So I'm working on a GUI and most of it I implemented with 1 window and used the code-behind for that window to handle most of the logic. The program is very GUI driven. Say there is a combo box, if you select something from the combo box, the data drastically changes and all the other GUI boxes/labels/grids change or clear ect ect.
I'm doing a lot of refactoring and I've been aware of MVVM, but I've never really seen the need for it. I understand what and why its used, but functionality its just easier to reference all the GUI components straight from the code behind I've found.
For example...
In my main window I have
<ComboBox x:Name="MyTitlesUI" ItemsSource="{Binding Titles}" SelectionChanged="MyTitlesUI_SelectionChanged">
So the ComboBox is tied to a List Titles in my MainWindowViewModel right?
Where should MyTitlesUI_SelectionChanged event go? It needs to go in the View correct? But what if the functionality of SelectionChanged has to do with data inside MainWindowViewModel?
Say you change the selection in MyTitlesUI and now the program has to look up up that Title string in a database. All of that database functionality is in DBClass which you declare in MainWindowViewModel. How do you access that functionality? Why would you have to do this:
In main window cs:
private void MyTitlesUI_SelectionChanged(object sender, EventArgs e)
{
viewModel.ConnectToDataBase((string)MyTitlesUI.SelectedItem);
}
In MainWindowViewModel.cs
private SelectedTitle;
public void ConnectToDataBase(string title)
{
SelectedTitle = title;
DBClass myDB = new DBClass(SelectedTitle);
.... //do stuff with myDB
}
That just seems kind of unnecessary no? This is just a mild mild example of course and maybe that seems pretty clean. But if you're doing really complex back and fourth between View and ViewModel, the reference to MyTitlesUI.SelectedItem in View may be needed in ViewModel for other functions to work hence the SelectedTitle private variable.
Now you have more assignments, more variables, more functions that just call other functions than just a simple MyTitlesUI.SelectedItem to deal with.
Why not bring the DBClass reference up to the View or similar?
Especially if you're doing a lot of UI manipulation that the information inside your ViewModel will be playing with. Say once I change the selection of Title, I need graph to clear. But my graph can't clear until my ViewModel has connected to the DB or something.
I'm going to have graphs or grids defined in my View that depend on dynamically created data in my ViewModel that needs to update. And I'm trying to wrap around what needs to be in View and what needs to be in ViewModel. It seems to be not proper to reference View from ViewModel, so something like MyTitlesUI.SelectedItem can't be called in ViewModel.
EDIT:
So going back to the Selected Item example, say I have a Treeview UI item. I want to bind that to a Treeview that I don't have yet. I create the data for it procedural with DB connect. So the user selects from the combobox the Title they want. Db Connect then creates, asynchronously, a TreeviewItem in some kind of data structure.
private SelectedTitle;
public void ConnectToDataBase(string title)
{
SelectedTitle = title;
DBClass myDB = new DBClass(SelectedTitle);
if(myDB.doneWorking)
{
myTreeView.ItemsSource = myDB.GetTree();
}
}
but functionality its just easier to reference all the GUI components
straight from the code behind I've found
Wrong. MVVM delivers a clean, Property-based approach that's much easier to work with than the txtPepe_TextChanged() winforms-like approach. Try to change the Text for a TextBlock buried deep inside a DataTemplate that is used as the ItemTemplate of a Virtualized ItemsControl using code behind... WPF is not winforms.
Where should MyTitlesUI_SelectionChanged event go?
Nowhere. MVVM works best with a property/DataBinding based approach, as opposed to a procedural event-based approach.
For instance, a ComboBox-based UI that "does stuff" when the user changes the selection in the ComboBox should be defined like this:
<ComboBox ItemsSource="{Binding MyCollection}"
SelectedItem="{Binding SelectedItem}"/>
ViewModel:
public class ViewModel
{
public ObservableCollection<MyItems> MyCollection {get;set;}
private void _selectedItem;
public MyItem SelectedItem
{
get { return _selectedItem; }
set
{
_selectedItem = value;
NotifyPropertyChanged();
DoStuffWhenComboIsChanged();
}
}
private void DoStuffWhenComboIsChanged()
{
//... Do stuff here
}
}
Now you have more assignments, more variables, more functions that
just call other functions than just a simple MyTitlesUI.SelectedItem
to deal with.
