it far better that I could so my advice is watch the video. After a few days of working like this, you'll wonder how you ever got by without it.
Good luck.
These are mine specific to MVVM
Increases the "Blendability" of your views (ability to use Expression Blend to design views). This enables a separation of responsibilities on teams that are lucky enough to have a designer and a programmer... each can work independent of the other.
"Lookless" view logic. Views are agnostic from the code that runs behind them, enabling the same view logic to be reused across multiple views or have a view easily retooled or replaced. Seperates concerns between "behavior" and "style".
No duplicated code to update views. In code-behind you will see a lot of calls to "myLabel.Text = newValue" sprinkled everywhere. With MVVM you can be assured the view is updated appropriately just by setting the underlying property and all view side-effects thereof.
Testability. Since your logic is completely agnostic of your view (no "myLabel.Text" references), unit testing is made easy. You can test the behavior of a ViewModel without involving its view. This also enabled test-driven development of view behavior, which is almost impossible using code-behind.
The other two patterns are really sort of separate in terms of the concerns they address. You can use MVVM with MVP and MVC (most good samples out there do some form of this).
In fact, MVP (w/ a Passive View, rather than a Supervising Controller) is really just a variant of MVVM, in my opinion.
WPF has better databinding than any other UI framework, which MVVM would be unruly without
MVVM provides unit testability and excellent view-agnosticism, which make it a good thing to use
Baked in support for ICommand and INotifyPropertyChanged are the two biggest benefits. Using MVVM makes it really easy to wire up the commands and plug data into the WPF UI. Things just work.
I personnaly see MVVM not as a benefit, but as an obligation for those who want to use WPF cool features.
WPF is very very heavily built with data binding at the core, to enable separation of UI from Model. But the way data binding is technically done in WPF is somewhat special, as it's tied to classes like:
DependencyProperty
INotifyPropertyChanged
ObservableCollection
Because of this you just can't really write a model the way you want using standard .NET technology. For example, the WPF TreeView is almost impossible to use w/o using data binding and templates. You just can't populate it simply like you would from a generic model in Winforms for example. It must be bound to a hierarchical model using ObservableCollection to represent a node's children.
So let's say V represents the XAML code and it's code-behind counterpart (so it's tied to WPF as a technology), and let's say M represents your model (so it's not tied to WPF UI technology in anyway).
Well, you'll never have this working properly under WPF with only these V & M.
You must add something between the two. Something that's WPF-compatible and understands your model. Something that speaks DependencyProperty, ObservableCollection and INotifyPropertyChanged. That's what's called VM.
As a side note, an alternative to MVVM is to build a V & M (w/o VM plumbing) combination with M being WPF-compatible but still with a reasonable UI independency. Historically, ObservableCollection was in the WindowsBase.dll assembly (that was shipped with WPF), so it really looked weird to bind a generic model to something tied to a UI technology. It's been moved back to System.dll since. Even then, it's sometimes hard to keep a pure VM model w/o tweaking the M specifically for WPF...
The ability of XAML code to databind, as well as the existance of triggers will break the MVP and MVC Patterns.