So I was looking at https://github.com/xamarin/Sport as an example I came across when googling something for my current project. It is similar to what Im working on because I use an azure backend as well.
I have a question about their mvvm layout. I thought that in mvvm the models were sort of POCOs and not supposed to implement INotifyPropertyChanged. Arent they acting as both a Model and a ViewModel in this case? Look at the Athlete model and the AthleteViewModel. The VM has a property for for Athlete and so the model is used as a VM as well.
In my project, if I had the same types, I would have an Athlete model, an AthleteViewModel and an AthletePageViewModel. Where the Athlete and AthleteVM would be automapped. The only reason to populate and or create the Athlete is to persist it to the service or local storage.
Is one way more "correct" than the other way? Or am I just doing it wrong and over complicating it? I almost don't want to continue with the way I'm doing it because I dont want to have a bunch of "extra" model files if I can just use some of my VMs as models.
Thanks.
There's no ultimate master set of strict rules that you need to follow in order to implement the MVVM design pattern. In fact, the guidelines are generally quite blurry.
From what I've seen, there are a couple of different methods of which a model may be exposed to the view. Here they are:
Method 1 - INotifyPropertyChanged in the Model
public class Car : INotifyPropertyChanged
{
private string _Model;
public string Model
{
get { return _Model; }
set
{
_Model = value;
NotifyOfPropertyChange();
}
}
...
}
public class CarViewModel
{
//The entire model is exposed to the view.
public Car Model { get; set; }
...
Method 2 - INotifyPropertyChanged in the View Model
public class CarViewModel
{
private Car _Car;
//The model property is exposed to the view, not the model itself.
public string CarModel
{
get { return _Car.Model; }
set
{
_Car.Model = value;
NotifyOfPropertyChange();
}
}
...
In terms of a preferred method, I would say method 2 is the better option. Why?
The Model object is not exposed to the view.
The View Model only exposes what the View needs.
Method 2 does have its downsides. Imagine if you needed to expose lots of model properties, or imagine if your model changes, it is certainly easier to simply implement INotifyPropertyChanged in the model and expose it to the view. Programmers are lazy by nature, therefore in order to save hassle, you'll see method 1 just as much as method 2.
But that isn't a bad thing.
Is one way more "correct" than the other way? Or am I just doing it wrong and over complicating it?
Remember, the MVVM design pattern is just a pattern. Neither options are correct, it's mostly down to the developers preference how they choose to approach the implementation of the pattern, as long as the main MVVM concepts are there, that's all that matters.
Related
I'm a bit confused about ViewModel, in particular I don't understand if is possible add method inside this or the scope of the ViewModel is include only the properties for bind the data of the model to the view. So consider this implementation of a CountryVM:
public class CountriesVM : ViewModel
{
public AsyncObservableCollection<CheckedListItem<Country>> Countries { get; set; }
public void GetCountries()
{
Countries = new AsyncObservableCollection<CheckedListItem<Country>>();
Task.Run(() =>
{
var countries = Soccerway.Scraper.Countries.GetCountries();
foreach (var country in countries)
{
var nation = new CheckedListItem<Country>();
nation.Item = new Country { Name = country.Name, Link = country.Link };
Countries.Add(nation);
}
});
}
so essentially in the ViewModel I've a metdho called GetCountries() that is called at application startup, this will populate the Countries ObservableCollection. But I notice that some examples on the net have only properties in the ViewModel.
In each ViewModels I inherit the ViewModel class that is the base class that contains the implementation of INotifyPropertyChanged.
So my question is:
is my implemenation correct? Or, where the method that I need to call should be placed for valorize the properties?
Thanks.
is my implemenation correct?
Yes, certainly as far as MVVM is concerned.
The VM is supposed to handle the UI logic, just like the Controllers in MVC.
You could consider moving this to a Services layer, but your GetCountries() is on the fence. It's mostly UI related, so I would leave it here.
Assuming you are referring to ViewModels in MVVM; these are two fantastic references that really helped me understand MVVM:
Understanding the basics of MVVM design pattern
Model-View-ViewModel (MVVM) Explained
To answer your question directly, yes you do place methods in the View Model that contain business logic.
Edit:
And then the shortest possible summary I can come up with:
View - UI
ViewModel - UI Logic and bindings between View & Model.
Model - Data models and data access.