Wrong. What you have now is a Strongly Typed property in the ViewModel of type MyItem that you work with instead of the horrible casting stuff (MyItem)ComboBox.SelectedItem or things of that sort.
This approach has the additional advantage that your application logic is completely decoupled from the UI and thus you're free to do all sorts of crazy stuff on the UI (such as replacing the ComboBox for a 3D rotating pink elephant if you wanted to).
Why not bring the DBClass reference up to the View or similar?
Because DataBase code does NOT belong into the UI.
Especially if you're doing a lot of UI manipulation
You don't do "UI manipulation" in WPF. You do DataBinding which is a much cleaner and scalable approach.
Related
What I want to do is access the database via a query (already have one made, but heres the issue:
namespace WpfApp3
{
/// <summary>
/// Interaction logic for Window1.xaml
/// </summary>
public partial class ViewData : Window
{
public ViewData()
{
InitializeComponent();
}
private void ShowData(object sender, RoutedEventArgs e)
{
CustomerEntities1 db = new CustomerEntities1();
if (report.IsInitialized)
{
var report = from values in db.Customers
select values;
valueGrid.ItemsSource = report.ToList();
}
else if (sortName.IsInitialized)
{
var sortName = from values in db.Addresses
select values;
valueGrid.ItemsSource = sortName.ToList();
}
}
}
}
I Do not understand how the bindings in WPF work, and Im having issues when running the code. It only runs the top portion of the if condition. Any ideas what binding I should use where if clicked, it is considered true, and runs the condition when met. Once the condition is met, it should remove the old one, and replace it with the new query for the DataGrid. Thank You
So in WPF the UI Components (valueGrid) in your case are usually notified if the Data changes (CustomerEntities1) on Initialization and IF and ONLY IF the Source implements INotifyPropertyChanged . you should therefore consider implementing this interface in ally or ViewModels, assuming you are using the MVVM pattern.
Any ideas what Binding I should use
The way your code is set up you can't use bindings at all but you would want a oneWay binding if all you do is populate your DataGrid from code.
If you want to use Data binding you need to set the DataContext of your Window to something like
DataContext = this;
This will allow you to bind properties that you declare in your code behind to UI controls in your .xaml code like this:
<TextBlock Text="{Binding PropertyName}"/>
Now this might work syntax wise but your DataGrid still won't update because your DataContext class or any of their parent classes need to Implement the INotifyPropertyChanged Interface.
I advise you to start reading about the basics of Data binding and work through a tutorial or two. Because even if you follow what I've written above and you get it to work it is not the ideal approach to this problem.
You'd want to set up a type of class known as ViewModel that will be have all the properties that hold the data you want to show in your View (the Window) and have it as the Datacontext.
This link will give you a good overview of what data binding is, what you can do and how get it to work.
https://learn.microsoft.com/en-us/dotnet/desktop/wpf/data/?view=netdesktop-6.0
In my program I'm trying to design an output window to display logging information. For right now I'm binding to a string and updating the info as soon as log data comes in.
Ex:
<FlowDocument>
<Paragraph>
<Run DataContext="{Binding}" Text="{Binding OutputText}"/>
</Paragraph>
</FlowDocument>
(The flow document is inside a rich textbox)
In my viewmodel this is how I'm updating my outputtext
OutputText += loggingInfoString;
The problem is that strings are immutable data types and I don't like the idea of creating a new string every single time new data comes in. It's an unnecessary overhead.
RichTextBox has a method called AppendText which I assume will use something like a Stringbuilder in order to add to the string. The problem I'm having is being able to access the AppendText in the ViewModel.
I thought about making an instance of RichTextBox inside the ViewModel and binding the RichTextBox to the one in the ViewModel. I'm thinking that this violates MVVM but I'm not entirely sure. Is there another way to go about this? Or should I just create the RichTextBox instance inside the ViewModel?
Ex:
// ViewModel
RichTextBox Output;
// Update Method
Output.AppendText(loggerInfoText);
// Xaml
<RichTextBox DataContext="{Binding Output}" />
Thanks in advance!
You create an interface like so
interface IAppender{
void Append(string appendText);
}
You inject this interface into your viewmodel and you implement it in your view.
This means that in your viewmodel code you simply do
appender.Append(loggerInfoText);
And in your view you implement the interface by adding text to the richtext.
