¿Qué son los eventos y comandos enrutados en WPF?

What are routed events and commands in WPF?

Even simple things like Microsoft® .NET Framework properties and events have new counterparts in WPF with added capabilities and associated complexity—specifically dependency properties and routed events. Then there is all the brand new stuff, such as animations, styling, control templates, and routed commands. There is a lot to learn.

Commands are very powerful tool in WPF. Moreover when it comes the question of MVVM pattern in WPF commands become great tool to execute an action defined in the ViewModel and triggered from the control in the view.

When the MouseLeftButtonDown event happens inside the Image element, a PreviewMouseLeft­ButtonDown event starts at the root and tunnels down to the Image. If no handlers set the Handled flag to true for the preview event, the MouseLeftButtonDown event then starts bubbling up from the Image element until it gets to the Button.

The simplest way to create a control in WPF is to derive from UserControl. When you build a control that inherits from UserControl, you add existing components to the UserControl, name the components, and reference event handlers in Extensible Application Markup Language (XAML).

A CommandBinding associates a command with the event handlers that implement the command. The CommandBinding class contains a Command property, and PreviewExecuted, Executed, PreviewCanExecute, and CanExecute events. Command is the command that the CommandBinding is being associated with.

Routed events support a RoutingStrategy of Bubble, Tunnel, or Direct. Bubble is the most common and means that an event will bubble (propagate) up the visual tree from the source element until either it has been handled or it reaches the root element.

Typically, a command source will listen to the CanExecuteChanged event. This event informs the command source that the ability of the command to execute on the current command target may have changed. The command source can query the current status of the RoutedCommand by using the CanExecute method.

ICommandSource exposes three properties: Command, CommandTarget, and CommandParameter: 1 Command is the command to execute when the command source is invoked. 2 CommandTarget is the object on which to execute the command. 3 CommandParameter is a user-defined data type used to pass information to the handlers implementing the command.

It is worth noting that in WPF the CommandTarget property on ICommandSource is only applicable when the ICommand is a RoutedCommand. If the CommandTarget is set on an ICommandSource and the corresponding command is not a RoutedCommand, the command target is ignored.