Is it possible to get the whole view model in tag helper Process method (.NET Core MVC)?
Everything passed to the tag helper is done via attributes. If you want the whole view model, then you'd simply so domething like:
<mytag model="#Model" />
And then you'd need a property on your tag helper to bind this to like:
public MyViewModel Model { get; set; }
The name of the attribute corresponds to the name of the property. There's nothing special about "model" here.
However, the utility of that is going to be limited. Tag helpers are intended to be somewhat generic. That's the point: encapsulating reusable logic. If you tie it to a particular view model class (based on the property), then it will only work with that particular view model. The only way to make it more generic would be to use a base class or to literally type it as object, so that anything could be passed. However, with a base class, 1) you need to have every view model inherit from this base class and 2) even then, you'd only be able to use properties on the base class. With object, you wouldn't really be able to reference any properties unless you downcast it to a particular view model class first. While that would allow you to handle any scenario, in principle, you'd be forced to have long blocks of switch or if statements in your tag helper to conditionally handle different scenarios.
Long and short, it's not a great idea for many reasons to pass the whole model. The tag helper should have one specific purpose, and you should only pass things that are specifically needed by it, which also allows you to be explicit about those needs.
If you're looking for something to handle a whole model, you're more likely looking for a partial view or view component, rather than a tag helper.
The viewmodel is actually available if you bind first the for element as :
[HtmlAttributeName("asp-for")]
public ModelExpression For { get; set; }
Then you can access it in your tag helper Process or ProcessAsync through:
For.ModelExplorer.Container.Model
Related
my question is, what is the best way to let a child class communicate with the parent class.
For example:
I have a main class simply called Main, and another class SomeClass.
Now the Main class creates an instance of SomeClass, once the state of the SomeClass-object changes, the Main class should execute different code, depending on what changed.
Yeah I know, that already sounds like the Observer design pattern, but how would I implement it with state changes treated differently?
I'm currently writing an Android app with a database to make it more specific.
In my project I have the main class, a class to connect, read from/write to the database and a GUI container class. (oversimplified, there are a few more)
The main class creates an instance of both the GUI and database class.
Now if I press a button A, it should write A-data to the database,
if I press button B, it should write B-data to the database.
As I think that a gui class shouldn't have direct access to the database, I tried other options, than just accessing the database from the gui-class
Currently, I defined a placeholder abstract class with only one method, that I am just overwriting with the functionality.
So right now I have to create a one-method-class A for the click of button A and a one-method-class B for the click of button B.
It doesn't sound like the best way to me, I mean It's working, but I'd like to improve my code, so if you have any idea, please write your solution. :)
As a good practice it is better to avoid write code in GUI class. So we can use MVVM pattern here.
Let me show a simple example for your case. This is a ViewModel class. View model does not have reference to view class:
public class YourViewModel
{
public void LoadA()
{
// here you can interact with your database
}
public void LoadB()
{
// here you can interact with your database
}
}
This is your view class. It handles button clicking, user interactions with view and forwards to the view model. It has a reference to view model.
public class YourView
{
YourViewModel yourViewModel;
public YourView()
{
yourViewModel = new YourViewModel();
}
public void ButtonA_Handler()
{
yourViewModel.LoadA();
}
public void ButtonB_Handler()
{
yourViewModel.LoadB();
}
}
If you want to handle many events, then you can try to use this approach How to: Handle Multiple Events Using Event Properties.
It seems like a good way to approach this would be to use a pattern like that described in this previous Stack Overflow answer.
They provide sample implementations there but to apply to your case, you don't need to give the GUI direct access, you can have a parent class which implements the "listener" functionality, and a child (GUI) class which just calls its parent, with those details abstracted away from the child.
If you feel like you need more details/examples on implementing this pattern see https://refactoring.guru/design-patterns/observer/java/example
I have searched many places and have not seen anything similar to what I am thinking.
Let's say I want to create a reusable container component, like a card, form, or a modal, and save that as a View Component. How would I add a new view components inside of the "body" of that main View Component in a way that would make it maximally reusable?
The syntax here is just to demonstrate the idea of course, but for example, something like this:
<vc:parent var1="x" var2="y">
<vc:child var3="a" var4="b"></vc:child>
<vc:child var3="c" var4="d"></vc:child>
</vc:parent>
Is anything like this possible?
This doesn't necessarily need to use View Components—maybe partial views?—so long as the primary goal of reusing the containers of other reusable elements is achieved.
I have looked into helpers, but they are no longer available in ASP.NET Core.
