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• How reusable should ViewModel classes be?

  - by stiank81

  I'm working on a WPF application, and I'm structuring it using the MVVM pattern. Initially I had an idea that the ViewModels should be reusable, but now I'm not too sure anymore. Should I be able to reuse my ViewModels if I need similar functionality for a WinForms application? Silverlight doesn't support all things WPF does - should I be able to reuse for Silverlight applications? What if I want to make a Linux GUI for my application. Then I need the ViewModel to build in Mono - is this something I should strive for? And so on.. So; should one write ViewModel classes with one specific View in mind, or think of reusability?

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• How Can I Add a New Command to a Control's Event

  - by Chris

  Hi, thanks for taking a look at this question. I have been building an application (VB/WPF) following Josh Smith's example of an MVVM pattern: Click here to view My Views have Controls which trigger a Command upon an event, such as a button click and these are bound to properties in the ViewModel. My ViewModel exposes properties of type ICommand which invoke the appropriate methods. This has worked well, until now! I now want to use a Thumb Control to allow a user to move an object within a Canvas. I don't believe that the Thumb has a Command capability 'out of the box'. A Thumb raises a 'DragDelta' event when being moved. So my question is as follows: How do I extend a Control's capability to issue a Command upon an event please? I wish to issue a new Command, let's say 'onDragDelta' so that I can bind it to a property in my ViewModel just like my Buttons. Thank you very much

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• Is it possible to have CommandManager requery only specific WPF command instead of all?

  - by aoven

  I'm trying to implement a highly responsive UI for my MVVM application, so I've chosen to have all command handlers automatically execute in a BackgroundWorker so they don't block the UI. But at the same time, I don't want the user to be able to execute the same command while it is still executing in the background. The solution seems obvious: When Executed handler is invoked, have the CanExecute handler return false Start the BackgroundWorker async When BackgroundWorker finishes, have the CanExecute handler return true again. Problem is, I need to notify WPF after step 1 and again after step 3 that CanExecute has changed and that it should be required. I know I can do it by calling CommandManager.InvalidateRequerySuggested, but that causes CanExecute handlers of all other commands to be requeried as well. Having a lot of commands, that's not good. Is there a way to ask for requery of a specific command - i.e. the one that is currently being executed? TIA

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• DataTemplate defautl visibility for ContentControls

  - by bitbonk

  In my MVVM based WPF application I have a lot of different ViewModel types that dynamically loaded into ContentControls or ContentPresenters. Therefor I need to explictly set what DataTemplate is to be used in XAML: <ContentControl Content={Binding SomePropertyOfTypeViewModel} ContentTemplate={StaticResource someTemplate} /> Now my problem is that the content control is displaying the UI of someTemplate even if the ContentControl is bound to nothing (i.e ViewModel.SomePropertyOfTypeViewModel is null) Is there a quick and easy way to make all ContentControls display nothing if they are currently bound to nothing? When I use implicit DataTemplates everything works as expected. Unfortunately I can't use this mechanism here.

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• PropertyChanged doesn't work properly

  - by Karen

  Hello, I have a Silverlight application in which I implemented MVVM pattern. In my application there is a child window on which I have ComboBox. I bound ItemsSource and SelectedItem of my combobox to a property (typeof ObservableCollection) and property of MyType appropriately. MyType is a "MODEL" derived from INotifyPropertyChanged. When my window is loaded I set values to this properties. But my combobox doesn't display selected item. I found that when I set property which is bound to selected item (in ViewModel), the PropertyChanged event is null. Can anyone help me. Thanks.

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• Can a DataTemplate be a Page?

  - by dthrasher

  I'm using the MVVM pattern to create a WPF standalone application. My program compiles in Visual Studio 2008, but I frequently get warnings in the editor for my DataTemplates. In my MainWindow.xaml, I've defined the following DataTemplate: <DataTemplate DataType="{x:Type ViewModels:TagViewModel}"> <Views:TagView /> </DataTemplate> Where "TagView" is derived from a Page, rather than an ordinary UserControl. This causes the following message to appear every time I reload the designer in Visual Studio: "Could not create an instance of type 'TagView'. Yet the solution compiles fine and the program seems to work properly. Is this a bug in the Visual Studio 2008 editor? Or am I doing something wrong?

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• TabItems from View collection

  - by byte

  I am using MVVM. I have a tab control. I will have a collection of items. I want to display each of this item in the collection as a tab item. The view in each tab item is different and may have its own viewmodel. How do I achieve this? E.g. I have 3 items in the collection. The Tab item template contains an ItemControl. I would like to now have 3 Tabs created and the ItemControls inside each tabitem may be showing different views. One way I could do is have a single view and viewmodel for each item. Now based on some condition the View will display different UI elements and behave differently. But I am afraide this will make the view quite complex over a period of time.