Based on the implementation of your view and viewmodel, depending on where the viewmodel is created you can inject the interface in the viewmodel. Suppose the viewmodel is created in the view you get something like this:
class View : UserControl, IAppender{
View(){
InitializeComponent();
DataContext = new YourViewModel(this);
}
void Append(string appendText){
//add text to richttext
}
}
public class YourViewModel : ViewModelBase{
private IAppender _appender;
public YourViewModel(IAppender appender){
_appender = appender;
}
}
This should get you started I hope
Note that this is pseudo code, not tested not even ran it through a compiler.
Passing the richttext to the viewmodel is indeed not MVVM. The idea is to have separation of concern. Via my interface approach this is not violated.
As alternatives to injecting a dependency (which even though decoupled, makes injection of views a pain in the ass) which is implemented by your control:
First, you can use StringBuilder internally to build your string.
public class MyViewModel : ViewModelBase
{
private StringBuilder sb;
private string Content
{
return sb.ToString();
}
protected Append(string text) {
sb.Append(text);
PropertyChanged("Content");
}
}
then bind Content property to your XAML element. This way you won't have allocations/deallocations in your ViewModel and you don't need to hassle with DI to inject your View/Control into the ViewModel (even though it's abstracted by the interface).
However, this assumes that you are going have avoid this allocations everywhere. If you only want it in a certain type of application (i.e. Windows Store App) but not in your ASP.NET or Desktop app, the second alternative may be much more attractive (especially for other similar scenarios):
You can use a custom Behavior to achieve this, as seen as answer in this question. Behaviors are reusable and you won't have to do this kind of decisions within the ViewModel, allowing you to differently handing this per platform or even per View.
And it simplifies your ViewModel and you won't have to bother how to inject the View that implements the IAppender into your ViewModel (can cause troubles in View-first approach, where the View gets resolved by the IoC and which gets the ViewModel injected into the View to be set as DataContext)
Or should I only create viewmodels for the domain data being represented? While reading on MVVM, I came across this:
"The ViewModel is responsible for these tasks. The term means "Model of a View", and can be thought of as abstraction of the view, but it also provides a specialization of the Model that the View can use for data-binding. In this latter role the ViewModel contains data-transformers that convert Model types into View types, and it contains Commands the View can use to interact with the Model. "
http://blogs.msdn.com/b/johngossman/archive/2005/10/08/478683.aspx
If the viewmodel is a model of the view, then doesn't it make sense to put properties of the view in the viewmodel rather than on the code behind of the view itself?
I guess in making a custom control I just have a hard time deciding when I should just add a property to the control's code behind and when it is worthwhile to make a viewmodel for the control to represent it. Honestly I kind of feel that moving all of the control's view related properties to the viewmodel would clean up the code behind of the control leaving only the control logic.
However, if I were to change things like this, then at times when an item needs properties from the control itself I can no longer use {Binding ElementName = control, Path=property} and have to instead get the data context of the parent (because the current datacontext would be on the individual subitem of the observable collection.
Basically I was considering whether I should move properties from Class GraphViewer into a GraphViewerViewModel and then just bind to it.
Code is worth a million words so:
public class GraphViewerViewModel :DependencyObject
{
private const int DEFAULT_PEN_WIDTH = 2;
private const int DEFAULT_GRAPH_HEIGHT = 25;
public SignalDataViewModel _SignalDataViewModel
{
get;
set;
}
public PreferencesViewModel _PreferencesViewModel
{
get;
set;
}
}
Meanwhile
public class SignalDataViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
ObservableCollection<SignalViewModel> _signals;
public ObservableCollection<SignalViewModel> Signals
{
get
{
return _signals;
}
private set
{
_signals = value;
}
}
ObservableCollection<SignalViewModel> _AddedSignals;
public ObservableCollection<SignalViewModel> AddedSignals
{
get
{
return _AddedSignals;
}
private set
{
_AddedSignals = value;
}
}
it is a pain to type:
PenWidth="{Binding RelativeSource = {RelativeSource AncestorType={x:Type DaedalusGraphViewer:GraphViewer}},
Path = _GraphViewerViewModel._SignalDataViewModel._AxisDivisionUnit.GraphPenWidth, Mode=OneWay}"
and I'm wondering if it is worthwhile to make the change or whether I'm misunderstanding what a view model should be used for in mvvm.
I guess in making a custom control I just have a hard time deciding when I should just add a property to the control's code behind and when it is worthwhile to make a viewmodel for the control to represent it. Honestly I kind of feel that moving all of the control's view related properties to the viewmodel would clean up the code behind of the control leaving only the control logic.
