I have a WPF application designed using MVVM. I have a view which will always be read only. It is a product result view. So a user will searh for a product in someother view and the search result will be displayed. So when the user double clicks a particular result record, the product details view will be opened which is a read only view. When the user double click another product record in the search result, the sam product details view will be refreshed(Not instantiated) with the new details data. I am thinking of binding the product details view to the viewmodel using OneWayToSource binding mode. Is this good?I believe this will result in reduced memory footprint.Pls confirm? Also, still, do i have to implement INotifyPropertyChanged in the viewmodel properties even the view is a readonly view?
I doubt the binding mode will make any difference to memory footprint, just extra stuff to type in. So long as you use read-only controls and textblocks and the like then keep it simple.
As for the NotifyPropertyChanged: No, it isn't required. However, if you have an Edit view sharing the same view model then it can be beneficial since then edits to those properties will automatically propogate to an open read-only view. Same goes if there are any commands or such that may result in a value of the viewmodel changing, without INotifyPropertyChanged, the view will not refresh those changes. If those types of refreshes aren't needed, then neither is INotifyPropertyChanged.
Related
I created a new entity object and bound it to controls in another window (edit window). After modifying and saving I assigned a new entity object into the one in the main window. The old entity object is bound into a datagrid, now I want the datagrid to display the data that I had modified and saved.
ObjectContext.Refresh Method (RefreshMode, Object) seems to be what I want but I don't know how to use it correctly.
In short :
I have a main window with datagrid displaying the whole data of the table. Users can pick one row and edit it in a edit window. After saving, the datagrid should display what has been modified.
Your best bet here is to use an ObservableCollection as your data source for the datagrid instead of the query.
And look at implementing INotifyPropertyChanged interface in your Customer class.
The ObservableCollection is initially populated by the database query. User changes are made to elements within the ObservableCollection and once complete you then just need to trigger transferring the changes to wherever you originally obtained your list of Customer objects
By doing this changes made both to the collection of Customers and to individual Customer objects (if present within the datagrid) will be automatically updated for you.
edit
I must admit that I'm a bit rushed to offer up any code at the moment, but here's a pretty good article that explains how to use ObservableCollections and classes that implement INotifyPropertyChanged. It also has code examples, which although in VB.NET should give you enough of an idea to get started.
In effect you separate your code into distinct layers UI (View), business logic (View Model) and data layer (Model where your entity framework resides).
You bnd your datagrid to the ObservableCollection type property in your Customers class and your edit csutomer window is bound to as instance of your Customer class.
I'm trying to build a data entry form in wpf. To perform validation I apparently need to have an object attached in the datacontext of my grid. But how can I have one when I didn't create one yet?
How does it work?
For example, I have a screen with a datagrid. The datagrid contains users that were obtained from membership. Above the grid is a button: add user. When clicked a new window appears and the following can be entered: user name, password, email. To perform validation on the textboxes to see if they aren't empty. Now, it is my understanding that the way this works is by having an object attached to the window (datagrid datacontext). But how can I have it attached when it doesn't exist yet?
This is a case where MVVM design patterns are very useful.
Every WPF view has a corresponding view model object that the properties in the view are bound to. So your window with the data grid has a view model - its DataContext - and the view model has properties that are bound to properties in the view - e.g. the ItemsSource in the data grid is bound to a collection (see note 1).
The "add user" command (which is implemented as a RelayCommand in the window's view model) creates a new view (the new window) and its corresponding view model object (the new user), sets the view's DataContext to the view model, and calls ShowDialog to show the window. (See note 2.) If the user accepts the new object, ShowDialog returns true, and the logic in the command takes the view model object (which now contains whatever changes the user made) and uses the information in it to create a new model object and add it to the model. If the user cancels, ShowDialog returns false, and the command discards the view model object without creating a new model object.
Note 1: The collection here may be a collection of model objects, or it may be a collection of view model objects. It depends on whether or not you need anything that's not in the model for displaying the model objects in a data grid. It's common, in this kind of scenario, for the objects in the grid to be view models for the dialog - that is, the view model objects have properties implemented for both display in the grid and modification in the dialog window. On the other hand, if all the grid is doing is displaying data from the model, there may be no need for an intermediary object.
Note 2: Having the command create a WPF window violates a central MVVM design principle, which is that view models shouldn't create WPF objects. The reason for this principle is pretty simple: you can't build an automated unit test for this command, since it's just going to throw up a dialog and wait. There are all kinds of different approaches to this - see, for instance, this question, and Josh Smith's blog post on the Mediator pattern - and all of them involve delegating the creation and display of the actual dialog window to a separate service that can be mocked out for unit testing. If you don't want to choose one of those approaches up front, you can retrofit one into your application once you get this thing working.