So I figured out how to do it.
I used part of this tutorial about helpers: Templates With Razor
And modified it so it works with ViewComponents.
So to get it working, in a minimal example, create a ViewComponent class as such:
[ViewComponent(Name = "Test")]
public class VCTest : ViewComponent
{
public IViewComponentResult Invoke(Func<dynamic, object> Content)
{
return View(Content);
}
}
Create the actual template that you want, in a cshtml file like this:
#model Func<dynamic, object>
<div id="SomeTemplateTest">
#Model(null)
</div>
In this very simple case I just used the Func as model since there is only one parameter, but for more parameters you'd just have to call #Model.funname(null) instead of just #Model(null). No big deal.
when calling this component from your view, create your child elements beforehand like so:
#{Func<dynamic, object> children=
#<div>
<vc:child var1="a" var2="b"></vc:child>
<vc:child var1="c" var2="d"></vc:child>
<vc:child var1="e" var2="f"></vc:child>
</div>;}
The div is there only to encapsulate all the elements. I haven't found a way around that but it has no major implications.
Then call the parent ViewComponent tag passing on the parameters accordingly:
<vc:test content="children"></vc:form-test>
And that's it, worked perfectly. It is unfortunate that I could not find a more seamless way. But this does the job.
If anyone knows of a better alternative I'd love to know more.
Those are three concepts on Yii that I really don't get what should we use, on what scenarios?
Can anyone be kind enough to clarify those Yii elements, and on what situation should we use them?
In documentation of CForm one can read the following:
...we can divide a form
in two parts: those that specify each individual form inputs, and
those that decorate the form inputs. A CForm object represents the former part...
...and CActiveForm represents the latter.
In other words, CForm specifies elements of the form but CActiveForm (being a widget) renders it.
Looking at the source code we state that CForm can also render() itself and its rendering relies on and is wrapped by CActiveForm widget by introducing its configuration property activeForm, though rendering input elements and buttons is implemented by its own methods renderElements() and renderButtons() relatively. By default their implementations rely on classes using CHtml's static methods what is exactly the same (or almost exactly the same) what CActiveForm's rendering methods do. Of course, default behavior can be overriden by extending the class.
That's why it's the question of a taste which technique to use: CActiveForm widget alone combining form fields' and buttons' declaration with their representation in a view file by calling convenient (required) methods of CActiveForm instance or CForm class declaring form's input specifications in a separate configuration file and customizing its rendering by pointing at appropriate active form widget and/or by overriding default rendering methods. The latter technique allows to reuse a form in several actions easily and is no more than using form builder.
Check here for live examples of ActiveForm, CForm, et cetera. You can also see the live Model, View & Controller files.
Is it correct to store all my ViewModels in SimpleIoc? For instance I am having three pages MainPage, Photos, Directories (therefore three ViewModels -> MainVM, PhotosVM, DirectoriesVM). Should I set DataContext in each page to View Model Property in ViewModelLocator or nest ViewModels as properties in MainVM and bind each page DataContext to Main.PhotosVMProperty, Main.DirectoriesVMProperty and so on? Could anyone explain me idea and purpose of IoC ?
First, lets look at what ViewModelLocator does and why we use it:
ViewModelLocator is declared as an object on our App.xaml page and is an application singleton. We're going to have one, and only one of them available to the application when it runs.
ViewModelLocator is the source for all our ViewModels in MVVM Light. For each ViewModel we'll have a property on the ViewModelLocator that allows us to get a ViewModel for a View. This code looks like this:
public class ViewModelLocator
{
public MainPageViewModel MainPage
{
get { return new MainPageViewModel(); }
}
}
This is a piece of my App.xaml:
<Application.Resources>
<vm:ViewModelLocator
x:Key="ViewModelLocator" />
</Application.Resources>
This is a piece from View.xaml
DataContext="{Binding MainPage, Source={StaticResource ViewModelLocator}}"
So far so good. To answer your first question, do you have to use Ioc in MVVM Light? No. There's no need as your viewmodel will be given to your view fully built and instantiated by the ViewModelLocator.
Now, onto your second question: What's the purpose of IoC?