In general, a custom control is 100% View layer code. As such, it really falls outside of MVVM entirely.
The main goal when making a custom control to be used within an application being designed with MVVM is to make sure that you design and build the custom control in a way that it is fully compatible with data binding. This will allow it to be used within your View layer of your application exactly like other controls.
As such, this pretty much guarantees that you'll have code behind, since implementing Dependency Properties really requires code behind. You also don't want to set the DataContext of a custom control within the control (since you want to inherit the data context of the user control or window using the control).
Basically I was considering whether I should move properties from Class GraphViewer into a GraphViewerViewModel and then just bind to it.
If the types are specific to your domain, then this is really typically more of a UserControl being used by your application. In that case, creating a ViewModel and just binding is likely good.
If this is, on the other hand, a true custom control that's made to be completely general purpose (ie: usable by anybody in any application), then keeping it as a "pure view" custom control typically means that you 1) won't take a dependency on any ViewModels or domain specific objects, and 2) not set the data context (which means no view model).
Currently I have 2 ViewModels in my project. MainViewModel is currently bound to my WPF window, so I have a command in my other ViewModel (AdventurerViewModel) that doesn't fire anymore. Is there a way to have that command fire without the ViewModel being bound or do all of my commands need to be in MainViewModel? And is there a good way to organize commands that might help me avoid this situation in the future? Any advice would be greatly appreciated.
EDIT: A bit of code
From my XAML
Button Grid.Column="3" Grid.Row="1" Content="IncreaseLevel" Command="{Binding IncreaseLevel}"
AdventurerViewModel.cs
public ICommand IncreaseLevel { get { return new RelayCommand(IncreaseLevelExecute, CanIncreaseLevelExecute); } }
private bool CanIncreaseLevelExecute()
{
return true;
}
private void IncreaseLevelExecute()
{
Level++;
}
First make sure you split the ViewModels correctly and have the commands in the correct ViewModels.
After that it might still be possible that you want to notify other ViewModels of changes in a current ViewModel. In that case you could have a look at a messaging system between ViewModels so a ViewModel can subscribe to a specific event and respond to it.
Use one ViewModel per View. If you use a Window then you should use one ViewModel. Right now in my project a have a Window which has 2 user controls in it, and each UserControl has its own ViewModel plus the ViewModel for the Window. I could put everything in one ViewModel but it is cleaner this way, but for your example I recomend you put everything in one ViewModel. It realy depends on situations or project requirements.
I've been doing the best I can to try to stay true to the separation recommended by the MVVM pattern. One thing I haven't figure out how to do correctly has to do with initializing my UserControls.
My most recent example of this has to do with a library that I wrote to talk to some low-level hardware. That assembly happens to have a UserControl that I can simply drop into any GUI that uses this hardware. All that is necessary for it to work is to set a reference to the object that has access to the low level methods.
However, that's where my problem lies -- currently, the UserControl is added to the GUI via XAML, where I define the namespace and then add the UserControl to my window. Of course, I have no control over its creation at this point, so the default constructor gets called. The only way to set the necessary reference for hardware control involves calling a method in the UC to do so. The ViewModel could feasibly call a method in the Model, e.g. GetController(), and then call the method in the UserControl to set the reference accordingly. The GUI can pass a reference to the UserControl to the ViewModel when said GUI creates the ViewModel, but this violates MVVM because the ViewModel shouldn't know anything about this control.
Another way I could deal with this is to not create the UserControl in XAML, but instead do it all from code-behind. After the ViewModel gets initialized and retrieves an initialized UserControl (i.e. one that has the low-level object reference set), it can set the Content of my Window to the UserControl. However, this also violates MVVM -- is there a way to databind the Content of a Window, TabControl, or any other element to a UserControl?
I'd like to hear if anyone has had to deal with this before, and if they approached it the first or second way I have outlined here, or if they took a completely different approach. If what I have asked here is unclear, please let me know and I'll do my best to update it with more information, diagrams, etc.
UPDATE
Thanks for the responses, guys, but I must not have explained the problem very well. I already use RelayCommands within the UserControl's ViewModel to handle all of the calls to the hardware layer (Model) when the user clicks in the control in the UserControl itself. My problem is related to initially passing a reference to the UserControl so it can talk to the hardware layer.