What is the best practice for binding a list of items to an ItemsControl when using the MVVM pattern?
1. Bind a list of ViewModels
Load the items from the database, create the models and all viewmodels and then bind a list of viewmodels to the ItemsControl.ItemsSource:
public class MyMainViewModel
{
public List<PersonViewModel> Persons { get; set; }
}
2. Bind a list of Models
Load the items from the database, create the models and then bind a list of those models directly to the ItemsControl.ItemsSource:
public class MyMainViewModel
{
public List<Person> Persons { get; set; }
}
I think the answer here is really it depends.
Firstly, you need to assess whether your view needs to interact with your model in such a way that it would make sense for a view model to be wrapped around a particular model. Let's look at an example:
public class WebsiteModel
{
public string URL { get; set; }
}
Here I have a very simple model which represents a website, nothing too fancy. I can create a view model which will encompass all websites, like a one to many relationship:
public class WebsitesViewModel
{
//A list of websites.
public List<WebsiteModel> Websites { get; set; }
//The command I will use to navigate, where the object parameter will be the WebsiteModel.
public ICommand NavigateCommand { get; set; }
...
public void Navigate(WebsiteModel model)
{
...
}
Here I want my view to be able to navigate to the URL using the browser. My view model holds a list of models and my command takes care of navigation.
The next method I can create a view model to represent a single model, I would say this is a SOLID approach:
public class WebsiteViewModel
{
//The website model
public WebsiteModel Website { get; set; }
//The command I will use to navigate, no parameters needed.
public ICommand NavigateCommand { get; set; }
...
public void Navigate()
{
...
}
In this scenario, I'll need another view model which will expose a list of WebsiteViewModel to my view.
public List<WebsiteViewModel> Websites { get; set; }
The truth is there isn't really a best practice. Neither method really trumps the other. There are benefits to each method, however the method to choose really depends on the implementation. In this scenario, method 2, I would say is overcomplicated. However it's not uncommon for a view model to become very large very quickly, and the need to separate concerns will force you to create smaller classes, and even view models to wrap your models inside, making method 2 a viable option.
So to wrap up. Neither method is best practice.
The only "correct" way to do it, is to use ViewModels all the way.
While initially more work, it gives you more flexibility and less bugs later on.
Don't for get, when you a model should only be valid in it's bounded context and when you bind your ViewModel to the view, you have a leaky abstraction. The View becomes aware of model and each change to the model, will affect your View.
Further more, refactoring doesn't work within XAML. So if you name a model property via refactoring, your XAML will still bind to the old property. This doesn't give you a compile error and your bounded element will just remain empty (in best case) or crash (in worst case).
This can be quite hard to figure out and fix. Also as Scroog1 commented, it introduces memory leaks. May not be noticeable in small applications, but ones working with big data sets it can lead to out of memory exceptions.
You should utilize an automapping library to map from Model to ViewModel in cases that allows it, this will reduce some boilerplate code. But remember to avoid ViewModel to Model automappings, as it's discouraged.
You want to avoid that a change in your model influences code in a different bounded context, i.e. you don't want to expose every database or model change in a rest service, even if the change do not influence the given rest action.
The same strategy can be applied to the n-layer model (View, ViewModel, (Domain) Model layer, Services and Infrastructure)
I think there is no correct way, using models is the pragmatic and simpler way, using view models is more time consuming but more decoupled...
You should have a look at this article:
http://blog.alner.net/archive/2010/02/09/mvvm-to-wrap-or-not-to-wrap.aspx
also: http://www.codeproject.com/Articles/61147/MVVM-Creating-ViewModel-Wrap-your-business-object
Assume that there's a PIModel (i.e. Personal Information Model) and a ViewModel (contains some information from PIModel and other).
public PIModel
{
private string firstName;
public string FirstName { get; set; }
private string lastName;
public string LastName { get; set; }
... // other
}
The FirstName and LastName properties need to be bound to View, so I have two questions:
Does the ViewModel have a property reference to a PIModel instance?
If so, does the ViewModel have property references to PIModel.FirstName and PIModel.LastName?
I learned that implementing INotifyPropertyChanged in the Model is not recommended.
After a few years practicing MVVM, I would nuance a bit the answer, even if it is not 100% MSDN compliant.
I would strongly agree with this reccomandation: do not implement the INotifyPropertyChanged in Model.