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• In Prism (CAL), how can I RegisterPresenterWithRegion instead of RegisterViewWithRegion

  - by Edward Tanguay

  I have a module in a Prism application and in its initialize method I want to register a presenter instead of a view with a region, i.e. I want to do this: PSEUDO-CODE: regionManager.RegisterPresenterWithRegion( "MainRegion", typeof(Presenters.EditCustomerPresenter)); instead of loading a view like this: regionManager.RegisterViewWithRegion( "MainRegion", typeof(Views.EditCustomerView)); The presenter would of course bring along its own view and ultimately register this view in the region, but it would allow me to bind the presenter to the view in the presenter's constructor instead of binding the two together in XAML (which is more of a decoupled MVVM pattern which I want to avoid here). How can I add a Presenter to a Region instead of a view? namespace Client.Modules.CustomerModule { [Module(ModuleName = "CustomerModule")] public class CustomerModule : IModule { private readonly IRegionManager regionManager; public CustomerModule(IRegionManager regionManager) { this.regionManager = regionManager; } public void Initialize() { regionManager.RegisterViewWithRegion("MainRegion", typeof(Views.EditCustomerView)); } } }

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• Limit the model data fields serialized by Web API based on the return type Interface

  - by Stevo3000

  We're updating our architecture to use a single object model for desktop, web and mobile that can be used in the MVVM pattern. I would like to be able to limit the data fields that are serialized through Web API by using interfaces on the controllers. This is required because the model objects for mobile are stored in HTML5 local storage so don't carry optional data while a thin desktop client would be able to store (and work with) more data. To achieve this a model will implement the different interfaces that define which data fields should be serialized and there will be a controller specific to the interface. The problem is that the Web API always serializes every field in the model even if it is not part of the interface being returned. How can we only serialize fields in the returned interface?

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• MVVM- View Model-View Model Communications

  - by user275561

  How do I go about having two view models communicate with one another using MVVM Light. I know how to use the messenger class and register etc.. Here is my Scenario A Settings View ---> a Settings View Model . . . A MainPage View ---> A MainPage ViewModel If something changes in the Settings View it will Message back to the Settings View Model. So then I want the Settings View Model to communicate to the MainPage View Model about what changed. THe MainPage ViewModel will then tell the View.

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• WPF passing the Int argument to a data object provider method

  - by SAD

  Hi, I'm trying to produce a master/detail datagrid view. I'm using object data providers. Now I have seen many examples when the argument of a method for returning the records for the detail view is a string, like in this example: <!-- the orders datasource --> <ObjectDataProvider x:Key="OrdersDataProvider" ObjectType="{x:Type local:OrdersDataProvider}"/> <ObjectDataProvider x:Key="Orders" MethodName="GetOrdersByCustomer" ObjectInstance="{StaticResource OrdersDataProvider}" > <ObjectDataProvider.MethodParameters> <x:Static Member="system:String.Empty"/> </ObjectDataProvider.MethodParameters> </ObjectDataProvider> But in my case, the argument that I want to pass is the int ID not the string. Can I still somehow use the object data provider to return all the records from the database for the detail view if the argument passed to a method has to be an int? How could it be implemented in MVVM? Thanks a lot for any advice1

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• Silverlight data-driven application with NHibernate

  - by Tigraine

  Hi Guys, this is more of a subjective Question, but I'll ask it anyway. I'm about to develop a very data-centric application that has to run inside the browser. The frontend will be Silverlight, backed by a Fluent NHibernate service that runs server side. The problem here is: Wherever I look for data-driven silverlight app I wind up finding Silverlight RIA services examples, but nothing on how to build this without some ADO.NET stuff involved. I have little to no knowledge in WCF so far, but from the limited research I did it seems like WCF is pretty much the only way to let the client talk to the server. Are there any tutorials/best practices on how to write a Silverlight MVVM app that provides CRUD for a non-EF database? Suggestions would be very much appreciated. Thanks PS: I can't use .NET remoting. The backend has to run on IIS6 :(

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• Can you use data binding with the Content property of a WPF Frame?

  - by dthrasher

  I can use data binding to set the initial Content of a WPF Frame, but subsequent changes to the the bound property (implemented using INotifyPropertyChange) do not seem to change the content. Also, does anyone know if binding directly to the Content property in this way will cause the bound item to appear in the Frame or NavigationWindow's journal? Some context: I realize that I should probably be using the NavigationService to interact with the Frame, but I'm attempting to follow the MVVM pattern. It seems like it would be much simpler to databind to the Content property...