If I create the UserControl directly in XAML, then I can't pass it this reference via a constructor because I can only use the default constructor. The solution I have in place right now does not look MVVM-compliant -- I had to name the UserControl in XAML, and then in the code-behind (i.e. for the View), I have to call a method that I had added to be able to set this reference. For example, I have a GUI UserControl that contains the diagnostics UserControl for my hardware:
partial class GUI : UserControl
{
private MainViewModel ViewModel { get; set; }
public GUI( Model.MainModel model)
{
InitializeComponent();
ViewModel = new MainViewModel( model, this.Dispatcher);
ViewModel.Initialize();
this.DataContext = ViewModel;
diagnostics_toolbar.SetViewModel( ViewModel);
user_control_in_xaml.SetHardwareConnection( model.Connection);
}
}
where the outer class is the main GUI UserControl, and user_control_in_xaml is the UserControl I had to name in the GUI's XAML.
Looking at this again, I realize that it's probably okay to go with the naming approach because it's all used within the View itself. I'm not sure about passing the model information to user_control_in_xaml, because this means that a designer would have to know to call this method if he is to redo the GUI -- I thought the idea was to hide model details from the View layer, but I'm not sure how else to do this.
You will also notice that the main GUI is passed the Model in the constructor, which I assume is equally bad. Perhaps I need to revisit the design to see if it's possible to have the ViewModel create the Model, which is what I usually do, but in this case I can't remember why I had to create it outside of the GUI.
Am new to MVVM myself but here's a possible solution:
Create a property in your VM that is of the object type (that controls the hardware) and bind it to an attached property on your UserControl. Then you could set the property in your VM using dependency injection, so it would be set when the VM is created. The way I see it, the class that talks to the hardware (hardware controller) is a service. The service can be injected to your view model and bound to your UserControl. Am not sure if this is the best way to do it and if it is strict enough to all the MVVM principles but it seems like a possible solution.
if your question is: How do i show my viewmodel in the view? then my solution is always using viewmodelfirst approach and datatemplates.
so all you have to do is wire up your viewmodel via binding to a contentcontrol.content in xaml. wpf + datatemplates will do the work and instantiate your usercontrol for your viewmodel.
You are right, the ViewModel shouldn't know about anything in the View - or even that there is such a thing as a View, hence why MVVM rocks for unit testing too as the VM couldn't care less if it is exposing itself to a View or a test framework.
As far as I can see you might have to refactor things a little if you can. To stick to the MVVM pattern you could expose an ICommand, the ICommand calls an internal VM method that goes and gets the data (or whatever) from the Model, this method then updates an ObservableCollection property of the data objects for the View to bind to. So for example, in your VM you could have
private ICommand _getDataCommand;
public ICommand GetDataCommand
{
get
{
if (this._getDataCommand == null)
{
this._getDataCommand = new RelayCommand(param => this.GetMyData(), param => true);
}
return this._getDataCommand;
}
}
private void GetMyData{
//go and get data from Model and add to the MyControls collection
}
private ObservableCollection<MyUserControls> _uc;
public ObservableCollection<MyUserControls> MyControls
{
get
{
if (this._uc == null)
{
this._uc = new ObservableCollection<MyUserControls>();
}
return this._uc;
}
}
For the RelayCommand check out Josh Smiths MSDN article.
In the View you could either call the ICommand in the static constructor of your UC - I am guessing youwould need to add an event in your class for this - or call the ICommand from some sort of click event on your UC - maybe just have a 'load' button on the WPF window. And set the databinding of your UC to be the exposed observable collection of the VM.
If you can't change your UC at all then you could derive a new class from it and override certain behaviour.
Hope that helps a bit at least, like I say, have a look at Josh Smiths MVVM article as he covers the binding and ICommand stuff in there brilliantly.
If you set the DataContext of the Window or UserControl containing thisUserControl to the main view model, the user control can call SetHardwareConnection() on itself in its Loaded event (or DataContextChanged event handler).
If that's not possible because you're saying the UserControl is 'fixed', you should derive from it or wrap it up in another UserControl, which would serve as a MVVM 'adapter'.
(In order to bind the window: you could make the MainViewModel a singleton with a static Instance property and use DataContext="{x:Static MyClass.Instance}". A nice way to get things going quickly)
Note; this is based on my understanding that MVVM works because of Bindings.. I always bind the control to a ViewModel, not pass a ViewModel as a parameter.
Hope that helps!