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Commanding Overview

  • Articolo

Commanding is an input mechanism in Windows Presentation Foundation (WPF) which provides input handling at a more semantic level than device input. Examples of commands are the Copy, Cut, Paste operations found on many applications.

This overview defines what commands are in WPF, which classes are part of the commanding model, and how to use and create commands in your applications.

The commanding system in WPF is based around the RoutedCommand and the RoutedEvent. A command does not have to be a RoutedCommand and can simply implement the ICommand, which will be discussed, but the bulk of this overview will focus on the RoutedCommand model.

This topic contains the following sections.

  • What Are Commands?
  • Simple Command Example in WPF
  • Four Main Concepts in WPF Commanding
  • Command Library
  • Creating Custom Commands
  • Related Topics

What Are Commands?

What makes commands different form a simple event handler attached to a button or a timer is that commands separate the semantics of an action from its logic. This allows for multiple and disparate sources to invoke the same command logic, and allows the command logic to be customized for different targets.

Examples of commands are the editing operations Copy, Cut, and Paste which are found on many applications. The semantics of a command are consistent across applications and classes, but the logic of the action is specific to the particular object acted upon. The key combination Ctrl-C invokes the Cut command in text classes, image classes, and web browsers, but the actual logic for performing the Cut operation is defined by the object or the application on which the cut is occurring and not on the source that invoked the command. A text object may cut the selected text into the clipboard, while an image object may cut the selected image, but the same command source, a KeyGesture or a ToolBar Button, can be used to invoke the command on both classes.

Simple Command Example in WPF

The simplest way to use a command in WPF is to use a predefined RoutedCommand from one of the command library classes; use a control that has native support for handling the command; and use a control that has native support for invoking a command. The Paste command is one of the predefined commands in the ApplicationCommands class. The TextBox control has built in logic for handling the Paste command. And the MenuItem class has native support for invoking commands.

The following example shows how to set up a MenuItem so that when it is clicked it will invoke the Paste command on a TextBox, assuming the TextBox has keyboard focus.

<StackPanel>
  <Menu>
    <MenuItem Command="ApplicationCommands.Paste" />
  </Menu>
  <TextBox />
</StackPanel>

// Creating the UI objects
StackPanel mainStackPanel = new StackPanel();
TextBox pasteTextBox = new TextBox();
Menu stackPanelMenu = new Menu();
MenuItem pasteMenuItem = new MenuItem();
           
// Adding objects to the panel and the menu
stackPanelMenu.Items.Add(pasteMenuItem);
mainStackPanel.Children.Add(stackPanelMenu);
mainStackPanel.Children.Add(pasteTextBox);

// Setting the command to the Paste command
pasteMenuItem.Command = ApplicationCommands.Paste;

Four Main Concepts in WPF Commanding

The routed command model in WPF can be broken up into four main concepts: the command, the command source, the command target, and the command binding. The command is the action to be executed. The command source is the object which invokes the command. The command target is the object that the command is being executed on. And finally, the command binding is the object which maps the command logic to the command.

In the previous example, the Paste command is the command, the MenuItem is the command source, the TextBox is the command target, and the command binding is supplied by the TextBox control. It is worth noting that it is not always the case that the CommandBinding is supplied by the control. Quite often the CommandBinding must be created by the application developer and the CommandBinding may be attached to an ancestor of the command target.

Commands

Commands in WPF are created by implementing the ICommand interface. ICommand exposes two methods and an event: Execute, CanExecute, and CanExecuteChanged. Execute performs the action of the command, CanExecute determines whether the command can execute on the current command target, and CanExecuteChanged signifies that the ability of the command to execute may have changed. The WPF implementation of ICommand is the RoutedCommand class and is the focus of this overview.

The main sources of input in WPF are the mouse, the keyboard, ink, and routed commands. All of these types of input use a RoutedEvent to notify objects that an input event has occurred. A RoutedCommand is no different. The Execute and CanExecute methods of a RoutedCommand do not contain the application logic for the command, but rather they raise routed events that tunnel and bubble through the element tree until they encounter an object with a CommandBinding. The CommandBinding contains the handlers for these events and it is the handlers that perform the command. For more information on event routing in WPF, see the Routed Events Overview.

The Execute method on a RoutedCommand raises the PreviewExecuted and the Executed events on the command target. The CanExecute method on a RoutedCommand raises the CanExecute and PreviewCanExecute events on the command target. These events tunnel and bubble through the element tree until they encounter an object which has a CommandBinding for that particular command.