The idea here is that you should attach an object which is slightly different from your business models. In your case it won't UserInfo (or whatever you have for users in grid). It will be some other class, more suitable for editing. In MVVM this class will be a ViewModel. This class will have some differences comparing to your regular user class, for example it may have some properties nullable (when you haven't set them yet). Also this class will handle validation. You should instantiate this class at the same time you're creating an editor window and put instance of this class into Window.DataContext.
Hmm, there is a lot in this question but I just created a screen with three data grids (I am using Telerik in this case) and under each datagrid is a button to add to the grid. No the window with the three datagrids has it's own view model. and each of the "pop up's" has it's own viewmodel, in this case all of these are user controls and I just create a new window and set window.content and call show dialog.
Communication is facilitated via "events" - not the standard events you are used to in .NET but in this case I am using Prism and it's CompositePresentationEvent class. When the user is done creating their new object they click add and I fire off this event with the "payload" being the object they created. The main window with the three grids listens for that event and has a method to handle it, in this case adds it to the ObservableCollection which is what I bind the grids to.
If I were you I would look into the various frameworks that are out there, Prism, MVVM light etc... Again, your question seemed rather broad, I tried to give an overview but I didn't go into detail, if you look into some sort of framework I think it will clear up a lot of these details for you.
When the users hit Add New, create a new blank copy of your object, and set the datacontext to that new object.
Set some kind of flag to identify that it is a New object. This can be the Id being NULL, 0, -1, etc or an ObjectState property set to New. That way all your validation rules apply, and once the user hits save you know to INSERT instead of UPDATE
I am new to MVVM, and I am trying to implement a simple application, following the pattern.
For simplicity, I am breaking the problem down to it's simplest form. If I manage to get this to work, I will have little trouble getting the application made.
The simple application consists of a tabcontrol. It is important that both tabs have their own ViewModel. However, they will get most of their data from the same source. The main issue is to get the second tab to know that the first have initiated a change on the datasource.
So, for simplicity, let's just say that the model is holding a single integer. This integer needs initially to be set to 1.
The first tab is holding a textblock and a button. The text of the textblock is bound to the integer in the datamodel. Upon pressing the button, the integer in the moddel should be incremented with 1.
The second tab holds only a textblock, also bound to the integer in the datamodel. The challenge is to get this textblock to update along with the first textblock, but still being it's own viewmodel.
I need somewhere central to store the values of the model, and in some way, let the viewmodels know that they have been updated, so their properties can be updated, and the Views therefore get's updated accordingly.
Can someone explain in as much detail as possible how this is done? I have tried a billion different ways, but I am not getting it to work.
Let me see if I have your question down right:
You have a data source (your model).
You have 2 view models.
View model 1 changes data in the model.
View model 2 needs to update with changes in the model.
If that all sounds right, here's one solution:
Have your model implement INotifyPropertyChanged. When the integer changes, raise the PropertyChanged event. In view model 2, listen for the model's PropertyChanged event. When it occurs, raise view model 2's property changed event, and its UI will get updated automatically.
I have no idea in which scenario you want to do that.
But a solution that crosses my mind is to have a "parent" ViewModel that holds instances of the two tab ViewModels.
e.g.
public class ParentViewModel{
private Tab1ViewModel viewModel1;
private Tab2ViewModel viewModel2;
}
Then the ParentViewModel can subscribe to the INotifyPropertyChanged event of the ViewModel1 and set the value on the ViewModel2.
I have recently implemented something similar to this. It was for implementing a wizard, consisting of:
7 Views
8 View models
1 Model
The main ViewModel created the Model and passed this on to all the other view models through their constructors.
In your scenario you could have a main ViewModel with an ObservableCollection of ViewModels. Each of these VM's would have the same instance of the model as their data source.
As previously mentioned, implement INotifyPropertyChanged on the model and bind the views directly to the model through a property on the ViewModel. I found this diagram very useful : http://karlshifflett.files.wordpress.com/2009/01/wpflobmvvm1.png
I'm building a simple financial record-keeping application. The main window view model holds a list of Accounts. The view shows this list (in a ListView) along with a panel showing details about the currently selected Account.
At first, I bound the details panel and ListView's SelectedItem to the same property (of type Account) on the view model. However, I quickly realized that the details panel needed to be bound to an AccountViewModel, not directly to an instance of Account.
There are several ways to providing this AccountViewModel:
Bind the details panel to a separate property on the view model. When ListView's SelectedItem changes, the view model should create and set this new property to an instance of AccountViewModel that is associated with the selected Account.
