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I have spent long time searching about MVVM in WPF. I understand why to use it. This is clear for me.
What I don't is, what is in ViewModel itself. I know it is the connection between the View and the Model. Also, I know (at least, what most of programmers suggest) that implement the INotifyPropertyChanged. Adding to this, the Commands.
Let say, that I have this Model (Code First with EF 6):
public class City
{
[Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int CityId { get; set; }
[DataType("VarChar"), MaxLength(25), Required]
public string CityName { get; set; }
}
My Questions are:
1- In many examples I've seen, they recreate the same fields again in VM. What is the purpose, is it enough to have the whole object (City in my example). How about if there are many fields!.
2- To be able, to use it in my View (CRUD), how is the CityVm() will be in my example?.
I appreciate any help.
The view model in the MVVM pattern encapsulates the presentation logic and data for the view. It has no direct reference to the view or any knowledge about the view's specific implementation or type. The view model implements properties and commands to which the view can data bind and notifies the view of any state changes through change notification events. The properties and commands that the view model provides define the functionality to be offered by the UI, but the view determines how that functionality is to be rendered.
The view model is responsible for coordinating the view's interaction with any model classes that are required. Typically, there is a one-to many-relationship between the view model and the model classes. The view model may choose to expose model classes directly to the view so that controls in the view can data bind directly to them. In this case, the model classes will need to be designed to support data binding and the relevant change notification events.
The view model may convert or manipulate model data so that it can be easily consumed by the view. The view model may define additional properties to specifically support the view; these properties would not normally be part of (or cannot be added to) the model. For example, the view model may combine the value of two fields to make it easier for the view to present, or it may calculate the number of characters remaining for input for fields with a maximum length. The view model may also implement data validation logic to ensure data consistency.
The view model may also define logical states the view can use to provide visual changes in the UI. The view may define layout or styling changes that reflect the state of the view model. For example, the view model may define a state that indicates that data is being submitted asynchronously to a web service. The view can display an animation during this state to provide visual feedback to the user.
Typically, the view model will define commands or actions that can be represented in the UI and that the user can invoke. A common example is when the view model provides a Submit command that allows the user submit data to a web service or to a data repository. The view may choose to represent that command with a button so that the user can click the button to submit the data. Typically, when the command becomes unavailable, its associated UI representation becomes disabled. Commands provide a way to encapsulate user actions and to cleanly separate them from their visual representation in the UI.
To summarize, the view model has the following key characteristics:
The view model is a non-visual class and does not derive from any WPF base class. It encapsulates the presentation logic required to support a use case or user task in the application. The view model is testable independently of the view and the model.
The view model typically does not directly reference the view. It implements properties and commands to which the view can data bind. It notifies the view of any state changes via change notification events via the INotifyPropertyChanged and INotifyCollectionChanged interfaces.
The view model coordinates the view's interaction with the model. It may convert or manipulate data so that it can be easily consumed by the view and may implement additional properties that may not be present on the model. It may also implement data validation via the IDataErrorInfo or INotifyDataErrorInfo interfaces.
The view model may define logical states that the view can represent visually to the user.
This is a pretty broad topic based a lot on opinion. The MVVM model isn't a set-in-stone thing, and there's lots of different ways of implementing it.
The ViewModel, in my opinion, has two main purposes:
1) To expose the Model to the View. There's two ways of doing this. Having your Model wrap your CRUD object and manage NotifyPropertyChange. In this case, it's common for your ViewModel to just expose the Model to the View and not do much else with it, the View binding directly to the Model, or via simple pass-through properties.
The other common scenario is just to have your model as a CRUD. In this case, the ViewModel manages the NotifyPropertyChange notifications and manages model updates, etc.
2) To manage Presentation logic and data. There's lots of things that you need to bind to and store that have no place in the Model. The Model is the data. It's most likely loaded from a database. Yet the View needs to track other properties. An example: say your city object is displayed in a list, and you want to show or hide it with a button. The logic for hiding it, and the boolean for it being hidden doesn't belong in the View, and has nothing to do with the base data in the Model, so it sits in the ViewModel.
Similarly, maybe you have a CityPopulation property. In your view you want to highlight cities that are larger than 1,000,000 population in red. Your View Model has the a property:
bool LargeCity
{
return CityPopulation<1000000;
}
And you render the label with a style trigger.
Another example, a ViewModel containing a Brush property to bind to, which I think is probably a common but bad practice. slugster also comments below that visual elements should not be present in the ViewModel.
Brush HighlightBrush
{
get
{
if (CityPopulation<1000000)
{
return Brushes.Red;
}
else
{
return Brushes.Black;
}
}
1- In many examples I've seen, they recreate the same fields again in
VM. What is the purpose, is it enough to have the whole object (City
in my example). How about if there are many fields!.
Since the view model is the responsible of implementing view's logic and it performs actions against the model, there're some cases where some properties wouldn't be necessarily part of the model, but they're just there to serve view's requirements.
For example, let's say your model has a Name and Surname, and you want to show a concatenation of both properties in your view. You may be able to implement this using binding expressions or implementing a property in your view model:
public string FullName => $"{Model.Name} {Model.Surname}";
2- To be able, to use it in my View (CRUD), how is the CityVm() will
be in my example?.
This question has no specific answer. But you'll need a Model property at least to store a given City.
I have 3 screens that share a section(with model data in it(#Html.TextBoxFor)). What is the best way to implement this screens?
What I tried:
1) Partial view for the common section(_ClientData). 3 views for the different screens. 3 view models that have common property(ClientData), that is the view model of the partial.
Problem: If I pass the model to the partial as #{Html.RenderPartial("_ClientData", Model.ClientData);} the data from the partial is not submited to the model.
If I pass the model to the partial as #{Html.RenderPartial("_ClientData", Model);} and reference the properties with a fill name the data is submited, but I can't pass models with different types to the partial view.
2) Use one big View model with all the data required by the 3 screens, one view and show/hide some elements depending on some flag.
Problem: I can't use ValidationAttributes(for example if one field is required in screen 1, but it's not shown in screen 2 and its value is null, the validation will fire). I can use some manual validation in the controller but the whole thing with the all in one view and viewmodel sounds very bad.
Partials are usually not the best choice in case you want to place them inside one form and submit together. In such scenario it is better to take advantage of EditorTemplates which will solve your problem.
Firstly you would have to drag your partials to the folder ~/Shared/EditorTemplates/ and rename them to match the model name.
Then you can call them in your view like this:
Html.EditorFor(model => model.ClientData)
Thanks to this your HTML code (the name attributes to be precise) will be generated in such a way that your default model binder will be able to bind this part of your view as well.
I have created a model with Name, Email, Phone no properties and also created views for create, edit, delete and index. All these are working fine.
Now I have added a property in model like Address. Now I want that, is there any method so that, after adding the property in model, the newly added property automatic add in all the respective views.
Can we do this?
There is a way but only applicable if the view is generated by using HTML helper classes. The view should be strongly typed.
#Html.EditorForModel()
This HTML helper will automatically update and genatrate required input fields but with this you might have limited flexibility.
Other way can be creating your own class that renders html page.
Use as below in your view. View should be strongly typed of the model you want to use:
#using(html.BeginForm()){
#Html.EditorForModel()
}
I have an enum view model property which I want to hide in the CREATE View but show in the LIST/Index View. The enum is Open,Failed,Succeeded.
For the CREATE View the editor/control should not be visible.
For the LIST View the editor/control should be a selectable combobox
Can this be done somehow with the HiddenInput attribute?
If this can not be done then I want at least to make it visible in both views, but disabled/another control (label instead of combobox).
I am using asp.net mvc 4.0
Actually you should be using different view models. Remember: the first word of view model is view meaning that you define a view model per view. So you will have a CreateViewModel that will be used for the Create view without the enum property in question and a ListViewModel with the enum property that will be used in the Index view.
When doing code reviews I see many developers trying to reuse the same models over different views and ending up with some horrible logic because those models simply are not adapted for those views. They try to hide, they try to write ifs and stuff to perform validation, usually end up in a maintenance nightmare.
Existing Model, Controller and View, all working well.
Due to a requirement change, there is suddenly a change in the db schema, need to add a few new fields.
Database is updated, model (ADO.NET Entity) is refreshed from database and new fields are picked up. Controller is OK since it is working with the "set" and keying off the PK. The view when originally created was strongly typed against the model (as it was at the time)....
Is there an easy way to "refresh" the existing view to pick up the new fields?
Or will this always be a manual addition or where you would need to create a "newer" view to replace the previous view?
Thanks!
On the slim chance you're using the exact view created by the "add view" command, you can just delete/re add it. Otherwise, you can add a new temporary view, copy over the new/changed fields and then delete your temporary view.