WPF supplies a set of common routed commands spread across several classes: MediaCommands, ApplicationCommands, NavigationCommands, ComponentCommands, and EditingCommands. These classes consist only of the RoutedCommand objects and not the implementation logic of the command. The implementation logic is the responsibility of the object on which the command is being executed on.

Command Sources

A command source is the object which invokes the command. Examples of command sources are MenuItem, Button, and KeyGesture.

Command sources in WPF normally implement the ICommandSource interface.

ICommandSource exposes three properties: Command, CommandTarget, and CommandParameter. The Command is the command to execute when the command source is invoked. The CommandTarget is the object on which to execute 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. If the CommandTarget is not set, the element with keyboard focus will be the command target. CommandParameter is a user defined data type used to pass information to the handlers implementing the command.

The WPF classes that implement ICommandSource are ButtonBase, MenuItem, Hyperlink, and InputBinding. ButtonBase, MenuItem, and Hyperlink invoke a command when they are clicked, and an InputBinding invokes a command when the InputGesture associated with it is performed.

The following example shows how to use a MenuItem in a ContextMenu as a command source for the Properties command.

<StackPanel>
  <StackPanel.ContextMenu>
    <ContextMenu>
      <MenuItem Command="ApplicationCommands.Properties" />
    </ContextMenu>
  </StackPanel.ContextMenu>
</StackPanel>

StackPanel cmdSourcePanel = new StackPanel();
ContextMenu cmdSourceContextMenu = new ContextMenu();
MenuItem cmdSourceMenuItem = new MenuItem();

// Add ContextMenu to the StackPanel.
cmdSourcePanel.ContextMenu = cmdSourceContextMenu;
cmdSourcePanel.ContextMenu.Items.Add(cmdSourceMenuItem);

// Associate Command with MenuItem.
cmdSourceMenuItem.Command = ApplicationCommands.Properties;

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. The command source can then disable itself if the command cannot execute. An example of this is a MenuItem graying itself out when a command cannot execute.

An InputGesture can be used as a command source. Two types of input gestures in WPF are the KeyGesture and MouseGesture. You can think of a KeyGesture as a keyboard shortcut, such as Ctrl-C. A KeyGesture is comprised of a Key and a set of ModifierKeys. A MouseGesture is comprised of a MouseAction and an optional set of ModifierKeys.

In order for an InputGesture to act as a command source, it must be associated with a command. There are a few ways to accomplish this. One way is to use an InputBinding.

The following example shows how to create a KeyBinding between a KeyGesture and a RoutedCommand.

<Window.InputBindings>
  <KeyBinding Key="B"
              Modifiers="Control" 
              Command="ApplicationCommands.Open" />
</Window.InputBindings>

KeyGesture OpenKeyGesture = new KeyGesture(
    Key.B,
    ModifierKeys.Control);

KeyBinding OpenCmdKeybinding = new KeyBinding(
    ApplicationCommands.Open,
    OpenKeyGesture);

this.InputBindings.Add(OpenCmdKeybinding);

Another way to associate an InputGesture to a RoutedCommand is to add the InputGesture to the InputGestureCollection on the RoutedCommand.

The following example shows how to add a KeyGesture to the InputGestureCollection of a RoutedCommand.

KeyGesture OpenCmdKeyGesture = new KeyGesture(
    Key.B,
    ModifierKeys.Control);

ApplicationCommands.Open.InputGestures.Add(OpenCmdKeyGesture);

CommandBinding

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. The event handlers which are attached to the PreviewExecuted and Executed events implement the command logic. The event handlers attached to the PreviewCanExecute and CanExecute events determine if the command can execute on the current command target.

The following example shows how to create a CommandBinding on the root Window of an application. The CommandBinding associates the Open command with Executed and CanExecute handlers.

<Window.CommandBindings>
  <CommandBinding Command="ApplicationCommands.Open"
                  Executed="OpenCmdExecuted"
                  CanExecute="OpenCmdCanExecute"/>
</Window.CommandBindings>

// Creating CommandBinding and attaching an Executed and CanExecute handler
CommandBinding OpenCmdBinding = new CommandBinding(
    ApplicationCommands.Open,
    OpenCmdExecuted,
    OpenCmdCanExecute);

this.CommandBindings.Add(OpenCmdBinding);

Next, the ExecutedRoutedEventHandler and a CanExecuteRoutedEventHandler are created. The ExecutedRoutedEventHandler opens a MessageBox that displays a string saying the command has been executed. The CanExecuteRoutedEventHandler sets the CanExecute property to true.

void OpenCmdExecuted(object target, ExecutedRoutedEventArgs e)
{
    MessageBox.Show("The command has been invoked.");
}