And I would explain why: if your model is nothing but properties and INotifyPropertyChanged, what is its role in term of responsability? (Think about Single Responsability Principle: http://en.wikipedia.org/wiki/Single_responsibility_principle)
Let's take your example: if you use INotifyPropertyChanged in PIModel, then the role of PIModel is to present your data to your view. And by the way, what's the role of a ViewModel in the MSDN definition? You got it: present your data to your view.
So in the end, if you use both Model and ViewModel to present your data, the role of each component will blur, and you will have some ideas like "well, I think here I do no even need a ViewModel".
The data presentation responsability will be set in different conceptual class.
In my opiniion if you have this kind of thought, you need ONLY ViewModel (but probably a bigger ViewModel containing beyond others PIViewModel). Do not build an anemic Model (model with only properties and no responsability at all), because it will complexify your code and add no value.
Use a Model only if you add some other responsability to your object, and not display responsability (because it belongs to a ViewModel) but rather real business responsability.
So if the most of data is get from server, and the most of business responsability is on the server, it will be logical for me to see mainly ViewModel in your client application.
Hope it helps.
There is no problem if your model is self notifying. You can have your model INotifyPropertyChanged implemented. Here is the msdn article, it will clarify all the doubts you have regarding model implementation and the work around if your model is not implemneting INPC.
http://msdn.microsoft.com/en-us/library/gg405484%28PandP.40%29.aspx
So if your models do not fall in category where you can implement INotifyPropertyChanged (e.g database autogenerated entities) then you will have to write wrapper properties over model proerties in VM.
But frankly if you can implement INPC you should, as it saves from unnecessary duplication and maintainance of code.
In an MVVM implementation, is every ViewModel coupled to just one Model?
I am trying to implement the MVVM pattern in a project but I found that sometimes, a View may need information from multiple Models.
For example, for a UserProfileView, its UserProfileViewModel may need information from UserAccountModel, UserProfileSettingsModel, UserPostsDataModel, etc.
However, in most articles I read about MVVM, the ViewModel only consists on one Model via Dependency Injection. So the constructor takes in only one Model.
How would the ViewModel work when it has to get information from multiple Models? Or would such a situation ever occur in MVVM?
PS: I am not using the Prism or Unity Framework. I am trying to implement similar patterns into a project that I am working on which doesn't use Prism or Unity. That's why I need to understand exactly how some of these things work.
In my understanding of the MVVM pattern, the only practical requirement is that the View gets all its data from the properties of a ViewModel (probably through a binding mechanism). The ViewModel is a class that you craft specifically for that view, and takes on the responsability of populating itself as required. You could think of it like ActiveRecord for the view.
As such, it doesn't matter what you do inside the ViewModel to obtain the data that its properties should show. You could get it by querying some services, reading one or more business entity models, generating it on the spot, or all of the above. It's perfectly normal to need a combination of all these things to make a functional view.
As in any presentation pattern, the point is just to separate the process of showing some data on the screen, from the process of obtaining that data. That way you can test each part of the process separately.
Edit: Here's a small but hopefully complete example of the flow of dependencies.
// Model/service layer
public class MyModelA
{
public string GetSomeData()
{
return "Some Data";
}
}
public class MyModelB
{
public string GetOtherData()
{
return "Other Data";
}
}
// Presentation layer
public class MyViewModel
{
readonly MyModelA modelA;
readonly MyModelB modelB;
public MyViewModel(MyModelA modelA, MyModelB modelB)
{
this.modelA = modelA;
this.modelB = modelB;
}
public string TextBox1Value { get; set; }
public string TextBox2Value { get; set; }
public void Load()
{
// These need not necessarily be populated this way.
// You could load an entity and have your properties read data directly from it.
this.TextBox1Value = modelA.GetSomeData();
this.TextBox2Value = modelB.GetOtherData();
// raise INotifyPropertyChanged events here
}
}
public class MyView
{
readonly MyViewModel vm;
public MyView(MyViewModel vm)
{
this.vm = vm;
// bind to vm here
}
}
// Application layer
public class Program
{
public void Run()
{
var mA = new MyModelA();
var mB = new MyModelB();
var vm = new MyViewModel(mA, mB);
var view = new MyView(vm);
vm.Load();
// show view here
}
}
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).
a viewmodel contains the "view logic" - so all you wanna show on the view is exposed through the viewmodel. if you wanna show data from diffenrent "models" then your viewmodel agregate this and the view can bind to.
the main purpose from mvvm was btw unit test. this mean easy testing of view logic without UI.
