The other two patterns are really sort of separate in terms of the concerns they address. Things just work. But the way data binding is technically done in WPF is somewhat special, as it's tied to classes like:.
Because of this you just can't really write a model the way you want using standard. NET technology. You just can't populate it simply like you would from a generic model in Winforms for example. It must be bound to a hierarchical model using ObservableCollection to represent a node's children.
You must add something between the two. Something that's WPF-compatible and understands your model. That's what's called VM.
Historically, ObservableCollection was in the WindowsBase. It's been moved back to System. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Why MVVM and what are it's core benefits?
Asked 12 years ago. Active 3 years ago. Viewed 38k times. Improve this question. Anderson Imes Add a comment. When the user creates a new customer and clicks the Save button in the CustomerView control, the CustomerViewModel associated with that view will add the new Customer object to theCustomerRepository.
In a sense, CustomerRepository acts as asynchronization mechanism between various ViewModels that deal withCustomer objects. Perhaps one might think of this as using the Mediator design pattern. I will review more of how this works in the upcoming sections, but for now refer to the diagram in Figure 11 for a high-level understanding of how all the pieces fit together.
After the user types valid values into the input fields, the Save button enters the enabled state so that the user can persist the new customer information. There is nothing out of the ordinary here, just a regular data entry form with input validation and a Save button.
The Customer class has built-in validation support, available through its IDataErrorInfo interface implementation. That validation ensures the customer has a first name, a well-formed e-mail address, and, if the customer is a person, a last name. If the Customer's IsCompany property returns true, the LastName property cannot have a value the idea being that a company does not have a last name.
This validation logic might make sense from the Customer object's perspective, but it does not meet the needs of the user interface. The UI requires a user to select whether a new customer is a person or a company. The Customer Type selector initially has the value " Not Specified ". How can the UI tell the user that the customer type is unspecified if the IsCompanyproperty of a Customer only allows for a true or false value? Assuming you have complete control over the entire software system, you could change the IsCompany property to be of typeNullable, which would allow for the "unselected" value.
However,the real world is not always so simple. Suppose you cannot changethe Customer class because it comes from a legacy library owned bya different team in your company. What if there is no easy way to persist that "unselected" value because of the existing database schema?
What if other applications already use the Customer classand rely on the property being a normal Boolean value? Once again,having a ViewModel comes to the rescue. The test method in Figure 12 shows how this functionality works in CustomerViewModel. It also exposes a CustomerTypeproperty, which stores the selected String in the selector. Figure 13 shows the two properties.
When the selected item in that ComboBox changes, the datasource's IDataErrorInfo interface is queried to see if the new value is valid. Since the data source is a CustomerViewModel object, the binding system asks that CustomerViewModel for a validation error on the CustomerTypeproperty. Most of the time, CustomerViewModel delegates all requests for validation errors to the Customer object it contains.
However, since Customer has no notion of having an unselected state for the IsCompany property, the CustomerViewModel class must handle validating the new selected item in the ComboBox control. That code is seen in Figure The key aspect of this code is that CustomerViewModel's implementation of IDataErrorInfo can handle requests for ViewModel-specific property validation and delegate the other requests to the Customer object. This allows you to make use of validation logic in Model classes and have additional validation for properties that only make sense to ViewModel classes.
That command uses the RelayCommand class examined earlier to allow CustomerViewModel to decide if it can save itself and what to do when told to save its state. In this application, saving a new customer simply means adding it to a CustomerRepository. Deciding if the new customer isready to be saved requires consent from two parties. The Customer object must be asked if it is valid or not, and theCustomerViewModel must decide if it is valid. This two-part decision is necessary because of the ViewModel-specific properties and validation examined previously.
The save logic forCustomerViewModel is shown in Figure The use of a ViewModel here makes it much easier to create aview that can display a Customer object and allow for things likean "unselected" state of a Boolean property. It also provides the ability to easily tell the customer to save its state.
If the view were bound directly to a Customer object, the view would require a lot of code to make this work properly. In a well-designed MVVM architecture, the code behind for most Views should be empty, or, at most, only contain code that manipulates the controls and resources contained within that view. Sometimes it is also necessary to write code in a View's codebehind that interacts with a ViewModel object,such as hooking an event or calling a method that would otherwise be very difficult to invoke from the ViewModel itself.