Give the main view model a list of AccountViewModels instead of an Accounts list. Both the ListView listing all accounts and the details panel could then be bound to the same property on the main view model.
Have one AccountViewModel, changing the Account it references with each change to ListView's SelectedItem property.
Are there other options? Which choice do you recommend?
Thank you,
Ben
This is what I do:
Give the main view model a list of
AccountViewModels instead of an
Accounts list.
This will serve you well in many ways. I always find that eventually, for one reason or another, I need to augment my Models in some way to support a View, so these days I just start out by creating ViewModels.
The way you can tell that this is going to be a better option is that it involves less code. Less code always equals less bugs, in my opinion.
i like this option
Bind the details panel to a separate
property on the view model. When
ListView's SelectedItem changes, the
view model should create and set this
new property to an instance of
AccountViewModel that is associated
with the selected Account.
then you can bind the selected item to the view model and the details pane as well. this can be tested independently of the view. when you create your list of accounts you can also create a list of account view models, so when you change selection you are not having to create anything its all sitting there. this way your viewmodel is comprehensive, representing the whole screen not just floating bits of viewModel.
So far, the idea I like best is #3 from the original post for two reaons:
It has a 1 view to 1 view model correspondence, which I think best fits the M-V-VM pattern.
Since the account view model is told when to change to display details of a different account, it can ask the user if he wants unsaved edits to be saved before changing to display the new account (etc.).
There are lots of great examples around on MVVM but I'm still confused.
Lets say you have a CustomerModel and a CustomerViewModel. It seems there would be a Name property on the CustomerModel and one on the CustomerViewModel. The setter on the CustomerViewModel will set the CustomerModel Name property and then call the OnPropertyChanged(PropName) so that the UI will update. Is this really right? Seems like the getter/setters will be defined twice. If you have a model with 50 properties then thats going to get real tedious.
Also, lets say I set a Qty property. The ViewModel updates the Model. The Model updates its Value property based on the new Qty. How does the ViewModel get notified that the Model property changed?
Your ViewModel doesn't have to encapsulate the Model that strictly. In your scenario, the CustomerViewModel might have a Customer property, which in the end means your View binds to the Model properties... it just does so through the ViewModel. That's perfectly legitimate. That said, however, there's often a benefit to encapsulating this. Your business model may not include change notification. You may not want the user interaction to modify the business model until the user clicks an OK button. Your business model may through exceptions for bad input, while you want to use another form of validation. I'm sure you can think of other things. In fact, I'd guess that most of the time you're going to want the encapsulation, so it's not really "tedious" in the sense of just writing a lot of pointless relay methods.
In the customer example that you give, the CustomerModel contains all the information that is stored by your database (or other backend). The CustomerViewModel contains similar information if it's going to be shown on the UI (Name etc., potentially 50 other properties if you have a large class) but as uses the INotifyPropertyChanged interface to show them as properties that the View (i.e. the XAML) can bind to.
e.g.
public int Name
{
get
{
return this.name;
}
set
{
if (this.name!= value)
{
this.name= value;
this.OnPropertyChanged("Name");
}
}
}
The ViewModel also contains other bits of UI state - Visibility flags, current Tab index, more complex bits of text built out of data in several fields, ObservableCollection<> of child items, etc. All are there to be bound to the XAML.
I have seen the ViewModel created from the Model as a one-time, one-way process, e.g. with a constructor:
CustomerViewModel viewModel = new CustomerViewModel(customer);
or as an extension method
CustomerViewModel viewModel = customer.ToViewModel();
I haven't seen any provision for updating a ViewModel for changes to the Model - the point of the ViewModel is that it's isolated from the model. It keeps a separate copy of the data. It does not propagate changes back to the model, not until you press a "save" button. So if you cancel instead, nothing in the model has changed and there's nothing to undo.
You may be trying too hard to keep the ViewModel up to date with the Model - most cases like save or load you can just throw away the current ViewModel and make a new one from the current state of the model. Do you need to keep the ViewModel's UI state and change the data in it? It's not a common requirement but it could be done with a method or two called when the save or load happens.
So there's also the assumption that this wire-up logic happens somewhere. This is why most patterns that involve views also involve controllers that are responsible for acting on commands (e.g. show a customer, save a customer) and setting up new UI state afterwards.
Exactly how this is done, will depend in part on your business model as wekempf has already stated.
Depending on how your displaying Customer info in your UI, you might have an ObservableCollection of Customer(your model) types in your ViewModel. If, for example, you're displaying a master/detail scenario, where you might have a list of customers and show details below when a particular customer is selected.