Contents tagged with WPF
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Review the WPF ICommand interface
I have not worked with WPF for a while. Recently, I need to make enhancement to a WPF project that I did 20 months ago. It is time to refresh my memory on several WPF unique things. The first thing that came to my mind is WPF Commanding.
In the old days when we create a VB 1-6 or Windows Forms applications, we just double click a button from the designer to create an event handler. With WPF, we can still do it the old way but there is a new way: we bind the Command property of the control to a property of the view model that implements ICommand. So what is ICommand?
The ICommand interface has 3 members: Execute, CanExecute and CanExecuteChanged.
Why the complications? That is because the WPF Commanding allows enable/disable the command sources such as button and menu item without resorting to spaghetti code.The basic workflow is simple:
- Whenever we change a state in the view model that we think that might affect CanExecute state of the command, we call the this.MyCommand.CanExecuteChanged() method.
- This will trigger the view to call the CanExecute method the command object to determine whether the command source should be enabled.
- When the command source is enabled and clicked, the Execute method of the command object is called; there is where the event handler would be implemented.
The prism project has a nice implementation of ICommand called DelegateCommand. Its declaration is fairly simple:
All we need is to instantiate it and give it a executeMethod call back and an optionally canExecuteMethod call back.
Noticed that both CanExecute and Execute functions allow one argument? We could use the mechanism to bind multiple controls to a single command on the view model. We can use CommandParameter of the control to differentiate the controls.
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Getting Started with Prism (aka Composite Application Guidance for WPF and Silverlight)
Overview
Prism is a framework from the Microsoft Patterns and Practice team that allow you to create WPF and Silverlight in a modular way. It is especially valuable for larger projects in which a large number of developers can develop in parallel.
Prism achieves its goal by supplying several services:
· Dependency Injection (DI) and Inversion of control (IoC): By using DI, Prism takes away the responsibility of instantiating and managing the life time of dependency objects from individual components to a container. Prism relies on containers to discover, manage and compose large number of objects. By varying the configuration, the container can also inject mock objects for unit testing. Out of the box, Prism supports Unity and MEF as container although it is possible to use other containers by subclassing the Bootstrapper class.
· Modularity and Region: Prism supplies the framework to split application into modules from the application shell. Each module is a library project that contains both UI and code and is responsible to initialize itself when loaded by the shell. Each window can be further divided into regions. A region is a user control with associated model.
· Model, view and view-model (MVVM) pattern: Prism promotes the user MVVM. The use of DI container makes it much easier to inject model into view. WPF already has excellent data binding and commanding mechanism. To be productive with Prism, it is important to understand WPF data binding and commanding well.
· Event-aggregation: Prism promotes loosely coupled components. Prism discourages for components from different modules to communicate each other, thus leading to dependency. Instead, Prism supplies an event-aggregation mechanism that allows components to publish and subscribe events without knowing each other.
Architecture
In the following, I will go into a little more detail on the services provided by Prism.
Bootstrapper
In a typical WPF application, application start-up is controls by App.xaml and its code behind. The main window of the application is typically specified in the App.xaml file. In a Prism application, we start a bootstrapper in the App class and delegate the duty of main window to the bootstrapper. The bootstrapper will start a dependency-injection container so all future object instantiations are managed by the container.
Out of box, Prism provides the UnityBootstrapper and MefUnityBootstrapper abstract classes. All application needs to either provide a concrete implementation of one of these bootstrappers, or alternatively, subclass the Bootstrapper class with another DI container.
A concrete bootstrapper class must implement the CreateShell method. Its responsibility is to resolve and create the Shell object through the DI container to serve as the main window for the application.
The other important method to override is ConfigureModuleCatalog. The bootstrapper can register modules for the application. In a more advance scenario, an application does not have to know all its modules at compile time. Modules can be discovered at run time. Readers to refer to one of the Open Modularity Quick Starts for more information.
Modules
Once modules are registered with or discovered by Prism, they are instantiated by the DI container and their Initialize method is called. The DI container can inject into a module a region registry that implements IRegionViewRegistry interface. The module, in its Initialize method, can then call RegisterViewWithRegion method of the registry to register its regions.
Regions
Regions, once registered, are managed by the RegionManager. The shell can then load regions either through the RegionManager.RegionName attached property or dynamically through code.
When a view is created by the region manager, the DI container can inject view model and other services into the view. The view then has a reference to the view model through which it can interact with backend services.
Service locator
Although it is possible to inject services into dependent classes through a DI container, an alternative way is to use the ServiceLocator to retrieve a service on demard. Prism supplies a service locator implementation and it is possible to get an instance of the service by calling:
ServiceLocator.Current.GetInstance<IServiceType>()
Event aggregator
Prism supplies an IEventAggregator interface and implementation that can be injected into any class that needs to communicate with each other in a loosely-coupled fashion. The event aggregator uses a publisher/subscriber model.
A class can publishes an event by calling eventAggregator.GetEvent<EventType>().Publish(parameter) to raise an event.
Other classes can subscribe the event by calling eventAggregator.GetEvent<EventType>().Subscribe(EventHandler, other options).
Getting started
The easiest way to get started with Prism is to go through the Prism Hands-On labs and look at the Hello World QuickStart. The Hello World QuickStart shows how bootstrapper, modules and region works.
Next, I would recommend you to look at the Stock Trader Reference Implementation. It is a more in depth example that resemble we want to set up an application.
Several other QuickStarts cover individual Prism services. Some scenarios, such as dynamic module discovery, are more advanced.
Apart from the official prism document, you can get an overview by reading Glen Block’s MSDN Magazine article.
I have found the best free training material is from the Boise Code Camp.
To be effective with Prism, it is important to understands key concepts of WPF well first, such as the DependencyProperty system, data binding, resource, theme and ICommand. It is also important to know your DI container of choice well. I will try to explorer these subjects in depth in the future.
Testimony
Recently, I worked on a desktop WPF application using Prism. I had a wonderful experience with Prism. The Prism is flexible enough even in the presence of third party controls such as Telerik WPF controls. We have never encountered any significant obstacle.