I have a View with a LongListMultiSelector. My ViewModel needs to access the SelectedItems property which is not available for binding. Without exposing my View to my ViewModel, how do I expose it?
This other question from someone also trying to accomplish the same thing has the solution.
Related
I have one controller, for which I'am binding context data. I have the context ID to get the binding context and want to get corresponding controller. Is it possible?
This is not possible as the binding context is not aware of the controller.
I have spent hours trying to use a set a tableViewCell from a picker selection in another view. I have posted two questions, that brought no answers. So i decided to approach it differently. I tired making a global variable, but then figured out, I had to make a dataClass file which wouldn't work for me because I have to use a viewController. Im beginning to lose hope. Is their any way to set the title of a table view cell from another view? Im not looking for a giant chunk of code, just a place to start. The way to actually do it, if its possible. Thanks in advance.
Yes, this is definitely possible. In Model-View-Controller systems all information sharing happens through your model class. Make it a singleton object (singleton is similar to global variables, but it has proper initialization).
Create a class with the data that must be shared. Create a class method of that class to produce the sole instance of that class. Define and initialize a static variable holding that instance. Use dispatch_once to initialize that instance. Here is an answer illustrating this approach.
With a singleton instance in place, all your view controllers can access the model as necessary. One view controller can set properties of the model, so that when the other view controller comes along, the data is ready for it to process.
I have just read this article about MVVMC pattern. Now I have a question. Should Controller be injected to ViewModel, or ViewModel should be injected into Controller?
The MVVMC is simply a MVC where the View is replaced by a ViewModel pair.
The View interacts ONLY with the ViewModel taking advantage of the powerful data binding mechanisms in XAML based technologies.
The ViewModel can notify the Controller but SHOULD NEVER inject a controller.
I have put together a simply sample based on the well know sample of Josh Smith on MSDN... where I have introduced a Controller.
It depends on what you are doing. I'm going to guess that most of the time the Controller would not need to be injected into either, but if it is needed, it is more likely to be needed in the ViewModel. Let me explain.
What are you doing with the controller? You must be doing something.. If that "something" is solely related to "what the data looks like", then it belongs in the View. If it is related to "what is be being shown to the user" then it belongs in the ViewModel.
I'm injecting a controller into one of my ViewModels. My ViewModel represents data which is then graphed in the View. I have a command which moves a data item from the current graph to a new graph. Since this changes "what is being displayed in the graph window" I implemented the command in my ViewModel. The ViewModel removes the data item from its own collection of items, and then uses the Controller to request a new view be created for that new data (it already had this functionality).
Looking at the article, I don't see arrows between the controller and the view
The ViewModel is a contract between the View and the Controller, and ideally does not need to know about (be dependent on) either.
So I definitely wouldn't be injecting a Controller into a ViewModel.
I'm not sure I'd be doing the opposite either: the controller is normally responsible for creating new ViewModel instances. If you want to go for a more loosely coupled implementation, you could go for injecting an abstract factory into the Controller, to avoid directly creating new instances of your ViewModel classes.
I believe the Controller should be injected as an abstraction IController. The ViewModel needs IController to be able to navigate to a different View/ViewModel.
For example, in ViewModel:
IController _controller;
public MyViewModel(IController controller){
_controller = controller;
}
void NavigateHome();
{
_controller.NavigateHome();
}
The abstraction IController is better than injecting the Controller itself for these reasons:
Testability. You can inject a mock IController and test the ViewModel
Decoupling. ViewModel doesn't have to know Controller.
I developed a lightweight framework for doing MVVMC in WPF.
It has a lot of resemblance to MVC in Asp.NET Core.
Check it out if you're looking for a WPF solution.
Blog post with documentation:
http://michaelscodingspot.com/2017/02/15/wpf-page-navigation-like-mvc-part-2-mvvmc-framework/
GitHub:
https://github.com/michaelscodingspot/WPF_MVVMC
I've been looking through the PRISM 2 samples for ideas about how to best approach a new application I'm working on, which will be a PRISM 2/WPF app. Looking in particular at the View Injection sample application that ships with PRISM I've noticed that all of the views implement an interface which allows the presenter (or ViewModel) to interact with the View.
