Commanding Overview

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, and 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.

This topic contains the following sections:

Commands have several purposes. The first purpose is to separate the semantics and the object that invokes a command from the logic that executes the command. This allows for multiple and disparate sources to invoke the same command logic, and it allows the command logic to be customized for different targets. For example, the editing operations Copy, Cut, and Paste, which are found in many applications, can be invoked by using different user actions if they are implemented by using commands. An application might allow a user to cut selected objects or text by either clicking a button, choosing an item in a menu, or using a key combination, such as CTRL+X. By using commands, you can bind each type of user action to the same logic.

Another purpose of commands is to indicate whether an action is available. To continue the example of cutting an object or text, the action only makes sense when something is selected. If a user tries to cut an object or text without having anything selected, nothing would happen. To indicate this to the user, many applications disable buttons and menu items so that the user knows whether it is possible to perform an action. A command can indicate whether an action is possible by implementing the CanExecute method. A button can subscribe to the CanExecuteChanged event and be disabled if CanExecute returns false or be enabled if CanExecute returns true.

The semantics of a command can be consistent across applications and classes, but the logic of the action is specific to the particular object acted upon. The key combination CTRL+X 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 application that performs the cut. A RoutedCommand enables clients to implement the logic. A text object may cut the selected text into the clipboard, while an image object may cut the selected image. When an application handles the Executed event, it has access to the target of the command and can take appropriate action depending on the target's type.

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;

// Setting the command target to the TextBox
pasteMenuItem.CommandTarget = pasteTextBox;


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.

  • 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 that is the command target class. Quite often the CommandBinding must be created by the application developer, or the CommandBinding might be attached to an ancestor of the command target.

Commands

Commands in WPF are created by implementing the ICommand interface. ICommand exposes two methods, Execute, and CanExecute, and an event, CanExecuteChanged. Execute performs the actions that are associated with the command. CanExecute determines whether the command can execute on the current command target. CanExecuteChanged is raised if the command manager that centralizes the commanding operations detects a change in the command source that might invalidate a command that has been raised but not yet executed by the command binding. 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. The more device-oriented inputs use a RoutedEvent to notify objects in an application page 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 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 generally implement the ICommandSource interface.

ICommandSource exposes three properties: Command, CommandTarget, and CommandParameter:

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)
{
    String command, targetobj;
    command = ((RoutedCommand)e.Command).Name;
    targetobj = ((FrameworkElement)target).Name;
    MessageBox.Show("The " + command +  " command has been invoked on target object " + targetobj);
}



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


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 decisions you will want to consider when you are creating 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;

// Setting the command target to the TextBox
pasteMenuItem.CommandTarget = pasteTextBox;


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.

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.

When you reference commands in the various command libraries using XAML, you can usually omit the class name of the library class that exposes the static command property. Generally, the command names are unambiguous as strings, and the owning types exist to provide a logical grouping of commands but are not necessary for disambiguation. For instance, you can specify Command="Cut" rather than the more verbose Command="ApplicationCommands.Cut". This is a convenience mechanism that is built in to the WPF XAML processor for commands (more precisely, it is a type converter behavior of ICommand, which the WPF XAML processor references at load time).

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 Create a Custom RoutedCommand Sample.

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