EDIT: why do you think:
ViewModel only has one single parameter for the View in its constructor
EDIT2:
there btw two main approaches to work with mvvm, first is "View First" second is "Viewmodel First" you can of course mix up both and choose the best approach for you needs.
A ViewModel may and in many cases does use multiple Models. It is itself a "Model" of your view.
Consider a profile screen that a user enters their personal information including address. If the address is stored in an "addresses" table and the rest in a "profile" table, then the ViewModel uses both the Profile and Address models to create a unified ViewModel.
As jgauffin mentioned in his answer, many times you can use user controls to achieve a one to one relationship, but you can also introduce needless complexity by trying for this 100% of the time.
I would make sure you understand the difference between view, viewmodel, and all other model classes. The ViewModel is the model object that is filled with data that the view can be bound to. It just exists to provide data to the view, which makes the ViewModel object unit-testable, and the whole business logic separate from the view. So, you can develop your business logic entirely without using the view itself, and can replace the view with just building or using another view and binding to the ViewModel object's properties. If a view is full of empty text fields for example, the contents of the text fields can be bound to different properties of the view model.
There usually really should only be one view model. BUT if it's too complex, you can use subproperties of the bound objects like described in Binding to ViewModel.SubClass.Property (sub-property)
The ViewModel can get the data it returns to the view from a lot of different sources, business objects, databases, whatever.
Usually there is one ViewModel per Model. These ViewModels contain the logic to handle the model's data. On the other side every view has it's own view model, too. So this means:
class ModelA
{
bool TestValue{get;set;}
}
class ViewModelA<ModelA>
{
ValueViewModel<bool> TestValue{get; private set;}
public ViewModelA(ModelA model)
{
base.Model = model;
this.Initialize();
}
}
class ModelB
{
string Username;
}
class ViewModelB<ModelB>
{
ValueViewModel<string> Username{get; private set;}
public ViewModelB(ModelB model)
{
base.Model = model;
this.Initialize();
}
}
These are the ViewModels that encapsulate the models. The views have their own ViewModels:
public ViewModelForExactlyOneView
{
public ViewModelA{get;set;}
public ViewModelB{get;set;}
}
To answer your question, ViewModel1 refers to ViewModelA and ViewModelB. The View therefore can get it's data from ViewModel1.ViewModelA.TestValue.
just use the User model in your view
public partial class User : Login
{
public string Password { get; set; }
public List<Customer> customer { get; set; }
}
in this the another model login inherited and the customer model also used in this model..
I've recently started develping an application using WPF and I'cant really wrap my mind around following thing:
I have a Domain Model of my application which is simple POCO objects serialized to/from harddisk. Then I have the WPF application and I'd like to bind it to various parts of the model. I need to be able to notify UI of changes to underlying model (eg. implement INotifyPropertyChanged) BUT I want to do that without interfeering with my model (read without modifying current implementation of the model). How can I implement changes notification other than modifying the model?
Reason why I want to do that is that I share the model among multiple projects, only one being WPF, and I don't want to add unncecessary code to the model.
One thing that came to my mind was to create "copy" of the model (with INotifyPropertyChanges and BindingLists etc.) but that seems to be hard to maintainn...
Thanks in advance.
Ondrej
Check this out MVVM
Download its source code see the hierarchy.
Basically you still keep simple POCO objects as your models. Then you create a ViewModel around the model like this:
public class CustomerViewModel : INotifyPropertyChanged
{
readonly Customer _customer;
public CustomerViewModel(Customer customer)
{
_customer = customer;
}
public string FirstName
{
get { return _customer.FirstName; }
set
{
if (value == _customer.FirstName)
return;
_customer.FirstName = value;
OnPropertyChanged("FirstName");
}
}
...
}
I see two possible solutions here:
Use a separate model for WPF screens only (MVVM pattern). This will require maintaining two different models, also be prepare for lots of mapping code.
Use PostSharp to "enhance" your model with all necessary boilerplate code. Here you can find example of automatic INotifyPropertyChanged implementation. Remember that introducing PostSharp to the project is an important decision, so I suggest getting familiar with it first.