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• WPF Drag and Drop - Get original source info from DragEventArgs

  - by Quinn351

  I am trying write Drag and Drop functionality using MVVM which will allow me to drag PersonModel objects from one ListView to another. This is almost working but I need to be able to get the ItemsSource of the source ListView from the DragEventArgs which I cant figure out how to do. private void OnHandleDrop(DragEventArgs e) { if (e.Data != null && e.Data.GetDataPresent("myFormat")) { var person = e.Data.GetData("myFormat") as PersonModel; //Gets the ItemsSource of the source ListView .. //Gets the ItemsSource of the target ListView and Adds the person to it ((ObservableCollection<PersonModel>)(((ListView)e.Source).ItemsSource)).Add(person); } } Any help would be greatly appreciated. Thanks!

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• Validation.HasError attached property

  - by Nima

  Did I miss something? 1- Style <Style TargetType="{x:Type TextBox}"> <Style.Triggers> <DataTrigger Binding="{Binding Path=Validation.HasError}" Value="true"> <Setter Property="BorderBrush" Value="Blue" /> </DataTrigger> </Style.Triggers> <Setter Property="MinWidth" Value="160" /> <Setter Property="Margin" Value="0 7 0 0"/> </Style> 2 - Viewmodel implement IDataErrorInfo 3- textBox in view <TextBox x:Name="FirstName" Text="{Binding Person.FirstName, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=true}"></TextBox> 3 - I use Caliburn MVVM I got " BindingExpression path error: 'Validation' property not found on 'object' ''PersonWindowViewModel' (HashCode=38783181)'. BindingExpression:Path=Validation.HasError; DataItem='PersonWindowViewModel' (HashCode=38783181); target element is 'TextBox' (Name='FirstName'); target property is 'NoTarget' (type 'Object')"S

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• Why state cannot be part of Presenter in MVP?

  - by rFactor

  I read http://www.codeproject.com/KB/architecture/MVC_MVP_MVVM_design.aspx and it said: As powerful as they are, both MVC and MVP have their problems. One of them is persistence of the View’s state. For instance, if the Model, being a domain object, does not know anything about the UI, and the View does not implement any business logic, then where would we store the state of the View’s elements such as selected items? Fowler comes up with a solution in the form of a Presentation Model pattern. I wonder why Presenter can't hold View state? It already holds all View logic. As far as I understand, in MVC and MVP the state is kept in View. In PM and MVVM the state is kept in the Presentation Model. Why can't Presenter follow PM in this particular case and contain the state of the view? Here is another article which says Presenter does not hold View state, instead the view does: http://www.codeproject.com/KB/aspnet/ArchitectureComparison.aspx

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• ICOmmand - canexecute can not disable Button with image content.

  - by Anish

  Hi , I have a button control in my wpf-mvvm application. I use an ICommand property (defined in viewmodel) to bind the button click event to viewmodel. I have - execute and canexecute parameters for my ICommand implementation (RelayCommand). Even if CanExecute is false...button is not disabled...WHEN button CONTENT is IMAGE But, when button content is text..enable/disable works fine. <Button DockPanel.Dock="Top" Command="{Binding Path=MoveUpCommand}"> <Button.Content> <Image Source="/Resources/MoveUpArrow.png"></Image> </Button.Content> <Style> <Style.Triggers> <Trigger Property="IsEnabled" Value="False"> <Setter Property="Opacity" Value=".5" /> </Trigger> </Style.Triggers> </Style> </Button>

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• CommandManager "cannot resolve symbol"

  - by Budda

  After adding "RelayCommand" class (that uses CommandManager class) into my Silverlight App I've got an error: The name 'CommandManager' does not exist in the current context This class is a member of System.Windows.Input namespace that is in PresentationCore.dll assembly. The problem is that I can't add this dll into my app... I am having a strange assumption: this class (CommmandManager) can't be used in the Silverlight. Am I right? How can I implement MVVM-pattern (http://msdn.microsoft.com/en-us/magazine/dd419663.aspx)? Thanks.

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• Set ListBoxItem.IsSelected when child TextBox is Focused

  - by jpsstavares

  Hi, I have a typical MVVM scenario: I have a ListBox that is binded to a List of StepsViewModels. I define a DataTemplate so that StepViewModels are rendered as StepViews. The StepView UserControl have a set of labels and TextBoxs. What I want to do is to select the ListBoxItem that is wrapping the StepView when a textBox is focused. I've tried to create a style for my TextBoxs with the following trigger: <Trigger Property="IsFocused" Value="true"> <Setter TargetName="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type ListBoxItem}}}" Property="IsSelected" Value="True"/> </Trigger> But I get an error telling me that TextBoxs don't have an IsSelected property. I now that but the Target is a ListBoxItem. How can I make it work?