The demo application also contains a workspace that displays all of the customers in a ListView. The customers in the list are grouped according to whether they are a company or a person.
The user can select one or more customers at a time and view the sum of their total sales in the bottom right corner. In the previous section, you saw how a CustomerViewModel can render as a data entry form, and now the exact same CustomerViewModel object is rendered as an item in a ListView. The CustomerViewModel class has no idea what visual elements display it, which is why this reuse is possible. AllCustomersView creates the groups seen in the ListView. When a CustomerViewModel is selected or unselected, that causes the sum of all selected customers' total sales to change.
The AllCustomersViewModel class is responsible for maintaining that value, so that the ContentPresenter beneath the ListView can display the correct number. Figure 17 shows how AllCustomersViewModel monitors each customer for being selected or unselected and notifies the view that it needs to update the display value.
The UI binds to the TotalSelectedSales property and applie scurrency monetary formatting to the value. The ViewModel object could apply the currency formatting, instead of the view, by returning a String instead of a Double value from theTotalSelectedSales property. WPF has a lot to offer application developers, and learning toleverage that power requires a mindset shift.
Itallows you to create a strong separation between data, behavior,and presentation, making it easier to control the chaos that is software development. Josh Smith is passionate about using WPF to create great user experiences. Josh works for Infragistics in the Experience Design Group. When he is not at a computer, he enjoys playing the piano, reading about history, and exploring New York City with his girlfriend.
You can visit Josh's blog at joshsmithonwpf. Skip to main content. This browser is no longer supported. Download Microsoft Edge More info. Contents Exit focus mode. Order vs. Chaos It is unnecessary and counter productive to use design patterns in a simple "Hello, World! The Evolution of Model-View-ViewModel Ever since people started to create software user interfaces, there have been popular design patterns to help make it easier.
Figure 1 Workspaces Only one instance of the "All Customers" workspace can be open at a time, but any number of "New Customer" workspaces can be open at once. Figure 2 New Customer Data Entry Form After filling in the data entry form with valid values and clicking the Save button, the new customer's name appears in the tab item and that customer is added to the list of all customers.
Relaying Command Logic Every view in the app has an empty codebehind file, except for the standard boilerplate code that calls InitializeComponent in the class's constructor. Figure 4 Inheritance Hierarchy Having a base class for all of your ViewModels is by no means a requirement. PropertyChanged; if handler! NewItems workspace. OnWorkspaceRequestClose; if e. OldItems workspace. AreEqual 0, target. Count, "Workspaces isn't empty.
Execute null ; Assert. AreEqual 1, target. Count, "Did not create viewmodel. Count, "Did not close viewmodel. The Data Model and Repository You have seen how ViewModel objects are loaded, displayed, and closed by the application shell. IsTrue cust. IsCompany, "Should be a company" ; target. IsFalse cust. In the above command code the view model object is passed via the constructor. Now go to the properties of the button, scroll to the command property, right click on it and click create data binding.
In other words the UI will not be automatically notified about the tax calculation. So we need to send some kind of notification from the object to the UI saying that tax values have changed and the UI has reload the binding values. To enable notification in your view model class we need to do three things. All three things are pointed in the below code with comments like point 1 , point 2 and point 3. So till now we have created a simple screen with MVVM which has both properties and command implemented.
We have a view whose UI input elements i. View model talks with model internally. If you remember the command class code i have pasted the same again below in the constructor we are passing the view model object , which means that this command class cannot be reused with other view model class.
So there should be a way to generalize these actions and attach it in a more general way to the view models. If you think logically actions are logics which are wrapped in to methods and functions. So in other words we need two delegates one a function which returns Boolean value and the other an action which returns void.
We have made three changes to the command class below is the code for the same and I have marked them as Point 1, 2 and Both these delegate values are passed via the constructor and set to private respective delegate variables internally. In the model we already knew what to execute i. This makes the architecture much better and decoupled as this command class can be attached with any view model in a generic way.
Below is the improvised architecture, do notice how the view model is talking with the command class via delegate func and action. Finally a framework would be great if it can bring down some work in our MVVM code. PRISM is one of those frameworks which comes to rescue.
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