Im new to MVVM and XAML. Currently the application that I have to extend is some kind of imageviewer. It uses MVVM with a RelayCommand. Currently the code behind file of my XAML-file is almost empty.
Data is displayed with databinding to the viewmodel and user interaction is bound with command to the model.
But now I need to display an image that is an custom control. In XAML I have to place a Border-Control and than the control must be initialized in the code behind file.
My question is how to do the setup when following MVVM.
PseudoCode:
View{
initPDFControl(){
borderControl.DoMagic();
};
openImage(String path);
}
ViewModel{
openImage(){
getMyView.openImage(pathFromModel);
}
}
You can help me by linking me to helpful tutorials or note down some pseudocode.
From the example you have given it looks like you are not aware of the MVVM pattern. The view (XAML) is linked to the viewmodel class. The viewModel should be unaware of any Views. But the view is aware of its viewModel. You should write the commands in viewModel and not in the model. You have to access model class from the viewModel. First understand the pattern and then go forward to coding. There are many MVVM Libraries (for eg. MVVMLight, Prism) available in Nuget, you can use them in your code.
You can look at these links for understanding the concept.
Model-View-ViewModel-MVVM-Explained
WPF-MVVM-step-by-step-Basics-to-Advance-Level
MSDN-Implementing the MVVM Pattern Using the Prism Library
Related
I have been following this tutorial on MVVM pattern using caliburn micro
https://www.youtube.com/watch?v=laPFq3Fhs8k
What is the difference between .xaml.cs and ViewModels/ViewModel.cs ?
I see we put our code for the shellview in ShellViewModel.cs, but when I, for example, choose an event handler for a Button, visual studio takes me to .xaml.cs and not to the ViewModels/ViewModel.
Do I have to stick to one or each file has a different role/responsibility? ( considering that I want my code to still be following MVVM pattern )
Files ending in .xaml.cs are what is called the View's "code-behind". Anything inside this file is tightly coupled with the View.
If you want to follow the MVVM pattern, you will code almost all of the interaction of the UI with your Models (example: click of a button change some data or get some data from the database) on the ViewModels (.cs files), which are independent from the Views.
The only link between the View and the ViewModel is that the ViewModel is the DataContext of the View. In other words, the View knows about the ViewModel, but that ViewModel does not know anything about the View.
Event Handlers are not generally used in a MVVM world. You need to use Commands. But that is a whole other question/answer.
The important thing to keep in mind is that MVVM comes to address, mainly, two things: the Separation of Concerns (UI vs UI Logic) and Testability of your software.
I'll start by suggesting that you learn the basics of the UI framework you're using (WPF, etc.) before adding an additional framework on top of it. I've seen that this only makes the learning curve steeper.
With that said, the .xaml.cs file is what is commonly referred to as the code behind. This is the code that "controls", so to speak, the UI. You'll note, for example, that these are all "partial" classes because there's going to be some auto-generated code when you compile that does view-specific things like bindings, etc.. The xaml.cs file should be used sparingly and only when you need to know or operate on specifics of the View itself.
With MVVM, you'll often see code behind classes that are almost completely empty, with just a constructor present. The logic in these cases is moved to a "View Model" class which provides a link between the View and the Model (take a look at this SO post for a more thorough description). You should bind to objects in the View Model, and use it as the data context for your view. It contains all the logic to define how the view effects your model.
So - applying this to event handlers... An event must live in the code-behind, because you can't explicitly bind to one in your VM. You could easily call the function you want by accessing the ViewModel right inside that event handler, and there's nothing particularly egregious about this approach. That said, there is another concept called a Command that will live in the View Model itself; a command, as opposed to an event handler, is a Property, so you can bind to that in your .xaml file using
...Command="{Binding FunctionNameHere}"...
It also has nice features like being able to define when the command is valid (so you can disable your button automatically when, for example, the form is missing a required field).
I've recently been learning the MVVM pattern in WPF and just started making my first proper, rather big application. So far it's all smooth sailing, and I'm liking what I'm seeing a lot. However I recently met something of a stumbling block.
The application is built with a main TabControl, each TabItem containing a pretty big details view.
TabControl inside main View, ItemsSource bound to MainViewModel.OpenTabs
TabItem with data specific View+ViewModel
TabItem with data specific View+ViewModel
TabItem with data specific View+ViewModel
etc...
The OpenTabs collection is an ObservableCollection<BaseViewModel> on MainViewModel, and the TabControl's SelectedItem is bound to MainViewModel.ActiveTab.
So far so good! However, what I'm not sure I'm getting is how to handle closing of tabs while at the same time following MVVM. If I wasn't trying to be strict with the MVVM (in order to learn it properly), I'd just bind a MouseDown-event on the TabItem-headers and thus get a reference to the clicked item in that event, removing it from the OpenTabs collection in that way. But - unless I'm mistaken - the interaction logic shouldn't need references to actual UI items in order to be effective and proper MVVM.
