The Prism Library provides the following communication approaches:
Solution commanding.
Use when there is an expectation of immediate action from the user interaction.
Event aggregation.
For communication across view models, presenters, or controllers when there is
not a direct action-reaction expectation.
Region context.
Use this to provide contextual information between the host and views in the
host's region. This approach is somewhat similar to the DataContext, but it does not
rely on it.
Shared services.
Callers can call a method on the service which raises an event to the receiver
of the message. Use this if none of the preceding is applicable.
In short
Composite command- communicate among the views in module and shell project
Delegate command-communicate among the views in the same module
Event aggregation-communicate the views in different modules.
Region context- communicate among the views in the same region
Shared services- Use this if none of the preceding is applicable.
Event Aggregation
|
Composite Command
|
You publish an event -> all subscribers receive notice and execute
code in response
|
You execute a composite command -> all registered commands get
executed and with it their attached code
|
Asynchronous
|
Synchronous
|
If you need the sending of a message to be initiated by a user gesture
such as a button click, Commands may be a good choice. If you need the
sending of a message to be initiated by business logic code, such as in a
controller or presenter, you should consider using the EventAggregator.
When to use:
There are two primary differences
between these two.
- CanExecute for Commands. A Command can say whether or not it is valid for
execution by calling Command.RaiseCanExecuteChanged() and having its
CanExecute delegate return false. If you consider the case of a "Save
All" CompositeCommand compositing several "Save" commands,
but one of the commands saying that it can't execute, the Save All
button will automatically disable (nice!).
- EventAggregator is a Messaging pattern and Commands are
a Commanding pattern.
Although CompositeCommands are not explicitly a UI pattern, it is
implicitly so (generally they are hooked up to an input action, like a
Button click). EventAggregator is not this way - any part of the
application effectively raises an EventAggregator event: background
processes, ViewModels, etc. It is a brokered avenue for messaging across
your application with support for things like filtering, background thread
execution, etc.
It's more difficult to say when to use each, but generally if its user interaction that raises the event use
a command, for anything else use EventAggregator.
Solution Commanding
if you need to respond to a user gesture, such as clicking
on a command invoker (for example, a button or menu item), and if you want the invoker to be enabled based on business logic,
use commanding.
DelegateCommand,
which allows you to call a delegate method when the command is executed.
The DelegateCommand will be used
just to connect the View with its Model.
CompositeCommand,
which allows you to combine multiple commands.
How does this help you with cross
module communication?
Applications based on the Prism Library may have global
CompositeCommands that are
defined in the shell that have meaning across modules, such as
Save,
Save All, and
Cancel. Modules can then
register their local commands with these global commands and participate in
their execution.
example code:
publish:
MobileNumber = SearchTextBox.Text;
ServiceLocator.Current.GetInstance<IEventAggregator>().GetEvent<SearchCustomerEvent>().Publish(MobileNumber);
subscribe:
private void Window_Loaded(object sender, RoutedEventArgs e)
{
ServiceLocator.Current.GetInstance<IEventAggregator>().GetEvent<SearchCustomerEvent>().Subscribe(this.Display, true);
}
private void Display(object val)
{
MessageBox.Show("Mobile number is" + val);
}
Msdn:
FAQ: Questions and answers:
Event Aggregator:
To understand event
aggregator:
Simple example using event
aggregator
Sample application using
event aggregator
Region Context:
Shared Services:
