Currently, we are investigating possibilities with ReactiveUI how to visualize content of data objects coming from a backend system.
In the ReactiveUI documentation it is mentioned, the recommended approach is using type-safe bindings.
Therefore,
the code behind consists of all bindings between view and view model
the view model is enriched with read-write properties or read-only properties necessary for the binding
the view model has to take the data object's content to make it accessible via the read-write or read-only properties
View:
this.Bind(ViewModel,
viewModel => viewModel.DiameterInInch,
view => view.MaterialDiameterInInch.Text)
.DisposeWith(disposableRegistration);
View Model:
constructor
{
// reading data object in constructor
this.WhenAnyValue(x => x.Material.MutableDataObject.DiameterInInch)
.Subscribe(diameter => DiameterInInch = diameter);
// writing to data object in constructor
this.WhenAnyValue(x => x.DiameterInInch)
.Subscribe(diameter =>
Material.MutableDataObject.DiameterInInch = diameter);
}
[Reactive]
public double DiameterInInch { get; set; }
With this approach, we see that we are facing a certain effort when we have to implement the complete binding chain in code behind and the view model for each content field / property of a data object.
What further approaches can you recommend to minimize or avoid such an implementation effort / code duplication?
Why not just bind directly to DiameterInInch?
The way I see it, Material is a property of your ViewModel and its property - MutableDataObject - has to be a ReactiveObject (or some other implementation of INPC; otherwise you wouldn't be able to use this.WhenAnyValue on it). Maybe try doing it this way:
View:
this.Bind(ViewModel,
vm => vm.Material.MutableDataObject.DiameterInInch,
v => v.MaterialDiameterInInch.Text)
.DisposeWith(disposableRegistration);
Please note that I'm not an expert user of ReactiveUI so I may be missing something important here.
Related
From what I've read, I'm supposed to be using ViewModels to populate my views in MVC, rather than the model directly. This should allow me to pass not just the contents of the model, but also other information such as login state, etc. to the view instead of using ViewBag or ViewData. I've followed the tutorials and I've had both a model and a viewmodel successfully sent to the view. The original problem I had was that I needed a paginated view, which is simple to do when passing a model alone, but becomes difficult when passing a viewmodel.
With a model of
public class Instructor {
public string forename { get; set; }
public string surname { get; set; }
}
and a viewmodel of
public class InstructorVM {
public Instructor Instructors { get; set; }
public string LoggedIn { get; set; }
}
I can create a paginated list of the instructors using the pure model Instructor but I can't pass InstructorVM to the view and paginate it as there are other properties that aren't required in the pagination LoggedIn cause issues. If I pass InstructorVM.Instructors to the view, I get the pagination, but don't get the LoggedIn and as this is just the model, I may has well have passed that through directly.
An alternative that was suggested was to convert/expand the viewmodel into a list or somesuch which would produce an object like this that gets passed to the view
instructor.forename = "dave", instructor.surname = "smith", LoggedIn="Hello brian"
instructor.forename = "alan", instructor.surname = "jones", LoggedIn="Hello brian"
instructor.forename = "paul", instructor.surname = "barns", LoggedIn="Hello brian"
where the LoggedIn value is repeated in every row and then retrieved in the row using Model[0].LoggedIn
Obviously, this problem is caused because you can only pass one object back from a method, either Instructor, InstructorVM, List<InstructorVM>, etc.
I'm trying to find out the best option to give me pagination (on part of the returned object) from a viewmodel while not replicating everything else in the viewmodel.
One suggestion was to use a JavaScript framework like React/Angular to break up the page into a more MVVM way of doing things, the problem with that being that despite looking for suggestions and reading 1001 "Best JS framework" lists via Google, they all assume I have already learned all of the frameworks and can thus pick the most suitable one from the options available.
When all I want to do is show a string and a paginated list from a viewmodel on a view. At this point I don't care how, I don't care if I have to learn a JS framework or if I can do it just using MVC core, but can someone tell me how to do this thing I could do quite simply in ASP.NET? If it's "use a JS framework" which one?