So, how do I handle this MVVM style? Do I use a command that sends a specific parameter with it to my MainViewModel? It seems like the preferred implementation of ICommand in MVVM doesn't take object parameters (looking at MVVM Light as well as some other tutorials).
Should I just create a CloseTab(int id) public method on my MainViewModel and call that from the view codebehind after catching the Click on my TabItem close button? This seems like MVVM-cheating. :)
Also a final note - this should work even if I click close on a TabItem that isn't the currently active one. Otherwise it wouldn't be hard to setup with OpenTabs.Remove(ActiveTab).
Thanks for any help! I'd also appreciate any links to recommended reading/watching regarding these problems.
Solution: It seems the best way is to use a command that can accept command parameters. I used the RelayCommand from MVVM Light framework:
In MainViewModel:
CloseTabCommand = new RelayCommand<BaseViewModel>((vm) =>
{
OpenTabs.Remove(vm);
});
In XAML:
<Button
Command="{Binding Source={StaticResource MainViewModel}, Path=CloseTabCommand}"
CommandParameter="{Binding}">
Note: Your binding paths may of course vary depending on how your Views and ViewModels are set up.
The best and the right way is to create the command. In different frameworks ICommand usually has two implementation, with the parameter and without one (as often you do not need it).
MVVM light has two ICommand implementation as well: RelayCommand and RelayCommand<T>
I suggest creating your own DelegateCommand implementation, a good example on how to this can be found here or here. Or use the Prism variant, you can download it here.
With a DelegateCommand you can pass arguments down to your ViewModel.
i'm trying to walk my way in the mvvm world,
i'm not suing any framework , only what i saw in
http://blog.roboblob.com/tag/viewmodellocator/
i really like that post and it works very well.
i'm trying to create a master details view that containes 3 user control.
one of them need to change depending on user inputs ( like a wizard)
so i have something like
MasterDetailsView
-> MenuView
-> FirstPageView
-> SliderView
by the post i read, each view create his own viewmodel and bind it "automaticly"
so FirstPageView got his viewmodel.
if SliderView change selection i want to bind to FirstPageView a new viewmodel
if FirstPageView got button (Next) click inside it i want to switch FirstPageView to SecondPageView and etc..
i'm a bit lost, anyone can please show me the way how to design and solve this ?
should i use mvvm light or some other framework ? i tryed to use mvvm light but that lack of examples holds me back.
any guidance will be welcome, Thanks.
The statement below is wrong:
each view create his own viewmodel and bind it "automaticly" so
FirstPageView got his viewmodel
Your views should not be responsible for the lifecycle of your view model.
One of the most important things to remember when using the MVVM pattern is that it should be possible to execute your application without your view. In your case in would create a view model that closely mirrors your desired view. Here is the pseudo-code:
MasterDetailsViewModel
{
MenuViewModel
CurrentPageViewModel
SliderViewModel
}
Notice the big difference here is that we have a CurrentPageViewModel, not a view model that is specific to a page. This property starts off as a FirstPageViewModel instance, then changes as the users advances from one page to the next.
For details on how to achieve this, I would thoroughly recommend the article by Josh Smith:
WPF Apps With The Model-View-ViewModel Design Pattern
I'm implementing my first WPF application using the MVVM pattern.
According to the tutorial I'm following, it is a bad practice to have code in your code behind class.
Now I want to implement sorting of a GridView when you click on a column header.
The usual way to achieve this, is implementing a column click handler in your code behind class, which seems to be a code smell (see above).
Is there a solution without using the code behind class?
One way I can think of is to provide a SortCommand : ICommand in my viewmodel that does the sorting in the model. But I'm not sure if this is a good and clean solution.
Thanks for any help
I would use the command approach.
Remember that the Model is your data, the ViewModel is an adaptor which makes the data available to the View, and the View is your GUI.
I was wondering what the best approach is for sharing a menu across all wpf windows/views.
My Application doesnt really fit the navigation model, so will probably use a Ribbon control.
I am leaning towards creating a user control for the menu and dropping it on each view, but have also seen Josh Smith's msdn article, where he loads user controls.
Are there any other options or established best practices?
Thanks in Advance.
I ended up implementing in a way similar to Josh Smiths; I have however simplified things a bit.
All views are usercontrols, except the MainWindow.
The MainWindow contains a ContentTemplate which is bound to a property holding reference to a single UserControl in the view model.
I then have an ApplicationController responsible for controlling the view and view model lifecycle.
The ViewModel base class used by each view contains a reference to the IApplicationController.
The MainWindowViewModel then makes calls to the ApplicationController to load a new view etc.
Still not 100% on this approach so would welcome any further suggestions.
Here's what I would probably try:
Define an ISharedMenu interface
Create a UserControl which uses the ISharedMenu as its DataContext.
For each ViewModel that you want to use the shared menu, implement the ISharedMenu interface.