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• Cannot seem to communicate between views

  - by devnet247

  Hi all I have written a MVVM prototype as a learning exercise and I am struggling to understand how I can communicate between views.Let me explain I have a treeview on the left (leftSideView) ListView on the right (rightSideView) MainWindow (includes the 2 views mentioned above and a splitter) What I have implemented does not work and I would like you if you can point out where I am going wrong or if there is a better way of doing it. you can download the quick prototype from here http://cid-9db5ae91a2948485.skydrive.live.com/self.aspx/.Public/WpfCommunicationBetweenViews.zip For sure I am doing something wrong with the bindind. Also it would be nice to learn how you do it. EG ListBox on left (one view) ListView On the right (another view) how u communicate between the two. Thanks a lot of any suggestions

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• Is it considered bad practice to have ViewModel objects hold the Dispatcher?

  - by stiank81

  My WPF application is structured using the MVVM pattern. The ViewModels will communicate asynchronously with a server, and when the requested data is returned a callback in the ViewModel is triggered, and it will do something with this data. This will run on a thread which is not the UI Thread. Sometimes these callbacks involve work that needs to be done on the UI thread, so I need the Dispatcher. This might be things such as: Adding data to an ObservableCollection Trigger Prism commands that will set something to be displayed in the GUI Creating WPF objects of some kind. I try to avoid the latter, but the two first points here I find to be reasonable things for ViewModels to do. So; is it okay to have ViewModels hold the Dispatcher to be able to Invoke commands for the UI thread? Or is this considered bad practice? And why?

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• Best way to attach row from datagrid to EF.

  - by AKoran

  Using MVVM and EF...I have a datagrid binding to a View Model (using ObservableCollection). The view model has a save command which simply calls the SaveChanges command of the Data Context. However, when a user adds a new row to the datagrid, the new entity is detached. Is there any easy way to automatically attach it when it gets created. Currently, I'm having to do this in the Save command of my View Model and it seems a bit clunky: foreach (var dataItem in _DataList) // where _DataList is the ObservableCollection { if (dataItem.EntityState == EntityState.Detached) { _DataContext.AddToTestTables(dataItem); } } _DataContext.SaveChanges();

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• Activate WPF command based on TextBox value

  - by zendar

  This is MVVM application. I have form and related view model class. There is TextBox, Button and ListBox on form. Button is bound to DelegateCommand that has CanExecute function. Idea is that user enters some data in text box, presses button and data is appended to list box. I would like to enable command (and button) when user enters correct data in TextBox. Things look like this now: CanExecute() contains code that checks if data in property bound to text box is correct. Text box is bound to property in view model UpdateSourceTrigger is set to PropertyChanged and property in view model is updated after each key user presses. Problem is that CanExecute() does not fire when user enters data in text box. It doesn't fire even when text box lose focus. How could I make this work?

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• WPF Prism deactivate ?

  - by 2Fast4YouBR

  Hi all, I have an problem and would like to know if it is common problem or jsut with me. I am using Wpf with Prism and Unity, all with the pattern MvvM. I am loading a viewModel that has a reference to a dropdown with few items, my idea is that each time that the user click in some place to open the view that has this dropdown is that the dropdown will be shown with diferent values. The problem is that I see is that after I show the view for the first time and after the first DEACTIVATE, when I try to load it again looks like it is already in the memory (was not deactivated/disposed), so as is already in memory, it not call the constructor again of the modelView and the dropdown is shown with tha same old values. public BranchSelectionViewModel(IUnityContainer unityContainer) { this.unityContainer = unityContainer; User user = this.unityContainer.Resolve<User>(); this.branches = new ObservableCollection<Department>(user.Departments .Where(department => department.DepartmentId != user.SelectedDepartment.DepartmentId)); }

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• Why doesn't my viewmodel properties get populated

  - by Jakob

  Hi. I've looked all over and I can't figure out why my viewmodel doesn't get populated. I have this code: Followers = new ObservableCollection<aspnet_User>(_followersRepo.aspnet_Users); _followersRepo.Load(_followersRepo.GetUsersFollowingIDQuery(CurrentUserId)); Following = new ObservableCollection<aspnet_User>(_followingRepo.aspnet_Users); _followingRepo.Load(_followingRepo.GetUsersFollowedByIDQuery(CurrentUserId)); CurrentUser = _fullUserRepo.FullUsers.SingleOrDefault(); _fullUserRepo.Load(_fullUserRepo.GetFullUserByIDQuery(CurrentUserId)); But when I debug, there is no data loaded to the Followers, Following and CurrentUser objects. I know that data should be returned because I'm trying to implement the MVVM pattern in my app, and haven't changed the domainservice. Also I can se debugging the CurrentUserId has a value.

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