Sub OpenCmdExecuted(ByVal sender As Object, ByVal e As ExecutedRoutedEventArgs)
    MessageBox.Show("The command has been invoked.")
End Sub

void OpenCmdCanExecute(object sender, CanExecuteRoutedEventArgs e)
{
    e.CanExecute = true;
}

Sub OpenCmdCanExecute(ByVal sender As Object, ByVal e As CanExecuteRoutedEventArgs)
    e.CanExecute = True
End Sub

A CommandBinding is attached to a specific object, such as the root Window of the application or a control. The object that the CommandBinding is attached to defines the scope of the binding. For example, a CommandBinding attached to an ancestor of the command target can be reached by the Executed event, but a CommandBinding attached to a descendant of the command target cannot be reached. This is a direct consequence of the way a RoutedEvent tunnels and bubbles from the object that raises the event.

In some situations the CommandBinding is attached to the command target itself, such as with the TextBox class and the Cut, Copy, and Paste commands. Quite often though, it is more convenient to attach the CommandBinding to an ancestor of the command target, such as the main Window or the Application object, especially if the same CommandBinding can be used for multiple command targets. These are design decision you will want to consider when you are created your commanding infrastructure.

Command Target

The command target is the element on which the command is executed. With regards to a RoutedCommand, the command target is the element at which routing of the Executed and CanExecute starts. As noted previously, 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.

The command source can explicitly set the command target. If the command target is not defined, the element with keyboard focus will be used as the command target. One of the benefits of using the element with keyboard focus as the command target is that it allows the application developer to use the same command source to invoke a command on multiple targets without having to keep track of the command target. For example, if a MenuItem invokes the paste command in an application that has a TextBox control and a PasswordBox control, the target can be either the TextBox or PasswordBox depending on which control has keyboard focus.

The following example shows how to explicitly set the command target in markup and in code behind.

<StackPanel>
  <Menu>
    <MenuItem Command="ApplicationCommands.Paste"
              CommandTarget="{Binding ElementName=mainTextBox}" />
  </Menu>
  <TextBox Name="mainTextBox"/>
</StackPanel>

// Creating the UI objects
StackPanel mainStackPanel = new StackPanel();
TextBox pasteTextBox = new TextBox();
Menu stackPanelMenu = new Menu();
MenuItem pasteMenuItem = new MenuItem();
           
// Adding objects to the panel and the menu
stackPanelMenu.Items.Add(pasteMenuItem);
mainStackPanel.Children.Add(stackPanelMenu);
mainStackPanel.Children.Add(pasteTextBox);

// Setting the command to the Paste command
pasteMenuItem.Command = ApplicationCommands.Paste;

The CommandManager

The CommandManager serves a number of command related functions. It provides a set of static methods for adding and removing PreviewExecuted, Executed, PreviewCanExecute, and CanExecute event handlers to and from a specific element. It provides a means to register CommandBinding and InputBinding objects onto a specific class. The CommandManager also provides a means, through the RequerySuggested event, to notify a command when it should raise the CanExecuteChanged event.

The InvalidateRequerySuggested method forces the CommandManager to raise the RequerySuggested event. This is useful for conditions that should disable/enable a command but are not conditions that the CommandManager is aware of. For an example of calling InvalidateRequerySuggested to force the CommandManager to raise the RequerySuggested event, see the Disable Command Source Via Dispatcher Timer Sample sample.

Command Library

WPF provides a set of predefined commands. The command library consists of the following classes: ApplicationCommands, NavigationCommands, MediaCommands, EditingCommands, and the ComponentCommands. These classes provide commands such as Cut, BrowseBack and BrowseForward, Play, Stop, and Pause.

Many of these commands include a set of default input bindings. For example, if you specify that your application handles the copy command, you automatically get the keyboard binding "CTRL +C" You also get bindings for other input devices, such as Tablet PC pen gestures and speech information.

Creating Custom Commands

If the commands in the command library classes do not meet your needs, then you can create your own commands. There are two ways to create a custom command. The first is to start from the ground up and implement the ICommand interface. The other way, and the more common approach, is to create a RoutedCommand or a RoutedUICommand.

For an example of creating a custom RoutedCommand, see the Create a Custom RoutedCommand Sample.

See Also

Tasks

How to: Implement ICommandSource
How to: Add a Command to a MenuItem

Reference

RoutedCommand
CommandBinding
InputBinding
CommandManager

Concepts

Input Overview
Routed Events Overview

Other Resources

Create a Custom RoutedCommand Sample
Disable Command Source Via Dispatcher Timer Sample