Thanks
I'm not exactly sure what the difficulty is here, as pagination and using a view model aren't factors that play on one another. Pagination is all about selecting a subset of items from a data store, which happens entirely in your initial query. For example, whereas you might originally have done something like:
var widgets = db.Widgets.ToList();
Instead you would do something like:
var widgets = db.Widgets.Skip((pageNumber - 1) * itemsPerPage).Take(itemsPerPage).ToList();
Using a view model is just a layer on top of this, where you then just map the queried data, no matter what it is onto instances of your view model:
var widgetViewModels = widgets.Select(w => new WidgetViewModel
{
...
});
If you're using a library like PagedList or similar, this behavior may not be immediately obvious, since the default implementation depends on having access to the queryset (in order to do the skip/take logic for you). However, PagedList, for example has StaticPagedList which allows you to create an IPagedList instance with an existing dataset:
var pagedWidgets = new StaticPagedList<WidgetViewModel>(widgetViewModels, pageNumber, itemsPerPage, totalItems);
There, the only part you'd be missing is totalItems, which is going to require an additional count query on the unfiltered queryset.
If you're using a different library, there should be some sort of similar functionality available. You'll just need to confer with the documentation.
Very simple question: in a strict MVC design pattern we want to keep Model, View and Controller can the View layer know which custom data classes are defined in the model?
As example:
I got a CarViewController in the view layer and a Car object in the model layer. Whenever the model layer changes the controller object that "sits" between the model and the view notifies the CarViewController and in my current implementation passes a copy of the updated car data as an instance of the Car class. Is this correct?
My gut instinct would have said no because I would not want the view layer to know the details of the model objects. It is not strict decoupling. However if I pass specific values instead of passing the custom data model I would need to stick to standard/primitive values (E.g. int as number of wheels) and it may require more coding.
Have I understood MVC correctly? Is there any reason why the view should not know the custom classes of the model layer?
If I am understanding your question correctly, I would say that your view needs to know the details of your Car object in most of the cases. You can utilize metadata in this way like this:
Model:
public class Car
{
[Display(Name = "Number of wheels")]
public int Wheels { get; set; }
}
View:
#model Namespace.Models.Car
#* This will display whatever your [Display(Name="Value")] decorator defines
as a display name, also the editor will respect the data type decorator. *#
#Html.LabelFor(m => m.Wheels)
#Html.EditorFor(m => m.Wheels)
In this case if you basically pass down a primitive then all metadata for your model is lost.
Ok!
I have to say both technology are great. Although there seems that something I do not get it.
You have a data in you database (and let say you want to show data from a table that has references to other tables).
I have a model with List or IEnumerable or IQueryable or whatever...
So in my view I want do foreach through the list of object and take advantage of cool feature of references to other tables. No problem in controller while you are in
using (var datatabse = new MyEntity)
{
}
But when you get out of using db has disposed and you get common error The ObjectContext instance has been disposed and can no longer be used for operations that require a connection.
So i do not see other options as creating in memory copies of entity objects...but you loose all cool EF4 references and you have to manually load data first in your model and then with foreach show it on the view.
So instead of List<(EF4Type)> or IEnumerable<(EF4Type)> or IQueryable<(EF4Type)>
you have to do List<(MyCustomHelperClass)> where MyCustomHelperClass represents a class with properties similiar to entity objects and probably some additional beacuse you do not have access to properties of referenced tables Then you have to do foreach and Load data into this List and the another #foreach on the view with Razor to show all.
Twice as much work and if project is big...you can see a bigger picture of how manny those helperClasses you need. Was all this cool new technology really meant to be used in that way?....or am I missing something.
You are probably getting that error when you reference a lazy loaded property in your view. You should eager load everything you need in the Controller before passing it to the View.
See Loading Related Objects (Entity Framework).
The following example will cause all courses to be retrieved with the departments in the same query. This is eager loading.
// Load all departments and related courses
var departments1 = context.Departments
.Include(d => d.Courses)
.ToList();
Without the Include() part, courses could be retrieved later (possibly after your context has been disposed in the view). This is called lazy loading.
Along with eager loading as remembered by jrummell, there's also another way of loading related entries, it's explicit loading. Let's suppose you have a User entity, with many Groups entities related to it. You can explicitly load them:
var user = context.Users.Find(id); // Load the user.
context.Entry(user)
.Collection(u => u.Groups)
.Load();
This way you don't have to use the .Include(), and you can even filter the Groups:
context.Entry(user)
.Collection(u => u.Groups)
.Query()
.Where(g => g.SomeProperty.Contains("something"))
.Load();
TheMentor,
Depending on whether you have a repository or a db context, this object should only live for the duration of the controller action (Request), so you should be able to do everything required within the confines of the action.
Maybe i've misunderstood, but based on your question, this is what your issue appears to be. If I have misunderstood, then I'd still suggest that the db repository or db context should be referenced across the controller, rather then invoking it inside the action each time.
so you should see something like this in your controller:
public class TasksController : BaseController
{
private readonly TaskService _serviceTasks;
public TasksController(IRepository repository)
{
_serviceTasks = new TaskService(repository);
}
//
// GET: /Tasks/
public ActionResult Index()
{
var viewModel = _serviceTasks.All<Task>();
return View(viewModel);
}
public ActionResult Details(int id)
{
var domainModel = _serviceTasks.GetById<Task>(id);
var viewModel = PopulateDetailsViewModel(domainModel);
return View(viewModel);
}
//.. rest of actions cut
}
I have a view model sent to the edit action of my controller. The ViewModel contains references to EntityObjects. (yea i'm fine with it and don't need to want to duplicate all the entities properties in the viewmodel).
I instantiate the view model and then call UpdateModel. I get an error that a property is "null" which is fine since it is a related model. I am trying to exclude the property from being bound during model binding. On debugging it I see in the entity where the model binder is trying to set the value of the property to null.
Here is my edit action:
var model = new SimplifiedCompanyViewModel(id);
var excludeProperties = new string[] {
"Entity.RetainedEarningsAccount.AccountNo"
,"Property.DiscountEarnedAccount.ExpenseCodeValue"
,"Entity.EntityAlternate.EntityID"
,"Property.BankAccount.BankAccountID"
,"Entity.PLSummaryAccount.AccountNo"
,"Property.RefundBank.BankAccountID"
,"Company.Transmitter.TCC"
};
try
{
UpdateModel<SimplifiedCompanyViewModel>(model, String.Empty, null, excludeProperties);
if (ModelState.IsValid)
{
//db.SaveChanges();
}
return RedirectToAction("Index");
}
catch
{
return View(model);
}
I have looked at a few other issues about specifying a "prefix" but I don't think that is the issue since I am telling it to bind to the viewmodel instance not just the entity object.
Am I excluding the properties correctly? Strange thing is is only seems to happen on this item. I suspect it may be an issue with the fact that there is actually no refund bank related to my entity. But I have other related items that don't exist and don't see the same issue.
More info... since I'm told me model isn't designed well.
The Company is related to a BankAccount. The Company view shows the currently related BankAccount.BankAccountId and there is a hidden field with the BankAccount.Key. I use jQueryUI autocomplete feature to provide a dropdown of bank account displaying the BankAccount.BankAccountId and when one is selected the jQuery code changes the hidden field to have the correct Key value. So, when this is posted I don't want the current bankaccounts BankAccountID modified, hence I want it to skip binding that field.
If I exclude BankAccountId in the model then on the BankAccount edit view the user would never be able to change the BankAccountId since it won't be bound. I'm not sure how this indicates a poor model design.
Use the Exclude property of the Bind attribute:
[Bind(Exclude="Id,SomeOtherProperty")]
public class SimplifiedCompanyViewModel
{
public int Id { get; set; }
// ...
}
This is part of the System.Web.Mvc namespace. It takes a comma-separated list of property names to exclude when binding.
Also you should consider using TryUpdateModel instead of UpdateModel. You can also just have the default model binder figure it out by passing it as an argument to the constructor:
public ActionResult Create([Bind(Exclude="Id")]SimplifiedCompanyViewModel model)
{
// ...
}
A very simple solution that I figured out.
try
{
UpdateModel<SimplifiedCompanyViewModel>(model, String.Empty, null, excludeProperties);
ModelState.Remove("Entity.RetainedEarningsAccount.AccountNo");
ModelState.Remove("Property.DiscountEarnedAccount.ExpenseCodeValue");
ModelState.Remove("Entity.EntityAlternate.EntityID");
ModelState.Remove("Property.BankAccount.BankAccountID");
ModelState.Remove("Entity.PLSummaryAccount.AccountNo");
ModelState.Remove("Property.RefundBank.BankAccountID");
ModelState.Remove("ompany.Transmitter.TCC");
if (ModelState.IsValid)
{
//db.SaveChanges();
}
return RedirectToAction("Index");
}
catch
{
return View(model);
}
Another option here is simply don't include this attribute in your view and it won't be bound. Yes - you are still open to model injection then if someone creates it on the page but it is another alternative. The default templates in MVC will create your EditorFor, etc as separate items so you can just remove them. This prevents you from using a single line view editor with EditorForModel, but the templates don't generate it that way for you anyways.
EDIT (adding above comment)
DRY generally applies to logic, not to view models. One view = one view model. Use automapper to easily map between them. Jimmy Bogard has a great attribute for this that makes it almost automatic - ie you create the view model, load up your Customer entity for example, and return it in the action method. The AutpMap attribute will then convert it to a ViewModel. See lostechies.com/jimmybogard/2009/06/30/how-we-do-mvc-view-models
Try the Exclude attribute.
I admit that I haven't ever used it.
[Exclude]
public Entity Name {get; set;}
I am sure there has to be an easy way to do this, but I just cant seem to get my head around it.
I am using the MVCContrib Grid control to display a number of grids in a 3 tier application I am working on (ASP.NET MVC3 PL -> BLL -> DAL). I also am using Ninject to automatically inject all my dependencies.
The problem I am having is that I am using a grid model to display grids in my Views like this:
#Html.Grid(Model).WithModel(new UserGridModel(Html)).Attributes(id => tableName)
and have the corresponding grid model defined:
public class UserGridModel : GridModel<User> {
public UserGridModel(HtmlHelper html)
{
Dictionary<int, string> userStatuses = /*TODO: GET ALL USER STATUSES*/;
Column.For(user => user.ID);
Column.For(user => html.ActionLink(user.Email, "Edit", new {id = user.ID})).Named(DtoResources.UserDto_Email);
Column.For(user => user.FirstName);
Column.For(user => user.LastName);
Column.For(user => userStatuses[user.StatusID]);
}
}
Now I need to inject a service into this model so it can pull in all of the applicable statuses from the service (BLL) level. Currently just to make sure this would work, I exposed the IKernel in the Bootstrapping code and just IKernel.Get() but I don't think that is the cleanest way to get it. I would use constructor injection, but if I put the IUserStatusService as a parameter in the constructor, I can't figure out how I would get Ninject to inject the correct parameter when I call new UserGridModel(Html) in the view without explicitly using the IKernel there.
I am either missing something or wiring this up all wrong. Either way I'm stuck ... any help? What is the proper way to get an instance of my service through Ninject
In my opinion the cleanest solution to your problem is to change your controller so that it creates a model that already contains the user status as string so that no convertions is required in the view. I would do as littel as possible in the view and grid model.
Another possibility is to property inject the service to your view an pass it to the grid model. But as I mentioned this way you are introducing logic to your view.