IoC is designed to allow you to do the following:
With Mvvm Light you do the above like this:
public class ViewModelLocator
{
public ViewModelLocator()
{
ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default);
if (ViewModelBase.IsInDesignModeStatic)
{
SimpleIoc.Default.Register<IDataService, Design.DesignDataService>();
}
else
{
SimpleIoc.Default.Register<IDataService, DataService>();
}
SimpleIoc.Default.Register<MainViewModel>();
}
public MainViewModel Main
{
get { return SimpleIoc.Default.GetInstance<MainViewModel>(); }
}
}
public class MainViewModel
{
public ObservableCollection<Foo> Foos { get; set; }
public MainViewModel(IDataService dataService)
{
_dataService=dataService;
Foos=_dataService.GetFoos();
}
}
When I resolve my MainViewModel when I call
SimpleIoc.Default.GetInstance<MainViewModel>()
what happens internally is that the SimpleIoc checks to see if the MainViewModel has any dependencies (parameters in its constructor). It then tries to resolve these parameters by looking at the interfaces that have been registered with it. It does this recursively, so if DataService had a dependency it would be instantiated and passed to the DataService constructor when it was being instantiated as well.
Why would I do all this work?
Make your classes easily unit testable
Make your code interface-driven. This means that you're referencing interfaces rather than concrete classes
Make your code loosely coupled. This means that someone can change the implementation of an interface and classes that consume that interface don't care and don't have to be re-coded.
Resolve your classes dependencies in an automated way.
In MVVM Light, you'll see that it can tell when it's running in design-mode (ViewModelBase.IsInDesignModeStatic), this means that you can create design-time services to provide your viewmodels data so your View in Visual Studio contains actual data.
MVVM Light has a lot of nice features but it appears to me that the Service Locator creates unwanted dependency of the views on the view models. Ideally, I would like to have the ViewModelLocator in Library A, the view models in Library B and the views in Library C. Then I can mix and match these as needed for future projects. However, in the design of MVVM Light, as far as I can see, the views (Library C) will always have a dependency on the ViewModelLocator (this is okay) but because the ViewModelLocator (Library A) will always have a dependency on the view models (Library B), then the views will always depend on the view models (this is not okay because a view now must include all the view model libraries it was ever used with across all products).
I believe that Prism gets around this problem by using string keys somehow. Am I missing something?
Oops! I think I just answered my own question. The solution is to make Library A, the ServiceLocator, specific to a particular solution (product). It then contains a reference to the view models only for that solution. Then the views depend on this ServiceLocator which in turn depends on all the view models for that product. The final result is that the views depend only on the views models that it will be used with for that product. There is no problem with thee fact that we are duplicating the ServiceLocator for each solution because this module contains only code that is specific to the solution. The components of the ServiceLocator such as the SimpleIoc class are, of course, common to all solutions, but these have been factored out into reusable classes that we invoke in ServiceLocator.
To summarize things, the problem I am trying to solve is suppose that a solution has 6 view models, four of which are closely related and two of which are closely related. We therefore create two assemblies, each containing the closely related view models. Suppose we design a product that uses one set of view models and the solution is designed to run Windows 8. Now the views are all different and we want to re-use only one set (assembly) of view models. So we just create a new ServiceLocator assembly that points to this assembly of view models and also any others that we need. Our new Windows 8 views now depend on this new ServiceLocator assembly and only the view models that are used in our new product (solution).
I'm new to Silverlight/MVVM. I tried some example of MVVM Light, it looks great.
For my scenario, I want to create a Panorama Page, for each Panorama Item, showing my usercontrol, a item list for a customer.
I've built usercontrol(view), viewmodel and WCF service model and works well in a single Panorama Item(Only use first customer).
Also, I use Locator of MVVM Light shown in MIX10 demo, it enables me to make design time data for Expression Blend.
My viewmodel will receive a parameter of customer ID then exchange data with WCF based on this ID.
And the customer list also comes from WCF. So I can't actually makes viewmodels in Locator's static constructor.
If viewmodels are built in runtime by calling Locator, how to make data binding?
The only way I think about is to make viewmodel object in usercontrol's constructor and make it datacontext.
Is there a better solution?
If you want to keep the same declarative model in the XAML, you can put a CurrentCustomerViewModel property on the locator and then set property to the right viewmodel before you navigate to the page.
Personally though for pages like that I typically put a viewmodel factory method on the locator (so it can cache them, etc) and call it from the OnNavigatedTo method, something like this.
protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
string id = NavigationContext.QueryString["customerID"];
vm = ViewModelLocator.GetCustomerViewModel(id);
DataContext = vm;
base.OnNavigatedTo(e);
}
Then I just use Blend's sample data capabilities for design time data. This way also helps support pinning the page to the start screen since that will be the entry point to the app and I won't necessarily get a good chance to set the "CurrentCustomerVM" property anyway.