In the past I've done this the opposite way round, I inject the presenter into the view so that the view can directly call to methods on the presenter a bit like this:
public partial class SomeView : ModuleBase
{
private ISomePresenter _somePresenter;
public SomeView (ISomePresenter somePresenter):this()
{
// Give the view a reference to the presenter
_somePresenter = somePresenter;
// Bind the View to the presenter
DataContext = _somePresenter;
}
private void btnSubmit_Click(object sender, RoutedEventArgs e)
{
// The view can call actions directly on the presenter (OK I should probably use a command for this)
_somePresenter.SomeAction();
}
}
The technique above seemed reasonable enough to me, but looking at the samples I'm starting to question the approach. Does anyone have views (no pun intended) on the best way to go about this?
Add the presenter to the view and get the view to interact with the presenter
Add the view to the presenter and get the presenter to interact with the view
Something totally different that I haven't thought of yet?
The most common approach to map a ViewModel to a View in MVVM is to use a DataTemplate :
<DataTemplate DataType="{x:Type vm:SomeViewModel}">
<v:SomeView />
</DataTemplate>
When you display a ViewModel instance in a ContentControl or ItemsControl, WPF will automatically instantiate the appropriate View for the ViewModel, and set the View's DataContext to the ViewModel instance.
That way, you don't have any reference to the View in the ViewModel, and the View only references the ViewModel through the DataContext property. In case you really need to access the ViewModel in the View's code-behind, you can always cast the DataContext (but this implies that the View knows about the ViewModel's actual type, which induces coupling)
I think it's all a matter of taste. Personally, I enjoy the way you see it in samples you are looking at. IView has one method, thats SetViewModel(...). IViewModel has a property called View of type Object, that essentially returns the DI instantiated IView.
The reason I like this way is I almost always want to create a ViewModel first and I want nobody in the code to be able to do anything with my IView, except get reference to the instance (for view injection or binding the view to say a ContentControl), which is why its of type object. If any code needs to talk to the View, for me, it's always via the VM...and even then the view gets updated usually via binding. It would feel odd to go from the View->ViewModel->UpdateBinding->View, than it is, VM->UpdateBinding->View
To answer the question, I generally don't need a reference to the presenter in the codebehind. Usually I can handle that with commands from the view being bound to the VM.
In some cases, you might want to keep reference to the presenter to do what you have in your example, but it is avoidable given the correct toolset (makes SL harder that it doesn't have built in commands).
Like I said, it's all a matter of taste...
-Jer
We are building an app using the MVVM pattern, we have controllers that wire up all the views and viewmodels using DI. All examples of MVVM I've seen are really simplistic and have 1 view. How do/should viewmodels talk back to the controller? The controller knows about the models and views, should the viewmodel send events back to the controller? Where should a save happen? Model? Controller?
Could your ViewModel not take a dependency on an IController or some other interface, so they can talk back to it? I try to keep as much application logic out of the ViewModel as possible, as these classes can easily become bloated.
MyViewModel(IController controller)
{
this.controller = controller;
}
void Save()
{
this.controller.Save();
}
I do agree that the MVVM frameworks tend to be too simplistic with their samples. In particular, moving between views/screens in your application is something I would like to see more examples of. I create an IViewManager interface, to allow my ViewModels to request that we move to another view.
We use Controllers too but in our case they are responsible for the application workflow. The Controller knows the ViewModel and the Model but not the concrete View because this will be injected by the IoC Container.
If you are interested in an example that shows more than just one UI (modal dialog, wizard with conditional workflow) then you might have a look at:
WPF Application Framework (WAF) - http://waf.codeplex.com
In case of an application that has multiple modules and requires separation of concerns I would recommend using prism framework.
http://msdn.microsoft.com/en-us/library/gg406140.aspx
I use a similar setup to you. In my controller, where my DI and view injection goes down, I sometimes keep reference to the ViewModel (which hold the View). Some cases I may have an event on the VM which is handled by the controller. In other, extreme cases (like if the VM/V was created outside the controller, say in another VM), I may even use the EventAggregator (with a strong ref) to listen to events that may be fired on the VM. In that case, a stored ref to the VM is not needed.
How about using events wherein the controller subscribes to VM events or using a mediator pattern where in a mediator is injected in a VM.